2010 Ifa Sulphur Agriculture

8/8/2019 2010 Ifa Sulphur Agriculture

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Sulphur and

Sustainable AgricultureA.R. Till

International Fertilizer Industry AssociationParis, May 2010


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The designation employed and the presentation of material in thisinformation product do not imply the expression of any opinionwhatsoever on the part of the International Fertilizer Industry

Association. This includes matters pertaining to the legal status of any country, territory, city or area or its authorities, or concerningthe delimitation of its frontiers or boundaries.

28, rue Marbeuf,

75008 Paris, FranceTel: +33 1 53 93 05 00Fax: +33 1 53 93 05 45/ [email protected]

Sulphur and Sustainable AgricultureBy A.R. TillFirst edition, IFA, Paris, France, May 2010Copyright 2010 IFA. All rights reservedISBN 978-2-9523139-6-4

The publication can be downloaded from IFAs web site.To obtain paper copies, contact IFA.

Printed in FranceLayout: Claudine Aholou-Putz, IFAGraphics: Hlne Ginet, IFA


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3

About the book and the author 5

Acknowledgements 6

Symbols, acronyms and abbreviations 7

1. Sulphur in agriculture: setting the scene 92. Introduction 12

3. The global sulphur cycle 13

4. Importance of sulphur in agriculture 164.1. Boundary conditions 164.2. Farming systems: sulphur cycling and balance 16

4.3. Forms of sulphur 194.4. Management of inputs and losses 204.5. Nutrient balances and sustainability 21

5. Soil sulphur 235.1. General description 235.2. Sulphur fractions 235.3. The carbon cycle and balanced nutrition 265.4. Sulphur mineralization and xation 27

6. Sulphur in plants 296.1. Needs and de ciencies 296.2. Uptake processes and metabolism 306.3. Plant sulphur and product quality 32

7. Sulphur de ciency detection and correction 347.1. Plant testing 347.2. Soil testing 35

Contents


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4 Sulphur and sustainable agriculture

8. Sulphur in animal production 398.1. Sulphur metabolism in ruminants 398.2. Body sulphur content and turnover in sheep 418.3. Wool production 428.4. Sulphur metabolism in monogastrics 428.5. Animal production systems 438.6. Fertilizers and ruminant diet quality 438.7. Protected protein in ruminant diets 448.8. Sulphur in excreta 45

9. Sulphur interactions and processes 469.1. General 469.2. Glucosinolates and cyanogenic glycosides 469.3. Micronutrient interactions in ruminants and man 47

10. Sulphur sources 4810.1. Application 4810.2. Sulphur fertilizers 48

10.3. Predicting the release of sulphate from fertilizers 5010.4. Effect of placement and form on plant uptake and residual value 5110.5. Organic sulphur sources 53

11. Concluding remarks 56

References and further reading 59References 59

Further reading 62

Appendix 2. Advantages and disadvantages of various sulphur-containing fertilizers 65

Appendix 3. Plates 68


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5

About the book and the author

Tis publication has brought together the many aspects o sulphur in agriculturalsystems. It is important to do this at this time as intensifcation o ood, fbre and animalproduction is escalating to eed the ever increasing world population. Te move romsulphur-containing single superphosphate and ammonium sulphate to high-analysis

ertilizers such as diammonium phosphate (DAP) and monoammonium phosphate(MAP), which contain little sulphur (S), has lead to increasing defciencies o S whichmust be overcome.

A.R. (Ray) ill is a retired agricultural scientist who holds a M.Sc. rom the University o Melbourne and a Ph.D. rom the University o New England (UNE). He commenced

his career in 1949 as a laboratory assistant in the racer Elements Investigation Unit inCSIR (later CSIRO - Commonwealth Scientifc and Industrial Research Organisation)in Australia, and joined the CSIRO Division o Animal Physiology at Prospect in 1957as an experimental o cer, his main role being the application o radiotracer techniquesto research in sheep and wool production. In 1963, he commenced studies on theapplication o radiotracers in grazing systems and, in 1967, he joined the CSIRO AnimalProduction Division at Armidale in New South Wales (NSW), as head o a researchgroup applying radiotracer techniques to grazing systems, particularly in studying thepool sizes and ow rates o S in soils, plants and animals.

Upon retirement rom CSIRO in 1991, he joined the Department o Agronomy andSoil Science at UNE in Armidale with a large commitment to international projectsthat utilized his considerable prior experience. He was also heavily involved in post-graduate training.

Ray ills name appears on a wide range o publications dealing with many aspectso S, and he has been a speaker at many national and international con erences dealingwith S.


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Acknowledgements

Te author wishes to thank the International Fertilizer Industry Association (IFA) andTe Sulphur Institute ( SI) or their initiation, stimulation and continuing supportor the production o this booklet. Tanks also to CSIRO, UNE and the Australian

Centre or International Agriculture Research (ACIAR) or their long term supportor the research which provided a signifcant background or this work. Special thanks

go to my long term colleague and riend Graeme Blair and the keen and challenginginteraction rom our many post-graduate students rom many countries. Also thanksto the experienced sta o IFA, particularly Patrick He er, who trans ormed some o the sections where my views on fnancial management and politics might have been

too pointed and critical. Tanks are also due to John Ryan or his editorial work on themanuscript.


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7

Symbols, acronyms and abbreviations

(as used in this publication)35S Radioactive S isotope where superscript is the mass numberAPS Adenosine-3-phosphosulphateAS Ammonium sulphateA P Adenosine triphosphateC CarbonCH3SH Methyl mercaptanCN- Cyanide

COS Carbonyl sulphideCP Crude proteinCS2 Carbon disulphideCSIRO Commonwealth Scientifc and Industrial Research OrganisationCu CopperDAP Diammonium phosphateDM Dry matterDMS Dimethyl sulphideEB Empty body GSH Glutathione (reduced orm)GSSG Glutathione (oxidized orm)ha HectareHI-S Hydriodic acid reducible S / mainly ester sulphateHS- Te charged part o H 2S taken up rom the rumenH2S Hydrogen sulphideH2SO4 Sulphuric acidIFA International Fertilizer Industry AssociationIFDC International Fertilizer Development CenterK PotassiumKCl Potassium chloridekg KilogramKMnO4 Potassium permanganateK2SO4 Potassium sulphateM MoleMAP Monoammonium phosphateMCP Monocalcium orthophosphatemM MillimoleM MicromoleMo MolybdenumMoO4

2- MolybdateMt Million metric tonne (= g)N Nitrogen


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Na2SO4 Sodium sulphateNH3 Ammonia(NH4)2SO4 Ammonium sulphate (=AS)

NO3-

NitrateOM Organic matterP PhosphorusPAPR Partially acidulated phosphate rock PAPS 5-phospho-adenosine-3-phosphosulphatePR Phosphate rock S SulphurS0 Elemental sulphurS2- Sulphide

Se SeleniumSO2 Sulphur dioxideSO3

2- SulphiteSO4

2- SulphateSO4

2--S Sulphate sulphurSOM Soil organic matterSR Specifc radioactivity SSP Single superphosphatet Metric tonne

Hal -li eg eragram (=Mt)SI Te Sulphur InstituteSP riple superphosphateVA ennessee Valley Authority

yr YearZn ZincZnSO4 Zinc sulphate


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9

1. Sulphur in agriculture: setting the scene

In nature, sulphur (S) occurs as inorganic and organic orms, ranging rom reducedstates such as hydrogen sulphide (H2S) gas through elemental sulphur (S0), to complex

organic compounds and enzymes that contain S in various valency states. ElementalS is widely used in industry, mainly or sulphuric acid (H2SO4) production, with asignifcant proportion o the acid used in various phases o ertilizer production. In2008, total S consumption was 74.1 million metric tonnes (Mt) o which 34.5 Mt wereused to produce ertilizers (IFA, 2009).

Te S-containing amino acids, methionine and cystine (Figure 1), are essential orproduction o proteins; without them and other S-containing materials, there would

be no li e as we know it. Higher animals cannot produce the essential amino acidsand depend on predation, plants and/or the micro ora and micro auna or theirsupply. Ruminants overcome this problem by having a symbiotic relationship withmicroorganisms in their gut.

A simplifed ow diagram illustrates the use o S in crop and animal production(Figure 2). Plants require S in inorganic orms, normally as sulphate (SO4

2-) rom thesoil, but can also use sulphur dioxide (SO2) rom the atmosphere. Sulphur is transportedto the leaves, where it is incorporated into essential amino acids, proteins, oils and otherorganic compounds.

Some o the plant products are used directly by man, while others are used by animalsto yield an extended range o products. Most o the S used by animals is excreted asSO4

2-, which is readily available or reuse by plants. Te remaining S, and that romcrop residues, is mainly organic and must be converted to SO42- by the micro ora andmicro auna to complete the cycle.

In some areas, the S input rom the atmosphere can be high enough to supply the Sneeded or growing crops. However, as more and more stringent limits are en orced on

Figure 1. Essential S-containing amino acids. Cystine is two cysteine molecules joinedby a SS bond. There is ready interchange between these two and it is often written ascyst(e)ine.

CYSTEINE HOOCCHCH2SH

NH2

METHIONINE CH3SCH2CH2CHCOOH

NH2

CYSTINE HOOCCHCH2SSCH2CHCOOH

NH2 NH2


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the emission o SO2 and other S-containing compounds by industry, and the changetowards the use o low-S uels progresses, S defciencies will inevitably increase. SomeS-containing ertilizers will be required to replace the S removed in or with the fnalproduct and other losses due to residue management and leaching, i current productionlevels are to be maintained or increased.

Several S-containing ertilizers are commercially available, and there is a potentialto develop more appropriate ones or specifc crops. Fertilizers containing solubleSO4

2- provide S that is readily useable by plants, but the plants have to compete withsoil microbes and loss processes or the pool o available S. echniques such as usingcontrolled-release ertilizers, and banding and split applications can improve the plantsability to compete and reduce leaching losses. Elemental S and organic orms o S in

ertilizers must undergo mineralization to SO42-, as does the crop residues, be ore being

available to plants. Tus, the soil biota play a crucial role in relation to S in agriculture.How a particular arm is managed can have a big e ect on the loss and recycling

o S and, consequently, on the ertilizer S requirements. Under low-intensity grazing,the requirement is 510 kg S/ha/yr, while a high S-demanding crop like rapeseedmight need 5080 kg S/ha/yr, depending on the crops used in the rotation and theirrequirements . Care should be taken to select the appropriate ertilizer and S applicationrate, timing and placement to minimize leaching losses and/or adverse interactionswith other nutrients.

Figure 2. Basic diagram of S use in farm systems (For simplicity, the pathways involvinginput, loss or recycling are not depicted).

Bio-processing / Competition / Losses

FERTILIZERSPlant available S(various release rates)

Organic and otherunavailable S forms

Plant available SLabile organic S

Resistant organic SUnavailable "inert" S

SOIL

PLANTS

MONOGASTRICS(including man)

RUMINANTS

Grain / Oil /Fibre / Residues

Meat / Wool / Hides /Excreta / Product / Waste

EXPORTS


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11

Soil and plant testing can be used to estimate the probability o a defciency occurringbut, in specifc locations, their most likely value will be in monitoring changes and thestability o the system. In rotations, especially those involving a pasture phase, there

may be di erent depths and volumes o soil explored by plant roots, and this has a directbearing on the soil testing method and its interpretation in relation to ertilizer use andthe residual value or subsequent crops.

In grazing systems, the interaction between animals and pasture in general and themanagement o the excreta in particular are important. For example, mature wool-growing sheep only retain about 10% o the S that they ingest, i.e. the ate o the 90%that they excrete can greatly in uence the amount o ertilizer S needed.

Likewise, the S in the marketable proportion o the crop is requently less thanhal o that in the harvested plant, and the ate o the residues a er processing may

signifcantly in uence total S losses. In addition, the management o crop rotations,including orage crops, and the interaction between any cropping phases and grazingand/or supplementary eeding o animals has signifcant e ects on the redistribution o S and the orm and timing o application o the ertilizer.

Te calculation o a simple nutrient balance gives a good indication o how muchertilizer is needed or a system somewhere near equilibrium and whether there are

potentially damaging e ects on the environment, but it cannot readily account orchanges in management.


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2. Introduction

Elemental S (brimstone, S0

) is o en ound in the vicinity o volcanoes and hot springs,and so has been known to man or centuries. ogether with other naturally occurringS-containing materials, S0 has been used or medicinal purposes, as a crop protectionproduct and in explosives, among others. Estimates o the amount o S in the Earthscrust range rom about the 10th to 16th most abundant element (95% as the 32S isotope).

Te issue o acid rains has led to many cities and countries introducing increasingly stringent controls on the amounts o SO2 that can be released into the atmosphere but,especially in the cases o poor and developing countries, there is no universally acceptedinternational standard. Te reduced emission has some benefts but it has contributed

to increasing S defciencies in many agricultural crops. I production levels are to besustained or increased, the S defcit must be made up by some orm o ertilizer.With the exception o ruminants, which have a symbiotic relationship with

microorganisms in their gut, no higher animals can metabolize inorganic S to synthesizethe essential S-containing amino acids. Consequently, the interactions between theenvironment, micro ora, micro auna and plants orm the basis o the initial part o the

ood chain or survival o all the higher animals.Excluding cataclysmic events, changes in nature take place slowly and the various

biological entities evolve and/or become extinct as the global and local environmentschange. In the evolution o man, critical changes have been the movement rom livingwithin the confnes o nature as nomadic hunter-gatherers, to establishment o smallcommunities with the development o primitive agriculture, ollowed by technologicaldevelopment, cities, population explosion and the ever increasing demand or ood andconsumer products.

Until recently, when serious problems have emerged, agriculture and industry havepaid scant respect to the global or local environment, and this is particularly true orcontrol o atmospheric emission o S and the use o ertilizers with little regard ortheir S content. However, agriculture is now battling to maintain ood supplies, usingnutrients e ciently and management strategies appropriate to sustainable agriculture.

One undamental requirement or a sustainable agriculture is that there mustbe at least a balance o nutrients between the inputs, removal in products and otherlosses. Any defcit between indigenous inputs and outputs must be corrected by changes in management and/or some orm o ertilizer. Animal manures and wastematerials contain various orms o S, and their value as ertilizers can be in uenced by management practices. Te wide range o S-containing compounds that exist in nature,and are involved in various biological processes, emphasize the potential or developingnew and more e cient S ertilizers.

Tere are ten isotopes o S, our o which are stable and occur naturally as32S, 33S, 34S and 36S, accounting or 95.02, 0.75, 4.21 and 0.014% o the total, respectively. Terest are radioactive orms with hal -lives ranging rom approximately 0.2 seconds to 3months. In many situations, isotopic tracers provide vital in ormation on the pathwaysand dynamics o S utilization, which cannot be studied by other methods. For suchstudies, the only isotopes that are o signifcant value are the stable isotope34S, and a lowenergy emitter 35S that has a hal -li e o 87.4 days.


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3. The global sulphur cycle

In a survey o 11 world regions or global emissions, SO2 levels were similar rom 1980to the present time, but showed marked di erences at the regional level. In 1980, theNorth Atlantic region contributed 60% o global emissions but, by 1995, this had allento less than 40%, and it will probably continue to decrease in the uture. Te total globalSO2 emissions in 1990 were estimated at about 72 million tonnes (Mt). Actual emissionsmay be higher, with 56% rom coal, 24% rom oil, 15% rom industrial processes, and3% rom biomass burning (Smithet al., 2001). China is believed to be the largestcontributor to global SO2 emissions.

Biomass burning is only a small part o the global emission but, together with losses

rom some o the industrial processes, it should be studied with a view to recyclingmore o the S directly back into agriculture.Tese fgures suggest that the overall amounts in the global SO 2

cycle have not changeddramatically as ar as agriculture is concerned, but that the distribution o anthropogenicemissions has changed and, especially in relation to agricultural production, there willhave to be thorough monitoring o all regions to check or changes.

Tere are many global cycle models with varying degrees o refnement, mainly aimed at predicting and/or interpreting the results o mans activities on variouscomponents o the cycle, and the changes that they may produce on the whole o theEarths environment.

Te very simple cycle (Figure 3) and gross uxes ( able 1) show approximateamounts and trans ers o S or some o the components. Tese estimates emphasize

Table 1. Examples of estimated S uxes between the global pools.

Pathway Action Flux1(Mt/yr) Flux2(Mt/yr)

Atmosphere to land Precipitation/deposition 86 49+4

Atmosphere to land Vegetation intake 24 18

Atmosphere to ocean Precipitation/deposition 68 56

Atmosphere to ocean Gas absorption 22 16

Land to atmosphere Volcanoes 10 3

Land to atmosphere Biological decay (H2S) 60 3

Land to atmosphere Pollution (SO2) 60 65

Ocean to atmosphere Sea spray 44 44

Ocean to atmosphere Biological decay (H2S) 26 28

River to ocean Runoff 75Ocean to land Spray via atmosphere 4

1Davey (1980);2Granat et al. (1976). Most of the values are similar and variations depend largelyon estimated S levels and the boundaries used.


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the magnitude o the amounts and ows with respect to the requirement in localagriculture, especially the inputs to and rom the atmosphere and the e ect it has on Savailability to plants.

In terms o agriculture, the most important in uence o the global S cycle is exchangeswith the atmosphere. Te major natural inputs to the atmosphere are the reduced Scompounds: dimethyl sulphide ((CH 3)2S; 75%) and hydrogen sulphide (H2S; 15%).Carbon disulphide, carbonyl sulphide and aerosols, mainly rom the sea as SO42-, makeup practically all the rest, with a very small input o natural SO2 (Noggle et al ., 1986;Ryaboshapko, 1983; rudinger, 1986; Kelly and Smith, 1989).

Figure 3. Simplified global S cycle showing the approximate amounts of S (Mt) and theannual flows (Mt/yr) between basic components. Based on data from Ericsson (1960),Granat et al. (1976), Davey (1980) and Smith et al. (2001).

OVER OCEAN

ATMOSPHERE3. 6

LAND &FRESH WATER

7 x 10 9

OCEAN &SEDIMENTS

4 x 10 9

OVER LAND

Precipitation& deposition

86

Precipitation& deposition

68

Gases

22

95

Mineral useRun o

75

95

Plants600

Plants24

24Gas & particulate

Sea spray4

Volcano &pollution

70

Gases

60

Dead OM5000

24

Sea spray

44

Gases

24

Deposition


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Te total SO 2 input to the atmospheric cycle is about hal o the total S input andis practically all anthropogenic rom combustion o ossil uels and smelting o ores.Te large amount o dimethyl sulphide is ormed and catabolised by many biogenic

processes in soil and water, but the largest contribution to the atmosphere is rom theocean. However, all the reduced S compounds are rapidly oxidized in the atmosphereand the S is recycled as sulphuric and sulphurous acids and sulphate salts ( rudinger,1986).

Relative to air, SO2 is a dense gas and does not mix rapidly. Tus, in combinationwith the non-uni orm distribution o anthropogenic emission sites, the direction o theprevailing winds and rain all, the return o S is very uneven. Tese actors result in Sinput levels ranging rom toxic acidifcation, through adequate supply, to virtually noin uence depending on the distance rom the emission source.

Te total estimated input to the atmosphere in 1976 was about 200 Mt/yr, comprising70 Mt/yr rom the ocean, 70 Mt/yr naturally rom the land, and about 60 Mt/yr romhuman activities ( able 1). Over the years, the anthropogenic input to the atmospherehas been greatly reduced in some regions, allowing or plants to recover in some areaswhere the deposition levels were leading to toxicity problems. In many other areas,reduced opportunities or plants and soil to derive su cient S rom the atmosphere,together with the use o S- ree high-analysis ertilizers, resulted in more widespread Sdefciencies


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4. Importance of sulphur in agriculture

4.1. Boundary conditions

Currently, the major consideration in agriculture is the overall proftability o producingsu cient crop and animal products to meet market demand. Tis is rarely the sameas global needs; management o surpluses, defcits and impacts on the environmentare the result o the current political and economic pressures on both producers andconsumers. Meeting demand generally involves highly improved systems using

management practices designed to give maximum economic return.Within these constraints, the system is largely regarded as a simple linear progressionrom ertilizer to crop, and then either to crop products processed directly or

consumption by man and/or ed to animals that are in turn used by man. All too o en,the ocus in these systems is the dry matter yield o plant products without due regard tothe appropriateness or quality o the product or the end user, or the cost to environmentstability.

Compared to the total amounts o S in the global cycle, the amounts in agriculture areminuscule, e.g. the S in plant and animal pools is a tiny raction o the global S pool andit is o en assumed that agriculture, unlike anthropogenic SO

2, would have no impact

on the global cycle. However, the rapid rates o turnover o some o the components inagriculture can cause signifcant changes to the local inputs and losses.

For example, 7x1015 metric tonnes (t) o metamorphic and igneous rocks with a hal -li e o 50 million years would release about 10

7 t S/yr, while a similar amount wouldbe recycled rom 108 t o plant material i it had a hal -li e o our years. I the plantmaterial occupied 50% o the land area, this release would be about 13 kg S/ha/yr, anamount readily exceeded by S applications and recycling in agriculture. Consideringthat many o the soil organic S materials have a hal -li e very much less than our years,it is important that S in agriculture, as well as that used in industry, is recycled e ciently and that signifcant amounts are not allowed to enter the atmosphere and waterwaysand alter the natural cycle.

4.2. Farming systems: sulphur cycling and balance

As with the global cycle, the amounts and orms o S in the component parts o a armingsystem vary enormously and do not necessarily give any indication o their relativeimportance. A simple representation o the relative amounts o S in some componentso a sheep grazing system is given in Figure 4.

In this example, only the top 10 cm o the soil are considered, and the amounts areor a wool-producing enterprise stocked at 10 mature sheep per hectare. Te important

soil bio-processing micro ora and micro auna are so numerous and varied that they


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cannot be readily grouped into a single pool and are assumed to be spread throughoutthe appropriate components. Individual animals only contain a very small proportiono the total S in the system, but their management can exert a strong in uence on theamounts in the other components and involved in the di erent processes.

Te sheep consume between 50 and 90% o the above-ground plant production andonly retain 10% o the ingested S. Te rest o the S is excreted and its managementstrongly in uences how much ertilizer is needed to maintain the level o production.In this situation, the available pool is too small to provide the amount o S needed orthe total plant uptake over the year, and the only way the system can be maintained witha low S input is by mineralization and recycling rom the residues and organic pools.Some o the numerous interacting pathways that are in uenced by management o suchan enterprise are shown in Figure 5.

Apart rom animal health control and the seasonal changes in pasture production/availability, the major di erences between pure grazing and cropping enterprises are thetimescale required to obtain a product and the e ect o crop residue management onthe amounts o S in some components and the overall S requirement.

Crops have higher short-term demands or S, with the plant pool going rom zero tomaximum in a ew months. otal plant uptake is determined by crop type, nutritionalS status, and crop yield. Reported values range rom a ew kilograms to almost 100

Figure 4. Relative sizes of S pools in a grazed pasture. The boxes show the approximatesizes of the S pools (kg/ha), their positions relative to the soil surface at the time of fertilizer application and, for some, the annual flows (kg/ha/yr). Most fertilizer S enters

the cycle via the soil-available pool, but some of it and other above-ground S is taken updirectly by the plant before it can enter the soil pools.

Soilsurface

Plant Stops 15 kg/haFertilizer S12 kg/ha/yr

Sheep S1kg/ha

20 kg/ha/yr

Very slowly cycling S110 kg/ha

Organic S140 kg/ha

Dung, urine, litterand invertebate S

8, 11, 20 and 50 kg/ha/yrRoots 6.4 kg/haTotal 50 kg/ha/yrAvailable S 10 kg/ha

25 kg/ha/yr


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kg S/ha but, in temperate regions, it is usually less than 50 kg S/ha (e.g. Duke andReisenauer, 1986; abatabai, 1984). Especially or crops with a high S demand, theuptake requently exceeds the amount o available SO4

2- in the soil. Consequently, i therate o mineralization is too slow, ertilizers must be used.

Te e cient use o any ertilizer requires that it must be able to supplement thecurrent soil-available S to a level that will be able to match the changing demand o theparticular crop as it develops and matures. As well as the wide variation in the total Suptake by di erent crops, the proportion o the total plant S that is in the fnal productalso varies. Management o the residual/waste S thus has a signifcant e ect on the need

or ertilizer.In the section on boundary conditions, it was shown that the turnover rates ( )

o the system components can be more important than the actual amount in thatcomponent and, again or the grazed pasture, the management o the amount o S thatgoes through the animal is ar more important in terms o production than the actualamount in the animal.

o really manage any o these complex multi-component agricultural systems, it isnot only essential to know how much S is there but also the rates with which the di erenttrans ormations and trans ers proceed along the multiplicity o interacting pathways.

Figure 5. A simple representation of the S cycle and the input and loss boundaries. Theboxes represent pools of S. The arrows show flows of material in cropping or animalproduction systems.The S cycle in that figure is essentially that used in the radiotracer and simulationstudies described by Till et al . (1970). In real situations, the pools do not have clearlydefined boundaries. The pools mainly contain a loosely grouped mixture of S materialsand the agents for S transformations. A brief listing of the important components andflows follows in section 4.3.

I N P U T S

Fertilizer

Atmosphere

Irrigation

Weathering

Products

Volatilization

Leaching

Immobilization

L O S S E SR E C Y C L I N G

Livestock andsoil biota S

Resistantorganic S

Inorganic S

Available S

Plant S

Labil organicS


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Tis requires an appreciation o the possible recycling pathways or the particularproduction system and identifcation o key processes that limit the availability o S tothe plant and/or lead to the loss o S rom the productive system.

Apart rom ertilizer, the major S inputs are largely controlled by the global cycle,but they can be in uenced to some extent by the local management. For example, any S input rom irrigation water depends on the S concentration in the water, which canbe measured. However, the overall S input as the result o irrigation may be positive ornegative because it also depends on other actors such as when and how much waterwas used, the solubility o the ertilizer, and soil properties such as infltration rate,adsorption and profle depth.

Sulphur losses are partly controlled by the environment and product removal, butthey are signifcantly modifed by other actors, e.g. ertilizer solubility and returns rom

crop residues and animal wastes.

4.3. Forms of sulphur

4.3.1. AvailableTe available S pool, consisting o the soil solution and adsorbed SO4

2-, is probably thesimplest in the cycle. It is usually 3-10% o the total soil S and is depleted by plants andother competing processes. Recharging this pool is mainly by mineralization o variousorganic materials and application o ertilizers. Soluble S ertilizers can increase thepool directly, but SO4

2- is susceptible to leaching. Sulphur as S0 or in organic ertilizers isless susceptible to losses because it must be oxidized, usually by microorganisms, be orethe SO4

2- becomes available to plants.

4.3.2. PlantTe plant pool is essentially a pool o organic S, in particular the S-containing aminoacids and proteins in plants. Plants compete with other processes to take up SO4

2- romthe available pool and translocate it to the leaves, where practically all o it is convertedinto a range o organic materials. Apart rom leaching and erosion, plants provide themajor route or removal o S rom the arm.

4.3.3. Livestock and soil biotaTis is the most complex and di use pool in the cycle. As well as domestic livestock, itincludes an enormous range o micro ora and micro auna, and other consumers thatoccupy various niches and are spread throughout the system. Te range o their specifc

unctions combines to provide all the necessary trans ormations and trans ers thatmaintain the S recycling through the other pools.

4.3.4. Labile organicTis consists o a pool o organic S, plus the labile ractions o the plant and soil biota.In some situations, it may constitute up to hal o the total S pool, and it has a airly rapid turnover (e.g. =35 days). Because o its large size and rapid turnover, the


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management o the labile organic matter pool is critical or the ow o SO42- into the

available pool. In the cropping system, a signifcant portion o the crop residues and soilbiota are included in this pool.

4.3.5. Resistant organicTe resistant organic pool is a mixture o materials derived rom the more resistantcarbon-bonded-S ractions in the residues o various plants and in the soil biota.Depending on the land use, the combined components o this raction is requently larger than the labile pool. It ranges rom about 50 to 90% o the total soil S. Te turnovero some components can be very slow but, under cropping, most ractions will have ahal -li e o 1-3 yr.

4.3.6. InorganicTis pool does not include the available pool, but contains materials rom many sourcesand with a wide range o turnover rates, most o which are very slow. Te sources aresmall amounts o strongly adsorbed S, leached SO4

2- that may accumulate as gypsum atsome depth in the soil profle, reduced S in anaerobic ooded soils, and various mineraldeposits that release S very slowly.

4.4. Management of inputs and losses

E ciency o ertilizer use depends on ertilizer placement, release rates o nutrients,timing o application and interactions between nutrients.

Agricultural products (grain, livestock, etc.) are unavoidable losses o S rom thecycle, while the management o residues removed with product, and those le in thefeld also in uence nutrient losses/requirements. Management o residues le in thefeld in uences water infltration, soil aggregate stability, erosion, and rates and amountso nutrients recycled and/or lost.

Consideration o the preceding fgures and outlines o pools and ows emphasizessome important points: In most circumstances, any S inputs to the plant are via the available SO42- pool. Te plant must compete with other processes or the available S. Very di erent rates o S supply are needed or the cropping and grazing situations. For intensive animal production systems such as eedlots, pigs and poultry, the

primary crop production sources should be geared to quality products having theappropriate balance o protein, energy and nutrients to suit the specifc animal needs.

Te management o residues and waste rom the various enterprises is critical inminimizing ertilizer requirements and reducing nutrient losses to the environment.


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4.5. Nutrient balances and sustainability

I the overall production system is to be sustainable, the minimal requirement is a

balance between the total S inputs and outputs. Whether bringing new land intoproduction or changing cropping systems, the initial problem that the armer aces is toknow i his system is, or will become, S-defcient what measures he must take to reachhis required level o production.

It is di cult to get an early enough warning o crop S defciency to take correctivemeasures based on plant visual symptoms or tissue tests, while problems exist withmany soil S tests or predicting relationships between ertilizer requirements and levelso production.

Te starting point in this assessment is an educated guess based on local experience

and backed up by whatever technical in ormation that can be obtained. Any ertilizertrials carried out on the way to reaching an acceptable level o production are valuablein the fnal estimation o ertilizer requirements to reach a sustainable S balance.

Te inputs rom the atmosphere are virtually beyond the control o the local armerand will come through rain, dry all and gaseous absorption. In the long term, they areunlikely to change signifcantly unless there are big changes in local industry and largescale burning o crop residues, ossil uels and organic waste.

Depending on the crop or arming enterprise, the atmospheric S input may beadequate or virtually insignifcant, but it is important to know how much S is supplied.I crops are irrigated, the potential input rom water can be calculated, but it will notalways be easy to estimate how much o the S is actually available to the crop. Te S indrinking water can also be a valuable input to ruminants.

Te ease o predicting removals with products is subject to seasonal variability andthe processing that separates the disposable product rom the residues, but long-termexperience may help in estimating the magnitude o errors in estimating S loss. Temanagement o residues rom processing o products and any le in the feld may signifcantly reduce S loss. Leaching and sur ace ows can remove signifcant amountso S rom the root zone, and there may be very small losses o volatile S compounds romsome soils and plants. Any discrepancy between the inputs and losses must be made upby inputs rom appropriate ertilizer materials (organic, inorganic or a combination o both).

Assuming that the release rate o SO42- rom the ertilizer is known, the amount o

ertilizer needed is determined by the e ciency with which the plants compete or theavailable SO4

2-, and by any losses o ertilizer S. Te e ciency with which plants competeor nutrients can be modifed by strategic placement o ertilizers and using ertilizer

sources that supply a balanced suite o nutrients at rates that match the demand by theplant. Tis stage requires an intimate local knowledge and/or an understanding o any other actors that in uence the loss, utilization and recycling processes.

I the S balance is negative, the system cannot be sustained, but the converse is notnecessarily true. Even with a positive balance over a short term, the system can still bein a decline i changes in the various processes allow the relative sizes o some pools tochange. For example, a change in management could change the relative amounts o labile and resistant organic matter in residues or soil. A build-up in resistant organic


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matter would send the system into a decline due to a all in the supply o S to theavailable pool, while an increase in labile organic matter could allow production to riseor a reduced ertilizer input.

A major problem in fnding some o the required in ormation is that once S rom aparticular source has entered the system, it becomes mixed with S already in the cycleand it is not possible to know where the resh S goes to or whether any response is duedirectly to the added S or some secondary change.

Various isotopic techniques have been used to study the input, recycling and losso S in sites ranging in size rom small pots to pastures o over one hectare (e.g. Blairand ill, 2003). Usually, in the smaller-scale studies, radioactive35S or stable 34S wereapplied to the system being studied, while, on a larger scale, it is sometimes possibleto exploit the naturally occurring di erences in the ratio o 34S to 32S (34S) in materials

rom di erent sources to estimate trans ers and turnover rate. While it is unlikely atpresent that isotopic studies will be developed on a commercial scale, they are essentialor scientifc validation o key processes and the development o testing methods.It is the unenviable job o the armer and his advisors to fnd answers to these

questions and integrate their e ects with the probability o having a good season andproftable markets. Various computer models ranging rom simple local regression typesto mechanistic ones including economics and risk analysis have been developed to helpanswer some o these questions. However, the wide range in the levels o sophisticationneeded to explain individual processes together with the enormous amount o data thatis still needed to drive the models makes the chances o an all-seeing model that spansall the critical variables very remote. Di erent types o models can help with specifctasks, but there needs to be great care to ensure that the results are realistic and not pre-determined by the model construction.

Te minimum requirements or any particular cropping and/or livestock productionenterprise to be sustainable are: Tere must be energy and nutrient balances between the inputs to, and losses rom,

the system as a whole. Some orm o ertilizer and/or a change in management mustmake up any S defcit.

It must be fnancially viable, i.e. the overall management and productivity must beproftable.

For e cient production o plants and animals, it is necessary to have a balancedsupply o all the essential nutrients or each process, and the quality o the productmust be aimed to suit the end-user.

I an overall S balance is achieved, the productivity o the system is controlled by the relative sizes o the pools in the cycle and the rates o trans er o S along thecompeting pathways between the pools.


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5. Soil sulphur

5.1. General description

Te soil is not just a collection o minerals but it is a living, breathing complex o inorganic and organic materials, plus the myriad o micro ora and micro auna thatcoexist within it. Te proportions in various components and the biological activity are dictated by the environment and agricultural practices. Biological processes areresponsible or most o the mineralization, essentially oxidation o organic ractions to

SO42-

.Te term fxation is used reely or di erent processes depending on the nutrientand the situation. In the case o S, it can be considered as removing S rom the soilavailable pool by physical or chemical processes such as adsorption and precipitation,but also incorporation into a wide range o organic S orms in nature (e.g. soil biota andplants). Te time scale over which the changes take place ranges rom a ew minutes toseveral million years. Biological processes can be very rapid and are mainly limited by temperature and the supply o substrates containing minerals and energy.

In conventional agriculture and natural systems with higher plants and animals,the soil is the primary source and/or intermediate reservoir o the raw materials thatprovide the energy and nutrients or the soil biota and o the available nutrients orplants. Plants take up virtually all their S rom the soil solution as SO4

2-.In all productive soils, the predominant S storage is in organic matter: about 95% in

temperate soils and somewhat lower in weathered tropical soils. In the soil solution,SO4

2-, like nitrate (NO3-), is highly mobile and, as it is only weakly adsorbed onto the

sur ace o soil particles. It is vulnerable to displacement by strongly adsorbed ions suchas phosphate and/or to leaching rom the soil layer explored by plant roots.

Tis process may result in precipitation o SO42- too deep or retrieval by plants and/

or complete loss rom the profle into groundwater and river systems. Te interactionsbetween the environment, microorganisms and plant uptake can cause very largeseasonal uctuations in the extractable SO4

2- levels. Consequently, the rates o thebiological mineralization and fxation processes are major determinants in the supply o SO4

2- to plants.

5.2. Sulphur fractions

Te complexity o trans ormations in the mixture o inorganic and organic componentsin the soil is not clearly understood, and detailed studies o the interactions are very di cult in real agricultural situations. Tere is a good understanding o some inorganiccomponents and processes such as adsorption, di usion and leaching. However, the


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very important, mainly biological, processes responsible or the two-way conversionsbetween SOM and the inorganic ractions are poorly defned.

In attempts to get more orderly studies o the pathways and processes, researchers

have used various approaches to group materials and/or processes into ractions withknown composition or common attributes. Based on simple chemical processes, thereare our most commonly determined ractions: Total Sis determined a er digestion o the soil with a mixture o nitric and perchloric

acid. Available Sis commonly considered as the raction extracted by various soil test

reagents, which remove mainly soluble SO42- plus di erent proportions o other S

ractions (See Figure 9). Results rom the di erent methods are then correlated withyield responses, and they give estimates o ertilizer requirements.

An example o a test that has been increasingly used in Australia is the KCl-40extraction (Blair et al ., 1991), which removes adsorbed SO42- and some labile organic

S. Hydriodic acid-reducible S (HI-S) is roughly 50% o the S in SOM, and is considered

as airly labile as it contains some very labile components.Te analytical method is based on the release o H2S rom certain classes o compounds and is adapted rom Johnson and Nishita (1952). Te organic matterreduced is comprised mainly o the ester sulphates (-C-O-S-) plus smaller amountso sulphamates (-C-N-S) and complexes with ester bonds (-C-S-S-). Depending onthe actual sequence or handling the soil analysis, the HI-S value may have to becorrected or H2S rom the available SO4

2-. Carbon-bonded Sis the rest o the organic S, comprising materials with S covalently

bonded to carbon (C) plus some small peptides and ree and adsorbed S-containingamino acids.Additional ractions that are sometimes used are:

Rainey-Ni reducible Sis a portion o the C-bonded S, probably rom the ree andadsorbed S-containing amino acids, which may be up to 30% o the C-bonded S.Soil solution Sis extracted either directly using porous cups or microtubes in thesoil, or by centri ugation o soil samples, to monitor the concentration o S in thesolution that is available to plants and soil biota.Cycling S pool:I isotopic tracers are being used in a system that has time toapproach equilibrium, it is sometimes possible to calculate the cycling S pool andinput rates or small inputs and very slowly cycling pools. Te S cycling pool hasno clearly defned boundary but shows the amount o S actually taking part in therecycling during the length o the study. Te di erence between the total S and thecycling S shows the amount o intractable S (i.e. S with very long turnover rates). Itmay also be possible to measure the S in the SOM labile C raction.

Alternative ractionation methods have been described (Janzen and Ellert, 1998),such as: Physical ractionation o soil based on particle size, and then chemical determination

o S in the ractions as outlined above (Andersonet al., 1981).


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into SOM and emphasized the role o organic matter as the major storage o S and thesupplier o mineralized S to plants.

Te SOM is composed o a wide range o materials rom inputs such as plant

residues and animal excreta, and the multitude o living and dead intermediates in theprocesses and pathways or recycling S via the soil biota. Although the importance o these processes is obvious, it is di cult to study them in a ully unctioning productionsystem.

Tere are countless micro auna and micro ora species that exist in the soil and,together with the macro auna and others which populate the organic matter-rich uppersoil horizon, they play key roles in mixing and metabolizing organic matter. In a sheepgrazing system stocked at 10 sheep/ha, illet al. (1980) estimated that there was some395 kg/ha o underground micro and macro organic matter consumers and that this

compared with a sheep biomass o 513 kg/ha.Te management o the particular production system interacts strongly with the soilbiota and has the potential to signifcantly in uence the productivity and nutrient storageor loss. In terms o S metabolism, some o the important species have been identifed andthere is also some in ormation on their numbers, and how management in uences thepopulations. However, the integration o these studies with S mineralization in variouscropping and grazing management systems is di cult and requires collaboration o researchers with a very wide range o skills.

5.3. The carbon cycle and balanced nutrition

Given the dominance o the organic matter in the storage and recycling o S and theneed or a balanced supply o plant nutrients, it is appropriate to consider the carboncycle in relation to nutrient release rates and balances (Figure 6).Te cycle shows therelease o N, P and S but, even i they were initially in per ect proportions in a particularSOM raction, the di erences between nutrients in their mineralization rates, and theiradsorption, uptake and loss processes will make it unlikely that the nutrients remainavailable to the crops in optimum proportions (e.g. ill et al., 1982). Some o theimportant actors are: Sulphate is only weakly adsorbed in most soils and may be displaced by applied

phosphate; Nitrate and SO4

2- are very mobile and are susceptible to leaching; I S is defcient, N cannot be used e ciently or protein production, andvice versa; Legumes cannot fx N e ciently without an adequate supply o S and P.

In studies o SOM, the separation into di erent classes or pools has been carried outon the basis o molecular weight, particle size, microbial biomass and their oxidationby reagents o graded strengths. At this stage, progressive oxidation with potassiumpermanganate (KMnO 4) appears to be the most use ul approach to assess the lability o OM and nutrient turnover rates, especially i it is combined with isotopic tracer studies.Such an approach might resolve some o the observed di erences in plant-available Sbetween the HI-S and C-bonded S ractions.


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5.4. Sulphur mineralization and xation

In order to restrict the number o variables, most S mineralization studies are carriedout as isolated incubations under controlled conditions. Te results obtained depend onthe di erence between the orward (k1) and backward (k2) process rates. Dependingon the duration and number o parameters in the experiment, these are very variable.Certain key actors such as pH, temperature and moisture play an important part inthe k1 and k2 rates but, in practice, the fnal equilibrium values are controlled by othersources and sinks as shown in Figure 7.

Another problem with many o these studies is that they are carried out in soils wherethe agents responsible or the competing processes o mineralization and fxation arenot clearly identifed. In such studies, it is essential that the soil biota is either as close aspossible to that existing in the feld or comprised o known organisms, and that plantsor sinks that compete or the liberated SO4

2- are used to obtain realistic estimates o theavailability o the mineralized S and o a net equilibrium value.

Te mineralization and fxation o other nutrients rom the SOM ollows similarprocesses to those shown in Figure 7, but the amounts in the di erent pools andthe processes rates will almost certainly not be the same. Te di erences in the netmineralization o the elements will alter the balance in the concentrations in the soil

Figure 6. The C cycle and nutrient recycling.

Erosion / leaching

Solidinorganic

phase

Fertilizer Plant biomass

Plant residues

Soil biota

Soluble ions

H2SO2CO2

N2

CO2

N

P S

Grain / AnimalsForages / Residues


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solution and, or optimum value, the ertilizer will need to have appropriate release ratesor each nutrient.Soil organic matter tends to have an N:S ratio o about 8:1 compared to about 15:1 to

17:1 in most plants and 10:1 or plants that have a high S demand (Cowling and Jones,1970). Te enrichment o S in SOM relative to plants is in agreement with the resultso illet al. (1982), who isotopically labelled clover, using14C, 15N and 35S, and ounddi erent release/re-utilization rates or N and S.

Other studies ound that organic N is o en mineralized aster than organic Sin the early stages o plant decomposition (Barrow; 1961, Freney and Spencer,1960). Presumably, the proteins, which comprise most o the S in plants and soilmicroorganisms, are more rapidly broken down than the S-containing compoundsin them, the amino acids being recycled. Tis suggestion is supported by the act thatRainey Ni-reducible S, mainly cysteine and methionine, represents only 20 to 30% o the organic S in soils (Williams, 1975) and the rest is roughly divided about equally between HI-S and C-bonded S ractions. Te HI-reducible S is a very diverse mixtureo organic materials that collectively incorporate applied35S at a greater rate than theC-bonded soil S raction (e.g. Freney and Swaby, 1975).

Improved ractionation procedures and a greater knowledge o what happens tospecifc S-compounds in soils are clearly prerequisites to a better understanding o thenature, role and dynamics o organic S in soils. Tere is a need or more isotopic studiesin normal unctioning systems, where specifc components are labeled and elementstraced.

Figure 7. Mineralization and fixation interactions.

The boxes and arrows represent pools and flows. Brown boxes and arrows show thesituation commonly studied, while green boxes and arrows give examples of some of the missing inputs and losses.

K1 mineralization

K2 fixationSoil solution SO 42-

SOIL ORGANIC SSoil microorganisms

Adsorbed S

K5 and K6 K4 lossesK3 inputs

Plant residues anddomestic animals

Inorganic saltsand minerals

Plants andother losses


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6. Sulphur in plants

6.1. Needs and de ciencies

Te concentration o S in plants is similar to that o P, but both nutrients have very di erent roles. Phosphorus is mainly involved in energy and lipid production. Both Sand N are necessary or the production o essential S-containing amino acids. Sulphuris also essential or many enzymes and other important S-rich products. Plants such asbrassicas and oil palm have high requirements or S, while others can accumulate SO4

2-.

Te total amounts o N and S in plants can vary widely, with common values o N:Sranging rom about 10:1 to 20:1.Sulphur is essential or processes such as:

synthesis o essential S-containing amino acids and proteins, synthesis o coenzyme A, as well as biotin, thiamine and glutathione, synthesis o chlorophyll, lipids and volatile oils, and biological N fxation by bacteria living in symbiosis with legumes.

For many years, little attention has been paid to S as a plant nutrient, mainly becauseit has been supplied incidentally to soil through rain all and volcanic emissions andas a component o some ertilizers. In some early studies, in order to correct what wasperceived as specifc nutrient defciencies using ertilizers such as ammonium sulphate((NH4)2SO4), sulphate o potash (K2SO4), zinc sulphate (ZnSO4) and superphosphate,the responses attributed to N, P, K and Zn may have been partially due to S, or itsinteractions with other nutrients. Any such unobserved interactions emphasize theneed to accurately detect defciencies and provide plants with a balanced supply o nutrients.

Plants predominantly use SO42- rom the soil, but some can utilize SO2 and/or H 2S

rom the atmosphere. In the past, atmospheric inputs have o en supplied su cient Sor plants and masked the need or S. Other reasons or the increasing incidence o S

defciencies are changes in the crops grown, increasing yields and more short-seasoncropping with decreasing recycling o residues, especially in developing countries. Forexample, just changing crops rom cereal to oilseed can increase S demand by 50 kg/haor more ( able 2).

Te amount o S applied to crops as ungicides and insecticides have also declined.Meat and wool production and greater emphasis on high protein crops have increaseddemand, while the use o high-analysis ertilizers reduces inputs o S relative tothe other nutrients. Crops can be classifed into three groups according to their Srequirement (Spencer, 1975): high S requirements: rapeseed, al al a and cruci erous orages; moderate S requirements: coconut, sugarcane, clover, grasses, co ee and cotton; low S requirements: sugar beet, cereals and peanut.


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Te S requirements o some crops and the proportion o the S that is contained inthe useable product ( able 2) show values in line with the broad grouping by Spencer(1975). Te proportions o S in the useable product emphasize the need or good

management o the crop residues in order to reduce losses.Table 2. Crop S uptake and amounts in harvested product and residues (adapted fromZhao et al. , 2002).

Crop Uptake Exported Residues

kg S/ha kg S/ha % uptake kg S/ha % uptake

Brussels sprouts 73.5 29.8 40.6 43.7 59.4

Broccoli 60.4 14.3 23.7 46.1 76.3

Oilseed rape 58.3 14.6 25.1 43.7 74.9

Cabbage 55.1 22.9 41.5 32.3 58.5

Swede 50.5 27.3 54.0 23.2 46.0

Onion 41.1 31.7 77.2 9.4 22.8

Leek 37.2 14.0 37.5 23.2 62.5

Red beet 19.9 8.3 41.4 11.7 58.6

Sugar beet 16.5 5.1 30.8 11.4 69.2

Wheat 15.9 9.5 60.0 6.3 40.0Peas 15.9 9.5 60.0 6.3 40.0

Beans 14.6 8.1 55.7 6.5 44.3

Carrot 11.3 5.3 47.2 6.0 52.8

Lettuce 11.0 4.3 39.1 6.7 60.9

6.2. Uptake processes and metabolism

Te usual source o S or plants is SO42-, which is actively taken up rom the soil solution

by the root cells. Sulphate is then moved by transporter proteins through the xylem tothe leaves in the water transpiration stream. Although proteins can be synthesized inthe roots, the common site or SO4

2- reduction to H 2S is in the chloroplasts in the leaves.In most plants, S is used mainly to synthesize cyst(e)ine and methionine, which areincorporated into proteins. Smaller amounts o S are needed or the synthesis o otheressential molecules such as coenzyme A, biotin, thiamine, glutathione and sulpholipids.

Some brassicas (e.g. rapeseed) use signifcant proportions o the S to produceglucosinolates, while onions and garlic ( Allium sp.) have high S-alkylcysteinesulphoxides (allins) contents. Te production o allins appears to be a sink or H2S andthe levels o glucosinolates and allins depend on the availability o S.

In situations o excess S supply, some plants accumulate modest amounts o SO42-

and, in a control mechanism, may eliminate excess to the atmosphere as H2S, while


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others such as tomato and cotton can accumulate large amounts o SO42-, which is

apparently held in relatively unavailable orms. Te total S:SO4-S ratio is sometimesused to indicate the su ciency o S supply, but the diversity o the S orms and their

di erences in mobility in the di erent crops make it very di cult to have generalguidelines or critical S levels.Te basic uptake o SO4

2- and sequence o metabolic processes in the plant (Figure 8)are as ollows: Sulphate is transported to the leaves. In the chloroplasts, it is trans ormedto adenosine 5'-phosphosulphate (APS) prior to its reduction. Te reaction is catalyzedby A P sulphurylase, but its a nity or SO4

2- appears to be rather low, and the SO42-

concentration in the chloroplasts is probably one o the rate-limiting steps in theregulation o the SO4

2- reduction pathway (De Kok and Stulen, 1993; Stulen and De Kok,1993). Te APS is frst reduced to sulphite (SO 3

2-) by APS reductase, which probably has

the lowest activity o the enzymes involved in the assimilatory SO42-

reduction pathway.It may be the key regulation point, responding rapidly to S concentration, with sulphide(S2-), O-acetylserine, cysteine or glutathione being the likely regulators. Sulphite is thenreduced to sulphide by sulphite reductase, with reduced erredoxin as reductant. Teincorporation o sulphide into cysteine is catalyzed by O-acetylserine(thiol)lyase, withsulphide and O-acetylserine as substrates (For detailed descriptions on S metabolism,see Tompson et al ., 1970, and De Kok et al., 2002).

Figure 8. Simple diagram of SO 42- uptake and metabolism by plants.

SO42- in soil solution

Chloroplasts

S2- o-acetylserine(thiol)lyase cysteineSO32- sulphite reductase S2-

APS APS reductase SO32-

SO42- ATP sulphurylase APS

Cysteine organic S, protein, GSH

FOLIAGE

Reduced S & SO42- uptake

regulationXYLEM

P

HLOEM

Active uptake of SO42-

SO42-transporter

proteinsROOT

Water transpiration

Products

Exudates


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Methionine, an essential S-containing amino acid present in plants, is the mostimportant compound derived rom cysteine. It is synthesized via the bi-directionaltrans-sulphurylation pathway between cysteine and homocysteine, the cysteine to

methionine direction in ungi and higher plants and the reverse in animals and ungi.Te homocysteine in plants is converted into methionine using the methyl group derivedrom serine (Tompson et al., 1970) as indicated: Cysteine + O-acetyl homoserine

cystathione homocysteine methionine.Most o the cysteine and methionine is usually incorporated into plant proteins

and lesser amounts in glutathione, enzymes, phytochelatins and other secondaryS-compounds, such as sulpholipids, glucosinolates and allins. Te ability o the SHside to cross link in SS bonds plays a signifcant role in the structure and propertieso proteins, while the SH is also important in binding enzymes and metals, e.g. in

thioredoxins and erredoxins.Glutathione (GSH) and its homologues have important roles as regulators andin redox reactions in plants and animals. Te glutathione homologues have thesame N-terminal y-glutamyl moiety and central cysteine residue but have a variableC-terminal amino acid. Tey are widely distributed in plant tissue in concentrationsranging rom 0.1-3 mM.

Glutathione is predominantly present in its reduced orm and the ratio o reducedglutathione (GSH) to oxidized glutathione generally exceeds a value o 7. Reducedglutathione may unction as a carrier and reductant in the assimilatory SO4

2- reductionpathway and it appears to be the major transport control or shoot to root trans er o reduced S via the phloem.

Unconsumed plant material and crop residues are the major suppliers o organic S tothe soil with signifcant inputs rom animal excreta in some areas. Tere are also additionso S to the soil rom root exudates, which may be signifcant (Lynch and Whipps, 1990).However, it is di cult to get details o the amounts and role o these sources, probably because o the di culties in separating roots rom soil and the recycling o S withinthe soil biota, although the presence o amino acids in these exudates (Rovira, 1969)suggests that some S may enter the soil via this route.

I a crop has a high demand or S, the application o a particular S ertilizer at highrates may not supply the S to the plants e ciently, and can lead to undesirable changesin the rest o the system, e.g. increased leaching losses and antagonism between di erent

orms and amounts o S, selenium (Se) and molybdenum (Mo). Te stage o the cropgrowth when S demand is highest, the proportions o nutrients in the ertilizers, their

orm, site o application and rates o nutrient release will all in uence the e ciency o S use by the crop.

6.3. Plant sulphur and product quality

When dealing with a large number o interacting processes, at any level o sophistication,there is the problem o where to break into the various interlocking cycles because,ultimately, a perturbation at one point will inevitably have some consequences in anotherpart o the metabolism. Considering the absolute dependence o all higher animals on


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microorganisms and plants or their supply o essential S-containing materials and theever-increasing demand or ood by man, this section concentrates on the e ects o Ssupply on the benefcial components o crops and orages.

Tere are other interactions between S, Mo and Se that occur to varying extentsand are very important to di erent processes in soils, plants and animals. Naturalsystems gradually adapt to changes in their environment but, in the quest or more

ood, agricultural production has been intensifed and this has resulted in large-scalechanges to nutrient balances over large areas. o maintain current levels o production,the nutrient balances must be restored and this can only be achieved by ertilizationwith the appropriate combination o inorganic and organic materials.

6.3.1. Sulphur responses in crops and forages

Te range o S uptake or various crops is rom about 11 to 74 kg/ha ( able 2).Maintaining levels o production requires a ertilizer to supply su cient S to balance theoverall inputs and losses imposed by the management system. Consequently, the ormand amount o ertilizer required or each production system will vary signifcantly.General or average recommendations can rarely be more than a rough guide. Tisbecomes even more uncertain and di cult when there are crop rotations with di erentS requirements.

Te values o residual S may change dramatically with ertilizer and residuemanagement. I an appropriate source o S is applied, then, provided other severeconstraints are not operating, there will be an increase in biomass production and/or inthe amounts and distribution o quality components such as proteins, essential aminoacids and vitamins. Exceptions to this may arise when excess S applications lead toaccumulation o SO4

2- and/or compounds with anti-nutritional properties, especially glucosinolates, and disproportionately low N:S ratios in animal eeds.

When the economics o applying average levels are considered, it o en appears to beworthwhile to add extra S to be on the sa e side, but such actions can be a dangerous ordetrimental practice. It is important to get the application right ( right product(s) at theright rate, right time and right place) in order to ensure that the product will be the best

or the next stage in the production chain and ultimately or the end user.

6.3.2. Bread qualityVariable results have been obtained in New Zealand or studies o the e ect o ertilizationon bread quality. A change rom ertilizers containing S to urea or wheat nutrition canlead to a decline in the quality o wheat our or bread-making, expressed in doughrheological properties by increased arinograph, dough work input and extensigraphresistance. Te choice o cultivar seemed to be the single best option or obtaining thequality desired, but each o the cultivars tested showed a linear increase in work inputwith increasing N:S ratio.

Under high N management, the application o S ertilizer can signifcantly improvedough and baking characteristics (De Ruiter and Martin, 2001). In Australia, Europeand the USA, the poorer bread-making qualities have been attributed to S defciency (Moss et al., 1981, Zhaoet al., 2002).


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7. Sulphur de ciency detection and correction

7.1. Plant testing

7.1.1. Visual SymptomsTe visual detection o S defciency is airly straight orward, although in young plantsit may be con used with N defciency. Symptoms include a yellowing o the youngerleaves, as a result o a low chlorophyll production and poor remobilization o S withinthe plant (Yoshida and Chaudhry, 1972) and a marked reduction in plant height and

tiller number in cereals (Blairet al., 1979). Photographs o S defciency in crops providean excellent re erence source or the visual detection o S defciency in plants (see platesin Appendix 3).

Tere are two main problems with the visual detection: By the time the defciency is obvious it is usually too late to correct it in that crop; Te observation does not give an accurate indication o how much S is needed or the

best orm to apply.

7.1.2. Chemical analysisMany chemical analyses o plant tissues have been used to detect and/or estimatethe degree o defciency such as: total S, SO4-S, total S:SO4S ratio and N:S ratio.Sometimes they are use ul, especially or particular crops in regions where there is agood agronomic background and experienced extension o cers but, similarly to visualsymptoms, by the time results are available, it is usually too late or the crop, and noprecise recommendation can be given on how much S is needed. Another problem withthese tests is actually sampling the precise part o the plant at exactly the right time and,then, using a standard method o preparation or analysis.

A rice growers cooperative in the Murrumbigee Irrigation Area o Australia providesan example o the coordinated use o modern technology and local experience/cooperation to manage crop nutrition, especially S. In conjunction with researchersand extension o cers, the cooperative has adopted a diagnostic package developed by the New South Wales Department o Agriculture. Tis involves specifed sampling at aknown stage o plant development, immediate drying in a microwave oven, same day dispatch to the laboratory and Near-In rared estimation o plant S. Te combination o standardized sampling methods and local experience provides good recommendations

or management and/or remedial treatment o the current crop.Recent work at Rothamsted Research in the UK resulted in a simple and rapid method

or determining the malate:sulphate ratio in a hot water extract o plant materials (Blake-Kal et al ., 2002). It has been tested on plants grown in the feld with di erent levels o applied S ertilizer and it appears that this test can give an early enough warning o Sdefciency or corrective measures to be taken. However, the developers point out thatthere are many actors involved in arriving at the required plant product and no precise


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recommendation o the amount o S needed or correction can be made at present.Te interpretation also needs in ormation on the stage o development o the roots inrelation to the distribution o S in the soil profle, and needs to be backed up by the

experience o the armer and the local agronomic adviser.In glasshouse studies o rice, S defciency was indicated when the grain S content wasless than 0.1% and the N:S ratio was wider than 14:1. Application o S a er anthesis, topreviously defcient plants increased grain S rom 0.08% to 0.2%, well above the highestlevel achieved by applying S at sowing, and suggests that the use o grain analysis orretrospective diagnosis warrants urther testing in the feld (Randallet al., 2003).

Much in ormation is available on critical nutrient concentrations and ratios o nutrients in di erent parts o crops and trees (e.g. Reuter and Robinson, 1997).However, in many cases it is probably better just to leave it up to readers to consult local

agronomists or books such as those in the re erences.

7.2. Soil testing

Over the years, many extractants have been used on soils to predict the likely occurrenceo a defciency in a particular cropping or pasture situation and, more importantly,estimate the amount o ertilizer needed to obtain optimum production. Te objectivehas usually been to estimate the amount o soil solution SO4

2- plus adsorbed SO42- that

make up the pool o plant-available SO4

2-, and relate that to the plant requirements.Tis approach has not been success ul, the main problem becoming clear i the

uptake and cycling o S is considered (Figure 9). Even in that simplifed representation,there are many interacting alternative pathways and processes, and the relative sizes o the S pools (e.g. the extracted plant-available pool) are only part o the story, and it isthe process rates that complete the picture.

Te proportion o the total S as available SO42- in the cycle is about 5% and, subject to

large seasonal variations; the rest is in organic orms such as ester and C-bonded S. Evenwithout competition, the available SO4

2- pool is usually less than the amount required by the plants throughout the growing season.

Te proportions o the SO 42- initially available and o SO4

2- mineralized rom theorganic matter, mobilized rom inorganic sources and applied in ertilizer that theplant can retrieve depend on such actors as the placement o the ertilizer and themineralization rates o the various organic ractions. Consequently, except in a ewspecialized situations, the various soil extractants either under- or over-estimate theavailability o S rom SOM. Without a better understanding o the importance o theprocess rates and o the critical actors which control them, universally applicable soiltests are unlikely.

Bardsley and Lancaster (1960) attempted to overcome the problem by estimatingreserve S but, as with most o the proposed methods, the main di culty in evaluatingthe soil tests is that, without isotopic tracers, there is no way o measuring the e ciency o the ertilizer in supplying S to the plant, and whether the extractant recovered S romthe same pool as that used by the plant.


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Using standardized soil sampling methods at appropriate times o the year, andintegrating these with local experience o the magnitude and persistence o plantresponses to particular ertilizers, their methods o application and other managementpractices, it has sometimes been possible to use regression relationships and statistics toestimate soil critical S levels, and to develop response curves suitable or recommending

ertilizer rates or specifc crops. Such regression relationships can work or particularareas where there is enough background experience and data, but none o them havehad rigorous investigation to establish what ractions, pathways and processes actually supply the S used by the plant, and the e ciency with which the plant competes withother users and loss processes. Tese di culties do not preclude the possibility o otherextractants being developed, but it is pre erable that uture work should pay moreattention to the dynamics, processes and management o the whole soil-plant system.

From pot studies in which rice was grown in 35S-labelled soil, the relationshipsbetween the specifc radioactivities (SR) o the S in the plants and those o S recovered

rom the soil by various extractants show the relative merits o the extractants ( able 3).

Figure 9. Diagrammatic representation of the main soil S fractions and of theproportions of them that are removed by different extractants. Based on isotopic Sstudies at the University of New England, Australia.

P L A N T S

S in resistant OM

Mainly carbon-bonded S

S in labile OM

Mainly ester-sulphates

Inorganic S

Solution SO4

2-

Deposited

Adsorbed

Minerals

Plantuptake, via

the soilSO4

2- pool, of S released at

various rates fromdifferent sources

TotalNaHCO 3

KCL-100

KCL-40

MPC


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Te KCl-40 test, where 0.25 M KCl is added to the soil and extracted at 40oC or threehours was the most promising (Blair et al., 1991).

Table 3. Degree of association between the SR of the extracted S and that in the S takenup by rice (The nearer to 1, the closer the S used by the plant is to that extracted).

Extractant System

Non- ooded Flooded

H2O 0.88 0.85

MCP 0.83 0.79

KCl-40 0.92 0.92

NaHCO3

0.27 0.19

Soils rom 18 pastures in northern New South Wales, Australia, were used in pot trialsto test the relationship between plant responses to applied S, expressed as percentageo maximum yield, and the available S determined by various extractants. Te resultspresented in able 4 show that the best relationship was with the available S extracted by the KCl-40 test. Te results rom the KCl-40 test are a pointer to what may be achievedthrough a undamental approach. However, they are by no means universally applicabletests and, in some situations, the locally tried and adapted tests will give better results.

Table 4. Coef cient of determination (r 2) between S recovered by various extractants andthe percent of maximum yield for 18 pasture soils collected from northern New SouthWales, Australia (Blair et al., 1991).

Extractant r2

H2O 0.47

MCP 0.48

KCl-40 0.74NaHCO3 0.15

Total S 0.03

A number o other actors, such as the orms and distribution o S in the soil profleand the changes in volume o soil explored by roots as the plant develops, also have tobe taken into account when using soil samples to assess the probability o S defciency being a problem or di erent plants.

Some examples o large variations in available S in the soil profles rom our sites inNew South Wales are shown in Figure 10. Te variation in S demand with plant growth,the type o crop and management would have a signifcant e ect on the occurrence andpersistence o S defciencies.


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Care must also be taken when collecting the samples because the results could bebiased due to previous treatments such as burning o crop residues and animal grazinghabits and management. For example, there are important di erences in the pathwaysand orms o S excretion by ruminants with inorganic S mainly in urine and organic

orms in dung.Reutilization o S by plants rom these materials occurs by di erent pathways at

di erent rates, which can be altered by management. In pastures, there can be highS concentrations under sites o earlier dung or urine excretion. Te redistribution by cattle is airly random, while the camping o sheep leads to the development o gradientso increasing concentration o S towards the camp.

Figure 10. Sulphur distribution down the profile for some New South Wales soil sites(Blair et al. , 1997).

KCl-40 S (mg/g)

Depth (cm)

0 5 10 15 20

120

100

80

60

40

0

20

160

140

KCl-40 S (mg/g)0 40 60 100 12020 80

N7 Toombah Park

S22 Lambert

Q1 Malanda

S23 Nangwarry


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8. Sulphur in animal production

Sulphur is essential or growth o virtually all organisms. Plants and microorganismscan utilize inorganic sources o S and N in the synthesis o S-containing amino acidsand many enzymes and vitamins. Higher animals cannot synthesize the essentialamino acids and, except or ruminants which have a symbiotic relationship with themicroorganisms in their gut, they depend on plants or predation or their supplies o energy, proteins and the essential amino acids, among others.

Te ocus o this section is on the metabolism o S in animals, the interactionsbetween animals and the rest o the S cycle, the metabolism o S in terms o the quality o an animals diet, the removal o S in products, and recycling rom excreta.

8.1. Sulphur metabolism in ruminants

Te ability o ruminants to utilize low quality roughage and inorganic orms o N, P andS to supply proteins and other products and services enable them to play an importantrole in most arming systems and ood chains. Te metabolism o S in ruminants hasbeen reviewed in some detail (Bray and ill, 1975; Underwood and Suttle, 1999), butthe ocus here is on the role that the rumen plays in dealing with diets o very di erentquality that result rom variations in S supply and management o pastures, crops and

eed supplements. In terms o S metabolism, the gut o a ruminant can be divided intotwo main systems: Te reticulo-rumen , in which S in eed and secretions is largely reduced to H2S and

converted into microbial proteins or absorbed directly as HS- (Figure 11 ); Te post-ruminal section , in which the overall processes are the digestion o proteins

and other S-containing materials, and the absorption o amino acids, peptides andinorganic and organic sulphates.In addition, there are several interactions in the animal that regulate the levels o S in

various body components. Te undigested organic S, plus a small amount o inorganicS are excreted in the dung, and the rest o the inorganic S is excreted as SO4

2- in urine.Te metabolism o S appears to be via A P (adenosine triphosphate) to APS

(adenosine-3-phosphosulphate), which either interacts with A P to produce PAPS(5-phospho-adenosine-3-phosphosulphate), or releases H2S by the action o dissimilatory-reducing bacteria, which derive reduced S rom PAPS. Tese processescan be disrupted by competitive inhibition o APS ormation by molybdate, selenate,tungstate or chromate and, although the results are variable, this inhibition has beenused to show that microbial reduction is the major, i not sole, method o SO4

2- reductionin the rumen.

Radiotracer studies have shown that 35S-labelled APS and PAPS are ormed romNa2

35SO4 in mixed cultures o rumen bacteria, and subsequent incubation o 35S-labelled

APS and PAPS with mixed rumen bacterial cultures results in the ormation o H235S.


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As with plants, which synthesize proteins rom inorganic sources, there aremany similarities between the metabolism o S and N in ruminants so, at times, it isadvantageous to consider the relative retention o N and S by the animals.

Bray and ill (1975) used data or 51 sheep with a wide range o N:S ratios in theirdiets rom ten di erent sites. Tey showed that N and S retention was related by N =10.37 S 0.038 with a correlation o r = 0.952.

Te desirable dietary N:S ratio is about 10 :1 or sheep and about 13.5 to 15:1 orcattle (Bird, 1974; Kennedy, 1974). Tus, the supplementation o diets with non-proteinN, e.g. urea, may entail the provision o additional S and energy. Sulphur supplementscommonly used are various SO4

2- salts, S0 and, to a lesser extent, S-amino acids and thehydroxy analogue o methionine.

In addition, eed additives that supply energy, such as molasses, may containappreciable amounts o S. Provided there is su cient energy available, the synthesis o microbial protein rom non-protein N and S will enhance the nutritive value o a poorquality diet but, on protein-rich diets, ermentation in the rumen can lead to wastage o N and S via the production and absorption o ammonia (NH3) and sulphide (S

2-).Te S content o drinking water rom di erent locations can vary widely and its

potential nutritional value is requently ignored. Its value can range rom insignifcantamounts to the situation where the S intake rom this source may be large enough toproduce adverse e ects due to microbial production and subsequent absorption o largequantities o S2-, which can result in loss o appetite (Bird, 1972).

Figure 11. Diagram of S utilization in the rumen.

Absorption

POST -RUMINAL

TRACT

DIETARY

AND

RECYCLED

SULPHUR

SulphideSulphateSulphite

ThiosulphateComplexes and

adsorbed sulphides

Cystine CH3SHEsters

Protein Methionine

SULPHIDE R

T

MICROBIAL

PROTEIN


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8.2. Body sulphur content and turnover in sheep

Te incorporation o S into wool, various body components, and its excretion in urine

and aeces has been studied using35

S to help in estimating the e ects o grazing onertilizer requirements and S recycling ( illet al ., 1970). Te concentration o S in various body components ranged rom 0.12% dry matter in blood to 0.23% in liver, whilethe total S in the components, as a percentage o the body S, ranged rom 0.3% in brain

Table 5. Composition of an "average sheep (Till et al. , 1970).

Sulphur

ComponentConcen-tration

(% DM)

Proportions(% emptybody S)

Pathway1(xy)

Run B(T days)

Run D(T days)

Blood (p) 0.12 5.3 ap 1.4 1.7

Skin (s) 0.19 12.3 pg 15 15

Liver (I) 0.23 2.6 gp 26 26

Heart (h) 0.16 0.7 ga 30 30

Lungs (m) 0.17 2.3 pl 31 31

Kidney (k) 0.17 0.4 lp 16 16Brain (b) 0.13 0.3 ph 286 338

Gut wall (g) 0.10 5.5 hp 38 38

Spleen (r) 0.17 0.3 pm 65 89

Carcass (c) 0.15 70.3 mp 24 33

Gut contents (a) 0.29 6.2 pb 877 877

Total 0.15 106.5 bp 49 50

Empty body 0.15 100.0 pr 582 647

rp 34 34pc 3.9 4.8

cp 25 31

ps 10 13

sp 20 24

pk 274 413

kp 24 35

1Sulphur content of an "average" 40 kg Merino wether sheep and component T estimatedusing a simulation model for the 35S data. T = 0.693/k, where k is the rate constant for the owof 35S from component x to component y, i.e. along pathway xy. The two runs (B and D) arepresented to show the variation of estimated T in the simulations.


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to 12.3% in skin and 70.3% in the carcasses. Te excretion pathway and proportionsdepend on the diet quality. In this study, it was 61% in urine and 21% in aeces.

Te turnover rates or S trans er between the di erent pools were estimated by a

computer analysis o the35

S measurements ( able 5). Te observed hal -li e or the35

Sow into wool was 28 days, and similar hal -li e (about 30 days) were estimated or theblood, carcasses and skin, which are the other pools o major in uence. Consequently, itappears that, in the feld, most o the sheeps S equilibrates with the pasture with a hal -li e o about 28 days and, although there were di erences between body components inS contents and concentrations, there were no large S-rich components that would belikely to produce unusual turnover rates within the sheep.

8.3. Wool productionProduction o wool and some hair has a high demand or S but, unlike hair, wool is acontinu

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