Soil Science

Lesson 10

Agricultural Chemicals and the Soil

Agricultural Chemicals

• 農業藥物– 肥料 (fertilizer)– 農藥 (pesticides) ：殺蟲劑 (Insecticides) 、除草劑 (Herbicides)

Nitrogen fertilizers

• 氮肥 (Nitrogen fertilizers)1. 形式

a. soluble forms ( 速效性 )b. slow-release fertilizers ( 緩效性 )

2. 施用方式a. Broadcast ( 撒施 )b. Band ( 條施 )

3. 施用時間a. 基肥b. 追肥

4. 在土壤中之反應a. 肥害阻止發芽b. pH effectsc. Volatilization, leaching, denitrification

P fertilizers

• 最有活性之兩種成分– Monocalcium phosphate(MCP) 磷酸一鈣， Ca(H2PO4)2 2H‧ 2O– Phosphoric acid ( 磷酸 )

• Superphosphates ( 過磷酸鹽 )– Ca10(P04)6F2+7H2SO4+3H2O→3Ca(H2PO4)2 2H‧ 2O+7CaSO4+2HF

• Rock phosphate→P→P2O5

– P2O5+5H2O→2H3PO4

Δ Poly-P fertilizers– 3P2O5+5H2O→2H5P3O10 (superphosphoric acid)Δ Insoluble phosphates– Rock phosphate ore

Flow diagram of P fertilizer manufacture

Dissolution of P fertilizers in soils

Successive stages in the dissolution of an MCP granule in soil (after Tisdale and Nelson, 1975)

K fertilizers

Sulfur fertilizers

• Elemental S– S+3/2 O2+H2O→2H++SO4

2-

Thiobacillas

• Superphosphate (CaSO4)

• Gypsum

Micronutrient fertilizers

1. Sparingly soluble salts or oxides

2. Chelates– foliar spray → immediate uptake

• e.g. Fe-EDTA

Percent by weight

Salt hazard

Acid-forming tendencyb Kg CaCO3 100Kg

Comments

Fertilizer N P K S Ca Mg Micronutrients

Primarily sources of other nutrients

Basic slag

1-7 3-30 3 10%F e,2% Mn Low +70 Industrial by-product; slow availability; best on acid soils.

Gypsum(CaSO4‧2H2O)

19 23 Low 0 Stabilizes soil structure; no effect on pH; Ca and S readily available.

Calcitic limestone(CaCO3)

Very low +95 Slow availability; raises pH.

Dolomitic limestone[CaMg(CO3)2]

36 Very low +95 Very slow availability; raises pH.

Epsom salts(MgSO4‧7H2O]

13 24 Moderate 0 No effect on pH; water soluble.

Sulfur, flowers(S)

95 2 10 - -300 Irritates eyes; very acidifying; slow acting; requires microbial oxidation.

Solubor 20.5% B Moderate - Very soluble; compatible with foliar sprays.

Borax (Na2B4O7‧10H2O]

11% B; 9% Na Moderate - Very soluble.

EDTA chelate

13% Cu or 10% Fe or 12% Mn or 12% Zn

- - One or more micronutrients; see label.

Organic fertilizers

Fertilizer application

• 粒狀 vs. 粉狀 (Granular vs. Powdered from)– 對於可溶性磷肥，製成粒狀可減低磷固定之速率，並可增進施用之方便。

– 對於不溶性磷肥，其粉狀形式效率較高，但施用不方便。

• 條施 vs. 撒施

• Residual Effect– Eutrophication ( 優養化作用 )

農藥• 按離子性分類

a. Ionic pesticide1. Cationic pesticide

2. Anionic pesticide

農藥• 按離子性分類

a. Ionic pesticide1. Cationic pesticide

2. Anionic pesticide

農藥• 按離子性分類

b. Nonionic pesticide

農藥在土壤中之主要動態1. Volatilization ( 尤其對 surface-applied

compound)

2. Chemical and biological decomposition

3. Adsorption by soil solids

4. Absorption by plants

5. Transport on eroded soil particles

6. In solution, by leaching or in surface runoff