Chemical Changes in Flooded Soil, Summaries of Organic Chemistry

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Chemical Changes

in flooded soil

FLOODED SOILS

Flooded soils, now known as HYDRIC Soils are characteristic of wetlands and irrigated field cropped to rice (paddy soils) IMPACTS OF FLOOD ON SOIL Flooded soils create significant challenges for agricultural lands. The floods have many direct impacts:

1. Deposition of sand and debris on productive lands
2. Erosion of agricultural soil,
3. Flooded soil syndrome

IRON TOXICITY

• (^) Ferrous iron concentration after submergence is positively correlated with clay and organic matter
• (^) Nature and content of the Fe(III) oxide hydrates, pH of the soil, and temperature.
• (^) Strongly acid soils with adequate OM and reactive iron oxide can build up toxic concentration of Fe(II) and cause iron toxicity in submerged Ultisols, Oxisols, and acid sulfate soils in tropics.

P availability is more in flooded soils

• (^) Mainly due to reduction of ferric phosphate to ferrous phosphate
• (^) Hydrolysis of ammonium phosphate
• (^) Dissolution of calcium phosphate resulting from the accumulation of carbon dioxide

Mn availability is more in flooded soils

• (^) Reduction of the oxides of Mn(IV)
• (^) Reduction may result from these compounds functioning as:  (^) Eleectron acceptors in the respiration of microorganisms (anaerobes)  (^) Chemical oxidants of reduction products.
• (^) Mn(IV) to more soluble forms due to anaerobic metabolism of soil bacteria
• (^) Mn is more readily reduced and rendered soluble than Fe
• (^) Release of Mn into soil solution precedes that of iron

Mo availability is more in flooded soils

• (^) The concentration of water-soluble Mo in soil increases after flooding, presumably as a result of desorption following reduction of ferric oxides

The Chemical Changes and transformation of nutrients in flooded soil The main transformation are accumulation of ammoinia, volatilization loss of ammonia, denitrification, nitrogen fixation and leaching losses of nitrogen.

Changes in pH

• (^) Within a few weeks after submergence the pH of acid soils increases and pH of sodic and calcarous soil decreases. Thus, the pH most acid and alkaline soils converge between 6 & 7 after flooding.
• (^) The rate and degree of the pH changes depend on soil properties and temperature
• (^) Low temperature retards pH changes in both acid and alkaline soils

• (^) The stabilization of the soil pH (6.5-7.0) at neutrally after submergence has several effects on rice growth:  (^) Adverse effects of low or high pH are minimized  (^) Excess Al and Mn in acid soils are rendered harmless  (^) Fe toxicity in acid soil is lessened  (^) Availability of P, Mo and Si is increased  (^) Mineralization of organic N is favored  (^) Organic acids decompose  (^) Lime is seldom necessary

Positive and negative effect of reducing Eh POSITIVE:  (^) Increase the supply of N, P, K, Fe, Mn, Mo, and Si NEGATIVE:  (^) Loss of nitrogen by denitrification  (^) Decreased the availability of S, Cu, and Zn  (^) Production of substances that interfere with nutrient uptake or that poison the plant directly

Depletion in Oxygen

• (^) When a soil is submerged, water replaces the air in the pore spaces
• (^) Except in a thin layer at the soil surface, most soil layers are virtually oxygen-free within a few hours after submergence
• (^) In the absence of oxygen, organic matter decomposition is slower and less complete in anaerobic than in aerobic soil
• (^) The thickness of the oxidized layer increases with duration of flooding