Properties of submerged Soils
Physical - O2 depletion, CO2 accumulation, compaction – high BD,puddling, gaseous exchange and movement of water Chemical - Changes in reduction & transformation processes Electro-chemical - Changes in Soil pH, Specific conductance, Eh as against normal soils Biological - Changes in decomposition and mineralization of organic matter Depletion of oxygen
- Water fills the pore spaces and replaces the air
- Oxygen diffusion in the water layer above the soil is very slow
- Rate of oxygen consumption in reduced soil is high
- Surface soil profile differentiates into two distinct layers are
- An oxidized or aerobic layer near soil surface
- Reduced or anaerobic layer below soil surface
- Accumulation of carbon dioxide and methane and may escape as bubbles if pressure builds up.
- Partial pressure of CO2 affect the solubility of the cations Ca2+, Mg2+, Fe2+ and Mn2+
- Partial pressure of CO2 in a soil determines its pH, Eh and solubility of cations and thereby affects the specific conductance and exchange reactions
- Kinetics of PCO2 varies with soil properties
- Decline in PCO2 after one to four weeks of flooding
- Diluting effect of CH4 produced in later stage of OM decomposition
- Bacterial reduction of CO2 to CH2
- Leaching losses
- Removal of CO2 as insoluble carbonates
Electrochemical changes
- Decrease in redox potential
- Increase in pH of acid soil and decrease in pH of alkaline soils
- Increase in specific conductance
- Cation exchange reactions involving Fe2+
- Sorption and desorption of ions
Reduction
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Eh
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O2 to H2O
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+ 380 to + 320
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NO3 to N2, Mn4+ to Mn2+
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+ 280 to + 220
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Fe3+ to Fe2+
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+ 180 to + 150
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SO42- to S2-
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- 120 to - 180
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CO2 to CH4
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- 200 to - 280
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Specific conductance
- Specific conductance is related to the ionic content
- Differ with respect to the type of soil
- Alkaline soils : increase in specific conductance by Ca2+ and Mg2+
- Organic matter enhances the solubility of Ca2+, Mg2+, and Fe3+ and increase specific conductance.
![Properties of submerged soils Properties of submerged soils](http://14.139.56.154:82/file.php/88/sss11.jpg)
Simplified N transformation in submerged soils
- Mineralization of N restricted to ammonification stage
- Ammonification of organic matter also proceeds at a low rate
- Intermediate products of decomposition
- Methane and organic acids accumulation and incomplete breakdown of carbohydrates.
- Release of ammonia at much wider C: N ratio (Low N requirement of anaerobic)
Phosphorus
- The mobility of P is higher (diffusion process gets increased).
- The P mobility is restricted to the top 30 cm layer of the soil even in submerged conditions.
- The P fixation is more (fixation/ chemisorptions of P on colloids will be more).
- At the same time, when the colloidal fractions of P get mineralized, it results in more availability.
- The solubility of P compounds gets increased due to the presence of CO2 and decrease inEh.
- Hydrolysis of P compounds and its solubility of will be more leading to the higher solution P (available P).
- The mineralization of organic P will be higher.
Potassium
- Increase in K availability due to Fe2+
- Release from micas
- However the availability of applied K decrease due to the formation of sparingly soluble Fe-K complex
Sulphur SO42- reduced to H2S and then to FeS. It has three implications. They are
- S supply may become insufficient
- Zn & Cu may be immobilized and
- H2S toxicity may arise in soils low in Fe
Manganese
( Insoluble ) (Water soluble, available to plant)
- The above reaction occurs almost coincident with the disappearance of molecular oxygen and NO3-N resulting in the formation of manganous compounds.
- Water soluble and exchangeable ion increases on submergence and ensures a better supply of Mn in flooded soils to rice.
Iron
- In waterlogged conditions, Fe3+ is reduced to Fe2+ with concomitant increase in Fe solubility by anaerobic bacteria, which uses iron oxides as electron acceptors in respiration. It produces toxic effect in rice plants called ‘bronzing disease’.
- Under anaerobic conditions, owing to the reducing process, hydrous iron oxides give rise to Fe2+ according to the following equation.
Fe(OH)3 + e- + 3H+ -----> Fe2+ + 3H2O.
Zinc
- Zn deficiency in flooded soils thought to be related to high pH or the presence of CaCO3. However, Zn deficiency also occurs in acid soils.
- When soils are submerged, the concentration of many nutrients increases, but not Zn.
- In acid soils, Zn deficiency may be attributed to the increases in pH under reducing conditions and subsequent precipitation of franklinite or sphalerite.
- Decreasing pH in submerged, calcareous soils would usually increase Zn solubility.
- However, higher soil pH - poorer the aeration and greater the Zn deficiency.
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Last modified: Wednesday, 30 May 2012, 6:13 AM