SOIL AND PLANT ANALYSIS (1+1)

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

1. Water fills the pore spaces and replaces the air
2. Oxygen diffusion in the water layer above the soil is very slow
3. Rate of oxygen consumption in reduced soil is high
4. Surface soil profile differentiates into two distinct layers are
5. An oxidized or aerobic layer near soil surface
6. Reduced or anaerobic layer below soil surface
7. Accumulation of carbon dioxide and methane and may escape as bubbles if pressure builds up.
8. Partial pressure of CO2 affect the solubility of the cations Ca2+, Mg2+, Fe2+ and Mn2+
9. Partial pressure of CO2 in a soil determines its pH, Eh and solubility of cations and thereby affects the specific conductance and exchange reactions
10. Kinetics of PCO2 varies with soil properties
11. Decline in PCO2 after one to four weeks of flooding
12. Diluting effect of CH4 produced in later stage of OM decomposition
13. Bacterial reduction of CO2 to CH2
14. Leaching losses
15. 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


Critical Eh values of important components in submerged soils

Reduction	Eh
O2 to H2O	+ 380 to + 320
NO3 to N2, Mn4+ to Mn2+	+ 280 to + 220
Fe3+ to Fe2+	+ 180 to + 150
SO42- to S2-	- 120 to - 180
CO2 to CH4	- 200 to - 280

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.


Nitrogen Transformation in submerged soils



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


1. Methane and organic acids accumulation and incomplete breakdown of carbohydrates.
2. 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

• In submerged soils,
• MnO₂ + 4 H⁺ + 2e^- (reduction)   Mn²⁺ + 2H2O
 ( 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.