Dept. of Soil Science & Agril. Chemistry Master’s Seminar Presentation on Chemical Equilibria Involving Nutrient Ions under Submerged Soil Condition. Department of Soil Science and Agricultural Chemistry College of Agriculture, I.G.K.V. Raipur Speaker: Mithilesh Chandra M.Sc. (Ag.) Pre. Dept. of Soil Science & Agril. Chemistry

Submerged Soils Submerged soils/ water logged soil are soil that are saturated with water for a sufficient long time in a year, to give the soil the following distinctive gley horizons resulting from oxidation-reduction process: A partially oxidized ‘A’ horizon having high in organic matter. A mottled zone in which oxidation and reduction process take place alternate A permanently reduced zone which is bluish green in colour.

+ 400 mV - 400 mV

Kind of submerged soil Alternate submerged : Up to 2.5 cm depth, more important for rice cultivation. Because alternate submerged provide better physical condition (soil aeration) for plant growth and development. Continuous submerged : At 2.5 – 7.5 cm depth, provides the potential to produce optimum rice yields.

Properties of Waterlogged Soil PHYSICAL 1. Oxygen Depletion 2. CO2 Accumulation 3. Compaction 4. Increasing BD 5. Massive structure 6. Lowering diffusion coefficient of gases ELECTRO-CHEMICAL 1. Soil pH 2. Increase Specific Conductance 3. Decrease Redox potential WATERLOGGED SOIL BIOLOGICAL 1. Reduced aerobic Microbial activity 2. Mineralization 3. Immobilization CHEMICAL 1. Soil Reduction 2. Micronutrient toxicity

Diffusion of molecular oxygen A. Physical properties Diffusion of molecular oxygen When a soil submerged, water replaces the air in the pore spaces. The Oxygen-diffusion in the water layer is very slow and the rate of O2 consumption is reduced. Some O2 is rapidly utilized by the facultative anaerobic organisms.

2. Accumulation of carbon dioxide Soil gases like CO2 and CH+4 accumulate due to submergence and also may escape as bubbles if pressure builds up. 3. Aeration status of submerged soil Immediately after submergence, the normal process of gaseous exchange between soil and air is restricted.

4. Soil compaction and puddling: submergence causes compaction of soil, in compacted soil, bulk density, thermal conductivity and diffusivity also nutrient mobility increases and the hydraulic conductivity and water intake rate decreased. Puddling: It decreases the apparent specific volume and hydraulic conductivity, creates an anaerobic environment and affects redox potential (Eh) and pH

a. Short term effect : i. Soil structure: Puddling destroys aggregates and peds. ii. Bulk density and soil strength: If puddling produces closely packed structure which increases the bulk density. But puddling can also produce a more open structure, and decrease bulk density. Puddling decrease the shear strength of surface soil.

iii. Gas exchange: O2 concentration decreases and CO2 concentration increases. iv. Water retention and movement: Water retention in puddled soils always exceeds that in unpuddled soil.

b. Long term effects: Long-term puddling forms a hard pan in the subsoil below the puddled layer. Subsurface hard pans develop from physical compaction and precipitation of Fe, Mn, Si. Ferrolysis is another long term effect of puddling that may lower soil productivity.

Ferrolysis Is the process in which periodic reduction of iron oxide (Fe3+) to ferrous ions (Fe2+). Ferrolysis comprise various component process. Some operating in reduces conditions (wet season), alternating with other operating in oxidising condition (dry season).

B. Electro chemical changes Decreases in Redox potential (Eh) : when soil is submerged the reduction of soil take place in a sequential form, nitrate is reduced first followed by manganic (Mn). Eh is the quantitative measurement of the system, It is postive (+ve) for strongly oxidising system negetive (-ve) for strongly reducing systems.

O2 NO3- Oxidized H2O Slightly Reduced MnO2 N2 Mn2+ Moderately Reduced Fe2+ NO3- MnO2 Fe3+ CO2 CH4 SO4-2 H2S H2O Oxidized Slightly Reduced Moderately Reduced The boxes represent different chemical compounds in the soil. When the soil is aerated, the chemical compounds in the soil will be represented by the top set of boxes. The bottom set of boxes represent the chemical compounds in the soil after they are reduced. As you click on the slide (in presentation view), you can see the order in which the compounds are reduced and the change in form. The sequence of the compounds from left to right is the order in which they will be reduced starting with oxygen and ending with carbon dioxide. When a soil is drained, oxygen begins to move into the soil and react with the reduced compounds. The reduced compounds are then oxidized in the reverse sequence starting on the right and moving to the left. Strongly Reduced Reaction sequence following submergence Reaction sequence after draining

Soil pH Submergence of soil causes a more neutral soil pH (pH of soil goes to the side of neutrality). This is a result of the change in chemical compounds when soil is reduced. The typical pH range for many submerged soils is 6.5-7 The increase in pH of acid soil The decrease in pH of alkali and calcareous soil

Effect OF SUBMERGENCE ON SOIL pH Source :Ponnamperuma, F. N., The chemistry of submerged soils. Adv. Agron.,1972, 24, 29–96.

Specific Conductance The Specific Conductance of the solution of most soils increases after submergence, attain maximum and declines to a fairly stable value, which varies with the nature and properties of the soil.

Source : Ponnamperuma, F. N. , The chemistry of submerged soils. Adv Source : Ponnamperuma, F. N., The chemistry of submerged soils. Adv.Agron., 1972, 24, 29–96.

Changes in organic matter and availability of plant nutrients in soils following their submergence under water Source: Sahrawat, K. L.,Organic matter accumulation in submerged soils. Adv. Agron.,2004,81,169- 201.

C. CHEMICAL CHANGES Nitrogen The transformation of nitrogen are largely depend on micro-biological interconversion.The main interconversion shown below. N2 N2 Proteins Amino Acids NH+4 No-2 No-3 In submerged soil, the main transformation are accumulation of Ammonia, Volatilization losses of Ammonia, Denitrification, Nitrogen fixation and leaching losses of nitrogen.

Transformation of Nitrogen in Submerged soil *Source: Ponnamperuma, F. N., The chemistry of submerged soils. Adv.Agron., 1972, 24, 29–96.

For submerged soil: The total N immobilized and total N mineralized are typically less compared to aerobic soil. Net mineralization is usually higher for submerged than aerobic soil. Following decomposition, there is typically more N available for a rice crop in submerged soil compared to aerobic soil. Ammonia accumulation mineralization Organic forms of N NH+4

Ammonia volatilization Once NH4+ is converted to NH3 gas, it can be lost into the atmosphere through volatilization This is a major cause of N loss for submerged rice fields Losses could even be as high as 60% Wind accelerates the transport of NH3 from the water surface and increases the loss of N

Denitrification NO3- is mobile because of its high solubility in water It may move via water flow or diffusion into anaerobic soil In anaerobic soil, NO3- may be reduced by bacteria to N2 or N2O Losses could even 40%

Nitrogen fixation via. cyanobacteria (blue-green algae ) Single cell organisms living on the surface of water or plants in a submerged environment Produce their own food through photosynthesis Often native to the paddy 20-30 kg N per hectare can be fixed per crop

Azolla fern with Anabaena azollae Some species of azolla fern grow in association with Anabaena azollae, a blue green algae which fixes N2 The azolla-anabaena combination has been used for centuries in rice paddies of China and Vietnam It can produce 30-40 kg N per hectare per rice crop It needs to be established each rice crop Can require additional P fertilizer for growth Susceptible to insect and fungal attack

Phosphorus Soil submergence is known to increase the availability of both native and applied phosphorus. Phosphate is chemically associated with Fe and Al as FePO4.2H2O and AlPO4.2H2O and as the more soluble phosphate compounds like Ca3 (PO4)2 When an aerobic soil submerged, the concentration of available phosphorus initially increased and there after declines with the period of submergence

Potassium In soils two important parameters influence the availability of K to plants these are (i) intensity factor (I) (ii) The capacity factor (Q). Potassium is present in soil in four forms, which are dynamic equilibrium as follows Soluble K Exchangeable K Non exchangeable K (Instantly available) (Easily mobilizable (Slowly mobilizable stock reserve of available K) of available K) Mineral K (Semi permanent reserve)

With submergence soluble Fe++ and Mn++ ions increase and exchangeable K+ is displaced in to the soil solution. The availability of applied potassium decreases in submerged soils due to formation of Fe-K sparingly soluble complexes. SUBMERGED SOIL Soil Reduction INCREASED CONC. OF Fe2+ IN SOIL SOLUTION SPARINGLY SOLUBLED COMPLEX OF Fe-K APPLIED POTASSIC FERRTILIZERS DECREASES K AVAILABILITY

Sulphur In submerged soils the main transformation of sulphur are reduction of SO4-2 to S-2 and the dissimilation of the amino acids to H2S. In Submerged soils following reaction are occurs. SO4-2 Submergence H2S Reduction FeS (Insoluble) (Unavailable to plant) Fe+3 Reduction Fe+2 Start first than

Iron The most important chemical change that takes place when a soil is submerged is the reduction of Iron and increase in its Solubility. Due to reduction of Fe+ 3 to Fe+2 on submergence, the colour of soil changes from brown to gray. The initial increase in the concentration of ferrous iron ( Fe+2) on soil submergence is caused by the reduction that are shown bellow. Fe(OH)3 + e- reduction Fe+2 +3OH insoluble Soluble

TRANSFORMATION OF IRON IN SUBMERGED SOILS Exchangeable Fe3+ High Partial Pressure of CO2 Ferrous ion (Soluble) Fe2O3 H2 Reduction due to Submergence Fe2O or Fe(OH)2 FeCO3 CO2 Fe(HCO3)2 H2O Low Partial Pressure of CO2 Insoluble Iron Fe3+ Soil Solution Fe2+ H2S ( Under intense reduction ) In presence Of Organic Acids Combined with H2O Organic Fe2+ Salts Source: Das, D.K., Introductory Soil Science, page no. 485

Manganese The main transformations of Mn in submerged soils are the reduction of manganic (Mn4 +) to manganous (Mn2+ ). The concentration of Mn2+ (water soluble) Increases initially and there after declines with the period of soil submergence. MnO2 + 4H+ +2 e- reduction Mn2 + +2H2O Water soluble

Zinc Zinc deficiency in submerged rice soils is very common owing to the combined effect of increased pH, HCO3- and S-2 formation. The availability of Zn decrease due to submergence may be attributed to the following reasons. Formation of Insoluble franklinite. Zn+2 + 2Fe+2 + 4H2O reduction ZnFe2O4(franklinite) Formation of very insoluble compounds of Zn as ZnS under intense Reducing conditions. Zn+2 + S-2 Zn S (Sphalerite) insoluble in water

Copper In submerged soils, Copper comes into the soil solution and becomes available to the plant as follows: Soil Cu +2 + H+ Cu +2 (Soluble ) Cu2 (OH)2CO3+ 4 H+ Cu++ CO2+3H2O (Mala chite, and abundant source of (Soluble form) cu in submerged soils)

Management of waterlogged soil Leveling of land Mechanical drainage Controlled Irrigation Flood control measures Plantation of trees having high transpiration rate Selection of crops and their proper varieties Sowing on bunds or ridges Nutrients management

Conclusion Waterlogging causes lowering of Redox potential, Neutralized soil pH, N P K deficiency and micronutrient toxicity. Expect Rice, yield of other crops severely affected by waterlogging and submergence. Waterlogging can be efficiently control by forming different land configuration, mechanical as well as bio-drainage, controlling irrigation and different flood control measures. Tolerant and resistant varieties and proper nutrient management would be much more effective during management of waterlogged soil.