Soil Ammendment

Soil amendment • Soil amendment: Any substance other than fertilizers such as lime, sulfur, gypsum and sawdust that is used to alter the physical & chemical properties of a soil, generally to make it more productive is known as soil amendment. • What are the problems associated with strong acidity in soil? Soils become acidic when considerable portions of the exchangeable cations are H+ and various forms of hydrated aluminium, although some acid soils develop from acidic parent materials. Strongly acid soils are not productive for most crops, because various problems are associated with strong acidity in soils. These are1. Aluminium toxicity 2. Manganese toxicity 3. Iron toxicity (in a few soils) 4. Calcium deficiency 5. Magnesium deficiency 6. Molybdenum deficiency 7. N, P &/or S deficiency (because of very slow organic matter decomposition). 8. Phosphorus deficiency due to the precipitation by Fe, Al & Mn ions. • Aluminium toxicity: Al toxicity is most common in acid soils. When Al is present in the growth medium, it may inhibit the uptake, transport & utilization of many other nutrients and induce nutritional deficiency of plants. Increased Al concentrations in the solution exert an inhibiting effect on the concentrations of N, P, K, Ca, Mg, Fe, Mn, Zn & Cu in the plant tops. Decreases in the uptake of these elements are mostly related to morphological, physiological & biochemical effects of Al (Fageria & Carvalho, 1982; Frageria et al., 1988). These effects can be explained by the following hypothesis: 1. Al inhibits root growth, thereby causing the uptake of nutrients to be reduced (Fageria, 1982). 2. Al reduces cellular respiration in plants, inhibiting the uptake of all ions. (Aimi & Murakami, 1964). 3. Al increases the viscosity of protoplasm in plant root cells, and decreases overall permeability to salts.

4. Aluminium blocks, neutralize or reverse the negative

charge on the pores of the free spaces & thereby reduces the abilities of such pores to bind Ca. 5. Aluminium may compete for common binding sites at or near the root surface and thereby reduces the uptake of K, Ca, Mg & Cu 6. Aluminium reduces Ca uptake by completely inactivating part of the Ca accumulation mechanism. 7. In general, aluminium interferes with cell division in plant roots, decreases root respiration, interferes with certain enzymes governing the deposition of polysaccharides in cell walls, increases cell wall rigidity & interferes with the uptake, transport & use of several elements such as K, Ca & Mg. 8. Aluminium injuries plant roots and reduces Ca uptake. 9. Aluminium decreases the sugar content, increase the ration of non protein to protein N, and decreases the P contents of leaves from several plants grown on acid soils. 10. Aluminium can reduce such trace elements like Mn, Cu, and Fe. • Manganese Toxicity: Manganese (Mn) can exist in three oxidation states: Mn²+, Mn³+ & Mn4+. The trivalent form usually exists as Mn2O3, which can be found in substantial amounts in acid soils. In extremely acid soils, manganese solubility can be sufficiently great to cause toxicity problems in sensitive plant species. Large amounts of soluble Mn in water cause Mn toxicity in plants; hence it is needed only as a micronutrient. The presence of excess Mn may inhibits the uptake, transport & utilization of many other nutrients and induce nutritional deficiency. • Iron toxicity: Under reduced condition, iron toxicity occurs in very acid soils that are poorly drained. In very acid soils, iron in more soluble. It has been found that, high Fe concentration in the growth medium reduced uptake of nutrients. Among micronutrients, uptake of P as highly affected, followed by K & N. among micronutrients, adsorption of Mn &

Zn most affected. These results suggest that when there is a higher concentration of iron in lowland or flooded rice, P, K & Zn deficiencies will be first to appear if concentrations of these nutrients in the soils are most sufficiently high. • Calcium deficiency: In acid soil, calcium is replaced by Al³+ and H+ from soil colloids & results Ca deficiency in soil. Due to Ca- deficiency, structural stability is reduced. H Ca + 2H+ + Ca²+ Colloid Colloid H • Magnesium deficiency: In acid soils, magnesium is replaced by Al³+ & H+ from soil colloid. As a result, soil is deficient in Mg. • Molybdenum Deficiency: Plants absorb Mo as MoO4²ˉ. In strongly acid soils, Fe & Al oxides are available & Mo is strongly adsorbed by Fe & Al oxides, a portion of which becomes unavailable to the plant as Al-molybdate is formed. Mo is a component of the enzyme nitrogenase, which is essential for the process of nitrogen fixation, both symbiotic & nonsymbiotic. It is also present in the enzyme nitrate reductase, which is responsible for the reduction of nitrates in soils & plants. • N, P &/or S deficiency because of very slow organic matter decomposition: Organic matter is the main source of N, P, & S. in extremely acid soil heterotrophic bacteria are not able to degrade organic matter. Other microbes such as fungi (e.g. Aspergillus sp.) decompose organic matter, but decomposing process is slow. As a result, availability of N, P & S is reduced. Besides, nitrification is hindered/interrupted in extremely acid soils due to the inactivation of bacteria. • P-deficiency Due to the precipitation by Fe, Al & Mn ions: Some soluble iron, aluminium & manganese are usually found in strongly acid mineral soils. Reactions with the H2PO4ˉions would immediately occur, resulting in the formation of insoluble hydroxy phosphates. This chemical precipitation may be represented as follows, using the aluminium cation as an example. Al³+ + H2PO4ˉ+ 2H2O 2H+ + Al (OH)2H2PO4

(Insoluble) In most strongly acid soils, the concentration of the iron & aluminium ions greatly exceeds that of the H2PO4ˉions. Consequently the reaction moves to the right, forming the insoluble phosphate. This leaves only minute quantities of the H2PO4ˉion immediately available for plants under these conditions. • Soil physical condition: In acid soil condition the structure of fine textured soil will not improve, organic matter becomes to decrease. The soil becomes crusting, increasing its bulk density. Decreasing its infiltration capacity, rate of percolation of water & high power is needed for tillage operation.