Influence of form of organic matter on the mineralisation of applied phosphorus in submerged rice soils

Abstract

A field experiment to study the effect of incorporation of different forms of organic matter on the transformation of native and applied phosphorous in submerged rice soils and their effects on the growth and yield of rice (Var. Triveni) was conducted in the Instructional Farm of the College of Agriculture, Vellayani during second crop season in 1986. The treatments included the application of three different form of organic materials (farm yard manure,greenleaves and composted salvinia) and two phosphate fortilizers (super phosphate and Mussooria phosphate). Practionation of soil inorganic phosphorous at fortnightly intervals for a period of 90 days and estimation of available P, exchangeable iron and soil reaction were carried out. Observations on the growth characters were made at active tillering, maximum tillering and flowering and yield of straw and grain were recorded. The chemical analysis of the grain and straw for N, P, K, Ca and Mg was also carried out. It was found that in general submergence of soil increased the transformation of both native and added P in to Fe-P, Al-P, Ca-P and saloid – P. Reductant-P and occluded-P fractions decreased with progressive submergence. Submergence of the soil markedly increased the available P status of soil. Phosphorous applied as super phosphate and Mussooria phosphate was recovered in different inorganic fractions in the order Fe-P > Al-P > Red-P > Occluded-P Ca-P >saloid-P. Organic matter addition suppressed the transformation of native and added inorganic phosphorous into different inorganic P fractions (Fe-P, Al-P, Red-P, Occluded-P and Ca-P) and maintained higher amount of added P in the soil in available form during the initial 75 days of submergence. A marked increase in exchangeable iron was also observed with continuous submergence. The process was accelerated by the addition of organic matter. Phosphate application significantly decreased exchangeable iron content in the soil. Submergence of soil resulted in a significant increase in soil pH upto 45 days beyond which it remained more or less constant. Organic matter treatment significantly decreased soil pH whereas addition of P in the form of Mussoorie phosphate increased the soil pH. The growth parameters (tiller count and plant height) recorded a significant increase due to the application of organic matter and phosphorous. The grain and straw yield increased significantly with the application of P fertilizers and organic matter. Application of Mussoorie phosphate plus green leaves recorded the highest grain and straw yield. There was a significant increase in the mineral contents in the plant by phosphorous and organic matter application. The study highlights the importance of organic matter in producing a better response to phosphatic fertilizers by rice in the submerged paddy fields.