Effect of application of three phosphate fertilizer sources on long term availability of different phosphorus fractions in lateritic soils in the intermediate zone of sri lanka

Abstract

The application of three different phosphorous (P) fertilizer sources, viz., Eppawela rock phosphate (ERP), Imported rock phosphate (IRP) and Triple super phosphate (TSP) on the long term availability of several different P fractions in Lateritic soils in the intermediate zone of Sri Lanka was investigated. Each P fertilizer was treated at three different levels: 25, 50 and 100 g / palm/ 6 months in triplicate over a period of 11 years to all 6 coconut trees grown in each of the 30 experimental plots. Three experimental plots with 6 coconut trees in each with no fertilizer application were used as controls. After 11 years of fertilizer applications, soils were sampled 90 cm away from the base of coconut palm at 2 depths; (0-25) cm and (25-50) cm from randomly selected three plants from each of the plot. Soils of each treatment, sampled in triplicate, for each of the two depths were composited separately. Different P fractions, viz., saloid bound P ( Sa-P), aluminum bound P (Al-P), iron bound P (Fe-P), occluded P (Occ-P) and calcium bound P (Ca-P) were determined in each composited soil. The results showed that, in the soil depth of (0-25) cm, the ability of different fertilizers in producing plant available P; Sa-P and Al-P vary in the order of TSP>> IRP>ERP. Further, it was found that, the availability of Sa-P and Al-P in the TSP treated soil increased with increasing level of TSP application to the soil. The concentration of different P fractions in the TSP treated were in the order of Sa-P>Al-P>Occ-P>Fe-P>Ca-P. The increasing level of IRP and ERP applications did not increase the Sa-P and Al-P concentrations in the soil. The trend of the availability of different P fractions in the soil depth of (25-50 cm) was the same for all three of fertilizer sources at all three levels applications. However, the concentrations of soil P fractions found in this depth was smaller than the amounts found in the (0-25) cm depth.