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.