Phosphorus sorption and external phosphorus requirement of soils: the Case of Metahara Sugar Estate, Ethiopia.

Abstract

Low availability of P limits crop production in the Ethiopian soils. There are many reasons for low P availability to soil. One of them is sorption. Soil phosphorus sorption and supply characteristics of the Metahara Sugar Estate soils were investigated using soil samples collected from soil fertility Class I, Class II and Class III of the farms. In this sorption study, three grams of soil from each sample was equilibrated with 30 ml of 0.01 M CaCl2 solution containing different amount of KH2PO4 and the concentration of P was determined by ascorbic acidmolybdate method. Eventually, the amount of adsorbed P by each soil was calculated and the data were fitted into the Langmuir and Freundlich equations. Results indicated that P sorption data of all soils fitted well with Freundlich equations but not with the Langmuir equation. Freundlich Kf and external P requirement (EPRF) of the soils ranged from 92.54-96.94 and 17.3- 23.72 mg kg-1 respectively. There is no significant correlation in their Kf and EPR. The pH, OC, exchangeable K and Mg contents of soil had significant influences on the sorption capacities of Metahara sugar estate soils. Since all fertility class soils had EPR < 150 mg kg-1, they were classified under the low P sorption category. The low P sorption nature of the soils permits the use of low P fertilizer dose in the range of 34.6–47.4 kg ha-1. This implies that high P sorption is not P limiting factor in the estate. The result further indicates that current P fertilizer application rate of 30 kg P ha-1 being practiced across all soil classes of the farm needs to be revised after validating the EPR values estimated in this study for each soil both in the greenhouse and in the field at Metahara Sugar Estate.