Degree of Phosphorous Saturation in Intensively Cultivated Soils in Sri Lanka

The loss of Phosphorous (P) from intensively cultivated soil is one of the major causes of reducing water quality. The Degree of Phosphorous Saturation (DPS) which relates the extractable P of a soil to its P adsorption capacity is a good indicator of the soils potential to release P. Therefore, the objective of this research was to determine the DPS in intensively cultivated soils in Nuwara Eliya region of Sri Lanka and to predict P losses with runoff water. Twenty seven surface soil samples from intensively cultivated fields were analyzed for pH, water extractable P (P_H20) and Mehlich 3 extractable P. Single point adsorption study was conducted to determine the P adsorption capacity of soil (P₁₅₀). The Degree of Phosphorous Saturation was calculated as the percentage ratio between Mehlich- 3 extractable P and single point adsorption capacity. The runoff potential of P from soils was evaluated using P_H20, where 20% DPS was considered as the cutoff point for runoff losses. The studied soils were acidic to strongly acidic in reaction. Very high Mehlich -3 extractable P levels ranging from 17.3 to 298.4 mg/kg P with the mean of 155.5 mg/kg P were observed. Phosphorous adsorption capacities were also high in soils with a mean of 664.7 mg/kg P. Calculated DPS values ranged from 1.7 to 80.4% with mean of 27.7%, while, 55.6% of soils had DPS values higher than 20%. DPS showed a considerably strong relationship (r² = 0.67) with P_H20 and the corresponding PH20 at 20% DPS was 8.8 mg/kg P. Along with high DPS values, 59.3% of the studied soils were observed to exceed critical P_H20 content and thereby they were at a risk of losing P from soil due to runoff. <br /><br /><strong>Key words:</strong> Degree of Phosphorous Saturation (DPS); Mehlich 3 extractable P; Water extractable P (P_H20). <br /><br />DOI: 10.4038/tar.v22i1.2676<br /><br /><em>Tropical Agricultural Research</em> Vol. 22 (1): 113-119 (2010)