WATER-SOLUBLE P AS AFFECTED BY FRESHLY APPLIED AND RESIDUAL P AND P FRACTIONS OF SOIL

Abstract

Water-soluble P content of soils is not regularly investigated in Hungary, but we need to understand its dynamics as influenced by previous and recent fertilization in order to predict dissolved P load from agricultural areas at the watershed of Lake Balaton, Hungary. Two sites were selected from a long-term fertilization trial representing characteristic soils around the lake. Soil samples from plots with different P fertilization were investigated in an incubation experiment in which fresh P was applied in a single dose or split and water-soluble P was measured. Available P, water-soluble P and modified Hedley P fractions were determined on the original samples. The soil at Keszthely has lower total P content, but higher rates of available (ammonium-lactate-soluble) P and mineralizable Hedley P fractions than the soil at Bicsérd. Because of these “light” fractions, the incubation produced temporarily high-water soluble P content even in the unfertilized soil. Initial quick fixation of freshly applied P has been higher in the soil at Bicsérd (65–75%) than at Keszthely (55–70%). Further, the soil at Bicsérd has additional capacity to fix P (approximately 1% per week) and that property was only slightly influenced by the fertilization history. However, only the soil from unfertilized plot at Keszthely has shown this ability. Soils like we have collected at Keszthely pose higher risk to pollute Lake Balaton with P than soils from Bicsérd. Regulations in order to reduce P load should be focused more on the previous soils.