Changes in the nutrient content of agricultural soil on conversion to organic farming in relation to farm-level nutrient balances and soil contents of clay and organic matter

Agricultural soil on 12 farms converting to organic farming was sampled in 1989 and 1995 at the same sample points. Concentrations of plant-available (-AL) P, K, Ca and Mg, HNO3-soluble K, total-N, total-C and pH were measured in two layers. The average K-AL concentration and pH were reduced in both layers. The average P-AL concentration was reduced in the topsoil. K-AL and P-AL increased in samples with low concentrations and decreased in samples with high and very high concentrations in 1989. K-HNO3 increased in the topsoil and total-N increased in the subsoil. Total-N and total-C increased in top- and subsoil with a low organic matter content. The net import of P, calculated by farm-level nutrient balances, was negatively correlated to the change in kg P-AL ha(-1) in the topsoil. No such correlation was found for K.