Identification and quantification of organic phosphorus forms in soils from fertility experiments

The effects of soil type, crop rotation, fertilizer type and application rate on the composition of organic phosphorus (P) compounds in soils from four sites in a Swedish long-term fertilizer experiment were investigated with 31P-NMR. Soil textures investigated were loamy sand, sandy loam, silty clay loam and clay. Phosphorus has been added to the soils since the 1950s and 1960s at four different rates in the form of either mineral fertilizer or a combination of manure and mineral fertilizer. Results show that in soils receiving no P addition, most of the soil P was present in the form of phosphate monoesters (6070%, depending on soil type). However, a P addition equivalent to the amount of P removed annually by harvest altered this relationship so that the soils were dominated by orthophosphate instead. This trend became more obvious with increasing P addition. At the greatest P application rate, orthophosphate comprised 70% or more of the total extracted P in all the soils. These changes in the soil were due entirely to increase in orthophosphate, because the amounts of monoesters did not change with increasing P additions. This was true both for mineral fertilizer and the combination of manure and mineral fertilizer P. Soil type and crop rotation did not influence the results. The results indicate that there is no apparent build-up of organic P in the soils, but that P addition mainly affects the orthophosphate amounts in the soils regardless of form or amount of fertilizer.