Organic matter-phosphorus associations as a sink in P-fixation processes in allophanic soils of Chile

In fertilized and unfertilized plots of ten Chilean allophanic soils the fraction of total-P associated with organic matter and the organic carbon content were determined. Cultivation with P fertilization for 40 yr or more increased the organic C and organic P in most of the soils. It is hypothesized that these effects are related to the high biochemical activity of these soil environments which are dominated by actinomycetes and fungal species as well as to the chemical and physical properties of the soils. It appears that, following its fast adsorption onto active allophanic surfaces, added-P is subjected to reactions resulting in the formation of organic matter-P associations or complexes possibly through Al bridges. The humus-P complexes compose the major portion of total-P in these soils and may be viewed as a P sink in the overall P-cycling process. There is a need for further research on the specific mechanisms by which plant roots utilize P-sinks as sources of P. Based on microbial activity a model of the P-cycle operating in these soils is hypothesized.