Chemical forms of phosphorus of volcanic ash-derived soils in Chile

The volcanic ash-derived soils have strong capacity to retain phosphorus (P ). The P in 10 virgin Ultisols and Andisols in Southern Chile and their cla y size fractions were fractionated into inorganic, fulvic, and humic fracti ons using an acid and base sequential extraction procedure. These soils had similar mineralogy, and were formed under different climatic conditions, a nd were under different native vegetation. They are representative of the m ajor agricultural areas. The total content ranges from 733 mg P kg(-1) to 3 470 mg P kg(-1). As groups, Andisols had higher total P concentrations than the Ultisols. The organic P content was linearly correlated to organic car bon (C). In general, soils with higher organic C content had higher percent age of its P in the organic form. Clay size fraction accounted for 35 to 60 % of the soil and had significantly higher concentrations of P in all fract ions. In the clay fraction, the organic P content was also influence by the ir organic C. Soil depth did not affect the P distribution neither in the s oils nor in the clay fractions.