Phosphorus forms and desorption patterns in heavily fertilized calcareous and limed acid soils

Phosphorus desorption from fertilized soils is significant for agriculture and the environment. This study was aimed at characterizing the forms in which P occurs in calcareous and limed acid soils and their influence on P desorption patterns. The P forms of two limed acid and three calcareous soils of Europe were characterized by chemical extraction, isotopic exchange, and sequential fractionation. Phosphate desorption was studied using resins and goethite as ion sinks. Total P ranged from 705 to 1400 mg kg(-1). Isotopic exchange revealed more surface-adsorbed P in the limed acid than in the calcareous soils. Calcium phosphates predominated im the calcareous and Al and Fe phosphates in the limed acid soils; one limed acid soil contained significant amounts of Ca phosphates. The mixed cation and anion exchange resins (ACER) were more effective in desorbing P than the anion exchange resin (AER) and goethite. The desorption curves indicated significant differences in the degree of homogeneity of the P desorption reactions, the greatest and lowest degree of homogeneity corresponding with calcareous soils. The proportion of ACER-extractable P that was plant-available P was > 50% for the limed acid and < 20% for the calcareous soils. The proportion of ACER-extractable P that was released to 0.002 M CaCl2 was > 60% for the limed acid and < 40% for the calcareous soils. Plant P availability was thus poorer in the calcareous soils than in the limed acid soils.