Phosphorus supplying capacity of heavily fertilized soils I. Phosphorus adsorption characteristics and phosphorus fractionation

Abstract

Studies were conducted to investigate the P sorption characteristics and P fractions in eight intensively fertilized soils collected from southern and central Norway. Adsorption of P at the initial P concentrations in the soil solution was very high in the Særheim clay loam soil which contained high amounts of organic C and clay. Adsorption data were fitted well to the classical Langmuir equation. The P affinity constant (k), adsorption maximum (b) and maximum buffer capacity (mbc) calculated from this equation differed considerably among soils. The P affinity constant (r=0.96,p=0.01) and maximum buffer capacity (r=0.97,p=0.01) were highly and positively correlated to organic C. None of the soil parameters were related to adsorption maximum. Phosphorus desorption from the heavily fertilized soils varied widely and depended on the initial P status of the soil and soil texture. The ratio between desorbed P and total P was significantly correlated to sorption parameters. Multiple regression analysis showed that total P positively and organic C negatively affected P desorption in the soils. Iron-P was a major P sink in these soils and it was related to clay content (r=0.69,p=0.1) and organic P (r=0.76,p=0.0.5), but it did not relate to average P removed per harvest (RPH). Calcium-P and Al-P were not related to any of the soil parameters but these fractions were the major contributors to RPH as expressed by a multiple regression equation: RPH=0.397+0.0016 × Ca-P + 0.0012 × Al-P (r=0.84,p=0.05). High content of inorganic fractions shows that most of the residual P may be plant available, albeit at reduced rate with time, in these soils but the availability will depend on soil types.