PHOSPHORUS SUPPLYING CAPACITY OF HEAVILY FERTILIZED SOILS .1. PHOSPHORUS ADSORPTION CHARACTERISTICS AND PHOSPHORUS FRACTIONATION

Studies were conducted to investigate the P sorption characteristics a nd P fractions in eight intensively fertilized soils collected from so uthern and central Norway. Adsorption of P at the initial P concentrat ions in the soil solution was very high in the Saerheim clay loam soil which contained high amounts of organic C and clay. Adsorption data w ere fitted well to the classical Langmuir equation. The P affinity con stant (k), adsorption maximum (b) and maximum buffer capacity (mbc) ca lculated from this equation differed considerably among soils. The P a ffinity 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. N one of the soil parameters were related to adsorption maximum. Phospho rus desorption from the heavily fertilized soils varied widely and dep ended on the initial P status of the soil and soil texture. The ratio between desorbed P and total P was significantly correlated to sorptio n parameters. Multiple regression analysis showed that total P positiv ely 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 n ot relate to average P removed per harvest (RPH). Calcium-P and Al-P w ere not related to any of the soil parameters but these fractions were the major contributors to RPH as expressed by a multiple regression e quation: RPH = 0.397 + 0.0016 x Ca-P + 0.0012 x Al-P (r = 0.84, p = 0. 05). High content of inorganic fractions shows that most of the residu al P may be plant available, albeit at reduced rate with time, in thes e soils but the availability will depend on soil types.