Phosphorus status of diverse soils in Finland as influenced by long-term P fertilisation I. Native and previously applied P at 24 experimental sites

Abstract

The native reserves of phosphorus (P) in Finnish soils are sizeable but poorly available to plants. The P status has been generally poor, but large rates of P applied since the 1940s have substantially enhanced the supply of P from the soil. As a means of optimising P fertilisation of crops grown in the improved soils and promoting agro-environmental sustainability of agriculture, long-term field experiments were established in 1977-1981 on various soil types around Finland, between the latitudes of 60 and 65 N. The experimental sites (8 Cambisols, 9 Regosols, 6 Histosols, 1 Podsol) were moderately acidic (pH w 5.6, range 4.6-6.6) and contained 1.6-31.2% organic C, and (mineral soils) 3-74% clay. The concentration of total P was 1.22 (0.66-1.87) g kg-1, and the inorganic fraction separated with fluoride and hydroxide extractions (Al,Fe-P) was 8.2% (4.5-12.4%) of oxalate extractable Al+Fe (P saturation index). In mineral topsoil the pool of Al,Fe-P was 850 kg ha-1, which is 300 kg ha-1 larger than was obtained in the early 1960s and 600 kg ha-1 larger than found in virgin soils. The soil test P value was 13.1 (3.7-60) mg dm-3 by the acid ammonium acetate method, 11.5 (1.6-42) mg dm-3 by a water extraction method (1:60 v/v) and 61 (23-131) mg kg-1 with a modified Olsen method. The intensity factor of soil P status has remained at a relatively low level in a major part of the cultivated area.