Agroscope, Institute for Plant Production Sciences IPS, 1260 Nyon, Switzerland

DOC-trial: Phosphorus budget and availability over 21 years

The aim of this work was to assess the long term effect of organic and conventional farming on concentrations of total and available phosphorus (P) in cropped soils. Soil samples were taken from a field trial comparing a non-fertilised control (N), two organically cultivated treatments (D2, O2) and two conventionally cultivated treatments (M2, K2). Samples were taken from each treatment at two depths (0-20 and 30-50 cm) before starting the field trial (1977) and at the end of every three crop rotations (1984, 1991 and 1998). They were analysed for total P and P isotopically exchangeable in 1 minute (E1) which represents plant avaiable P. Over 21 years, the average P input by fertilisers was lower than P output by harvested products in all treatments except K2. This resulting average P balances ( input minus output) were: N, -21, D2, -8, O2 , -6, M2, -5, K2, +4 kg P/ha/year. In addition, over 21 years, on average between 5,5 and 10,9 kg P/ha/year was lost from the topsoil of fertilised treatments. In the same time, P contents in the subsoil increased between 7,0 and 8,7 kg P/ha/year. Topsoil, E1, values decreased from an initial value of 12 mg P/kg to 11 in K2, 8 in M2, 6 in O2, 5 in D2 and 2 in N in 1998. Because of negative P balances, E1 continues to decrease in D2 and O2 soils. Whether the applied fertilisation strategy will limit crop growht remains to be investigated.

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