Phosphorus budget and phosphorus availability in soils under organic and conventional farming

The aim of this work was to assess to which extent organic farming practices would affect the accumulation of total and available phosphorus (P) in a cropped soil in comparison to conventional practices. In order to achieve this, soil samples were taken from a long-term field trial comparing a non-fertilised control (NON), two conventionally cultivated treatments (MIN, CON), and two organically cultivated treatments (ORG, DYN). Soil 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 then analysed for total P (P-t), total inorganic P (P-i), total organic P (P-o) and isotopically exchangeable P-i. After 21 years, the average P input-output budget reached -20.9 kg P ha(-1) a(-1) for NON, -7.8 for DYN, -5.7 for ORG, -5.0 for MIN and +3.8 for CON. Total P, P-i as well as the amount of P-i isotopically exchangeable within 1 minute (E-1) were positively correlated to the P budget. Comparison between P budget and P-t in the top- and subsoils of the fertilised treatments suggested a net transfer of P from the 0-20 to the 30-50 cm layers between 13 and 26 kg P ha(-1) a(-1)during the first rotation and between 3 and 12 kg P ha(-1) a(-1)during the second rotation. During the third rotation a net upward movement of P from the subsurface to the topsoil ranging between 3.7 and 10.5 kg P ha(-1) a(-1)was estimated. In the topsoil, E(1)decreased from an initial value of 12 mg P kg(-1) to 11 in CON, 8 in MIN, 6 in ORG, 5 in DYN and 2 in NON after 21 years. In the subsoil, E-1 increased from an initial value of 2 mg P kg(-1) to 4 in MIN, ORG, DYN and NON and to 6 in CON. These results show that, with the exception of NON, all treatments had still an adequate level of available P after 21 years of trial and that, in this low to moderately P sorbing soil, an equilibrated input-output budget allows to maintain P availability at a constant level. In the organic systems, yields have so far partly been attained at the expense of soil reserves or residual P from earlier fertiliser applications.