Effect of phosphate fertilization on crop yield and soil phosphorus status

To evaluate the effect of three phosphorus (P) fertilization regimes (no P, P input equivalent to P off-take by crops, P input higher than P offtake) on crop yield, P uptake, and soil P availability, seven field experiments (six in crop rotations, one under permanent grassland) were conducted in Switzerland during nine years (six trials) or 27 years (one trial). Soil total P (Pt), inorganic P (Pi), organic P (Po), and the amount of isotopically exchangeable soil P were measured in the 0-20 cm and 30-50 cm layers of the arable soils and in the 0-10 cm layer of the permanent grassland soil. Omitting P fertilization resulted in significant yield decreases only in one field crop trial as the amount of P isotopically exchangeable within one minute (E-1min) reached values lower than 5 mg P (kg soil)(-1). In the absence of P fertilization Pi decreased on average from 470 to 410 mg P (kg soil)(-1) in the upper horizon of 6 sites while Po decreased only at two sites (from 510 to 466 mg P (kg soil)(-1) on average). In all the treatments of the trials started in 1989 the E-1min values of the upper horizon decreased on average from 15.6 to 7.4 mg P (kg soil)(-1) between 1989 and 1998. These decreases were also observed when P inputs were higher than crops needs, showing that in these soils the highest P inputs were not sufficient to maintain the high initial available P levels. Finally for the six arable trials the values of the isotopic exchange kinetics parameters (R/r(1), n, C-P) and P exchangeable within 1 minute (E-1min) at the end of the experiment could be estimated from the values measured at the beginning of trial and the cumulated P balance.