Wole Akinremi¹ C. A. Grant², D.V. Ige¹. X.Gao², D. N. Flaten¹, M. Tenuta¹, S. Malhi³

¹Department of Soil Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 email: akinremi@cc.umanitoba.ca

²Agriculture and Agri-Food Canada, Brandon Research Centre, Box 1000A, R. R. #3, Brandon, MB, R7A 5Y3, Canada phone: 204 578-3570  Facsimile 204 728-3858

³Agriculture and Agri-Food Canada, Melfort Research Farm, Melfort, SK, Canada

Key words:  STP, Mehlich-3 P, Olsen-P, Water-P, phosphate fertilizer

The application of phosphate  (P) fertilizer is known to influence soil test P (STP). If the rate of change of soil test P following application of phosphate fertilizer can be estimated, this will help to better tailor fertilizer application to meet soil test P thresholds that are set for environmental purposes.  Also, how changes in soil test P are influenced by soil factors is difficult to predict. The study took advantage of field plots that were established in 2002 at 7 sites across the Canadian prairies to evaluate the impact of repeated applications of 0, 20, 40 and 80 kg P ha^-1 applied as monoammonium phosphate (MAP) fertilizer containing 0.38, 70 or 210 mg kg^-1 of Cd. The sites were planted each year, using a durum wheat-flax cropping sequence. Soil samples were taken from each site in the fall at 0-7.5 and 7.5 to 15 cm depth. Archived samples for the plots with the highest rate of P (80 kg P ha^-1) from 2002 to 2008 were analyzed using Olsen, Mehlich-3 reagents and water.  The changes in STP were related to cumulative P applied using linear, quadratic and exponential functions. Changes in the parameters of these functions as related to different STP method, soil depth and site will be presented and discussed.