Toxicity of Seed-Placed Phosphorus and Sulphur Fertilizers for Canola Production.

The rate of fertilizer applied with canola seed is limited by crop safety.  Rates of phosphorus (P) and sulphur (S) fertilizers applied to maximize yield potential and maintain soil fertility could reduce plant populations below the critical threshold, potentially reducing yield.  Differences in soil properties affect the toxicity of P and S fertilizers and, therefore, the maximum rate of fertilizer that can be safely applied with canola seed.  The objective of this study was to determine the safe rates of seed-placed blends of monoammonium phosphate (MAP) and ammonium sulphate (AS) fertilizer across a range of soil types.  MAP was applied at rates of 0, 20, and 40 kg P2O5/ha and AS was applied at rates of 0, 9, and 18 kg S/ha.  The treatments were replicated under field conditions at six sites across Canada and in the growth chamber using soil from the knoll and mid-slope positions collected from two fields in western Manitoba.  Seedlings were counted at two and four weeks after crop emergence.  Seedling damage was highly variable, depending on year, site, landscape position, soil properties and source and rate of fertilizer.  In situations where damage was observed, damage was greater for AS than for MAP, especially in calcareous soils.