Are we mining soil phosphorus?

March 11, 2015 - Issue 4

Phosphorus removal

Phosphorus fertilizer rates have, pretty generally, lagged crop removal for some time. As crops remove the soluble phosphorus in the soil, the soil’s large reserve of less soluble forms will transfer some more phosphorus into the available “labile” pool. However, this labile pool may be losing its ability to keep up, resulting in a “hidden hunger” for phosphate in many crops in western Canada.

Yield loss is likely occurring — even if growers and agronomists don’t have a good handle on how big and how widespread that loss is. All we know is that if the trend continues, this will get worse — especially with high acreage of P-hungry crops like canola.

Phosphate fertilizer is seen as an investment in the soil. Return on investment may not be obvious every year, thanks to the labile pool coming to the rescue, but the labile pool is not an infinite resource, and sucking down the labile pool does have implications for soil health and long-term productivity.

ACTION: Soils now in the “low” range for phosphate will need rates that at least match crop removal so the situation does not get worse. If these rates exceed safe seed placed rates for canola and growers do not want to add phosphate into their banded fertilizer blend, then consider increasing phosphate rates for cereal crops to provide an overall replacement rate across the whole rotation. Note that safe seed-placed rates of phosphate for cereals, while higher than for canola, are 50-60 lb./ac. If that will not match removal across the whole rotation, banding phosphate with nitrogen and sulphur is another option.

Phosphorus Recommendation Strategies for Manitoba

Phosphorus calculator: P fert’n for rotation interactive v4

Canola Watch