Fertilizing with phosphorus serves a dual purpose. It supplies the crop’s phosphorus needs for the year of application. And it provides “residual” P to maintain soil P levels for future crops.

With higher prices for fertilizer these days, growers may be looking for ways to trim their fertilizer bill. You certainly don’t want to eliminate P fertilizer entirely, but you can probably cut the P you apply for “residual” purposes.

Westco data shows that crops given 15 to 20 pounds per acre of P2O5 will yield about the same crops given higher rates. (See figure 1.) Westco also found that yields improve with an application of phosphorus fertilizer, no matter what the soil test results tell you.

The bottom line is that you definitely want to apply some P fertilizer, but the rates do not have to be sky high.

 

Soil testing has been a pillar in deriving fertilizer recommendations. But a 1993 analysis of soil testing data from the Prairie provincial labs showed that average P soil test levels did not change much from 1965 to 1990, in spite of continuous fertilization with P. That suggests that the “residual” recommendation isn’t reflected in the soil test P levels with current soil testing methodologies. 

Westco also compiled yield data from 222 of its own experiments in Western Canada and concluded farmers should apply some P no matter what your soil test P levels. Although the frequency of responses was higher at lower soil test P levels, the data showed no clear trends. This suggests that response to phosphate fertilizer is affected by factors other than the soil test level. As a rule, the greater the severity of phosphorus deficiency, the greater the recommended rate of phosphate and the greater the response to its application.

 

First, phosphorus fertilizer use efficiency is fairly low and is crop dependent. For cereals it can range between 15 and 30 per cent. In other words, only 15 to 30 per cent of the P you apply will actually go to that year’s crop. For tap rooted plants such as canola, P use efficiency can be as high as 50 per cent.\

Second, prolonged P fertilizer use will result in well-supplied labile P pool (which is the pool containing all forms of potentially available P). So even if you forgo the “residual” part of your recommended P rate for 2008, you may want to bump it up again in the future.

 

 

Westco experiments show you get the greatest agroeconomic benefit from the first 15 to 20 pounds per acre of applied P₂O₅. This graph is based on results from a canola experiment carried out at Olds, Alta. A canola hybrid obtained 60 bushels per acre with 18 pounds of P₂O₅. Applying an additional 18 pounds of P₂O₅ per acre did not influence the yield. (In addition to the fertilizer, the field had 25 pounds of available P per acre, according to soil tests.)

Source: February 25, 2008 issue of Grainews- reprinted with permission