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Phosphorus gets short shrift

While neighbouring southern Saskatchewan canola fields were being swathed, one Southey-area farmer’s crop was far behind, and not nearly ready to be cut. Andrew, who farms 2,600 acres of barley, peas, wheat and canola, thought the variety could be at fault for the crop’s delayed maturity, stunting and bluish tinge.

“Why is my crop so far behind my neighbour’s?” he asked me. “Could it be the variety? And why is my crop so short?”

An initial examination of the plants in the field also revealed the number and size of the plants’ pods were reduced when compared with neighbours’. The root systems were also smaller than average.

However, I didn’t think the variety was the problem because Andrew had used one of the best hybrids for the area with a proven track record for vigour, yield and quality. Andrew and I also eliminated any issues surrounding the seeding of the crop, such as timing, rate and depth, as well as insect feeding and herbicide injury as possible causes of the crop’s delayed development. It was the field’s history and fertility records that ultimately provided the key information to solving the puzzle of the crop’s slow maturity.

Yields of 75 bushels per acre, 30 bu./ac., and 35 bu./ac. of barley, peas and wheat, respectively, were typical for this field. Last spring, Andrew had applied 80-0-0-20 side-banded with three gallons per acre of seed-placed orthophosphate fertilizer. Like many growers on the Prairies, Andrew thought he was taking care of the fertility needs of his plants, but this was not the case. Past soil test results revealed declining phosphate levels. Therefore, the cause of the stunted, short canola crop was a phosphate deficiency!

Like many farmers in the West, Andrew had underestimated how much nutrient, particularly phosphorus, was being removed by his harvested crops. Previous barley, pea and wheat crops had removed 31.5 lbs./ac., 21 lbs./ac. and 21 lbs./ac., respectively, of phosphate from the field’s soil. Andrew had been replacing only eight lbs./ac. with the application of the liquid orthophosphate fertilizer, which led to declining levels of plant-available phosphate.

When considering phosphate nutrition, it is important to replace what is being removed from the soil in order to avoid depleting the phosphate available to subsequent crops. Knowing what each crop removes and replacing that amount is essential for healthy plant growth and development. Western Canada is fortunate to have fertile soils that can supply a lot of nutrients; however, over time yields will eventually be affected if the plants’ actual nutrient requirements are not being met.

Unfortunately, it was too late to do anything for Andrew’s crop that growing season. The field yielded only 22 bu./ac. — well below the area average. However, this year, Andrew will seed-place from 35 to 40 lbs./ac. actual of 11-52-0 to restore the depleted soil phosphate to appropriate levels. In the future that rate will be decreased slightly to a minimum of 30 lbs./ac. actual of 11-52-0.

Understanding the amount of phosphate each harvested crop removes from the soil and what the different forms of phosphorus in the soil (organic, available inorganic and unavailable inorganic) actually supply to the crop, and applying this information to his phosphorus management should maximize Andrew’s yield and economic returns in the future. †

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