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How To Stretch Your Phosphorus Dollar

When fertilizer prices climb many farmers begin to look at ways to get more out of the fertilizer they are using. There are now a number of different products on the market which claim to help them do this. In reality, however, research suggests that good old-fashioned recommendations may work just as well as any fancy new product. Keeping tabs on what is in your soil and applying the appropriate rate in the rate spot may be all you need to get the most out of phosphorus.

Fertility research across the U.S. and Canada has led many in the scientific community to question whether some of the current recommendations for fertilizer applications are still relevant in view of the better-yielding crop varieties and modern cropping systems. There’s also discussion about whether fertilizer use efficiency, in some cases, is being underestimated.

Cynthia Grant, a soil scientist with Agriculture and Agri-Food Canada, has been conducting extensive studies into how to improve phosphorus use efficiency at AAFC’s Brandon Research Centre. She has examined most of the different products and methods of P fertilization in comparative trails to try and determine which give better results in terms of yield response and P retention in the soil.

Grant has come to the conclusion that there may be a general tendency to underestimate the efficiency of P utilization (and other nutrients, such as nitrogen) in crop systems in Western Canada. She believes that has a lot to do with the way that use efficiency is calculated

The current means of calculating P use (in studies), she says, neglects to take into account the way in which P accumulates in the soil over time. “We neglect to recognize the P that is there in (a) zero (pounds applied) treatment was largely coming from applications of fertilizer that was applied previously,” she says. “The P the crop has accumulated and is extracting from the soil is actually P that has been put on before.”

But P use efficiency really only considers the year of application, says Grant. Studies assume that what isn’t taken up by the crop in that year is lost, but long-term studies that looked at P recovery seem to refute this common wisdom.

“Recovery over a number of years is much higher than we talk about,” says Grant. “I think the 20 to 30 per cent efficiency rate that we think we are getting from the fertilizer we are applying may be too low and we need to take a longer term look at the input versus the off-take balance over time.”

GETTING THE MOST OUT OF PHOSPHORUS

Maximizing your phosphorus invesment likely require any sort of fancy products. It may just be a case of returning to some fairly straightforward fundamentals and having a little better understanding of the nutritional requirements of the crop throughout its growth stages and figuring out how to meet them.

Grant, after evaluating most of the current P fertilizer treatments and various rates, applications methods and timings comes back to an already accepted conclusion; that P should always be banded very close to the seed in the row. “If you are side banding or seed-placing P fertilizer you are putting it in a position where the plants can contact it earlier,” says Grant. “The plant will extract the P from the soil and use it for growth.”

Banding also helps slow the process of tie up of P in the soil by calcium (tying up makes it less available to the plant) especially if ammonia is also present in the band, she adds.

DON’T SKIMP ON P APPLICATIONS

What is equally important, however, is the rate of application, which should be adequate to meet the needs of the developing plant. Reducing the application rate below 15 to 20 lbs. per acre, says Grant, may restrict availability due to the very low mobility of P in the soil. “What scares me is when people say, ‘if you use my wonderful P product you can cut the rate down,’ but a molecule of P is still a molecule of P and the plant needs so many molecules to grow,” she says. “It doesn’t matter how efficiently the plant takes it up, if it doesn’t take up enough it won’t grow as well.”

And even when there may be enough P present in the soil, the cold conditions of early spring will lower the solubility of that P and enough of it may not become readily available early enough.

“It’s really important that there is an adequate amount of P that the plant can access really early in the growing season,” says Grant. At this stage of growth the plant will still need a boost from some closely banded fertilizer. “It’s good to have an adequate amount of P from residual fertilizer that is there in the soil, but you also need a quick start pop up to get the plant growing energetically.”

Fertilizer rates also affect proximity to the root zone of the emerging plant. “The rate of P fertilizer must be high enough to allow each seedling access to a P granule or droplet early in the season,” says Grant. “Cutting the fertilizer rate too low may restrict plant access to P because there are not enough fertilizer granules or droplets present to provide physical proximity to each germinating seedling.”

For example, the granules of a dry P fertilizer applied in seven-inch row spacings at 10 pounds per acre are spaced at around 7.6 inches apart, whereas at a rate of 38 lbs./ac. the spacing is reduced to 1.9 inches apart, giving more likelihood that the seedling’s roots will find the phosphate. For more information on the physical distribution of fertilizer granules or droplets at various fertilizer rates visit: http://www.agviselabs.com/tech_art/starterfertilizer. php.

AngelaLovellwritesfromManitou,Man.

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Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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