Catching and keeping the elusive phosphorus

To simulate snowmelt runoff, researchers used slabs of soil with snow on top of them. The slabs were then removed from the field plots for analysis.  |  T. King photo

What form phosphorus will take in a particular soil depends on the chemistry of that soil. The transformation can do strange things to fertilizer.

Soils with a lot of calcium, such as those in eastern Saskatchewan and Manitoba, will force the fertilizer to reside in a form known as calcium phosphates. These alkaline soils vary in their plant availability and solubility according to the pH.

In soils with higher pH, the calcium phosphate reaction products in the fertilizer become less soluble and less available to plants, according to Saskatchewan farmer Jeff Schoenau.

Schoenau, a soil scientist at the University of Saskatchewan, says acidic soils have a different chemical reaction, but the result is the same. Phosphorus is absorbed by iron and aluminum in the soil, making it less available. The lower the pH, the more Phosphorus gets tied up in the soil.

“The availability of Phosphorus in the soil is maximum around a neutral pH, around 6.5. That’s ideal,” says Schoenau.

“At that point, we don’t see much fixation by calcium, nor do we see much fixation by iron and aluminum.”

For the most part, the soil pH we had when we took over the land we farm is the soil pH we’ll have to live with forever. The exception is if you’re on highly acidic soil. A few Canadian prairie farmers have begun rejuvenating low pH soils by adding lime.

“Increased acidity is a consequence of decades of nitrogen fertilizers, many of which are acidifying. They produce protons when the ammonium converts to nitrate. So we’re seeing some of our prairie soils becoming more acidic, particularly the poorly buffered sandy soils. We’re seeing a drop in pH in the zone where that nitrogen fertilizer is applied every year.

“That brings into concern some issues about declining pH and will this dramatically reduce the availability of phosphorus. Or might we even see increased solubility of micro-elements such as manganese to the point of crop toxicity.

“We are becoming aware of issues relating to micro-scale or small zone acidity that may ultimately necessitate the application of lime down the road. As time goes on, we’re obviously going to apply more nitrogen fertilizer to grow crops, it’s something we definitely have to monitor. It will be especially significant for farmers on those light sandy poorly buffered soils with low organic matter.”

Schoenau says these soils lack the ability to resist the decrease in pH resulting from the protons formed by the nitrification process. He says it affects most nitrogen fertilizers we use. It’s not out of the question to assume that there are already areas on the Prairies that need lime.

“Liming is not something we think about much here on the Prairies, but it may be necessary. It’s not widely available and it’s not something we’re set up to do like they are in the mid-western states. Down there it’s part of a normal practice. They’ve been dealing with acidic soils for a long time.

“I’m thinking of strategies to address the issue before it gets out of hand. For instance, in the semi-arid regions, some farmers have gone to nitrate fertilizers instead of ammonium fertilizers. The nitrate fertilizers don’t produce acidity. Under low moisture conditions, the nitrate fertilizer may not be susceptible to loss.

“Another low-cost option is a strategic tillage operation to mix some of that B horizon with the A horizon. This can bring some of that deep carbonate up to the surface. Just a shovel type tool to mix some of that carbonate-enriched B horizon with the surface soil. We’ve eliminated tillage and moved toward very low disturbance, so we’ve lost that mixing of the carbonates we used to have.”

Many farmers and soil scientists now advocate periodic strategic tillage on zero till land to mix that valuable surface organic matter into the soil where they say it will do more good. That same operation can do as Schoenau suggests by bring B soil to the surface.

The obvious problem is any such tillage will bury residue and leave the soil susceptible to wind and water erosion for a period of time.

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