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Holding on to phosphorus

Plants can’t always use the phosphorus in your soil. Here are six factors that determine available levels

Most soils on the Canadian prairies are phosphorus deficient — 80 per cent of the fields in Saskatchewan, according to Dr. Don Flaten, soil science professor at the University of Manitoba. In Manitoba, 60 per cent of fields were found to be lacking in phosphorus; 50 per cent in Alberta.

There are two major causes for this deficiency, says Flaten. “One is that most of our soils are naturally fairly low in phosphorus and what phosphorus they have is often tied up with chemical and biological processes that don’t let the phosphorus be easily taken up by crops.”

The other factor lies in the large amount of crop production on the Prairies. Farmers don’t always match removed phosphorus with supplementary application. Regular soil testing will give farmers an idea of how much phosphorus should be applied.

Besides crop removal, here are six factors that influence the availability of phosphorus in your soil.

1. Form

Phosphorus availability is greatly determined by its solubility — its ability to dissolve. Solubility relates to the form of the phosphorus (dissolved or particulate).

Dr. Jeff Schoenau, a professor in the Department of Soil Science at the University of Saskatchewan and the Saskatchewan Agriculture chair in soil nutrient management, says phosphorus in the soil is most commonly found as “orthophosphate that is adsorbed onto soil particles or part of minerals or part of organic matter; that is to say in the solid phase, as opposed to in the soil water.”

Because most soil phosphorus is in the solid phase, it is less susceptible to leaching loss than other elements like nitrogen, and can be considered relatively immobile.

2. Soil pH

The plant-available form of phosphorus is orthophosphate. There are two forms of this: primary and secondary.

You’ll find primary orthophosphate in acidic soils. In alkaline soils, secondary orthophosphate dominates. In soil with a neutral pH, you’ll find both in approximately equal amounts.

Soil pH is a primary factor in determining phosphorus availability. In low pH soil, orthophosphate will react with aluminum and iron and form a solid. In this form, the orthophosphate is no longer available for plant uptake.

In high pH soil, orthophosphate reacts with calcium and magnesium to form a solid, again leaving less orthophosphate available for plants.

Soils with a neutral pH have more phosphorus in the soil solution.

3. Microorganisms

Phosphorus available in the soil is determined by soil biology, particularly the actions of microorganisms in the soil around the roots.

4. Minerals

Over the long term, weathering of minerals such as apatite will release phosphorus into the soil.

5. Erosion

Most phosphorus is found in the top six inches of the soil, so whenever topsoil is lost, so is phosphorus.

On most fields, tillage has little effect on phosphorus erosion — more phosphorus is lost in the dissolved form than the particulate form. However, erosion is an important factor on slopes.

6. Water

Dr. Rigas Karamanos, manager of agronomics solutions with Viterra, says, “If you can control the movement of water, you can control the movement of phosphorus.”

Typically, there are three main ways to lose phosphorus from a field to a nearby water body: leaching, erosion, and runoff.

Flaten says leaching rarely results in significant phosphorus loss on the Canadian Prairies.

Runoff is the major form of phosphorus loss in Prairie watersheds. In most soils, the amount of phosphorus lost in soil water would be very small, even considering runoff in manured soils.

Schoenau says, “environmentally, the amount of phosphate that would be moved is significant enough to be of concern, but in terms of pounds per acre, it’s typically less than one.”

Phosphorus that finds its way to water bodies can seriously deteriorate water quality, interfering with the nutrient balance in the water and encouraging the growth of algae.

Applying phosphorus

Farmers may not always see a response to applied phosphorus. Phosphorus is a very reactive compound — crop growth and phosphorus supply are very sensitive to environmental conditions. Crop response to phosphorus can vary greatly in the same field in a given year.

There is a finite supply of phosphorus available for farm use. There is a debate about how much easily accessible phosphorus can be mined. Lobbyists are working to stop corporations from expanding existing mines and starting new ones

More research is needed into finding new ways to use insoluble phosphorus in our soils, and breeding new crops that can more effectively scavenge nutrients in the soil.

Schoenau says farmers “always need to be looking at ways to maintain phosphorus.” There is a finite supply in the soil. Once phosphorus is washed out into oceans, it’s no longer available to farmers. †

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