Where Has All The Phosphorus Gone?

By Joanne Thiessen Martens

Phosphorus (P) is an essential nutrient for crop growth. Studies from western Canada and around the world tell us that in many cases soil on organic farms is deficient in available P. Is this a serious problem, or are low levels simply a reflection of the way we measure P?

Cathy Welsh, a recent M. Sc. graduate from Department of Soil Science, Soil Ecology Laboratory at the University of Manitoba, devoted some of her research to this question. Working with Dr. Mario Tenuta, Welsh studied the size of the various pools of phosphorus in the soil and how they were affected by crop rotation and organic vs. conventional management systems.

Available P, sometimes called “soil test P”, is only a small portion of the total P in the soil. Large amounts of phosphorus exist in the soil, but in a variety of forms, that range from moderately available to highly unavailable to plants. The less available P is not detected by standard soil test procedures, which evaluate only the P usable by plants and not other forms of P in the soil.

What happens to the rest of the phosphorus in the soil when the available P is used up? Is P from the less available forms converted to a more available form? If so, at what rate is P made available? Are the less available forms of P also being depleted? These are some of the questions Welsh set out to address.

As part of Welsh’s research, she collected soil samples in fall of 2004 from the Glenlea Long-Term Rotation Study south of Winnipeg, which is headed up by Dr. Martin Entz. Samples were taken from annual and forage-based crop rotations which had been under both organic and conventional management systems since 1992.

Phosphorus was extracted from the soil one fraction at a time, allowing Welsh to separate it into four pools based on availability to plants. The first fraction was extracted with water. This portion is known as orthophosphate and is the form most easily taken up by plants. The second fraction, extracted with sodium bicarbonate, is a form plant roots also utilize and is what soil test laboratories measure. It contains inorganic phosphorus that is weakly bound to aluminum and iron in the soil, as well as organic phosphorus weakly associated with soil organic matter. The third fraction, extracted with sodium hydroxide, is slightly available to plants. This fraction is made up of phosphorus tightly bound to aluminum, iron, and soil organic matter. The fourth fraction, extracted with hydrochloric acid, consists mainly of apatite-type P, which is the form of P found in rock phosphate. This form is highly unavailable to plants.

When the four fractions were added up, the total extractable P ranged from 259 to 345 parts per million (ppm). In comparison, the soil test P for the same soils ranged from 6 to 26 ppm. Soil test values under 10 ppm are commonly considered very deficient.

Welsh found that management system (organic vs. conventional) affected the sizes of the first three fractions of P in the soil, but had no significant effect on the least available fourth fraction of P. This suggests that as available P was used up, moderately available P was converted into the more available form.

Since the fourth fraction was not affected, Welsh concluded that this highly unavailable form of phosphorus was not being depleted – at least not yet.

According to Tenuta, it is important to continue studying the rate of P movement from unavailable to available fractions to determine which forms of less available P are being depleted in the long term.

Phosphorus depletion is a valid concern on organic farms. While the moderately available pools of P can feed the more available pools for a time, depletion is bound to occur when nutrients are exported annually from the system and are not replaced.

What can be done about P depletion on organic farms? Management options are available to recycle exported nutrients back into the system and help crops make the best use of the phosphorus that is present in the soil. Livestock manure, green manures, and mycorrhizal fungi are all effective P management tools available to organic farmers. A future article will consider these options.


Joanne Thiessen Martens is a Research and Extension Associate with the Organic Agriculture Centre of Canada working in collaboration with Dr. Martin Entz at the University of Manitoba. She welcomes your comments at 204-474-6236 or by email at j_thiessen_martens@umanitoba.ca.


The production of this article was supported in part by funding from Agriculture and Agri-Food Canada.


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Posted July 2008