Soil on organic farms is often deficient in available phosphorus. Is this a serious problem, or are low levels simply a reflection of the way we measure the nutrient?
Cathy Welsh, a recent masters of science graduate from the University of Manitoba’s soil sciences department researched this question. Working with 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 versus conventional management systems.
Available phosphorus, sometimes called soil test phosphorus, is only a small portion of the total phosphorus 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 phosphorus is not detected by standard soil test procedures.
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What happens to the rest of the phosphorus in the soil when the available phosphorus is used up? Is phosphorus from the less available forms converted to a more available form? If so, at what rate is phosphorus made available? Are the less available forms 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 by Martin Entz. Samples were taken from annual and forage-based crop rotations that had been under 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. Known as orthophosphate, it is the form most easily taken up by plants.
- The second fraction, extracted with sodium bicarbonate, is a form plant roots also use 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 phosphorus, which is the form of phosphorus found in rock phosphate. This form is highly unavailable to plants.
When the four fractions were added, the total extractable phosphorus ranged from 259 to 345 parts per million. In comparison, the soil test phosphorus for the same soil ranged from six to 26 ppm. Soil test values under 10 ppm are commonly considered extremely deficient.
Welsh found that soil management (organic versus conventional) affected the sizes of the first three fractions of phosphorus in the soil but had no significant effect on the least available fourth fraction of phosphorus. This suggests that as available phosphorus was used up, moderately available phosphorus was converted into the more available form.
Because the fourth fraction was not affected, Welsh concluded that this highly unavailable form of phosphorus was not being depleted – at least not yet.
Tenuta said it is important to continue studying the rate of phosphorus movement from unavailable to available fractions to determine which forms of less available phosphorus are being depleted in the long term.
Phosphorus depletion is a valid concern on organic farms. While the moderately available pools of phosphorus 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 phosphorus 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 manure and mycorrhizal fungi are all effective phosphorus management tools available to organic farmers.
Joanne Thiessen Martens is a research and extension associate with the Organic Agriculture Centre of Canada working in collaboration with Martin Entz at the University of Manitoba.
