Facts about phosphorus you should know

Highly-manured soils, wet growing conditions and lodging in cereal crops

Phosphorus or phosphate (P) is the most complex of the big four macronutrients in crop production. When you buy phosphate fertilizer, you are actually buying P2O5 the oxidized version, which is 62 parts actual P and 80 parts oxygen. Your actual P is only 43 per cent by weight.

The phosphate in all soils is absolutely essential for plant growth and energy exchange. This plant nutrient is key for plant cell biochemical functions and is a component of the DNA and RNA for cell division. Phosphate is needed in large quantities for seed germination, root development and seed or fruit production.

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Agricultural phosphate is mined from mineral rock and the amount available worldwide is limited. Five countries hold about 95 per cent of known P reserves — Morocco, China, South Africa, Jordan and the United States.

Phosphate fertilizer is usually available as super phosphate, monoammonium phosphate (MAP) and diammonium phosphate. Phosp­hates are extracted from rock phosphate by treating it with sulphuric acid in order to render the phosphate soluble.

Rock phosphate itself, sometimes called black gold as well as the “organic” bone meal, are very highly insoluble forms of phosphate. They are only sparingly useful in acidic soils below pH 5.5 and conditions of high rainfall. Bone meal and rock phosphate are totally unsuitable for neutral or high pH soils.

With P deficiency, plant growth is retarded, roots are stunted and leaves are usually red/purple pigmented — at this stage it’s usually too late to apply P.

When soluble P is applied to cropland it does not move very far in the soil from where it was applied. It quickly binds with aluminum, iron and micronutrients, especially at soil pH below 5. Remember, an average soil has something around five per cent of the soil weight in iron (Fe) and seven per cent of a typical soil is aluminum (Al) by weight. Since P is so easily fixed when it is applied to soil perhaps only 10–20 per cent is taken up by crop plants in the year of application. P is most readily available at pH 6.5 to neutral, and at a higher soil pH of 7.5 and up it again becomes less plant available.

In mineral soils, the total P in soils that is immediately plant available is around one per cent at any given time. In most soils, phosphorus can be found in both inorganic -P and organic -P forms.

Thus, while the majority of soil P is pretty stable, it is largely unavailable to plants. The small amount of P that is water soluble is replenished slowly during the growing season following plant uptake. This P is made available in a slow process from organic matter decomposition and by interchange with soil mycorrhiza for most crops, canola and beets excepted.

Adding P fertilizers frequently to cropland will maintain those P reserves and build up the soil P. In soils that test very low for P, the reserves will be insufficient to meet the crop demand over the growing season, resulting in lower yields and significantly delayed crop maturity.

A significant proportion of P in mineral soils is stored in the soil organic matter. This organic P is made plant available by microbial activity, providing that moisture, warm temperatures and the soil pH is suitable.

Intensification of the livestock industry in many areas of Prairie Canada has created national and regional soil P imbalances. This is particularly so with the poultry, beef, milk and hog livestock industries. Today, only a small percentage of farm-produced grain and pulse crops is fed on the farm where it is grown, very unlike the situation of 80 to 100 years ago.

This evolution in agriculture is causing a huge transfer of P from grain-producing areas to the livestock-intense areas resulting in huge P and sometimes nitrogen overloads on relatively restricted areas of cropland. The transfer of livestock manures to nearby cropland continues to add N and in particular P to be applied to this cropland, sometimes way in excess of crop needs. This leads to leaching of P, crop yield problems and significant environmental concerns leading to surface and groundwater pollution.

Where I live, west of Edmonton, I have over the years investigated sand loam cropland in particular that has received major and repeated quantities of either cattle manure, both beef and dairy, and poultry manure.

Soil analysis of the top six inches (0–15 centimetres) would often show Bicarbonate and Bray soil available P to be in the low hundreds for the Bicarbonate test, and 300 parts per million and up to over 600 parts per million for the Bray test. Organic matter would usually be five to seven per cent. Zinc, manganese and iron levels would be high to very high yet copper levels would be moderate to very low — i.e., often less than one part per million for these highly-productive soils.

In wet summers, these soils would have wheat crops that lodge badly, yield poorly, are late maturing and are adjudged to be copper deficient. Reading the literature, I believe or hypothesize that the following may be happening on these highly-manured soils. The abundance of P may be chelating the available copper and the high P levels in grain and legume crops have been shown to strongly discourage mycorrhizal associations with these crop plants.

Mycorrhizal linkage to crop plants in normal soils have been shown to supply phosphate, copper and zinc to growing crops. The discouragement of mycorrhizal association therefore may result in a failure of the crop plants to receive copper in particular, resulting in severe stem weakness, lodging and delayed maturity. Mycorrhizal failure in high P soils could be likened to growing field peas in soils high in nitrogen, resulting in nodulation failure.

I do not profess a 100 per cent accuracy but over the years and even up to now highly-manured soils and in particular the high P levels may be directly responsible for severe lodging of cereal crops with huge loss of yield and quality particularly in wet growing seasons. In dry seasons in these areas, deep rooting of the crops usually results in exceptional yields of 100 bushels or more of high-grade wheat, indicating that deeper rooting in the subsoil supplies the missing copper nutrition resulting in bumper yields.

About the author

Contributor

Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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