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Exploring phosphorus uptake

Soil phosphorus (P) occurs in many inorganic and organic forms. Only a very small portion of inorganic soil P is available for plant uptake, with none of the organic forms taken up directly by plant roots. Phosphorus is the most challenging of all the plant nutrients to understand, as it can occur in numerous inorganic and organic forms, and its availability is strongly influenced by various soil chemical and physical factors.

October 23, 2017
By Ross H. McKenzie PhD P.Ag.


Inorganic soil P
Inorganic P can be divided into three pools: soil solution P; labile P; and non-labile P. Soil solution P is the dissolved P in soil water. Phosphorus is absorbed by plant roots from the soil solution, mainly as primary or secondary orthophosphate (H2PO4- and H2PO42-). The amou  nt of each form depends mostly on the pH of the soil solution. Primary orthophosphate dominates in more acidic soils and secondary orthophosphate dominants in alkaline (higher pH) soils. Inorganic P concentration in soil solution is normally very low. The quantity of P in the soil solution, even at relatively high levels, is only in the range of 0.3 to three pounds of P per acre (lb P/ac). Rapidly growing crops will absorb about one lb P/ac per day from the soil solution.

The labile pool of soil P is the solid P that is held on the surfaces of soil colloids. The non-labile P is the more strongly held P and crystalline P in the soil. The labile P is in rapid equilibrium with soil solution P. As soil solution P is taken up by plant roots, the equilibrium is disturbed and labile P shifts into solution rapidly. This, in turn, disturbs the equilibrium between labile and non-labile forms, resulting in P moving very slowly from non-labile to labile pools. The labile pool of soil P must replenish soil solution P rapidly in order to keep the solution P recharged for plant uptake. Labile P is a pool of soil P that is less available to plants but can undergo rapid chemical or biological changes to recharge or replenish the soil solution P.

Organic soil P
In Western Canada, about 25 to 55 per cent of the total P in topsoil is in organic form. The P in soil microbes accounts for about 10 per cent of the organic P. Organic P can be divided into labile P and non-labile P.

Soluble organic P can be dissolved in the soil solution and be rapidly hydrolyzed by an enzyme called phosphatase to become plant available as inorganic P in the soil solution. The P in soil microbes represents a small but important labile pool of organic P. Much of the soil organic P is in non-labile form. It is complex and many of the forms have not been chemically identified.

Plant uptake of soil P
Soil P is very immobile. Most of the soil P taken up by plants is from the root rhizosphere zone – the soil-root interface. As soil P is taken up from the rhizosphere zone, soil P concentration declines. This causes soil P in the bulk soil to move by diffusion to the rhizosphere. Soil P will only diffuse very short distances.

Root hairs, phosphatase enzymes and mycorrhiza
Root hairs are fine tubular extensions of the root epidermal cells and can dramatically increase the surface area of roots to increase nutrient uptake. They can vary in length from 0.1 to 1.5 millimetres in length and increase the rhizosphere zone. Most crop types have root hairs, however, some plant roots have a small number or none at all. A greater abundance of root hairs will aid plants in increasing uptake of immobile plant nutrients like P. Labile organic P can be rapidly mineralized in soil by phosphatase enzymes that catalyze the reaction. Phosphatase can be released from the roots of some crops to enhance the uptake of soil P.  

Vesicular arbuscular mycorrhiza (VAM) is a type of fungus that infects the roots of some plants. This symbiotic relationship means roots of specific crops are infected by hyphae. The plant provides carbon to the VAM, and the hyphae become extensions of the root, exploring the soil to aid in uptake of P and micronutrients like copper and zinc. Crops like wheat are mycorrhizal and use this mechanism to enhance P uptake. Other crops, like canola, are non-mycorrhizal. Soil mycorrhizal populations are reduced after a canola crop is grown. Crops such as wheat and barley, which utilize VAM to aid in P uptake, are often more responsive to phosphate fertilizer following a non-mycorrhizal crop.  

Soil P management
Soil P tests cannot predict with 100 per cent accuracy when crops will respond to added phosphate fertilizer. There are a number of crop and soil factors that influence soil P availability and uptake of P.  

The frequency of crop response to added phosphate fertilizer can be strongly influenced by environmental conditions, particularly soil temperature and moisture. Wetter, cooler spring soil conditions reduce availability of soil P. Research in Western Canada has shown the response to phosphate fertilizer (particularly with wheat, barley and canola) tends to be greater in wetter, cooler conditions versus at sites with warmer, drier spring soil conditions. Environmental conditions are a confounding factor affecting prediction of when crops will respond to phosphate fertilizer.

Soil test P levels in some soils have increased over the years as a result of repeated annual commercial fertilizer P application or livestock manure application. In recent years, with production of higher yielding crops and new crop types, some farmers have not been applying sufficient P fertilizer to keep up with crop removal of soil P. As a result, soil test P levels have been slowly declining. It is important to balance P additions with crop removal to maintain healthy soil P levels. Finally, factors like rate of P fertilizer applied and method of application used can all affect P uptake by a crop.