Soil phosphorus, mobility, availability, deficiency, and
application strategy
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Soil chemicals react and bond with phosphorus forms (The orthophosphate anion, PO43−, HPO42−, and, H2PO4−) making them immobile in soil and restricting soluble phosphorus forms movement to the plants' root surfaces through diffusion. Although the concentration of soluble phosphate in the soil solution is very low, the soil solution levels for phosphorus considered critical for plant growth range from 0.2 to 0.3 ppm (mg L-1).
The Phosphorus availability is controlled by several factors such as soil organic matter levels, soil pH, and soil aluminum and iron contents.
When P becomes a soluble form, which is part of the available phosphorus, it can include small amounts of organic particulates and purely inorganic forms of orthophosphate anion (PO43-). The orthophosphate anion, PO43−, acidifies rapidly in soil to hydrogen phosphate, HPO42−, and dihydrogen phosphate, H2PO4−, depending on soil pH.
Phosphorus (P) mobility in soil and availability to plants has created debate on the fate of phosphorus. P ionic transformation in soil, uptake by the plant, and fixation represent significant challenges for efficient phosphorus fertilizer use and application, increasing plant production, socio-economic impacts, and preserving the environment.
The main mechanisms related to increased acquisition efficiency of phosphorus include more favorable root architecture and morphology, microbial associations, enhanced expression of high-affinity transporters, rhizosphere alteration, and secretion of organic compounds into the rhizosphere such as phosphatases and organic acids. Applying phosphorus according to the 4-R principles, the right form, right time, right place, and right amount, is also a vital approach. The excess application of phosphorus to the agricultural soil for crop requirements and the related dynamic saturation of a soil’s phosphorus sorption capacity and concentration has risen to high levels.
The application of phosphorus fertilizers on drained agricultural lands raises concerns about the impairment of surface waters worldwide. By creating preferential water flow conditions, tile drainage removes P from soils in amounts sufficient to accelerate eutrophication in lakes and rivers. And numerous studies have documented the importance of overland runoff in dissolved and particulate P transport from agricultural fields to streams.
Six basic strategies that plant roots may employ to enhance their uptake of various forms of phosphorus from soils.
(1) Increased root absorptive surface area.
(2) Chelate iron or aluminum to release P.
(3) Dissolve Ca–P compounds with acid exudates.
(4) Exude phosphatase enzymes to release P from organic compounds.
(5) Exude substances to stimulate P-solubilizing rhizobacteria.
(6) Encourage colonization by mycorrhizal fungi that help plants take up P. (Image source: Ray R. Weil, 1999).
The efficiency of utilization of fertilizer phosphorus is debated from socioeconomic, environmental, and biological perspectives. Improving phosphorus use efficiency for crop growth requires enhanced phosphorus acquisition efficiency by plants from the soil and enhanced utilization of phosphorus in processes that lead to faster growth and a greater allocation of biomass to the harvestable plant components.
The figure shows that a 5% probability does not mean how big the yield will be increased but how frequently might expect the yield to increase. However, the chance of Zero increase is low, also a little response could be economic.
Phosphorus (P) Build-up and Maintenance:
The build-up and maintenance recommendations framework is most suitable for immobile nutrients like P and K, strives to build soil test levels up to and beyond P and K critical levels, then maintain these levels over time.
That means P and K would be built up a slow- consistent process with avoidance of a one-time high application rate, based on (crop removal + additional fertilizer to build soil test levels) if soils test below the critical level or are “deficient”. Once the critical soil test level is reached, fertilizer recommendations will be based on maintenance.
By building up P and K, soil test level increases and fertilizers recommendations will be based on maintenance.
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