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Phosphorus is an element that is indispensable to life. We consume two grams of it every day. Soils contain varying quantities of phosphorus depending on their geological origin. The solubility of phosphorus concentrated in sedimentary rock is increased by reaction with acids, improving its efficiency as a fertilizer.
The element is not very mobile in the soil solution, and it is hard to harness its bio-availability. It is a crucial nutrient, determining growth at the early stages such as rooting. This makes it especially important, since only with proper rooting can the plant be fed throughout its whole cycle.
IMPORTANCE FOR PLANT LIFE
Phosphorous is needed throughout the life cycle of the plant, for respiration, photosynthesis and cell multiplication. It is particularly important in the early stages for the growth of the root system and strengthening young seedlings. A shortfall at that early stage, reduces root development, making the plant less resistant to stress, particularly water stress, and leads to delayed maturity.
Phosphorous is not mobile in the soil and only diffuses slowly into the soil solution. The root needs to develop and seek phosphorous, expending energy to absorb it, often with the support of microbes and mycorrhizae in the rhizosphere.
INTERACTIONS AND SPECIAL FEATURES
Phosphorus in the soil is divided into different fractions where most it is fixed and released with different degrees of ease into the soil solution, from where it can be taken up directly. In acidic soils, it is strongly fixed by iron, aluminium and manganese. In calcareous soils, it is fixed by calcium. It is released regularly, but in small quantities, by the mineralisation of organic matter. Repeated fertilization without phosphorus or insufficient fertilization (considering nutrient export with the harvest) deplete reserves and jeopardise the fertility of the soil. It is important that phosphorus is fertilized in a plant available form in spring.
1. Nutrients contained in organic matter of all types, including livestock effluent, crop residue and other organic by-products of human activities, are an important resource for fertilization.
2. Phosphate rock is extracted from open-cast mines and is processed using acids to produce more soluble forms that can be assimilated by plants.
3. Mineralisation of organic matter in the soil releases soluble mineral phosphorus (phosphate).
4. Phosphorus is continuously changing between fixed, adsorbed and soluble forms.
5. The leaching of soluble phosphorus (removal to deeper layers of soil by water) is not ver common.
6. Not much phosphorus is carried away outside the field; it may happen due to runoff and erosion (phosphorus linked to solid particles).
7. Plant roots only absorb phosphorus from the soil solution.
8. The harvest is used as food (for humans or livestock), which is the fundamental objective of farming.
There are many methods for analysing phosphorus, which illustrates how difficult it is to interpret. The Olsen extraction method is widely used because it comes close to indicating the actual availability of phosphorus for the plant.
Sensibility table & Symptomes
Phosphorus-deficient plants show purple and reddish coloration on young leaves, and on sheaths.
Excess & Needs
In the soil, it can block the availability of zinc. It may lead to eutrophication due to runoff into water courses.
A small part of phosphates are derived from magma, but most deposits are of sedimentary origin, through the precipitation of marine micro-organisms in shallow seas.
Providing indications of appropriate P contents in soils is not an easy exercise and multiple extraction methods exist to estimate the proportion of available P in soils. Measuring phosphorus in soil samples is the best way to assess the availability of phosphorus. It is necessary to refer to national classification systems.
ORGANIC MATTER CONTENT
With close to 50% of phosphorus in organic form, mineralisation of organic matter allows improved
flowing of phosphorus into the soil solution. Organic matter can replace phosphorus in fixing sites, on
calcium, for example, making the phosphorus more available.
In clay soils, clay sheets with a positive charge tend to block phosphate ions. In sandy or filtering soils, phosphorus is diffused more easily.
Dry periods lead to the oxidation of iron ions, increasing the ability of the iron to block phosphorus. Temperature has a direct influence on biological activity in the soil, lowering the availability of phosphorus. Applications at the end of winter when vegetation restarts are therefore recommended.
In acidic soils, the availability of aluminium ions (Al3+), and iron ions (Fe3+) fixes phosphorus anions, making them unavailable for the crop. In alkaline soils, calcium (Ca2+) precipitates phosphorus into apatite rock. The availability of phosphorus is highest at a pH of 6-7.
ACTUAL COEFFICIENT OF PHOSPHORUS USE
The proportion of phosphorus supplied through fertilizer that can be absorbed by the plant is fairly low. Only 20% in silty soil with pH 6.5 and even less in calcareous soil with pH 8 are used by the crop in the year of application. Applications that are as closely adjusted to the needs of the crop as possible are the key to successful and efficient phosphate nutrition.