Phosphorus in the soil
The total phosphorus content of soils is generally high. However, only a fraction of it is available directly to the plant and the majority is adsorbed to the soil.
The availability of phosphorus can be categorised as follows:
- soluble state (directly available to the plant), such as:
- Orthophosphate in the form of H2PO4- and HPO42-.
- unstable state (available after mobilisation), such as:
- P fraction which has been adsorbed onto oxides and hydroxides of iron or aluminium as well as clay minerals
- calcium-, magnesium-, potassium-, sodium- and ammonium phosphate depending on the concentration of cations in the soil solution.
- easily soluble organically bound phosphorus.
- stable state (difficult or often not at all available to the plant) such as:
- calcium-, iron- and aluminium phosphate (inorganic)
- Phytate (organic).
Phosphorus dynamics in soils
Effect of pH on P availability:
The availability of phosphorus in the soil depends largely on the pH value. The greatest mobilisation occurs at a pH value between 6 and 7. The danger of phosphorus fixation is greater with an increasing soil pH. However, the availability can be improved at a relatively high pH (7.5-8) through addition of organic matter and at a high pH (>8) from addition of S or gypsum.
Increasing acidity of the soil results in the development of aluminium and iron phosphate. The availability of phosphorus can be improved by liming of the soil.
Effect of phosphorus fertilisation on the soil:
- Creation of stable soil crumbs and improved structure
- Proliferation of micro-organisms in the soil and support of their activity
- Increased humus content as a result of greater root growth
Soil fertility status
The fraction of phosphorus that is easily taken up from the soil solution is the important fraction for plant nutrition. Analysing soil for its plant available nutrients is a useful tool in calculating fertiliser requirements. Most countries have a scale of available P in soils and different crops require different P levels according to the responsiveness of that crop to P. It is important that P is neither limiting nor in excess since an excess of P not only increases the risk of leaching into the environment but also can cause problems with micronutrient availability.
Phosphorus in the plant
P is mainly taken up from the soil solution in the orthophosphorus form by the root hairs. These hairs are also able to solubilise a proportion of the unstable phosphate fraction through the excretion of acids. Therefore, a well developed root system is essential for the uptake of phosphorus.
Phosphorus is irreplaceable as a main nutrient for the plant. It is the constituent part of many plant compounds and affects the entire plant metabolism.
Functions of phosphorus in the plant:
- Important for the transfer of chemically bound energy in various processes in the plant metabolism.
- Central function in synthesis, breakdown and conversion of fat, proteins, carbohydrates and vitamins.
- Important component of biological membranes.
- Supports root and shoot growth of crops.
- High demand of phosphorus during ear development and flowering and for the development of fruits and seeds (development of phytin as a P reservoir for germination).
- Improves processing characteristics and the contents of bio-active substances in food crops. the practical value and the biological value of the products.
Phosphorus deficiency symptoms
- Plants are small and show a stunted, upright growth habit with rigid leaves.
- Root growth, as well as shoot growth (particularly in cereals) is reduced.
- Flowering and ripening are delayed.
- The energy transfer system does not proceed as normal and therefore the entire plant metabolism is comprimised
- Initially the older leaves may darken in colour, later often turning red and eventually dying. The cause of this is the accumulation of chlorophyll and an increased anthocyanin content
- Often older leaves are dropped prematurely.
- Plants may become less resistant to frost.