Phosphorus (P)

Phosphorus is the constituent part of many plant compounds and affects the entire plant metabolism. Plants take up phosphorus in the form of phosphate. The availability of phosphorus in the soil depends largely on the pH value.  

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 categorized as follows:

• Soluble state (directly available to the plant), such as:
  
  • Orthophosphate in the form of H₂PO₄- and HPO₄^2-.
• Unstable state (available after mobilization), 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).

Acidic pH further reduces phosphorus availability

The availability of phosphorus in the soil depends largely on the pH value. The greatest mobilization occurs at a pH value between 6 and 7. The danger of phosphorus fixation is greater with an increasing soil pH. The availability, however, 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. Analyzing soil for its plant available nutrients is a useful tool in calculating fertilizer 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.

The plant root mainly absorbs the orthophosphate present in the soil solution. Yet it is also able to dissolve the labile phosphate found in the soil via excreted acids. Therefore, a well-developed root system is essential for P uptake.

Phosphorus is irreplaceable as a macronutrient for the plant. It is a component of many important compounds and therefore 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 as well as for the development of fruits and seeds (development of phytin as a P reservoir for germination).
• Improves the utility and biological value of the products.

• The plants are small, have poor upright growth and rigid leaves. This is referred to as "stiff fruit".
• Root growth but also tillering (cereal plants) is reduced.
• There is delayed flowering and ripening of the crop.
• The entire metabolism is disturbed because the energy transfer does not function properly.
• The oldest leaves are first dark green, later often reddish in color, and eventually die. This is caused by chlorophyll accumulation and increased anthocyanin content.
• Older leaves are often shed early.
• The plants are less resistant to frost.