- IMPORTANCE OF PHOSPHOURUS
- Phosphorus is one of the major plant nutrients and of great importance both in the soil and plant and crop nutrition.
- Phosphorus is an essential part of the protoplasm in plant cells and is necessary for cell division. It is absorbed by the young plant and transferred to the seeds during ripening. Two thirds of the total uptake of phosphorus occurs in the first three weeks of growth of the seedling.
- It stimulates the development of tillers which means the production of more fruiting heads per plant and so gives a higher yield: see diagramme below:
Figure 1: Shows improved tiller production as well as better root development.
- Phosphorus has a marked effect on the early maturing and ripening of the plant and can shorten the growing period by up to 10 days. Contrast this with potassium which lengthens the growing period.
- It stimulates root development which is important on heavy clay soils and dryland farming.
- It improves fertility and the formation of flowers and seeds.
- It has a special effect in improving the growth of Legumes, particularly clovers.
AGRONOMY & PLANT NUTRITION –LEARNER GUIDE
DEFICIENCY OF PHOSPHORUS
A shortage of Phosphorus in the soil can cause the following:
- Stunted root development in plants;
- A lack of tillers;
- Severe shortage can cause a total crop failure;
- Poor clovers in pastures;
- Phosphorus deficiency causes purple patches or stripes on the leaves of plants; and
- A shortage in pastures can lead to a deficiency in the grazing animal causing bone abnormalities and infertility, especially in milking cows.
An excess of phosphorus in the soil is of little importance.
Figures 2 and 3: Shows phosphorus deficiencies in maize (left) and tomatoes (right).
- 2. PHOSPHORUS IN THE SOIL
- The Phosphorus content of soils is always referred to in terms of Phosphoric Acid (H3PO4).
- The availability of Phosphorus in the soil depends on its solubility. Plants can use any nutrient only when it is in solution (dissolved in water).
- Under Acidic conditions, phosphorus unites with iron and aluminium in the soil thereby forming insoluble phosphorus which is not available as a plant food. If Lime is added to the soil it becomes more Alkaline, some of this phosphorus is dissolved and becomes available to the plant. An acidic soil can have high phosphorus content, but is of no use to the crop.
Table 1: Fertiliser Phosphorus occurs as four different forms of Phosphoric Acid.
|This form is soluble in water.
|This is insoluble in water but is dissolved in dilute acid. The acid used to measure this type of phosphorus is 2% Citric Acid. Mono-calcium Phosphorus changes to Di-calcium in the soil.
|This dissolves only in very strong acids unless very finely ground which will slowly dissolve in Citric Acid. This is the form in which it occurs in rock phosphate and bones.
|Soluble in 2% Citric Acid.
Note: Citric Acid (2% solution) is used because this is thought to be very close to the acidity in normal soil. If the phosphorus will dissolve in this solution it will dissolve in the soil.
- When phosphorus fertiliser is applied to the soil the Mono-calcium Phosphorus is immediately available to the plant together with some of the other types which become slowly available. Much of the Phosphorus in the soil is permanently fixed in forms that are insoluble and unavailable to the plant and not washed out of the soil (leached).
- Between 10 and 15% of Phosphorus applied as fertiliser is recovered by the crop and used. In most soils there is no residual effect.
- Fertiliser applied one year will not be available to the crop the next year therefore phosphorus needs to be applied in the form of fertiliser every year. These applications of fertiliser top up the phosphorus in the soil and provide some water soluble phosphorus for the immediate use to the growing plant.
- As there is very little movement of phosphorus in the soil, e.g. it is not washed down through the soil by rain water; the fertiliser has to be placed at root depth for the best effect.
|Solubility: able to be dissolved, especially in water.
SOURCES OF PHOSPHORUS
All phosphorus fertilisers are sold on the basis of the percentage of citric soluble P2O5 contained, because this is the actual nutrient value.
This is manufactured by treating natural Rock Phosphate, which is mined from the ground in North Africa with Sulphuric
Acid make it more soluble. When applied to the soil it quickly becomes insoluble but the young growing plant gets some soluble phosphorus very quickly. It is sold either in powder or granular form and contains 16 – 20% P2O5.
Manufactured from Rock Phosphate but is treated with Phosphoric Acid and gives a higher content of soluble phosphorus. It is sold as a powder or as granules and the P2O5 content is 36 – 38%.