After reading this article you will learn about the fertilizers containing phosphorus.
(A) Fertilizers Containing Water-Soluble Phosphorus:
1. Phosphate is water-soluble and quick acting (available).
2. Very less leaching loss in this group of fertilizers.
3. Should be applied in neutral and alkaline soils.
4. Superphosphate and, soil reaction. When superphosphate is applied to neutral or alkaline, the soluble mono-calcic phosphate combines with lime in the soil and di-calcic and tri-calcic phosphates are produced. The phosphates are then said to be reverted.
Since the reverted phosphates are chemical precipitates, they are in a fine state of division and offer a large surface for the soil solution to act. As a result, even tri-calcic phosphate is rendered soluble slowly and gradually, and made available for plants.
When the soil is acidic and the pH is 5.5 or lower, the iron and aluminium in the soil become soluble and combine with the soluble phosphates in superphosphate. Iron and aluminium phosphates that are formed, as a result, are very insoluble and do not become available for the use of plants ordinarily. The phosphates are then said to be fixed or tied up in the soil.
When the pH of the soil ranges from 5.5 to 6.5, the light acidity of the soil assists greatly in dissolving the tri-calcic phosphate present in the soil and making it available for crops. The organic matter in the soil effectively prevents applied phosphates from being tied up (fixed) in the soil, in insoluble forms and even releases phosphates that had already been fixed, is likely to influence phosphate application practice in the future.
The phosphorus in bones and rock phosphates is in the form of tri-calcium phosphate (insoluble) and therefore, does not become readily available to plants. The insoluble phosphate in rock phosphate is converted into a soluble form treating with sulphuric acid and the resulting product is called superphosphate (ordinary).
When phosphoric acid instead of sulphuric acid is used (for treating rock phosphate) in preparation of superphosphate, a high grade material containing 40-48 per cent phosphoric acid is obtained. This is called concentrated or triple superphosphate.
(B) Fertilizers Containing Citrate-Soluble Phosphorus:
(1) Insoluble in water but soluble in citric acid, so that it does not become readily available to plants.
(2) No leaching loss.
(3) These are slow acting fertilizers therefore, applied in the soil 15-30 days before sowing.
(4) These type of fertilizers should be used in natural and acidic soils
It is more effective in soils with high rainfall and that are neutral to acidic in soil reaction. It is advisable to apply it in heavy applications and about a month before sowing the crop to compensate for its slow action. It is alkaline in reaction.
It has high amount of citrate-soluble phosphate and has an excellent physical condition. Due to its phosphate in di-calcic form, it is cheaper and there are less chances for fixation. It is effective on soils of acidic, neutral and alkaline reaction.
(C) Fertilizers Containing Insoluble Phosphorus:
(1) Because of their insolubility and their slow availability, fertilizers of this group should be applied in the soil about 2 months before sowing, Phosphorus is available in the form of tri-calcium phosphate.
(2) Generally, deep placement is done in highly acidic soils.
For effective results, it should be used in heavy application, particularly with organic matter. Its availability is increased by the presence of decaying organic matter. Rock phosphate must be finely ground before application in acid soils.
Bone meal is very safe and slowly available in the soil. Bone meal can be applied in acidic soil with light, medium and heavy texture. Its efficiency increases when organic matter is present in adequate quantities.
Phosphorus Use Efficiency:
In the case of phosphorus fertilization, fixation of phosphate is the main problem. Water soluble phosphatic fertilizers soon after application to the soil react preferably, called the initial phosphate reaction products. The efficiency of phosphatic fertilizers depends primarily upon the release of phosphorus.
Measures to improve phosphorus use efficiency are as follows:
1. To improve phosphorus use efficiency, phosphatic fertilizers should have minimum contact with the soil.
2. In acid soil, phosphorus use efficiency can be improved by raising pH with the application of time.
3. Surface broadcast, following by mixing during puddling, has the highest phosphorus use efficiency for rice, being even better than placement at various depths. Phosphate placement is more beneficial than broadcast application in wheat. Application in seed furrows or by drilling just below the seeds is quite efficient methods for wheat.
1. Recovery of fertilizer phosphorus in single season is very low. Residual effect of phosphatic fertilizer is quite high in the next succeeding crop. To cite an example, the yield response percentage and recovery of fertilizer phosphorus by rice and gram crop are better when superphosphate is applied to gram crop and its residual amount is allowed to be utilized by the succeeding rice crop than when it is applied directly to rice crop.
2. Phosphorous use efficiency is increased by application of phosphatic fertilizers with organic manures.
3. Because of fixation, crop may not use more than 10 percent of phosphorus in the fertilizers applied broadcast and incorporated to the soil. However, up to 30 percent or higher may be used when applied as concentrated band along the plant row.
While mixing with the soil increases fixation, localized placement of banding allows the phosphatic fertilizer to react with only a much smaller portion of the soil in the immediate vicinity of the band. Thus more phosphate remains in a available form for crop use. For banding, water soluble phosphate are preferable. Clay soils have greater phosphate fixing capacity than sand soils.
4. Fixation of P in citrate soluble form in less that of water soluble form. Hence, phosphatic fertilizers in citrate soluble form may be broadcast. Ground rock phosphate which is neither water not citrate soluble should preferably be applied to acid soils and thoroughly mixed with the soil. This ensures reaction with the soil acids which bring phosphorus into available form.