APPLICATION OF FERTILIZERS AND CHEMICALS THROUGH DRIP IRRIGATION SYSTEMS
Water soluble fertilizers can be effectively and efficiently applied through drip irrigation systems (Fertigation). Reduced labour, equipment and energy costs and higher fertilizer use efficiency are the major benefits of fertigation, compared to the conventional methods of water application. The success of drip irrigation, to a good degree, is due to the improved supply of nutrients to plants, which is a unique capability of fertigation. Proper control of the time of application, the concentration of fertilizers/ chemicals, proportion of the nutrients and the location of the point of application are possible with the fertilization systems used in pressurized irrigation systems. Plant protection chemicals can also be applied effectively using the same facility. However, materials such as inorganic forms of phosphorous that form chemical precipitates and cause clogging should not be injected into drip systems.
A detailed treatment on the subject of fertigation is presented in Figure 3 illustrates the layout of a drip irrigation system with fertilization unit.
The requirement of fertilizer application, including the type of fertilizers to be injected should be considered in designing a drip irrigation system. Some types of fertilizers are not suitable for application through drip systems, because of volatilisation of gaseous ammonia, low water solubility and problems with the chemical quality of irrigation water. Therefore, fertilizer injection equipment should be designed with an understanding of the chemical composition of the fertilizer to be used. Nitrogen is relatively problem free.
Nitrogen. Anhydrous ammonia and aqua-ammonia can be injected into the irrigation water, but volatilisation is a major problem. Another problem with ammonia injection is the rise of hydroxide ion concentration in water. Ammonia increases the pH, which causes soluble calcium and magnesium to precipitate in irrigation water and coat the inside of the pipes and plug emitters. This problem, however, can be solved by injecting a water-softener ahead of the ammonia gas. However, the process may increase the cost of fertilization.
Ammonium sulphate and ammonium nitrate are very common fertilizers. In the former, all the nitrogen is in the ammonium form, and in the latter about 28% by weight of the fertilizer is ammonium nitrogen and 8% is nitrate nitrogen. Urea is a soluble nitrogen fertilizer. It does
not react with water to form ions. Urea and ammonium nitrate are mixed in water to give a fairly concentrated liquid fertilizer mixture. All of these nitrogen materials can be injected with no side effects in the water or irrigation system.
Both urea and nitrate nitrogen stay in solution and move with the soil water. However, ammonium nitrogen behaves differently. Because it is a positively charged ion, it enters into cation exchange reactions in the soil. A small change in either soluble constituents alters the relative amount of the ions in exchangeable form. In the exchangeable form, ammonium is immobile. Because, cation exchange reactions are very rapid, ammonia applied in irrigation water is immobilized almost instantly on contact with soil and remains on or near the soil surface (US Soil Cons. Service, 1984).
Ammonium applied in water readily converts into exchangeable ammonium and generates an equivalent amount of cations in solution. In semi-arid and arid 'regions, soils are usually neutral to alkali (pH 7 to 8.20) depending on how much free lime or calcium carbonate is present. In such soils, any exchangeable ammonium that exists at the soil surface is likely to volatize. Ammonium is sensitive to temperature and moisture. Water evaporates rapidly from soil after irrigation, and ammonium is susceptible to gaseous loss at this time (US Soil Cons. Service, 1984).
Fig. 3. Schematic sketch illustrating the layout of a drip irrigation system with fertigation unit. (Source: Adapted from Jyoti Limited, Vadodara)
Phosphorous: Phosphorous is difficult to apply by injection into the drip system. Treble-superphosphate, which is commonly used is usually classified as water soluble, but is only moderately soluble. Hence, it is usually not recommended for injection in irrigation water, unless special precautions are taken. Several kinds of ammonium phosphates are soluble in water. Ammonium phosphate sulphate, ammonium phosphate and di-ammonium phosphate are suitable for injection in drip systems when the soil requires nitrogen and phosphorous. Phosphoric acid is another form of soluble phosphorous. The quality of irrigation water is to be determined before injecting phosphorous into the drip system. If the irrigation water has a pH above 7.5 and a high calcium content, the injected phosphorous will precipitate as calcium phosphate and may clog the emitters and restrict the flow in the pipeline. Under such situations, phosphoric acid could be used to meet phosphorous needs. Flushing the drip irrigation system with a solution of either sulphuric acid or hydrochloric acid immediately after applying phosphoric acid prevents clogging. Organic phosphate compounds like glycerol-phosphoric acid can also be injected into the drip irrigation system without any precipitation problem.
Fig. 4. Drip Irrigation Systems (a) Sketch of the tank assembly, (b) The tank to which are fitted inlet and outlet tubes. (Source: Jain Irrigation
Potassium: Potassium is easy for injection through drip irrigation systems. The commonly used potassium oxide is very soluble. The fertilizer moves readily into the soil and is not easily leached away.
Trace Elements: The common trace elements like magnesium, zinc, boron, iron, and copper can be applied through drip irrigation systems. The application rates of trace elements should be based on proper analysis of soil and water, as trace elements applied in excessive quantities can react with salts in the water and can be toxic to plants.