CHEMICAL INJECTION METHODS

Selection of a Pump for the Injection of a Chemical

While selecting an injection pump, one must consider the parts that will be in direct contact with the chemical substances. These parts must be of stainless steel or of ma-terial that is corrosion resistant. The injection method consists of basic components such as: Pump, pressure regulator, the gate valve, the pressure gage, the connecting tubes, check valve, and the chemigation tank. The injection pump should be precise, easy to adjust for different degrees of injection, corrosion resistant, durable, recharge-able, with availability of spare parts. The recommended materials are: Stainless steel, resistant plastic, rubber, and aluminum. Bronze, iron, and copper are non acceptable materials. The injection pump must provide a pressure on the discharge line greater than the irrigation pump. Therefore, the operating pressure of the irrigation system exerts a minimum effect in the endurance of the injection pump.

Injection Methods

The efficiency of chemigation depends on the capacity of the injection tank, solubility of a chemical in water, dilution ratio, precision of dilution, the potability, the costs and the capacity of the unit, the method of operation, the experience of the operator, and the needs of the operator. The chemical compounds for the chemigation process must be liquid emulsions or soluble powder. The injector must be appropriate to introduce the substances in the system. In addition, it must be of an adequate size to supply necessary amount of chemical at a desired flow rate. Normally, the injection is carried out with an auxiliary electric pump or by interconnecting the injection pump to the irrigation pump. The most common methods for chemigation are described in Figures 8.1 and 8.2.

Fertilizer Injector.using Venturi system.

Chemigation by Venturi injection

Figure 8.1a. Chemical injection methods: (a) Pressure pump and (b) Venturi type injector.

Figure 8.1b. Chemical injection methods: (c) Pressure difference method, and (d) Using the suction line of an irrigation pump.

Figure 8.2. Chemigation process. Top: The installation of a check valve (the nitrogen and pesticide tanks are behind the check valve). Bottom: Bypass Venturi system.

Injection by a pressure pump

A rotary, diaphragm or piston type pump can be used to inject the chemicals to flow from the chemical tank towards the irrigation line. The chemigation pump must de-velop a pressure greater than the operating pressure in the irrigation line. The internal parts of the pump must be corrosion resistant. This method is very precise and reliable for injecting the chemicals in the drip irrigation system.

Injection by pressure difference

This is one of the easiest methods to operate. In this method a low-pressure tank is used. This tank is connected with the discharge line at two points: one which serves as a water entrance to the tank and the other is an exit of the mixture of chemicals. A pres-sure difference is created with a gate valve in the main line. The prespres-sure difference is enough to cause a flow of water through the tank. The chemical mixture flows into the irrigation line. The concentration of chemical in the water is difficult to calculate and control. Therefore, it is recommended to install an accurate metering valve to maintain a precalibrated injection rate.

Injection by Venturi principle

A Venturi system can be used to inject chemicals into the irrigation line. There is a decrease in the pressure accompanied by an increase in the liquid velocity through a Venturi. The pressure difference is created across the Venturi and it is sufficient to cause a flow by suction of the chemical solutions from a tank.

Injection in the suction line of the irrigation pump

A hose or tube can be connected to the suction pipe of the irrigation pump to inject the chemicals. A second hose or tube is connected to the discharge line of a pump to sup-ply water to the tank. This method should not be used with toxic chemical compounds because of possible contamination of the water source. A foot valve or safety valve at the end of a suction line can avoid the contamination.

FERTIGATION

All fertilizers for the chemigation purpose must be soluble (see Table 8.1). The par-tially soluble chemical compounds can cause clogging and thus can create operational problems [2, 4].

Table 8.1. Solubility of commercial fertilizers.

Fertilizer Solubility (grams/liter)

Nitrogen

Nitrogen is an element that is most frequently applied in the drip irrigation system.

The principal sources of nitrogen for chemigation are: anhydrous ammonia, liquid ammonia, ammonium sulfate, urea, ammonium nitrate, and calcium nitrate. The an-hydrous ammonia or the liquid ammonia can increase the pH of the irrigation water, thus a possible precipitation of calcium and magnesium salts. If the irrigation water has high concentration of calcium and magnesium bicarbonates, then these can result in precipitation of chemical compounds. This enhances the clogging problems in the drippers, filters, and laterals. The ammonium salts are very soluble in water and cause less problems of clogging, with the exception of ammonium phosphate. The phosphate salts tend to precipitate in the form of calcium and magnesium phosphates, if there is an abundance of Ca and Mg in the irrigation water. Ammonium sulfate causes little obstruction problems or changes in pH water. The urea is very soluble and it does not react with the irrigation water to form ions, unless the water contains the enzyme urease. This enzyme can be present if the water has large amounts of algae or other biological agents.

The filtration system does not remove urease. This can cause hydrolysis of the urea. Because the concentrations of the enzyme are generally low compared to those in the soil, the urea will not hydrolyze to a significant degree in the irrigation water.

The nitrate salts (e.g., calcium nitrate) are relatively soluble in water and do not cause large changes in the pH of the irrigation water. The nitrogen fertigation is more effec-tive than the conventional methods of application, especially in sandy soils. In addi-tion, the nitrogen fertigation is more efficient than the conventional methods in fine textured soils.

Phosphorus

The phosphorus can be applied in the irrigation system, as an organic phosphate com-pound and glycerophosphates. The organic phosphates (orthophosphates) and urea phosphate are relatively soluble in water and can easily move in the soil. The organic phosphates do not precipitate, and the hydrolysis of an organic phosphate requires large lapse time.

The glycerophosphates react with calcium to form compounds of moderate solu-bility. The application of phosphorus can enhance obstructions in the drip irrigation system. When phosphoric fertilizers are applied in the irrigation water with high con-centrations of Ca and Mg, then the insoluble phosphate compounds are formed that can obstruct the drippers and lateral lines. The phosphorus moves slowly in the soil and the root zone. In addition, the moist soil particles absorb phosphorus to form insol-uble compounds. It is not recommended to fertigate phosphorus fertilizers during the growth period of a crop. Instead it should be applied before seeding, during seeding, and during fruit formation. The plant uses phosphorus early in its growth. In the drip irrigated crops, the fertigation of phosphorus can be combined with traditional meth-ods of application. The drip irrigation system is efficient in the application of soluble phosphorus compounds, because the water is applied in the root zone; that facilitates the availability of phosphorus.

Potassium

The potassium can be fertigated in the form of potassium sulfate, potassium chloride, and potassium nitrate. Generally, potassium salts have good solubility in water and cause little problems of precipitation.

Micro Nutrients

The micro nutrients are supplied in the form of chelates. Thus, its solubility in the water is increased and these do not cause any problems of obstruction and precipita-tion. If the micro nutrients are not applied as recommended, then iron, zinc, copper, and magnesium can react with the salts in the soil causing precipitation. This enhances the clogging of the emitters. The chelates should be dissolved before fertigation. The chemigation of micro nutrients benefits the plant to accomplish a good development and growth. In addition, the operational cost is lower compared to the foliage applica-tion.

PESTIGATION

Although, sufficient information is available on the application of pesticides through the drip irrigation system in different regions of the world, yet the data does not neces-sarily adapt to the weather and soil conditions of all regions of the world.

Insectigation

When dispersed emulsions or formulations are used in water, these comprise of the liquid phase and therefore can be distributed uniformly. In order to control foliage insects, the insecticides applied through the drip irrigation system must be systematic and of high solubility.

Fungigation

The fungi dragged by the wind are difficult to control. These fungi produce numerous spores that often are located on the leaves which are not easy to control. By means of injection of soluble and systematic fungicides through the drip irrigation, the effective-ness of certain fungicides can be increased.

Nemagation

The application of fumigants and nematicides through the drip irrigation system is convenient and safe. Using this method, the soil can be fumigated in an efficient way to control nematodes and other detrimental organisms. The success of the operation will depend on many conditions such as: Temperature, soil moisture, soil aeration, content of organic matter, and uniformity of irrigation.

Herbigation

In places where the rainfall is limited, the application of herbicides through drip ir-rigation serves to activate the applied herbicides. This action eliminates the need for mechanical incorporation and reduces the cost of weeding operation. As the drip irri-gation system directly takes the water with herbicides to the place where the weeds are

to be controlled, a uniform distribution is obtained. There is a significant reduction of losses of non-available chemicals as a result of the inactivation by rubbish or organic matter. Thus, the irrigation efficiency can be maximized and the efficiency of the her-bicide application can be increased.