9. Fertigation System
Fertigation is the injection of fertilizers into an irrigation system. It is a combination of the words “fertilizer” and “irrigation”. The fertigation system is connected to the irrigation head.
Fertigation is practiced extensively in commercial agriculture and horticulture and is mainly used to spoon-feed additional nutrients. It is usually practiced on high-value crops such as vegetables and fruit trees. Drip irrigation systems in particular are well suited to fertigation because of their operation frequency and since water application can be easily controlled by the producer. Liquid fertilizers often play hazard to clogging attributable to the frequent occurrence of a chemical reaction between the organic and inorganic matter in the irrigation water. To reduce this clogging effect, it is advisable to flush the system with pure water after using liquid fertilizer. Fertilizers delivered as a solution can be injected directly into the irrigation system, while those in a dry granular (e.g. urea) or crystalline form need to be mixed with water to form a solution. Considering the relatively low operating pressure (0.2–0.5 bar) of SPIS, the following fertigation options can be considered:
- differential pressure tanks;
- Venturi nozzles;
- electric metering pumps; and
- water-powered dosing pumps.
- Differential pressure tanks, often referred to as “batch tanks”, are simple injection devices where the amount of fertilizer injected slowly decreases over time, as if a bucket empties. If the chemical concentration needs to be kept relatively constant during injection, batch tanks are not appropriate.
- The Venturi nozzle makes use of the “Venturi hydraulic effect”. Because of the high pressure loss of Venturi nozzles and the fact that the pressure provided by a photovoltaic water pump is not constant (this would cause a strong fluctuation in the fertilizer concentration), the Venturi nozzle is not recommended for SPIS.
- Electric metering/dosing pumps are the most expensive injection devices but they provide a constant and precise injection concentration and are robust systems.
- For off-grid applications, water-driven dosing pumps installed directly in the water supply line are suitable for use in SPIS. They work at comparably low system pressures and the dose will be directly proportional to the volume of water entering the dosing pump, regardless of variations in flow and pressure that may occur in the main line.
In addition, the simplest form of fertigation is to mix soluble fertilizer (e.g. urea) into the elevated reservoir of a low pressure drip irrigation system.
The challenge with fertigation is to control the concentration over time. Overdosing will harm the environment and the production cost budget.
If surface pumps are used for fertigation, it is recommended to inject the fertilizer on the intake/suction side, as it is a cheap, easy and reliable way to fertigate.
Chemigation is used as an overall term for the injection of fertilizers, soil amendments, and other water-soluble products into an irrigation system. In a drip irrigation system, chemigation can be used to inject chemicals to dilute debris and other materials that tend to clog the outlets or narrow bends. After a certain period of time, the treated water including the dissolved material is flushed out of each drip line. The assistance of qualified water engineers is required to develop a chemigation plan that suits the producer needs and matches the water quality situation at the farm.