In fertigation, fertilizers are delivered through the irrigation water. The use of fertigation enables for highly accurate nutrient supply to plants. Small and frequent applications of fertilizers, in exact amounts that meet crop requirements, increase the efficiency of nutrient uptake and minimizes nutrient loses. However, using fertigation requires careful management and many factors must be taken into consideration.
Basically, there are two fertigation methods: proportional fertigation and quantitative fertigation.
A bypass fertilizer tank Venturi Injector
Quantitative fertigation – specific amounts of fertilizers are applied to a given area through the irrigation water. Nutrient requirements are usually expressed in amount/area units, such as Kg/ha, lbs/acre etc.
A by-pass fertilizer tank is the simplest way to apply fertilizers through the irrigation water. Injection of the fertilizer is not proportional to the water discharge rate. As the dilution ratio and rate of injection are not constant, fertilizer concentration is high at the beginning and decreases as irrigation progresses.
Proportional fertigation – The application amount of nutrients is proportional to the water discharge rate. Therefore, nutrient requirements and fertilizer application rates can be expressed as concentrations in the irrigation water. For example, mg/liter (=ppm), mmol/liter etc.
In proportional fertigation, fertilizer injectors are used, such as venturi and positive displacement pumps. The total amount of a nutrient applied per unit area can be evaluated by multiplying the concentration of the nutrient in the irrigation water by the total volume of water applied.
For example, if irrigation water contains 50 mg/liter of nitrogen and the volume of water applied is 30 m3/ha, then the total amount of nitrogen applied per hectare is:
The fertigation method used and the way it is practiced affect the uniformity of the fertilizer application. Uniformity allows for better distribution of the nutrients in the root zone and better efficiency.
For example, applying most of the fertilizer in the beginning of the irrigation results in fertilizer losses, as the fertilizers will leach below the root zone as irrigation continues. In contrast, applying the fertilizer amount at the end of the irrigation might result in salt accumulation in the root zone. Higher concentrations of fertilizers might accumulate at the top-soil or media, not being uniformly distributed throughout the root zone.
It is easier to achieve uniformity using fertilizer injectors, i.e through proportional fertigation.
Another factor that must be considered is the length of the pipelines. The pipeline may contain a considerable amount of water. Fertilizers that are injected at the beginning of the line might not reach the emitters if the pipeline is too long.
Here is an example: Assume that the length of the main line from the injection point to the plot is 300 meters and that the pipe diameter is 8 inches (20.32 cm). The pipe volume is then 8.3 m3.
If the irrigation discharge is 30 m3/ha, it would take the water, with the fertilizers, 16 minutes to reach the plot. This means that the actual application will reach the plot only 16 minutes and 8.3 m3 AFTER irrigation started.
Such an amount of water may be significant in relation to the total amount irrigated. For example, if the total irrigation amount for that plot is 40 m3, about 30% of the fertilizer will remain in the pipeline and never reach the plot.
The best practice would be to inject the fertilizers uniformly throughout the entire irrigation. Flushing the irrigation line at the end of the irrigation is also a good practice. However, the pipeline length and volume should be considered.