Chapter 17

Fertigation in Greenhouse Production

Criteria for Selecting Fertilizers for Fertigation

A large range of fertilizers, both solid and liquid, are suitable for fertigation depending on the physicochemical properties of the fertilizer solution. For large-scale greenhouse operations, solid fertilizer sources are typically a less expensive alternative to the commonly used liquid fertilizers. The solubility of these fertilizers does vary greatly. When switching to a solid fertilizer source, problems can be avoided in the nurse tanks by ensuring that ample water is added to the stock solution. Four main factors in selecting fertilizers for fertigation should be considered:

Fertilizer Formulations

Fertilizers applied in irrigation water may be purchased dry or as liquids. Liquid fertilizers are available as fertilizer solutions and suspensions, both of which may contain multi-nutrient or single nutrient materials. Solutions are defined as liquids that have all the plant nutrients in a solution while suspensions hold part of the plant nutrients suspended in the liquid by a suspending agent.

Liquid Fertilizers

Preparation of nutrient stock solutions from dry fertilizers may require considerable time and effort and can generate sediments and scums as waste products. Therefore, commercially prepared liquid fertilizer solutions (true solutions, not suspensions) that are completely water soluble are often used. Liquid fertilizers are available in a variety of grades and can be purchased with or without micronutrients.

Granular Fertilizers

As mentioned, dry granular formulations must be mixed with water to form a stock solution. When mixing granular fertilizers, it is better to start with about half the required amount of water in the tank. Then, while continuously stirring/agitating the water, begin adding fertilizer (in small increments) and water until the desired quantity of fertilizer is dissolved in the stock solution. Do not add fertilizer so fast that the material begins to settle to the bottom. It is often necessary to let the mixture stand for several hours and warm to a temperature that will allow all the mixture to dissolve. If the fertilizer does not readily dissolve, add more water while mixing. If solubility is a problem while mixing, consider: 1) making a less concentrated (more dilute) solution and increasing the duration of injection or 2) using a more soluble fertilizer.

Fertilizer Compatibility

Most soluble fertilizers suitable for liquid feeding are compatible at their dilute concentration. However, certain chemicals will react at higher concentrations to form insoluble precipitates. These precipitates can tie up the intended nutrients and clog the irrigation equipment. For example, mixing ammonium sulfate [(NH4)2SO4] and potassium chloride (KCl) in a tank reduces the solubility of the mixture due to the potassium sulfate (K2SO4) formation. When precipitate forms irrigation systems may clog-up and stop working properly. Other forbidden mixtures are:

Fertilizer Solubility

Highly soluble fertilizers are required for fertigation systems in order to minimize potential plugging problems. Dry fertilizer materials differ widely in water solubility, with solubility depending on the physical properties of the fertilizer as well as the irrigation water temperature and pH. Solubility generally increases with temperature, but some fertilizers actually cool the solution such as urea, ammonium nitrate, calcium nitrate, and potassium nitrate when dissolving. Consequently, it may not be possible to dissolve as much fertilizer as needed to achieve the desired concentration.

Jar Test

Before adding fertilizer to the supply tank, test the compatibility of fertilizers using the “jar test.” Take a clean jar and fill it with water from the irrigation system water supply. Add a small amount of the chemical to be injected so that the concentration is slightly higher than anticipated for injection, than shake well.

Application Techniques

To prevent emitter plugging, fertilizer must be injected upstream from the filter so all undissolved fertilizer material and precipitates will be contained by the filter. It is recommended that the fertigation run should be shorter than the normal irrigation run time, with the fertilizer applied towards the end of the run.

Water Quality

For successful fertigation, careful attention must be given to water quality. The quality of water required depends on several factors, but foremost is the type of fertigation. Drip fertigation requires the highest quality water, i.e., free of suspended solids and microorganisms that plug small orifices in emitters. Only liquid solutions should be used for drip fertigation. As the orifice size increases (e.g., on sprinkler systems) suspensions can be tolerated. Precipitation of fertilizers within the irrigation systems can be a very serious problem if solubilities are exceeded. One such problem commonly encountered is with calcium concentrations exceeding about 100 ppm.

Water Treatment

Routine system maintenance may include treating the water with chlorine to control organic growth. Chlorine is an effective biocide which will prevent microorganic growth in a micro-irrigation system and prevent emitter clogging. Since chlorine is most effective at lower pH's, it is important that the pH of the water be monitored and, if necessary, controlled during chlorination. At times, chlorine is used to precipitate iron, manganese and sulfur out of solution prior to filtration.

Click on the following topics for more information on fertigation in greenhouse production.