Chapter 20

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. Some fertilizer materials are not compatible with concentrated solutions, forming insoluble compounds and precipitates. The precipitates tie up the nutrients, make them unavailable to the plant, and cause clogging in the irrigation equipment. The solubility of a fertilizer is determined as its maximum amount that can be fully dissolved in a determined volume of water. Exceeding this maximum amount will result in precipitation of the fertilizers in the irrigation system and can be a very serious problem. The solubility of fertilizers varies greatly.

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. Suspension fertilizers contain undissolved constituents, whereas the constituents of solution fertilizers are completely dissolved. Dry or suspension fertilizers are mixed with enough irrigation water prior to application to ensure that the fertilizer material dissolves completely and forms a solution (no undissolved constituents).

Solution versus Suspension Formulations

The majority of liquid fertilizers injected through irrigation systems are solutions, which by definition have no solid particles. All of the fertilizer has been dissolved in water. Suspension fertilizers are sometimes called “liquid fertilizers,” and while they are fluid in nature, they are not true fertilizer solutions. They consist of solid fertilizer materials suspended in a liquid mixture of water and perhaps other fertilizer nutrients.

Granular Fertilizers

Some dry solid fertilizers are manufactured by coating them with a special conditioner to keep the moisture from being absorbed by the fertilizer pellets. If dry fertilizers are used containing non-dissolving filler or coating materials, they should be allowed a settling period (6-8hrs) before injection, so sediments can settle to the bottom of the tank. Prior to injection, adjust the injection pump intake so it is 8 to 10 inches (20–25cm) above the surface of the sediments to prevent the sediments from clogging the lines and emitters. A flush valve should be located at the bottom of the tank to facilitate the removal of sediments after the injection of the stock solution. Surface scums may form when dissolving fertilizers containing fillers or coatings, which will need to be skimmed off too.

Chemical Precipitate Clogging Caused by Chemicals

Certain dissolved fertilizers react with water to form precipitates that can clog emitters. The following are some of the reactions that may occur:

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 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, such as urea, ammonium nitrate, calcium nitrate, and potassium nitrate, cool the solution when dissolving. Consequently, it may not be possible to dissolve as much fertilizer as needed to achieve the desired concentration.

Fertilizer Solubility Considerations

Because solubility is reduced when water cools, it is not a good practice to heat water to dissolve extra fertilizer (more than is soluble at normal temperatures). As the solution cools, this extra fertilizer will come out of the solution (precipitate or salt out) and possibly clog drip emitters. Growers routinely make large quantities of fertilizer stock solutions for injection for fertigation over a period. When making stock solutions that will not be injected soon after preparation, keep in mind that solubilities decrease when the solutions are cool.

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.

Specific Fertilizers

Nitrogen Fertilizers

Ammonium nitrate, urea, calcium nitrate, potassium nitrate, ammonium sulfate, ammonium phosphate 21-53-0, and ammonium phosphate 12-61-0 are very soluble in water. While ammonium is water-soluble, it readily attaches to organic matter particles, which helps prevent it from being leached. Fertilizers with a moderate to high percentage of ammonium nitrogen are labeled as having a potential acidity because they decrease the substrate pH. Urea is another form of nitrogen commonly found in water-soluble fertilizers. Urea-based fertilizers are typically more affordable than other forms of nitrogen. Urea-nitrogen and ammonium-nitrogen are almost identical in how the plant responds and utilizes nitrogen.

Phosphorus Fertilizers

Monoammonium (MAP) phosphate 12-61-0, diammonium phosphate (DAP) 21-53-0, monobasic potassium phosphate 0-52-34, phosphoric acid, urea phosphate, liquid ammonium polyphosphate 10-34-0, and long-chain linear polyphosphates represent several water-soluble or water-miscible phosphate fertilizers. Nevertheless, they can still have precipitation problems when injected at high application rates into hard irrigation water.

Potassium Fertilizers

All potassium fertilizers are water soluble. The most common source of potassium in liquid feed programs is potassium nitrate, however other sources may be used. Potassium sulfate is another popular fertilizer. Potassium thiosulfate is compatible with urea and ammonium polyphosphate solutions in any ratio.

Calcium Fertilizers

Fertilizers containing calcium should be flushed from all tanks, pumps, filters, and tubing prior to injecting any phosphorus, urea-ammonium nitrate, or urea sulfuric fertilizer. The irrigation lines must be flushed to remove all incompatible fertilizer products before a calcium-containing fertilizer solution is injected. Calcium should not be injected with any sulfate form of fertilizer. It combines to create insoluble gypsum.

Micronutrient Fertilizers

Several metal micronutrients are relatively insoluble and, therefore, not used for fertigation. These include the carbonate, oxide, or hydroxide forms of zinc, manganese, copper, and iron. The sulfate form of copper, iron, manganese, and zinc is the most common and usually the least expensive source of micronutrients. These metal sulfates are water soluble and are easily injected into irrigation water. However, using these materials for fertigation is not very successful in alleviating micronutrient deficiency when applied through sprinklers.

Chelates.Chelated micronutrients are fertilizers where the micronutrient ion (for example, Fe or iron) is surrounded by a larger molecule called a ligand or chelator. Ligands can be natural or synthetic chemicals. These compounds, combined with a micronutrient, form a chelated micronutrient. Chelation keeps a micronutrient from undesirable reactions in solution and soil. Chelated fertilizers have been developed to increase micronutrient utilization efficiency. The chelated fertilizer improves the bioavailability of micronutrients such as Fe, Cu, Mn, and Zn and, in turn, contributes to the productivity and profitability of commercial crop production.

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