Chapter 17

Fertigation in Greenhouse Production

Fertilizers for Fertigation

There are a number of fertilizers specifically developed for fertigation. Some of these fertilizers have characteristics that lend themselves to specific soil conditions, while others are suitable for a wide range of soils. For example, certain soils have an overabundance of sulfur, but require potassium, calcium, and/or magnesium. Whereas acidic soils require potassium, calcium, and magnesium but these soils preclude the use of acidifying fertilizers.

Nitrogen Sources

Nitrogen is the most fertigated element in greenhouses due to high plant nutritional needs, to its great mobility in soil following chemical and biological transformation to nitrate (NO3¯), and because of the many soluble sources of nitrogen fertilizers available for fertigation. Many forms of nitrogen have been used in fertigation. Urea and urea ammonium nitrate solutions are probably most commonly used, but most any soluble form of dry fertilizer can be used. Anhydrous ammonia or any other nitrogen fertilizer that has free ammonia should not be applied through sprinkler systems. The major nitrogen sources are presented below, along with information on their use in fertigation.

Ammonium Nitrate (AN)

Ammonium nitrate is a liquid fertilizer widely used as a source of nitrogen in greenhouses. It contains two forms of nitrogen—the nitrate-nitrogen form that is mobile and instantly available and the longer lasting ammonium-nitrogen, which is gradually changed by soil microorganisms to the nitrate form.

Ammonium Phosphate

Ammonium phosphate can also cause soil acidification. If calcium or magnesium levels are high enough in the irrigation water, precipitates may also form, which can clog the drip emitters.

Ammonium Sulfate (AS or AMS)

Ammonium sulphate fertilizer is a widely used, inorganic soil supplement that has particular benefits when applied to alkaline soils. The active ingredients in it are nitrogen and sulfur. It dissolves readily in water, and is convenient to use for fertigation. It tends to be acid forming, which could be a disadvantage if greenhouse media is acidic.

Ammonium Thiosulfate (ATS)

Ammonium thiosulfate is used both as a fertilizer an acidulating agent. When ammonium thiosulfate is applied to the soil through fertigation, sulfur-oxidizing bacteria, Thiobacillus spp., oxidize free sulfur to form sulfuric acid. The acid then dissolves lime in the soil and forms gypsum. The gypsum helps to maintain a good, well granulated, aerated, and porous soil structure.

Anhydrous Ammonia or Aqua Ammonia

Anhydrous ammonia under normal air pressure is a gas but is kept in a liquid form under its own vapour pressure in closed tanks. When dissolved in water anhydrous ammonia forms aqua-ammonia. Anhydrous ammonia is classified as a hazardous material and is not recommended for fertigation.

Calcium Ammonium Nitrate (CAN)

Calcium ammonium nitrate is high in fast acting nitrate-nitrogen, low in longer lasting ammoniumnitrogen, and supplies calcium. CAN can be combined with ammonium nitrate, magnesium nitrate, potassium nitrate, and potassium chloride.

Calcium Nitrate (CN)

Calcium nitrate is relatively soluble in water and causes only a slight shift in the soil or water pH. If the water is high in bicarbonate, however, the calcium content may lead to precipitation of calcium carbonate (lime).

Urea Ammonium Nitrate (CAN)

Nitrogen is available in three forms—nitrate nitrogen, urea nitrogen, and ammonium nitrogen. The nitrate portion is immediately available as soon as it reaches the root zone. The urea portion moves freely with the soil water until it is hydrolyzed by the urease enzyme forming ammonium nitrogen.

Urea Sulfuric Acid

Urea sulfuric acid is well suited for fertigation. Urea sulfuric acid is an acidic fertilizer, which combines urea and sulfuric acid. The nitrogen and sulfuric acid contents of these products vary depending on their specific formulation. By combining the two materials into one product, many disadvantages of using these materials individually are eliminated. The sulfuric acid decreases the potential ammonia volatilization losses from the soil surface.

Phosphorus Sources

Monoammonium phosphate, diammonium phosphate, monobasic potassium phosphate, ammonium polyphosphate, urea phosphate, and phosphoric acid represent several water-soluble fertilizers. Nevertheless, they can still have precipitation problems when injected into water with high calcium or magnesium concentrations. These precipitants are very stable and not easily dissolved. Phosphoric acid is sometimes injected into the fertigation system, not only for phosphorus, but also to lower the pH of the water, which can prevent the precipitation problems previously mentioned. This practice will be effective as long as the pH of the fertilizer-irrigation water mixture remains low. As the pH rises (due to dilution with the irrigation water) the phosphate precipitates with the calcium and magnesium. One approach that is sometimes successful is to supplement the phosphoric acid injections with sulfuric or urea sulfuric acid to assure that the irrigation water pH will remain low. The major phosphorus sources are presented below, along with information on their use in fertigation.

Ammonium Polyphosphate

Ammonium polyphosphate can be used as a fertilizer as long as the injection rates are not too high. However, if the irrigation water has a high buffering capacity (high carbonate/bicarbonate content associated with a high pH, i.e., > 8.0) along with a high calcium and/or magnesium content, highly insoluble precipitates will form which can plug the emitters.

Diammonium Phosphate (DAP)

Diammonium phosphate is completely water-soluble fertilizer, a highly efficient source of phosphorus and nitrogen for plants. MAP, with its low pH levels, is best used for high-alkaline soils.

Mono-Ammonium Phosphate (MAP)

Ammonium phosphate is completely water-soluble fertilizer, a highly efficient source of phosphorus and nitrogen for plabts. Nitrogen in this product is in the ammonium form, immediately available to the plant.

Monobasic Potassium Phosphate

Monobasic potassium phosphate, commonly referred to as monopotassium phosphate, is a source not only phosphorus but potassium too.

Phosphoric Acid

Phosphoric acid can be used in many formulations of nitrogen, phosphorus, and potassium mixtures. Phosphoric acid should never be mixed with any calcium fertilizer. Phosphoric acid is also a good source of phosphorus for fertigation and has the added benefit of keeping the pH of the injected solution low enough to reduce the potential of dicalcium phosphate or dimagnesium phosphate from forming.

Urea Phosphate

Urea phosphate is not only a good source of phosphorus but nitrogen too. Its nitrogen is in the urea form. It is acidic and thus suitable for acidifying water and soil.

Potassium Sources

Most potassium fertilizers are water soluble, and injection of potassium through drip irrigation systems has been very successful. The problem most often associated with potassium injection is solid precipitants that form in the supply tank when potassium is mixed with other fertilizers. The potassium sources most often used in drip irrigation systems are potassium chloride (KCI) and potassium nitrate (KNO3). Potassium phosphates should not be injected into drip irrigation systems. The major potassium sources are presented below, along with information on their use in fertigation.

Potassium Chloride

Potassium fertigation is usually accomplished with potassium chloride due to its solubility and relative low cost per pound of potassium.

Potassium Nitrate

Potassium nitrate is expensive, but a source of both the nitrogen and the potassium in the product. Potassium nitrate is an excellent choice for operations where irrigation water salinity problems are present because of its low salt index.

Potassium Sulfate

Potassium sulfate is one of the best alternatives to potassium chloride in high-saline areas and simultaneously presents a source of sulfur, if that is required in your fertility or soil management program.

Potassium Thiosulfate (KTS)

Potassium thiosulfate is marketed in two grades and is a neutral to basic, chloride-free, clear liquid solution. This product can be blended with other fertilizers, but KTS blends should not be acidified below pH 6.0. The proper mixing sequence for KTS is first water, then pesticide (if any), and then KTS and/or other fertilizer.

Other Macronutrients

Sulfur (S), when needed, can also be provided as ammonium thiosulfate, ammonium sulfate or flowable S. It can be readily mixed with UAN and several other soluble fertilizer grades and injected. Magnesium sulfate (Epson salts) is often used to supply magnesium and sulfur.

Micronutrients

Micronutrients can be applied readily through the drip system. Sulfates of copper, iron, manganese, and zinc dissolve readily in water, and move well through the drip system. However, they are easily oxidized or precipitated in soil, and their utilization is, therefore, not very efficient. Chelated fertilizers have been developed to increase micronutrient utilization efficiency. Chelated forms are more likely to be effective, and can be easily injected. Chelates are generally highly water-soluble and consequently will cause little clogging and precipitation.

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