Chapter 17

Fertigation in Greenhouse Production

Calculating Fertilizer Stock Solutions for Fertigation

Commercially available water soluble fertilizers are combinations of fertilizer salts called fertilizer carriers. A portion of fertilizer carriers is made up of essential nutrients for plant growth while the rest is non-nutritive. Commercial fertilizers used in greenhouse production come in a wide variety of formulations. Complete fertilizers contain the three primary macronutrients: nitrogen, phosphorus, and potassium in various proportions, such as 20-10-20. Other commercial fertilizer formulations may lack one of the three primary macronutrients, most commonly phosphorus, e.g. 15-0-15. Micronutrients may or may not be included as part of a commercial fertilizer. Fertilizer recommendations for greenhouse crops are given as either parts per million (ppm) of a specific fertilizer nutrient or as pounds and ounces (weight basis) of a fertilizer formulation per 100 gallons of water. Growers must accurately determine the amount of fertilizer needed to mix stock solutions of fertilizers. Most of the manufacturers of commercial fertilizers and fertilizer injectors have produced tables (See Table 17.2) that simplify this task. Information is also provided on fertilizer bags. Without recourse to tables or bags, growers can use formulas to calculate the amount of fertilizer needed as demonstrated below in the following examples.

Parts Per Million Basis

Rates of fertilization are often given in parts per million (ppm) of nitrogen. Parts per million is a convenient unit of measurement for indicating the concentration of fertilizer solutions. For example, it is often recommended that 150 to 250 ppm nitrogen be applied in the irrigation water on a “constant feed” basis for fertilizing many floricultural crops. The advantage of parts per million terminology is that we can state the concentration of a fertilizer solution independent of the fertilizer analysis. Therefore, 200 ppm nitrogen may be applied using either a 20-10-20 or a 15-15-15 formulation. This is important for standardizing fertilizer recommendations since the percentage of nitrogen varies markedly among the water-soluble N-P-K fertilizers sold commercially. In addition, parts per million is a metric unit that can easily be converted to English units.

Weight Basis

Recommendations reported as weight of a fertilizer formulation per 100 gallons in final solution are easy to prepare as long as the stock tank size and injector ratio are known. However, one problem with recommendations on a weight basis is that the concentration of a specific fertilizer nutrient is not readily apparent. Therefore, comparing fertilizer recommendations or utilizing different fertilizer formulations for the same recommendation are difficult. Recommendations based on parts per million specify the exact concentration of a specific fertilizer nutrient to apply.

Incomplete Fertilizer Calculations

We can also use these calculations to determine the content of other fertilizer components in our fertilizer solution or we can develop a fertilizer solution based on an element other than nitrogen. This is important when using incomplete fertilizers to develop your own tank mixes or to supplement a complete fertilizer. 

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