Chapter 20

Fertigation in Greenhouse Production

Fertilizer Calculations for Greenhouse Crops

Proper fertilization of greenhouse crops is essential for producing high-quality plants. Some nutrients (such as calcium and magnesium) may be mixed into the growing medium prior to planting, but most of the nutrients are applied after planting using water-soluble fertilizers. Fertilizer injectors are used by most growers to apply water-soluble fertilizers to plants. These devices "inject" a small quantity of concentrated fertilizer solution (stock solution) into the irrigation line so that the water leaving the hose (dilute solution) supplies the proper concentration of fertilizer. Applying fertilizers in a liquid form with a fertilizer injector is more convenient than broadcasting or top-dressing with dry fertilizers. Additionally, most growers apply water-soluble fertilizers at a dilute concentration on a “constant feed” basis (with every watering) to insure an adequate supply of the essential elements for plant growth.

Rates of Fertilization

Rates of fertilization are often given in parts per million (ppm) of the required nutrient (e.g., nitrogen, N). 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 greenhouse crops. Fertilizer stock solutions are mixed according to the fertilizer injector ratio: each injector will deliver a certain amount of stock solution for each increment of irrigation water that passes through the injector. For example, a 1:100 injector will deliver 100 gallons of dilute fertilizer solution for each gallon of concentrated stock solution. A 1:200 injector will deliver 200 gallons of dilute fertilizer for each gallon of concentrated stock solution (or 100 gallons of dilute solution per ½ gallon of stock).

Stock Solution Calculations

Most of the manufacturers of commercial fertilizers and fertilizer injectors have produced tables 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. Without recourse to tables or bags, growers can use formulas to calculate the amount of fertilizer needed. If the rate of fertilization to apply to the crop (in ppm), the percentage of nitrogen (N), phosphorous (P), and potassium (K) in the fertilizers, and the injector ratio are all known, then calculations are simplified by the following formula:

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