Chapter 16

Fertilizers for Greenhouse Crops

Methods of Fertilizer Application

The best method of fertilizer application depends on the crop, available equipment, fertilizer-pesticide combination, labor, irrigation and tillage practices, and type of fertilizer. A major goal of many producers is to improve fertilizer efficiency (that is, greater crop yield per unit of fertilizer applied). The choice of method, however, also must meet future agronomic and environmental requirements. There are three methods of fertilizer application: 1) fertilizers can either be added during the pre-plant phase as a granular or slow- or controlled release fertilizer to the potting media; 2) fertilizers can be applied to the plant’s growing media via the irrigation system using a water-soluble fertilizer known as fertigation; or 3) applied as foliar sprays directly to the plant foliage.

Adding Dry Fertilizers to Growing Media

Growers can apply granular fertilizers or slow- and controlled-release fertilizers to container-grown plants in different ways: 1) top-dresses; 2) “dibble planting”; 3) incorporated into the media; and 4) layered. Applying dry fertilizers directly to the tops of the containers (“top-dressing”), should never be attempted with granular dry fertilizers because of the possibility of “burning” plants with succulent tissue. Slow- or controlled-release fertilizers can be top-dressed making sure that each container or cell receives an equal number of prills.

Fertigation

Most greenhouse operations apply soluble fertilizers through their irrigation systems, a process known as fertigation. This is accomplished in drip (trickle) by using some type of injector to meter 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. In addition to greater flexibility in application timing and optimal placement, fertigation increases the rate of nutrient uptake and predictability of plant response to fertilization compared applying dry fertilizers to the growing media. Consequently, it is normally the most efficient fertilizer application method. The fertigation method varies depending on the type of irrigation and the size and sophistication of the greenhouse. The simplest method is to combine soluble fertilizers in a watering container or use a hose injector and water plants by hand. This method can be tedious and time consuming but may be best when growing a variety of species with different fertilizer needs in small areas. Typically, fertilizer injectors are used when growing large numbers of plants with the same fertilizer requirements. The simplest injectors are called siphon mixers and the more complicated but more accurate fertilizer injectors of the water pump type that is installed directly into the irrigation line and pumps the fertilizer solution into the irrigation pipe at a range of injection ratios. For a more in depth discussion on fertigation refer to Chapter 17, Fertigation of Greenhouse Crops.

Foliar Application

Fertilizers can also be applied directly to the plant foliage where nutrients are taken up by the leaves. Nutrient foliar sprays are most commonly used to correct micronutrient deficiencies. Foliar application of micronutrients is preferred for three reasons: 1) micronutrients such as zinc, boron, manganese, and iron are required in relatively small quantities by plants, 2) many micronutrients are readily fixed by most media, so they soon become unavailable to the plant with fertigation, and 3) the elements form insoluble precipitates at neutral to alkaline pH. Copper, zinc, and manganese are present in many fungicidal sprays, which, if used, remove the need for dedicated foliar applications. Macronutrients are generally not effective or practical as foliar fertilizers and should be directly applied to the media by fertigation. There are several reasons for not foliar feeding macronutrients. Although plants can absorb macronutrients through the leaves, it is often not possible to supply sufficient amounts to correct any deficiency for very long, if at all. Leaves cannot absorb enough of these nutrients (without burning the leaves) to correct any significant deficiency. For a more in depth discussion on foliar application refer to Chapter 18, Foliar Fertilization of Greenhouse Crops.

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