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 Sprays

Foliar fertilization (or foliar feeding) entails the application—via spraying—of nutrients to plant leaves and stems and their absorption at those sites. Used in conventional greenhouse production systems, it is a viable means of enhancing crop nutrition. While plant root systems are in the most part efficient at absorbing mineral nutrients, certain conditions can prevent optimal uptake rates of some of the elements plants require. When plants are stressed for some reason, have suffered root death or damage, are showing a nutrient deficiency or are being established from cuttings, then foliar feeding becomes a particularly useful method of nutrient application. Supplying a plant’s major nutrient needs (e.g., nitrogen, phosphorus, and potassium) is generally not effective or practical as foliar fertilizers and should be directly applied by fertigation. However, foliar application has proven to be an excellent method of supplying plant requirements for secondary nutrients (e.g., calcium, magnesium, and sulfur) and micronutrients (e.g., zinc, manganese, iron, copper, boron, and molybdenum).

Advantages and Disadvantages in Foliar Fertilization

Foliar fertilization provides more rapid utilization of nutrients and permits the correction of observed deficiencies in less time than would be required by fertigation. The media applied nutrient has long influence on plant growth. However, plant response to foliar application is often only temporary. This means in case of severe nutrient deficiency several foliar applications are necessary adding to the cost of application. Foliar applications are most successful for micronutrients, whereas fertigation is effective for both macro and micronutrients.

Proper Timing of Foliar Applications

Foliar applications should be timed to provide needed nutrients during the yield potential determining time frame of plant development, which will in turn favorably influence the post-reproductive development stages. Multiple, low rate applications may show the most favorable responses within these time frames, whether a crop is nutritionally sound or not. Careful crop growth stage monitoring on a weekly, and sometimes a daily basis, is essential.

Proper growth stage. Foliar feeding can by carried out on a regular, weekly basis, or can be limited to the times when the crop comes under high nutrient demand such as early fruit set and heavy fruit loading. Often the greatest response to foliar feeding will occur during the active growth phases of plants.

Proper crop conditions. Generally speaking, crops that are nutritionally sound will be most likely to respond to foliar feeding. This is due to better tissue quality (allowing for maximum absorption of nutrients into leaf and stem) and better growth vigor (allowing for translocatable nutrients to be rapidly moved to the rest of the plant).

Proper environmental conditions. Environmental factors such as relative humidity and temperature will play a role with regard to the performance of a foliar sprays and the uptake of leaf-applied solutions. Relative humidity is a major factor influencing foliar uptake of nutrient sprays since it affects the permeability of the plant surface and the physico-chemical responses to applied compounds.

Foliar Fertilizer Formulations

Not all fertilizers are suitable for use as a foliar spray. The primary objective of a foliar application is to allow for maximum absorption of nutrients into the plant tissue; therefore, foliar fertilizer formulations should meet certain standards in order to minimize foliage damage. Qualifications for fertilizer materials for foliar application are discussed as follows:

Salt index. Tolerance of foliar applied fertilizers varies considerably by crop. The fertilizer salt index (SI) was developed to classify fertilizers according their potential to cause salt injury to plants with sodium nitrate as the standard with an index of 100.

Solubility. Before applying a foliar spray formulation, it is crucial that the compounds it contains are appropriately dissolved or suspended. Foliar fertilizers are commonly dissolved or suspended in water and contain as active ingredients chemical compounds as salts, chelates or complexes of mineral nutrients.

Adjuvants

Commercial foliar nutrient sprays are generally composed of at least two major components, namely: the active ingredient(s) and the inert material(s) or adjuvant(s). Adjuvants help to improve the spreading (wetting) and persistence (sticking) of the active ingredient(s) or mineral element(s) on the leaf surface as well as promote the rate of uptake and bioactivity of the mineral element(s) applied.

Foliar Fertilization Equipment

The most important goal in the application of fertilizers is to get uniform distribution throughout the crop foliage. Under dosing may not give the desired coverage and desired results needed while over-dosing is expensive as it wastes fertilizers and increases the potential for groundwater contamination. Two general types of sprayers are available for greenhouse application of fertilizer: hydraulic and low-volume. In addition, the proper selection of a nozzle type and size is essential for proper fertilization application.

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