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, must also 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 as previously discussed; (2) applied as foliar sprays directly to the plant foliage; or (3) fertilizers can be applied to the plant’s growing media via the irrigation system using a water-soluble fertilizer known as fertigation.
Foliar Fertilization
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, for 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.
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.
Foliar Application Considerations
Foliar feeding can be 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. During these active growth stages, leaves show a particularly high efficacy for absorbing nutrients. Research has shown that as leaves or fruits age, cuticles thicken, and these thicker cuticles absorb significantly lower amounts of nutrients such as potassium. However, younger plant tissue is also the most susceptible to potential fertilizer burn. 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 the leaf and stem) and better growth vigor (allowing for translocatable nutrients to be rapidly moved to the rest of the plant). Crops under heat or moisture stress show less response to foliar applications due to lower leaf and stem absorption rates and/or poor vigor. Foliar solutions should not be applied during hot, bright conditions or if the plants are wilting or under water/osmotic stress, as the plant’s stomata are likely to be closed, making the application ineffective.
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 to applying dry fertilizers to the growing media. Consequently, it is normally the most efficient fertilizer application method used in greenhouses.
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