It is neither practical nor safe to apply major elements to leaves directly. Foliar spraying is often used to correct deficiency problems of trace elements. Since trace elements are required in minute amounts, it is safe to administer them by foliar application. Fertigation Fertigation, also called chemigation, entails applying fertilizer to crops through the irrigation water. This method is usually used in the greenhouse. The drip irrigation method is particularly suitable for fertigation. In effect, it is a kind of topdressing application. Critical issues of concern in fertigation include the solubility of the fertilizer and the quality of irrigation water. Water-soluble fertilizers should be used in fertigation. Hard water with excessive amounts of dissolved calcium can be problematic if the irrigation system is incapacitated through blockage of holes by calcium deposits. The advantage of this method is that in soils that drain very freely (e.g., sandy) and for fertilizer elements that are prone to leaching (e.g., nitrogen), irrigation can be controlled to deliver the right amounts of moisture and fertilizer at rates that the plant can utilize efficiently. Small-scale fertigation systems are available for home use. They usually consist of a handheld unit containing a dry formulation of the fertilizer. The unit is connected to a watering hose that runs water through the container before sprinkling on the area to be irrigated. These units are used for watering vegetable gardens, flower beds, lawns, and other plants in the landscape. 4.4.6 TIMING OF APPLICATION Fertilizers are applied supplementarily to the nutrition plants obtain from the soil. Plants go through growth phases, each phase with special nutritional needs to support the growth and development activities taking place. Fertilizers are subject to a variety of environmental factors that cause them to be depleted in the soil. For example, under aerobic conditions, nitrogen is lost through denitrification (a process whereby bacteria convert nitrates into unusable nitrogen gas). Nitrogen fertilizers are readily leached or, in some cases, volatilized. Phosphorus and potassium are prone to fixation. Appropriate rate and type of fertilizer must therefore be chosen, and the application must be timely. Fertilizers are most beneficial to plants when applied as close as possible to the time of maximum need. Fertilizers release their nutrients at various rates and over varying periods of time. Organic fertilizers release their nutrients as the material decomposes; consequently, they are not useful when immediate supplemental nutrition is required. Dry application of fertilizers requires moisture availability to enable use of the nutrients. Rain or irrigation is needed after application of dry fertilizer. On the contrary, irrigation or rain is undesirable after foliar application because it will wash the nutrients into the soil. 4.4.7 FERTILIZER RECOMMENDATIONS After a soil test, fertilizer recommendations are made based on a number of factors, including the following: 1. The regional information on weather and soils. 2. The cropping history of the land. 3. The crop yield target or goal desired by the grower. 4. The crop to be fertilized (in terms of how it responds to fertilization). For small-scale growers such as home gardeners who have no set yield and profit targets, the general goal of gardening is to produce a good, healthy, and attractive crop. Homeowners and gardeners often overfertilize their gardens and lawns. For some, it is a seasonal ritual or tradition to fertilize the lawn and garden crops. Many people do not conduct soil tests on their plots and hence may not derive optimum benefits from their efforts. Since no specific yield goal is set in home gardening projects, a moderate application of fertilizer is all that may be needed. A general application may be 10 to 20 pounds per 1,000 square feet (4.5 to 9 kilograms per 92.9 square meters); a complete fertilizer (10:10:10) is often sufficient for gardens and lawns. 4.4 Fertilizers 125
4.4.8 GREENHOUSE FERTILIZATION Most greenhouse production occurs in containers, which restricts the amount of soil or growth medium from which a plant can obtain nutrition. Further, greenhouse media used in greenhouses for plant cultivation are often artificial and have little or no nutrition to support plants. As a result, fertilization is critical in greenhouse production. Fertilization programs should include minor or trace elements in addition to major elements. Liquid fertilizers, applied through irrigation water, are important in greenhouse fertilization. The subject is discussed further in Chapter 12. 4.4.9 FERTILIZER CALCULATIONS Following a soil test, recommendations are made as to the type and amounts of fertilizer the producer may apply. An amount of a certain nutrient may be purchased. However, the weight of fertilizer applied depends on the source, since fertilizers come in all kinds of grades. The three common calculations involving dry fertilizer formulations are (a) nutrient percentage, (b) amount or weight of source (commercial) fertilizer to apply, and (c) the amounts or weights of component materials to use in preparing a bulk of mixed fertilizer. 1. Nutrient percentage Problem: Solution: 2. Simple fertilizer mixture Problem: Solution: What is the percentage of nitrogen in the fertilizer urea? Urea has a formula of (NH 2 ) 2 CO, and molecular weight of 60.056 g Molecular weight of nitrogen (N 2 ) = 28.014 Percentage of nitrogen = [28.014/60.056] × 100 = 46.6 or 46% approximately (round down to nearest whole number). Given ammonium nitrate (34-0-0) and treble superphosphate (0-45-0), prepare 1,000 kg of fertilizer of grade 15-10-0. Final mixture will contain 150 kg of nitrogen (i.e., 15% of 1,000 kg). It will also contain 100 kg P 2 O 5 (i.e., 10% of 1,000 kg). Amount of ammonium nitrate needed (p.s., it contains 34 kg of N per 100 kg): [100 × 150]/34 = 441 kg of 34-0-0 Similarly, for phosphorus (contains 45 kg of P 2 O 5 per 100 kg)[100 × 100]/45 = 222 kg of 0-45-0 Total nutrients = 441 + 222 = 663 kg (leaving 337 balance of the desired 1,000 kg). 126 Chapter 4 Plant Growth Environment The balance is satisfied by adding a filler (inert material) or lime. The procedure is the same for compounding a mixture of three components (i.e., N-P-K). What if calculated proportions of nutrient components add up to more than total desired weight? In this example, what if the two amounts exceeded 1,000 kg? The component amounts cannot exceed the total desired weight. If that happens, the desired grade should be lowered or a source with higher analysis (e.g., for nitrogen, use urea with 46% N) should be used. 3. Amounts (weights) of sources (fertilizers) to apply Problem: It has been recommended that a producer apply 80 kg of nitrogen and 40 kg of phosphorus per acre to his field.
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