4.4 FERTILIZERS Fertilizer sources may be organic or inorganic. Most of the fertilizers applied in horticulture are inorganic in nature. 4.4.1 ORGANIC FERTILIZERS Organic fertilizers are derived from plant and animal residues. Sources of organic fertilizers include animal droppings (or manure from the barnyard or poultry house, bird droppings, and other animal wastes), dried blood, and bonemeal. Guano is an organic fertilizer consisting of bird droppings collected off the shores of South American islands. Other sources of organic fertilizers are cottonseed meal and liquid waste from meat- and poultry-processing factories. One of the commonly used nitrogen fertilizers, urea (46-0-0), is actually a synthetic organic fertilizer. Organic fertilizers are not commonly used in horticulture for several reasons, including the following: Fertilizer An organic or inorganic material of natural or synthetic origin applied to the soil or plant to supply elements essential to plant growth and development. 1. They are bulky to handle, requiring large amounts of space to store the required quantity. 2. Their nutrient content is low (low analysis). Large quantities are thus required to provide appreciable amounts of the nutrients needed. 3. They are difficult to quantify and apply according to a specified rate of desired amounts of nutrient elements. 4. The nutrients they contain are released too slowly. They are released as the material decomposes, and decomposition is variable and dependent on the environmental conditions. 5. They are difficult to apply uniformly. 6. They are applied only to the soil. 7. Their use is often accompanied by foul odor that could be slight or very intense as is the case of fish emulsion. 8. Fresh manures need to be composted to avoid burning plants from the high salts they contain. 9. Some sources such as sewage sludge may contain heavy metals (e.g., cadmium) that are injurious to plants. Although these disadvantages usually disallow organic fertilizers in horticulture, the following advantages for use do exist: 1. Organic fertilizers provide nutrients as well as increase organic matter content of the soil, thereby improving soil physical structure and microbial activity. 2. Composted manures are often marketed as soil conditioners. 3. They supply micronutrients in addition to macronutrients. 4.4.2 INORGANIC FERTILIZERS Inorganic fertilizers are most widely used to provide supplemental nutrition to horticultural plants during production in the field or in containers indoors. They are popular for reasons including the following: 1. They are easy to store. 2. They have higher analysis than organic fertilizers. 3. They can be custom formulated for specific purposes. 4. Inorganic fertilizers are easy to apply and can be applied uniformly. 5. They are available in liquid and solid forms. 4.4 Fertilizers 117
6. They can be applied to both soil and leaves. 7. Accurate quantitative application is facilitated. 8. Nutrients are readily available to plants. 9. Growers can mix their own formulations accurately. Complete Fertilizer A fertilizer formulation that supplies nitrogen, phosphorus, and potassium. Fertilizer Analysis The proportions of nutrient elements supplied by a fertilizer formulation. Fertilizer Formulations Commercial fertilizers formulated to supply one nutrient element are called straight (or simple or incomplete) fertilizers (e.g., urea supplies only nitrogen). Some formulations supply two or three major elements and are called compound (or mixed, balanced, or complete if all major elements are present) fertilizers. A grower may produce homemade mixed fertilizers from straight ones. Fertilizer Nomenclature The major elements (nitrogen, phosphorus, and potassium) are considered in this order in describing fertilizers. Technically, phosphorus is P 2 O 5 and potassium is K 2 O. A label on a fertilizer bag that reads 60:0:0 means that the fertilizer supplies only nitrogen, at the rate of 60 percent by weight of the total contents of the bag, the remainder being inert fillers added during the manufacturing process to aid in fertilizer handling. Similarly, a label reading of 0:15:0 means that the fertilizer supplies only phosphorus, at 15 percent by weight. A label reading 14:14:14 indicates that this mixed fertilizer supplies 14 percent each of nitrogen, phosphorus, and potassium. Except for nitrogen, the other major nutrients are not used in the elemental form. To convert phosphorus from its elemental form to phosphate, the conversion calls for multiplication by 2.29; to achieve the reverse (convert phosphate to phosphorus) the weight of phosphate is multiplied by 0.49. Similarly, to convert potash to elemental potassium, the weight is multiplied by 0.83; to obtain the reverse, elemental potassium is multiplied by 1.20. The proportion of the elements as illustrated earlier (e.g., 15:30:15) is called the fertilizer analysis or fertilizer grade. A fertilizer analysis is the minimum guaranteed analysis; an analysis of 60-0-0 means that a 100- pound bag supplies 60 pounds of nitrogen, not 59.9 pounds. (However, the weight could be higher than 60 pounds.) When the grade is divided by the highest denominator for the grade, the result is called the fertilizer ratio (e.g., 15:30:15 divided by 15 becomes 1-2-1). 118 Chapter 4 Plant Growth Environment Fertilizer Forms Mixed fertilizers are obtained in the form of granules (granular—all components are processed together in the factory into one compound), bulk blends (similar to granular, but components are physically blended such that they may segregate, leading to nonuniform application), or fluids. Fluid fertilizers allow micronutrients and pesticides to be incorporated into one application. They may be applied through irrigation water, directly to leaves (foliar application). Slow- or Controlled-Release Fertilizers Ordinary fertilizers dissolve quickly to release their nutrients into the soil, necessitating frequent application of fertilizers. It is estimated that one month after application of the regular type of nitrogen fertilizer to a lawn, little remains. In greenhouses where watering is frequent, regular fertilizers quickly leach out of the pots into the drain. Industrial processes enable fertilizers to be specially coated to release nutrients at a slow rate. For example, instead of “naked” urea, this fertilizer is coated with formaldehyde or sulfur. Such fertilizers are called slow release and are expensive. Sulfur-coated fertilizers may be designed for various rates of release. For example, sulfur-coated urea (SCU 10 ) releases 10 percent of the nitrogen in the first seven days, whereas SCU 40 releases 40 percent of the nitrogen in seven days. Slow-release fertilizers are used widely in the horticultural industry in greenhouses and for lawns and golf courses.