Organic Fertilization

In order for crops to grow and develop, soil nutrients (chemical elements) need to be absorbed by roots and moved into the plant. These nutrients, coming from soil parent material or from added fertilizers, function in structural and metabolic systems to enable the plant to carry on processes necessary to living. To obtain the full yield potential and produce a high?quality product, the soil must contain enough nutrients to support vigorous plant growth throughout the life cycle. Since the weathering of soil parent material is a slow, long?term process, assuring proper nutrition for crop plants generally requires the addition of fertilizers to the soil.

When a grower decides to fertilize, he is faced with the choice of which fertilizer to use and the task of determining how much fertilizer to apply. No universal guidelines are available for answering these questions as soils differ in fertility, and crops differ in their nutritional requirements. The grower must apply what seems to be the correct amount of fertilizers based on soil tests, plant analyses, previous experience and advice from others. Modifications can be made during the season or in the following year based on how the crop is growing, and on the amount of time remaining in the current season.

Personal beliefs often enter into the selection of a fertilizer with many individuals choosing to use organic fertilizers rather than manufactured products. Both types of fertilizer can be used to supply soil nutrients to the plant, but with few exceptions, natural or organic fertilizers contain nutrients in low concentrations and relatively insoluble forms as compared with synthetic fertilizers. The concentration and solubility of the nutrients govern how much fertilizer to apply because the nutrients must be dissolved in soil water before they can be absorbed by plants.

Nutrients in organic fertilizers become soluble through a process termed mineralization or weathering where the complex molecular structure is broken into smaller water?soluble mineral ions that can be utilized by the plant. Some organic substances have a rapid rate of mineralization and release all of their nutrients during the first growing season with little to no residual value for crops in subsequent years. Other organic substances have a slow rate of mineralization and release portions of their nutrients over several years and may be considered soil?building materials. Materials slow to mineralize generally have limited value for growth of plants in the year they are first applied. Some natural rock materials that are very slow to weather may have little value in supplying nutrients to a crop or in enhancing the fertility of a soil.

In the fertilization of crops, growers must first be concerned with supplying the primary macronutrients—nitrogen, phosphorus and potassium. Since organic or natural fertilizers have variable chemical composition, a balanced or adequate supply of all of the primary macronutrients from one organic fertilizer is unlikely. Therefore, more than one kind of organic fertilizer is usually needed to provide sufficient crop nutrition in any system of organic gardening. This situation differs from that of synthetic fertilizers that are manufactured to contain from one to all three of the macronutrients and can be purchased in practically any formulation.

Nitrogen, one of the most likely nutrients to be deficient in most soils, is seldom lacking in organic fertilization programs because of the relative availability and slow release of this source of nutrient in organic materials. Some nitrogen accumulates in the soil from rainfall and from nitrogen fixation (the conversion of gaseous nitrogen in the atmosphere to organic nitrogen) by free?living microorganisms. This accumulation rarely exceeds 10 pounds per acre per year, however, and cannot support much crop production. A grower who relies on precipitation for sources of nitrogen is unlikely to have much production success.

Manure, one of the oldest known fertilizers, can serve as a source of organic nitrogen. The actual nitrogen content of manures varies with the type of animal and feed given to the animal. Poultry manure is considerably higher in nitrogen than the manures from larger farm animals, and the better any livestock is fed the richer the nutrient content of the manure. Bedding, such as straw, wood chips, or sawdust, that is added to manure greatly reduces the nitrogen value. The nutrients are diluted and, if the manure is incorporated into the soil, nitrogen is immobilized and unavailable as microorganisms "tie?up" this nutrient in the process of decomposing the bedding. Although nitrogen content is reduced by one-half, manures with a high proportion of bedding should be composted before they are mixed in the soil to prevent immobilizing soil nitrogen.

Fresh manures with low bedding content should be turned into the soil as soon as possible after spreading to prevent volatilization and the loss of nitrogen from the manure to the atmosphere in the form of ammonia. Manures that are left on top of the ground for 2-4 days have only half of the nitrogen value of those that are plowed or tilled in immediately after spreading. Manures incorporated into the soil release approximately 50 percent of their nitrogen content for plant growth the first year. The rest of the nitrogen is retained in the soil and builds the soil fertility for subsequent cropping years. The nitrogen in organic fertilizers such as dried blood, alfalfa meal and seed meals is released almost entirely in the first season they are applied to the soil, leaving little residual nitrogen available for the following season unless the application was in excess of the requirements for the crop.

The organic grower has only a limited number of phosphorus fertilizers from which to choose. Plant residues, farm manures, and composts are, practically, too low in phosphorus analysis to be considered for any purpose other than maintaining soil fertility following a build?up of this element in the soil through the use of more concentrated materials. Residues from the bodies of animals are excellent sources of phosphorus with bone meal being the most significant among available animal residues. Unfortunately, bone meal, the oldest phosphorus fertilizer, is expensive, so its use is generally limited to garden?sized plots.

Rock phosphate, mined from deposits, has a high phosphorus analysis, but the material is of such low solubility that special application techniques are required to achieve any benefits in the soil. Colloidal rock phosphate, taken from a lower grade ore than regular rock phosphate, is claimed to release phosphorus more readily than regular rock phosphate. Although colloidal rock phosphate costs the same as regular rock phosphate, the phosphorus content is only about 2/3 that of rock phosphate. The low pH of the soil helps release the phosphorus from the phosphate source. Super-phosphates are manufactured by treating rock phosphate with sulfuric or phosphoric acid, simulating the action of acid soil on the rock. Unless restricted by marketing or philosophy, organic growers may want to use the super-phosphates during an initial build?up of phosphorus reserves in the soil. Once a soil test indicates an adequate level of phosphorus, organic materials can be used to maintain phosphorus fertility.

Most plant residues and farm manures can serve as sources of fertilizers in building and maintaining the available potassium reserves of a soil. Except for rinds and peelings, the vegetative portions of plants are higher in potassium than are the fruits and seeds. Hay, straw, hulls (or shells) and any other plant residues contain from 2-9 percent potash with the exact amount depending on the plant product and on the fertility of the soil in which the plants were grown.

Dried manures are about 2 percent available potash, and fresh manures with bedding have about 10 pounds of available potash per ton. Actually, soils already contain thousands of pounds of essentially unavailable potassium in the form of primary minerals.

Fertilization programs to provide other plant nutrients such as calcium, magnesium, sulfur, and the minor elements (iron, zinc, copper, manganese, boron, molybdenum, and chlorine) are usually not necessary in an organic system. Other minor elements come as impurities in limestone.

The application of farm manures, composts, or plant residues to meet the nitrogen requirements of a crop will satisfy the calcium, magnesium, sulfur and most of the minor element requirements of the crop. Other than the reserves in the soil, this type of organic matter is the most important source of micronutrients and sulfur. Organic matter produced from soils deficient in minor elements, however, will be deficient in those nutrients. In these situations, organic matter should be brought in from outside sources to enrich and maintain the soil or minor elements must be added.