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Organic and Inorganic Sources : Fertilizers may be divided into two broad groups: 1. organic, and 2. inorganic, or chemical.
An organic fertilizer is derived from a living plant or animal source. Nitrogen in an organic fertilizer is slow in becoming available for plant use because the organic nitrogen (NH2) must be reduced by micro-organisms to ammonium (NH4) or nitrate (NO3). Generally, home gardeners tend to use organic fertilizers more than commercial producers do because they are more willing to pay the high cost per pound of actual nutrient element. Fish, meat and bone scraps, cottonseed products and seaweeds are used in organic fertilizers. Animal manures* are organic fertilizers that are sometimes available free or low in cost. Another organic fertilizer type is processed from sewage sludge. Some newer organic fertilizers contain corn gluten or soybean hulls which are byproducts of food production. Composts are made from various materials partially broken down by natural processes into a soil amendment that can be worked into soil, blended into potting mixes or used as mulch. Composts have great value in improving soil tilth, that is its workability, or structure, but generally have a low nutrient content. The nutrients in composts and other organic fertilizers are valuable, including slowly-released Nitrogen and often additional micronutrients, but much more material must be applied in order to meet plant needs.
Chemical fertilizers are either mixed or manufactured and have the advantage of low cost. Consequently, most fertilizers used today are from chemical sources. Urea is about 45 percent N, a concentration which makes it a very economical source. In moist media at a temperature above 60 degrees F., it takes only about three to five days for the complete conversion of urea to ammonium. High analysis, water soluble, chemical fertilizers like or containing urea will injure plants if not washed or brushed off the foliage, or if overapplied. Consumers may not have adequate equipment or skill to apply concentrated products accurately or evenly, so lower concentrations are easier and safer for them to use.
Slow Release Fertilizers: Slow release fertilizers may be either inorganic or organic. They are characterized by a slow rate of release, long residual, low burn potential, low water solubility and they cost more than water soluble fertilizer. The most common element in a slow release fertilizer is nitrogen. Several categories of slow release nitrogen fertilizers are commercially available, including:
· Urea-formaldehyde (UF) (38-0-0). Released by microbial degradation.
· Isobutylidene diurea (IBDU) (31-0-0). Released by soil moisture and particle size.
· Sulfur coated urea (SCU) (36-0-0). Release rate controlled by coating thickness.
· Plastic coated fertilizers (various formulations). Release dependent on temperature and coating thickness.
· Natural organics--some types listed above.
*Animal Manure in the garden:
Animal manure is a secret ingredient which many home gardeners overlook. Manure offers many benefits, the most notable being increased available soil phosphorus and enhanced water infiltration rates. Chemical reactions with soil-and-water calcium alter the phosphorus to forms that are not available or only slowly available to plants. When manure decays, organic phosphates are released and the total phosphorus level available to plants increases. Organic phosphates may persist for many years after manure applications have stopped. The organic phosphorus also moves much deeper into the soil than do inorganic forms of phosphorus fertilizers.
Manure application also increases water infiltration dramatically compared to non-manured plots. In fact, studies indicate that manured plots can be properly irrigated five times faster than non-manured plots. Manure also keeps soils from sealing tightly while crops are growing.
There are three problems which can cause manure to be more of a problem than an asset.
First, manures must be handled carefully so that potential E. coli bacteria or other pathogens are not applied or splashed onto edible portions of plants, nor food contaminated from unwashed hands that touched manure or tools. The best practice is to plow manure into the soil a season or two before planting food, or to compost it thoroughly before use. Purchased, bagged manure is composted and often tested for viable weeds and excess salt.
Second, one must consider the source of the manure because undigested weed seeds, and weed seeds from hay or straw bedding, cause problems in a garden. Also, large amounts of salts in the manure can kill garden plants if large quantities are used. Salt content can be a problem in using sewage sludge (too many water softeners polluting a potentially excellent manure source). Saline plant toxicity can also be a problem when using feedlot manure if cattle are fed high salt rations.
Third, how much manure should be added to a garden? Bag instructions recommend 40 pounds per 100 square feet of garden area. That is a good, safe recommendation; however, some very successful gardeners apply the manure at 80 pounds per 100 square feet (8 pounds per tomato plant located 3 feet apart). Actually, the type of manure determines the rate which can be safely used.
Manure containing bedding such as sawdust or straw can be used in greater quantities because it does not contain as much ammonia. Vapors of ammonia in fresh manure, especially rabbit or poultry manure, can severely damage plants. Thoroughly incorporate all fresh manure into the soil several weeks before planting.
Certain tomato varieties, such as Merced and Surefire, and most peppers respond well to heavy manure applications. However, adding excessive amounts of manure on a yearly basis is not encouraged since nutrient release occurs over a period of time and nutrient toxicity could eventually occur.