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 Fertilizing the Organic Garden
By C.W. Basham and J.E. Ells, Colorado State University Cooperative Extension
Quick Facts...
• Organic materials generally contain all the nutrients essential to plants, but they may not be present in the ratio organic gardeners would like.
• Nitrogen and phosphate content are most important to the organic gardener.
• Never use fresh manure on food gardens.
• Some organic materials, such as sawdust or straw, may require special handling in the form of composting before use in the garden.
• Direct application of high nitrogen materials, such as dried blood or poultry manure in large amounts, may "burn" plants.
Organic gardening emphasizes the use of organic soil amendments to improve the nutrient content and physical characteristics of the soil. Synthetic fertilizers are not used. The presence of decomposing organic matter in soils has long been recognized as a nutrient source for plants and useful in maintaining and improving structure and tilth in clay soils and in improving the water-holding capacity of sandy soils. Moreover, organic matter contains natural organic complexes that make micronutrients, such as zinc and iron, more available to plants.
A wide variety of plant and animal organic materials are useful. The organic gardener often is able to productively use materials that would otherwise be discarded, thus eliminating some potential environmental pollutants.
Major Nutrients
Of the three major nutrients, crops respond most often to the addition of nitrogen and phosphorus; a response to the addition of potassium is rare. Although organic materials contain all of the major nutrients, they are present in widely varying ratios. Thus, few organic materials can be regarded as complete or balanced sources of plant nutrients. For example, even though manures are good nitrogen sources, they are relatively low in phosphate. Therefore, supplement manures with steamed bone meal. Continued heavy applications of manures may increase soil salts to harmful levels, a condition that can be avoided by periodically testing the soil. Because of potential food safety problems, never use fresh manure on food gardens.
A gardener always should have some idea of the content and availability of plant nutrients in the materials that are being added to the soil. Many materials contain nutrient elements in forms plants can't use and these may be very slowly converted to available forms. (For example, 50 percent to 75 percent of the nitrogen in cow manure may be available in the first growing season.)
Some organic materials may require special handling, most often in the form of composting, before use in the garden. For instance, sawdust, straw or other stemmy plant tissue with low nitrogen content or slowly available nitrogen may actually cause a temporary deficiency of nitrogen in crops if directly incorporated into the soil. This happens because the microorganisms decomposing this material require nitrogen for their own tissues and thus compete with crops for it. Nitrogen deficiency can be avoided by composting the low nitrogen material (with added nitrogen in the form of dried blood or poultry manure) before adding it to the soil. Coarse material, such as corn stalks and other plant residues, decompose slowly and also may present problems in soil preparation and cultivation if added directly to the soil. Shredding alone is not as effective as composting to handle these materials, but it is helpful.
Because direct application of high nitrogen materials, such as dried blood and poultry manure in large amounts may "burn" plants, use them in moderate amounts. Composting with low nitrogen materials may be advisable. Mix high nitrogen and low nitrogen materials together when building the compost heap.
The simplest method of composting is to build a heap by alternating layers of organic material and soil; the layers of soil may be 1/2 to 2 inches thick and the layers of organic material 6 to 12 inches thick. If the organic material is composed of more than 1/2 manure, no additional nitrogen is required, although 1/2 cup of bone meal per bushel may be useful. If the organic material is primarily vegetation, nitrogen may be added. As the heap is built, water the material until it is moist but not soggy. Two to 4 inches of soil should cover the final heap. After a few weeks, turn the material for aeration and mix well to move the outer parts to the center. The whole composting process should be complete in about three months in warm weather.
Nitrogen becomes available to plants in the form of nitrate or ammonium by decomposition of organic matter, the resulting nitrate is either taken up by plants (vegetables typically remove 30 to 100 pounds of nitrogen per acre), leached from the soil, or lost to the atmosphere as gaseous nitrogen. In the intense production typical of gardens, it is essential to supply decomposable organic matter to replace these losses.
A typical fertilizer recommendation for garden soil might call for 100 pounds nitrogen and 120 pounds phosphate per acre. A manure containing .25 percent nitrogen and .15 percent phosphate applied at the rate of 20 tons per acre furnishes 100 pounds nitrogen and 60 pounds phosphate. Adding steamed bone meal containing 25 percent phosphate at the rate of 240 pounds per acre will supply the remaining 60 pounds of phosphate. Twenty tons of manure per acre is equivalent to about 900 pounds per 1,000 square feet or about three bushels per 100 square feet; 240 pounds of bone meal per acre is equivalent to about 5 1/2 pounds per 1,000 square feet, or about 1 1/2 cups per 100 square feet.
Phosphate may be added to soil or compost heaps by using steamed bone meal. Although rock phosphate and colloidal phosphate often are recommended to organic gardeners, they become available slowly.
Phosphate made available by decomposition of organic matter is generally either removed by plants or fixed in slowly available mineral complexes in the soil. Vegetables typically remove 10 to 50 pounds of phosphate per acre per year from the soil. Soil retains excess phosphate, unlike nitrogen, making it available to future crops. As the phosphate level of the soil is built up, the organic gardener may decrease or stop supplementing the manure with bone meal.
When organic matter is added to the soil, the gardener may assume the potassium needs are being met. There is no need to add greensand, wood ashes, granite meal or kainite, which are potassium sources.
Micronutrients
Iron and zinc are the only micronutrients verified as deficient in Colorado soils. Vegetables most likely to exhibit these deficiencies are corn, potatoes and beans. Many woody plants are also sensitive to lack of iron and zinc. Iron deficiency commonly appears as yellow areas between greener veins of young leaves. In zinc deficient plants, leaves and stems often fail to grow to normal size. There also may be yellowing between the veins of leaves, but usually on older leaves. Decomposing organic matter supplies these necessary nutrients. In addition, organic complexes (chelates) may be formed. These hold the nutrients in a form available to plants and protect them from fixation in the soil in unavailable forms.
Since the requirements of plants for iron and zinc are very small, they almost certainly will be met by any program adding a variety of organic materials to the soil. Of course, materials slow to decompose, such as peat and sawdust, will be less effective than manures or dried blood.
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