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Index : Soils and Fertilization
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- 8. Organic Fertilizers Work Well in Warm Weather - Top
- Now is the time to apply organic fertilizer to vegetable and flower gardens. That's because temperatures have warmed sufficiently to allow organic matter is broken down by soil micro-organisms. In the process, nutrients are released to be absorbed by plant roots.
Animal manures are probably the best source of fertilizer and organic matter for the organic gardener. Readily available sources of manure can be found in and around Tucson. Horse manure is most available, and many stable owners are happy to provide it to gardeners. For greater convenience, bagged manure is also available at local nurseries and garden centers.
Fresh manures must be aged for a minimum of 90 days, or composted. These manures have a very high salt content that must be allowed to leach out before it is safe for plants. Compared to chemical fertilizers, manures are relatively low in nutrient concentration. At best, they provide 5 percent nitrogen and 2 percent each of phosphorous and potassium. But manures have the advantage of providing the soil with much needed organic matter. In addition, they provide most of the micro-nutrients needed by plants.
When using horse, cow or hog manure before planting, mix in a minimum of 25 pounds of manure per 100 square feet. For best results, supplement each 25 pounds of manure with 2 to 3 pounds of ground rock phosphate or raw bone meal. Poultry and sheep manure should be applied at the rate of 12 pounds per 100 square feet, and supplemented with 1 pound of rock phosphate or bone meal.
For garden plants that are up and growing, manures can be used as a side dressing. Applications are made, in the case of row plantings, by scattering a band of manure down each side of the row. Place each band at the edge of the root zone and work lightly into the soil. For individual plants, a furrow a few inches deep should be formed around the plant and filled with manure, then covered with soil or mulch.
Keep in mind that manures vary significantly in their quality and nutrient content. It's a good idea to supplement them with a complete organic fertilizer.
In addition to manures, many natural and organic materials are available which yield plant nutrients. They can be purchased separately or in combination. These materials may be applied directly to the garden, used in the compost pile, or mixed with manure.
Since nitrogen, phosphorous and potassium are used in the greatest quantities by plants, these are the most important when selecting a fertilizer.
Nitrogen is primarily responsible for leafy plant growth. It is the nutrient most readily leached through the soil and must be added to garden plants on a regular basis. With fruiting plants such as tomatoes, a little nitrogen goes a long way. Too much nitrogen and plants get big and leafy, but don't produce fruit.
Good organic sources of nitrogen include; fish meal, blood meal, guano (bat and bird droppings), cotton seed meal, and horn and hoof meal. Sewage sludge, such as Milorganite, is also a good source of nitrogen but should not be used on edible plants because of the danger of heavy metal contamination.
Phosphorous (phosphate) is the nutrient most responsible for the promotion of fruiting, flowering and rooting. It is essential for vegetables, annual and perennial flowers, bulbs and young transplants. But unlike nitrogen, phosphorous does not move much in the soil. That means it must be mixed into the soil prior to planting, or at least scratched into the top inch or so of soil to have a benefit to plants.
Probably the richest organic source of phosphorous is raw bone meal. It contains about 22 percent phosphorous. Other good sources include; ground rock phosphate, guano, fish meal and cotton seed meal. Apply bone meal and ground rock phosphate at the rate of 2 to 5 pounds per 100 square feet of garden area. Follow label directions for other materials.
Potassium (potash) is the nutrient most responsible for providing general plant vigor and hardiness. It moves less in the soil than does nitrogen but more so than phosphorous. As such, it can be worked into the soil or lightly scratched into the surface. Good organic sources of potassium include; ground rock potash, green sand, seaweed, and potash salts. Cotton seed meal also contains a small amount of potash.
Finally, organic sources of fertilizer also contain micro-nutrients such as iron and manganese. In combination with organic matter, such as manure, these nutrients which are usually insoluble in our alkaline soils, become readily available to plants.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: April 28, 2002
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