Organic fertilizers are complex chemical substances containing carbon. Generally, they are naturally occurring and are derived from living things such as barnyard manures, bone meal and composted materials. However, a few organic fertilizers are synthetic-urea, for example.

Most organic fertilizers become available to plants slowly. Before organic nutrients can be absorbed by plants, they first have to be broken down to an inorganic form. This has some advantage for perennial crops (plants that live for more than two years), in that one application, repeated annually, is adequate over a long period. Rapidly growing annual vegetables, such as lettuce, require a high level of available nutrients that most organic fertilizers cannot provide quickly enough.

Organic matter improves that structure of the soil and its ability to hold nutrients, water and air, and is generally more useful for these purposes than as a source of nutrients. This is especially true with the very low nitrogen organic materials, such as plant residues, which require supplemental inorganic nitrogen to avoid reduced plants growth due to competition from nitrogen-consuming decay organisms in the soil.

Organic fertilizers can be expensive if applied in amounts adequate to supply sufficient nutrients for good plant growth and yield unless you have a free source. They can be considerably more expensive than inorganic fertilizers that provide equivalent amounts of available plant nutrients; and they can be difficult to find, especially in large quantities. However, the value for improved soil structure must also be considered.

Time of application is usually early spring or late fall. Fall-applied organic fertilizers will be available to plants earlier the following season than those applied in spring.

How much to use varies with the nutrient content of the particular organic material, especially its nitrogen content, as well as the results of a soil test. The age of the material is also a factor - when organic fertilizers decay and are rained on, they lose nutrients, especially potassium. Five hundred to 1,000 pounds of moist horse manure and 250 to 500 pounds of moist poultry manure are suggested for 1,000 square feet of garden.

Composition of organic fertilizers varies considerably. Most are relatively low in nutrients but are especially valuable as a source of organic matter for improving the physical condition of soil. Some organic fertilizers do not contain balanced amounts of nitrogen, phosphorus and potassium. Therefore, the nutrient content may need to be supplemented with other organic or inorganic fertilizers. Fresh animal manure may cause plant injury, especially when large quantities are used.

When using mulches, such as straw, leaves, sawdust, grass clippings and other organic materials, you will usually find it beneficial to add nitrogen fertilizer while the material is decomposing. Plants fertilized or mulched with organic materials containing less than 1.5 percent nitrogen usually benefit from the addition of inorganic nitrogen to prevent nitrogen deficiency while the organic materials decompose.

Curt Peterson, Ph.D. is a professor of landscape horticulture on faculty at Florida Southern College. He resides in Union Springs during the summer months. He can be reached at curt.peters@hotmail.com