Organic fertilizers comprise a variety of plant-derived materials that range from fresh or dried plant material to animal manures and litters to agricultural by-products.
The nutrient content of organic fertilizers varies greatly among source materials, and readily biodegradable materials make better nutrient sources. Nitrogen and phosphorus content is lower, often substantially lower, in organic fertilizers compared to chemical fertilizers. Moisture content is another factor that reduces or dilutes the nitrogen and phosphorus concentrations of organic fertilizers. Thus, it can be cost-ineffective to transport high-moisture organic fertilizer long distances. However, the use of locally available sources is perfectly reasonable if its use is consistent with the production strategy. The nutrient value of animal manures is more variable than that of agricultural by-products. The animal’s diet, the use and type of bedding material, manure age, and how it was stored are factors that affect manure nutrient value; these factors can vary seasonally on and among farms, regionally or on a larger geographic scale. In contrast, the nutrient content of agricultural by-products is less variable but can be affected by the industrial process used to produce the by-product. However, it always is advisable to analytically determine the nutrient content of the organic fertilizer.
The organic carbon content of organic fertilizer can be of equal or greater importance than its nitrogen and phosphorus contents. Application of organic fertilizer promotes increases in heterotrophic bacterial biomass, which stimulates other secondary productivity and mineralizes nutrients to stimulate primary productivity
