Another disadvantage of organic manures is the wide variability in their nutrient content. The nutrient content of livestock manure varies with the amount and type of animal waste, straw, and sawdust or other bedding material mixed with it. Thus, the nutrient content of chicken manure is significantly different to that of steer, pig or horse and must be applied differently. This variability in nutrient content creates no small measure of difficulty to the grower in determining the exact amount of nutrients to apply. On the other hand, the nutrient content of these materials is often so low that large amounts of materials are needed to supply the required plant elements.
Organic manures can also contaminate produce or burn plants.
As such, fresh manure should never be applied to actively growing plants. Instead, apply composted manure and make sure it is mixed well with the soil. Research has shown that plants produced with commercial inorganic fertilizers are identical in appearance, taste and food value to plants produced with organic materials, and are just as safe. As compared to organic fertilizers, commercial fertilizers are easy to apply because one needs only a relatively small amount. Also, commercial fertilizers have a guaranteed content analysis, and as such, one can measure exact amounts of a nutrient.