Experts and BHG readers answer.
Which fertilizers are considered organic? Are they safer to use than other fertilizers?
Organic fertilizers may be derived from plant materials (compost, alfalfa meal), from animal sources (manure, bonemeal), or from naturally occurring minerals (greensand, rock phosphate). Most release their nutrients slowly, so there is little danger of burning plants by applying excess organic fertilizer. However, most have a low nutrient content per pound of material, so large volumes of them must be used, and they generally cost more per pound than manufactured fertilizers.
The specific nutrients in organic fertilizers are identical to the ones in manufactured fertilizers. For example, an ammonium ion from manure is no different than one produced at a fertilizer factory. However, the side benefits from the organic matter carrier are numerous. Organic matter loosens clay soil, helps retain moisture in sandy soil, and acts as a reservoir of nutrients in any soil. Manufactured fertilizers are usually high in nutrient content per pound of fertilizer, and the cost is low compared to organic materials.
Several synthetic fertilizers contain slow-release forms of fertilizer. Examples are urea-formaldehyde (UF), isobutylidene diurea (IBDU), sulfur-coated urea (SCU), and plastic-coated fertilizers, such as Osmocote. These slow-release forms are more expensive, but they reduce the potential of burning plants from overfertilization. They also cut down on the number of applications of fertilizer you'll need to make.