Many farmers often ask what the difference is between commercial fertilizers and certified organic fertilizers. Both can be used effectively as fertilizers, said Tom Jensen, Ph.D., director, IPNI North American Program. However, they usually come from different sources, and will have different physical and chemical properties, so they may need to be applied differently, or at much different rates.
Commercial fertilizers generally are processed and or manufactured compounds that contain concentrated and soluble forms of inorganic salts of plant nutrients. For example, a common phosphorus fertilizer is monoammonium phosphate (MAP or NH4 H2PO4) that contains 11% nitrogen in an ammonium form (NH4+), and 22% P (or 52% P2O5 equivalent) in a phosphate form (H2PO4-). This compound is the result of treating mined and finely ground rock phosphate with sulfuric acid, to first produce phosphoric acid (H3PO4) that is afterwards reacted with ammonia (NH3). Interestingly, the same rock phosphate processed to become MAP can be mined, ground, granulated, and sold as granulated rock phosphate and used as a certified organic fertilizer.
The difference between rock phosphates and processed MAP is that the rock phosphate is much less soluble. For example, in an evaluation of seven different rock phosphates sourced from different parts of Africa, their solubility averaged 8.6%. The same test for MAP is at least 91% soluble. This means you would need to apply ten and a half times more rock phosphate, compared to MAP, to supply an equal amount of soluble P to a crop.
The list of organically approved fertilizers is too long to be shown here, but there are certain guidelines as to what is accepted. It can either be sourced from recycled plant or animal parts, or manures. For example, harvested seaweed can be applied as a source of nitrogen, phosphorus and potassium, and most other plant nutrients. Also, natural ore minerals can be mined and used as fertilizer.
Some fertilizers are commercial fertilizers, and also certified organic fertilizers. In this instance they are usually mined minerals that require no processing, other than crushing and screening to obtain a granule of suitable size for application. They may also be precipitated out of evaporated water sourced from a saline lake, and granulated into fertilizer size granules. An example is potassium sulfate (K2SO4) that can be mined, or precipitated out of saline water containing dissolved potassium and sulfate. It is used as a source of both potassium and sulfur.
It's important to note that certified organic fertilizers are much less concentrated in nutrient content, and they cost much more per unit of nutrient applied. This is not a problem if a farmer can successfully market the organically certified crop produce at a sufficiently greater price compared to commercial farm produce.
So, both commercial fertilizers and certified organic fertilizers are suitable given they are managed right. An effective way to manage all nutrient sources is to use 4R Nutrient Stewardship to apply the right nutrient source at the right rate, time, and place. This allows needed nutrients to be available to crops in adequate, soluble amounts, and supplied so the crop can utilize the nutrients when needed. Whether a farmer uses commercial fertilizers or certified organic fertilizers, applying 4R management is most important.