Last winter we were parked in Florida for a couple of months. One day I happened to notice that the gravel in the RV park was made up of tiny shells and that reminded me that Florida is one of the world’s greatest repositories of rock phosphate. Mining in “phosphate alley” or “bone valley” in central Florida dates back to the late 1800’s and it is still one of the largest sources of phosphate in the world. Rock phosphate that accumulated in the bones and shells of ancient animals is the necessary precursor for all phosphate fertilizers.
Rock phosphate isn’t available to plants, despite what some “natural” phosphate snake-oil salesmen may tell you. Once it has been processed into various phosphate fertilizer forms it becomes more available but it is a difficult nutrient for many people to understand.
Phosphorus is a highly reactive metal that is never found by itself in nature. Rock phosphate can be smelted to produce phosphorus or it can be beneficiated to produce phosphoric acid. Phosphoric acid can be subsequently reduced with anhydrous ammonia to produce ammoniated phosphates such as MAP, DAP or ammonium polyphosphate solutions.
The phosphate ion (P2O5) is a cation – a positively charge ion. Soil has what is known as cation exchange capacity which is simply the ability to retain cations. At a microscopic level soil has many negatively charged surfaces. The negatively charged soil attracts the positively charged cation and holds it. Some soils hold cations better than others but that isn’t the point today – the point is that soil hangs onto cations. Plants are able to take up free phosphate ions but those ions first need to be pried loose from the soil exchange sites and that isn’t easy because the positive-negative attraction is relatively strong. When you flood the soil with a large amount of phosphate ions in close proximity (a fertilizer band in the seedrow for example) you temporarily disrupt that soil to phosphate balance thereby freeing some phosphate ions for crop uptake. Over time though the soil tries to get things back in balance by tying up the free phosphate ions.
The fertilizer phosphate has to come from somewhere. In North America that somewhere is from open pit mines in either Oregon or more likely Florida. Open pit mines anywhere tend to attract negative attention and the US is no exception. Florida has done a good job of reclamation on their old mining properties but new sites are increasing difficult to develop.
The International Fertilizer Association (IFA) is starting to deal with the concept of peak phosphate. Readers may be familiar with Peak Oil which is the theoretical point where we can no longer find any additional oil reserves. Some of us who are longer in the tooth can remember that we were supposed to run out of oil a long time ago. So far higher prices for a barrel of oil have stimulated exploration which has revealed new reserves but it seems logical that any resource must have some finite limits, whatever those may be.
Phosphate is no different in terms of the absolute quantity available. Increasing prices tend to stimulate discovery of new reserves but it seems logical to assume that at some point we will have found everything there is to find, whatever that amount may be.
The difference between phosphate and oil is that phosphate cycles in a useful time frame. Once we extract the energy from a barrel of oil it is for all practical purposes gone. We don’t get to reuse that energy. Phosphate however is different because once added to the soil it enters a renewable cycle. There are some removals from the cycle – human bones for instance – but most of the phosphate stays within the system once it is added. Animal & human waste is high in phosphates as are animal carcasses. I suppose our human carcasses are a good phosphate source but I think we are a long way from using them as fertilizer.
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