Phosphorous (P) is a primary plant nutrient. It stimulates root, flower, fruit development and overall crop maturity. P is necessary for energy transfer and the formation of RNA and DNA. Most plants require additional phosphorous during cold weather, in areas of limited root growth, during rapid vegetative growth, and since phosphorus is very reactive, in highly calcareous or acid soils because of tie-up with other elements such as calcium or, in the case of acid soils, aluminum and iron.
Because phosphorus is reactive, it quickly forms compounds with other elements in the soil. Therefore, phosphorous has been thought of as immobile, and not leachable. However, it is being discovered that P does move, at least through macro pores, wormholes and cracks, as well as possibly making its way to drainage tiles and ditches.
Two forms of P can leave the field: Soluble phosphorous is lost with water runoff; insoluble (or particulate) phosphorous is lost with erosion. The insoluble form of P is complexed with soil minerals, and while it is not as available as soluble P, this form of phosphorous can be released slowly in water. Then, just as any nutrient, phosphorus begins moving from higher areas to lower areas and ends up in bodies of water.
“Keeping P in the right place » means placing it where the plant can most readily access it, in the most efficient manner. The best way to keep phosphorous in the field is placing it below the surface. In order for a plant to benefit from applied phosphorus, the nutrient needs to be kept in the growing environment. The above-mentioned management practices provide the best opportunity for the plant to utilize the nutrient and prevent off target movement.
Obviously, phosphorous has been demonstrated to play a role in algae and water plant growth. Despite the best efforts of farmers, heavy rains can and do occur when the field is most vulnerable, washing away both valuable topsoil and applied fertilizer or manure. Most producers have adopted management practices, such as no-till or reduced till, to keep residue on the field, which will in turn function as a filter to keep soil on the fields. The farmer is the first to suffer from lost nutrients because the economic impact to replacing nutrients can impact profitability.
In snowmelt studies, there are large spikes of dissolved phosphorous discharge in the spring, diminishing with successive events. These studies demonstrate how important it is to not surface apply P late in the fall, and into the winter. Farmers have often applied phosphorus in the fall because of convenience and because, historically, they have been provided information that stressed that P is immobile. However, newer studies show this management practice leaves soluble P vulnerable to runoff. The same is true of manure applied when the ground is frozen; with spring snowmelt or early rains, soluble mineral P is likely to escape from the field. About 60% of the total phosphorous in manure is inorganic, soluble orthophosphate. While orthophosphate is 100% available to the crop, it is also 100% available to algae and water plants if it makes its way to watersheds.
Thus lies the conundrum. Phosphorous is an integral part of crop production. However, algal blooms and media coverage has given agricultural phosphorous use a bad name in recent years. Phosphorus isn’t cheap, so farmers are motivated to keep this nutrient on the field. But the public scrutiny has pushed the issue to the forefront. The perception is that those involved in agriculture don’t care, which couldn’t be more untrue. Everyone is seeking solutions. The motivation may be different, such as keeping the nutrient investment available to a growing crop because that is the only way to get a return on the investment, but in the end, everyone wants the same thing. That is for phosphorus to stay where it is intended rather than moving to unknown destinations.
There are a lot of variables that work to move nutrients. It is hard to isolate each piece so that we can find a quick and certain solution. Many improved practices have already been put in place, but the results can’t be determined immediately. However, no one will play ‘wait and see’, the industry continues to take steps to keep all nutrients, including phosphorus, on the growing environment. In the past, producers my have been lulled into believing that phosphorus wouldn’t move, because many publications indicated that was the case. New information, though, is leading to new practices and that will benefit all.