Open to your periodic table of the elements and you'll find phosphorus (P) in the number 15 position. It comes into the game with an atomic weight of 30.97 and bats in the second position as a listed primary nutrient on a fertilizer analysis --- right after nitrogen (N) and before potassium (K).

     Back in my high school chemistry class days, P was one of many elements to memorize on the periodic table. In recent years it has been thrust into my professional life by way of fertilizer regulation and environmental scrutiny. Phosphorus, both rightly and wrongly, has become the Asama bin Laden of fertilizer nutrients.

     Phosphorus is an essential nutrient for normal plant growth. It's required for photosynthesis, respiration, seed production, root growth, and other critical functions. Supplemental additions of phosphorus beyond naturally occurring soil levels are needed in productive cropping systems. However, like deep-fried cheese curds, too much of a good thing can lead to problems. That's where we find ourselves today.

     In Fond du Lac County, the average soil test P level has risen from 31 parts per million (ppm) in the mid-1970's to about 50 ppm today. In recent years, there are clear signs that it is starting to retreat. Reducing soil test P levels is a slow process. The reasons for increased P levels are two-fold. First, for many years purchased fertilizer was relatively cheap. That scenario has changed and the amount of purchased P fertilized being applied to crop fields has been drastically reduced.

     The second reason for the rise in soil test P can be traced to manure management. When manure is applied to crop fields to meet plant N needs, this generally results in an over application of P because crops such as corn need far more N than P. Hence it becomes a "Catch 22" situation whereby if manure is applied at a rate to only meet crop P needs additional N has to be purchased and manure has to spread across far more acres. Presently farm nutrient management plans dictate the amount of P that can be applied to crop fields based on current field soil test levels.

     The problem with high soil test P levels is that more P finds its way to surface waters. This occurs in two ways. As water from rain or snowmelt moves across the landscape it interacts with the topsoil and some P is detached from the soil and dissolved into the water (called soluble P). Further, if the water movement is harsh enough to detach and move soil (erosion), the P-rich soil particles are deposited directly into lakes and streams (called particulate P).

     Once in surface waters, P promotes the growth of aquatic vegetation such as algae and expedites the natural aging process of lakes. Further it reduces the recreation and aesthetic value and can be detrimental to aquatic life. Where this occurs it quickly gets the attention of nearby residents, lake users, and ultimately politicians.

     Earlier this year, the focus on P extended to turf and lawns when Governor Doyle signed the state phosphorus ban bill into law. This legislation makes it illegal to purchase and apply P fertilizers to established lawns (except municipal organic products) unless it can be shown that a soil test warrants such an application. It also allows for P applications for new lawn establishment. The effectiveness of such a law is certainly open for debate, but at the same time it will also probably have little impact on turf productivity and health. Like most farm fields, few lawns are deficient in P.