If you are planning on selling your farm and removing it from agricultural production, you probably don't need to worry about this issue. If you plan to farm for more years, pass the farm down to the next generation, or sell it to another farmer, there are good reasons to consider the impact of soil nutrient balance on the long-term sustainability of your livestock and cropping operation. Phosphorus is the main nutrient of concern. Due to binding tightly to soil particles, phosphorus, when applied in excess of crop need, will build up in the soil over time. If the soil never left the field, this might not be a concern. But any form of erosion via water or wind can move soil particles that carry the phosphorus with it, and they may end up in surface waters. Tile drains can also move the nutrient to surface waters. Once in surface waters, the soil and the nutrients will degrade water quality. The higher the phosphorus soil test and the more risk of erosion, the greater the potential for it to reach surface waters. Once a soil tests high in phosphorus, it declines slowly. For a rough example, it could take 20 years for soil test phosphorus levels to decline a hundred pounds per acre on a soil test. This is obviously dependent on the soil types, climate, and cropping system, but the point is that once soil tests are built to a high level, they decline much more slowly than other common nutrients such as nitrogen or potassium. Most states have a P-Index to assist producers in evaluating their risk of off-site movement of phosphorus. In some instances, these indexes are mandatory or regulated. No one can predict the future, but it is safe to think that standards for phosphorus will only become more restrictive over time. It will behoove producers to review and track their whole farm soil nutrient levels, especially phosphorus, now to better prepare for the future.
Author: Natalie Rector, Michigan State University, Extension Nutrient Management Field Specialist
