Managing Phosphorus for Crop Production

Phosphorus is a macronutrient that plays a number of important roles in plants. It is a component of nucleic acids, so it plays a vital role in plant reproduction, of which grain production is an important result. It is also critical in biological energy transfer processes that are vital for life and growth. Adequate phosphorus results in higher grain production, improved crop quality, greater stalk strength, increased root growth, and earlier crop maturity. For over one hundred years, phosphorus has been applied to crops as fertilizer—first as ground bone and now as some chemical reaction product of ground rock. Yet, for all that experience, its management cannot be taken for granted. Phosphorus is not lost into the atmosphere—rarely does it leach beyond the reach of roots—and its availability to crops can be accurately estimated by soil testing. The challenge is that phosphorus is a macronutrient in plants but behaves somewhat like a micronutrient in soils. The concentration of soluble phosphate in the soil solution is very low, and phosphorus is relatively immobile in the soil. That is important because crops take up phosphorus only from the soil solution. The crop depends on replenishment of the soil solution with phosphate from the other forms existing in the soil. The rate of replenishment, which determines the availability of phosphorus, is related to soil pH, phosphorus levels in soil, its fixation by the soil, and placement of added phosphorus. The crop manager must deal with each of these factors to avoid crop phosphorus deficiency. Phosphorus deficiency symptoms include reduced growth and yield, delayed maturity, and generally purple coloring along the edge of the lower plant leaves, especially on younger plants. In addition, the manager needs to consider possible “side effects” of crop production; specifically, nutrient pollution of streams or other surface water near crop fields. Water can be polluted with phosphorus primarily as a result of erosion and runoff of phosphorus in the soil or phosphorus applied either from fertilizer or manure. The amount of phosphorus lost due to runoff of manure, fertilizer, or soil may be relatively small as far as fertilizer costs are concerned. However, these small losses may have serious effects on the quality of water. The main problem with phosphorus pollution is eutrophication resulting in excessive growth of plants and algae in the water. This can seriously limit the use of the water for drinking, industry, fishing, or recreation. Pollution reduction may not be simply a direct economic problem for the farmer, but a responsibility that extends beyond the farm fence.