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. For more information see the Penn State publication “Agricultural Phosphorus and the Environment.”
Availability of phosphorus to crops
In general, crop use of any nutrient depends on a two-step process: soil supply of that nutrient in an available form, and uptake of that available nutrient by the crop. There are certain constants involved that the crop manager cannot change. Selecting among the options presented by nature constitutes management.
Figure 1 shows an overview of the behavior of phosphorus in the soil. The soil solution is the key to plant nutrition because all phosphorus that is taken up by plants comes from phosphorus dissolved in the soil solution. Because the amount of soluble phosphorus in the soil solution is very low, it must be replenished by as many as 500 times during a growing season to meet the nutritional needs of a typical crop. Although very little phosphorus is in the soil solution at any time, there is a large amount of phosphorus in most soils. The bulk of the soil phosphorus is either in the soil organic matter or in the soil minerals. A large proportion of the phosphorus in both of these fractions is in very stable, unavailable forms, while a much smaller proportion is in available forms that can dissolve in the soil solution and be taken up by plants. The dynamic and available phosphorus phosphorus in these fractions, such as phosphorus added in fertilizer or manure, can be quickly fixed into stable, unavailable forms in the soil. This is why, even with optimum management, the efficiency of plant uptake of phosphorus is very low—usually less than 20 percent. At the same time as the soil solution phosphorus is depleted by crop uptake, unavailable phosphorus can slowly be released to more available forms to replenish the soil solution. This slow release can sustain plant growth in many natural systems, but is usually not rapid enough to maintain adequate phosphorus availability in intensively managed cropping systems without some supplemental phosphorus in the form of fertilizer, manure, or crop residues.
Crop response to phosphorus depends on the availability of phosphorus in the soil solution and the ability of the crop to take up phosphorus. The availability of phosphorus in the soil solution has already been discussed. The ability of a plant to take up phosphorus is largely due to its root distribution relative to phosphorus location in soil. Because phosphorus is very immobile in the soil, it does not move very far in the soil to get to the roots. Diffusion to the root is only about 1/8 of an inch per year, and relatively little phosphorus in soil is within that distance of a root. Thus, the roots must grow through the soil and basically go get the phosphorus the plant needs. Therefore root growth is very important to phosphorus nutrition. Any factor that affects root growth will affect the ability of plant to explore more soil and get adequate phosphorus. Soil compaction, herbicide root injury, and insects feeding on roots can all dramatically reduce the ability of the plant to get adequate phosphorus. Young seedlings can suffer from phosphorus deficiency even in soils with high available phosphorus levels because they have very limited root systems that are growing very slowly in cold, wet, early early-season soil conditions. This is why some crops respond to phosphorus applied at planting in starter fertilizers even in relatively high phosphorus soils.