Understanding Phosphorus Versus Nitrogen Manure Management
In most cases, nutrient management plans are based on balancing nitrogen. This comes from our scientific understanding of nitrogen behavior and generally greater concerns with the environmental fate of nitrogen and the potential negative human health effects of nitrates in water.
However, phosphorus has also received significant attention because of the potential for accelerated eutrophication in water bodies receiving excess P. Phosphorus is an essential nutrient for crops. A critical component of crop production is managing phosphorus for optimum economic benefit.
However, if phosphorus is allowed to move off the land and get into water, it can be a pollution problem. Unlike nitrogen, excess phosphorus is not toxic but can result in eutrophication of water bodies. Eutrophication is the increased growth of undesirable algae and other aquatic plants, which limits the use of the water for drinking, fisheries, recreation, and industrial use. Historically, a major source of phosphorus in water was wastewater treatment plants.
But as the amount of phosphorus getting into water from this source has been reduced, agricultural sources of phosphorus have gotten more attention. Manure in particular has been identified as a source of phosphorus to the environment. Agriculture has evolved toward greater and greater specialization. Consequently, animal feed is often produced in a different location from where the animals are fed. Feed containing phosphorus is transported to the animals. However, the manure containing much of the phosphorus that was in the feed is spread locally and not recycled back to where the crops were produced. This has resulted in phosphorus accumulating in excess amounts in some areas of high animal concentration.
At the same time, we have long recognized that one of the consequences of basing nutrient management plans on nitrogen is that, in most cases, excess phosphorus will be applied to the soil because the nutrient content of manure does not match the nutrient requirement of most crops. On the other hand, if the phosphorus requirement of the crops is balanced with the phosphorus in manure, two to four times as many acres are required to utilize the manure, and nitrogen fertilizer will be required to meet the needs of the crop.
This is not an ideal situation, but from our understanding of phosphorus behavior we know that there are things that we can do to minimize the environmental impact of this excess phosphorus in many cases. Consequently, we have consistently emphasized that, while the official guidelines for manure management have been based on nitrogen, we cannot ignore phosphorus. This is the reason for including practices such as soil and water conservation, balancing manure applications over crop rotations, cover crops, etc. in nutrient management plans.
Research has shown that a majority of the phosphorus from agricultural fields lost to water comes from a limited area on most farms and from a few storm events. There are two factors that must be considered in developing effective management strategies. For nutrient loss to occur, there must be a source of phosphorus and there must be a mechanism for transporting it to the water. A key concept to effectively managing nutrient pollution is to focus on where these two factors overlap; i.e., a high source coupled with high transport.
Tools to help identify where these critical source areas are, and to suggest manure management strategies to address these areas, have been developed. See the Penn State Cooperative Extension publication, Managing Phosphorus for Agriculture and the Environment, The Pennsylvania Phosphorus Index v 2.0 for more details.
Since agriculture is changing rapidly, farm classifications may change with time. Therefore, farmers periodically should evaluate their farm’s nutrient management status to adjust their strategy as appropriate. More detailed evaluation of fields and practices will be necessary to identify specific management options or best management practices (BMPs) for each individual situation.