Manure Phosphorus and Surface Water Protection I: Basic Concepts of Soil and Water P

Phosphorus is an essential nutrient for the growth of aquatic vegetation. In many fresh water bodies, P is the most limiting nutrient to the growth of vegetation. Therefore, as P concentration in the water increases, the growth of aquatic vegetation increases. The direct effects of the excessive growth of aquatic vegetation can include a reduction in the transmission of solar radiation and production of toxins (Fig. 1).  A secondary effect is a decrease in the dissolved oxygen in the water, when bacteria utilize the oxygen while decomposing the increased amounts of dead aquatic vegetation. These effects associated with increased aquatic vegetation growth often have detrimental effects on fish and other aquatic life (Fig. 2).  This process is a form of eutrophication. In basic terms, eutrophication refers to the excessive growth of aquatic vegetation in surface waters due to nutrient enrichment. In most fresh water bodies, P is the nutrient limiting aquatic vegetation growth. In salt waters, N is often the most limiting nutrient; although P, as well as N, may be contributing to the hypoxia zone in the Gulf of Mexico.
 

Figure 1.  Algae growth stimulated by phosphorus from agricultural runoff. (Photo courtesy of David Tarkalson)

Fig. 2.  Water P concentrations higher than the critical P concentrations can lead to eutrophication. (Graphic by Bahman Eghball)

Eutrophication is a naturally occurring process that is often accelerated with intensification of agriculture or other practices that result in increased flows of nutrients to water bodies. With intensive agriculture involving heavy P applications, eutrophication may occur over periods of years to decades, while it may require centuries to occur under natural conditions.

 

Phosphorus levels in surface waters are often monitored and compared to critical P concentrations. Measured levels of P that are above the critical levels are considered excessive of an optimal aquatic ecosystem. Critical P levels have been set at 10 ppb (parts per billion) for dissolved P in lakes, 50 ppb for total P in lakes, and 100 ppb total P in streams. If levels of P in lakes and streams are above critical levels, efforts are needed to prevent further increases of P and, if possible, to reduce P concentrations in order to prevent or reduce the effects of eutrophication. This can be a daunting task, and we need to recognize that, in many cases, the P levels were above these critical levels under natural conditions prior to the implementation of intensified agricultural systems.

 

In agricultural land, high risk of P loss in runoff is often the result of an imbalanced P distribution cycle. Figure 3 shows a common example of an imbalanced P distribution cycle. This cycle begins when P is mined from deposits, such as those found in Florida.

Fig. 3.  Phosphorus is mined, distributed in fertilizer to crop producing areas, and then in grain to animal feeding areas where P becomes concentrated. Environmental problems often result, especially when there is little cropland available near the feeding operation for manure P application. (Graphic from Koelsch, Livestock and Poultry Environmental Stewardship Curriculum)

The P is transferred as fertilizer to agricultural lands, such as in the Midwest. Much of the grain or forage crops are harvested and transported to animal feeding operations. Most of the P in feeds grains consumed by animals is excreted in manures, which is spread on local fields. The imbalanced cycle is a result of the P in manure not being returned to the sources of P (mines and most farmland) and being concentrated in areas close to the animal feeding operations. This imbalance results in P levels rising in these soils to a level that P loss in runoff to surface waters is a concern. If enough cropland near animal feeding operations is available for manure application, the manure P might be safely applied to meet, but not exceed, crop needs. Often, however, when the cropland available for manure application is insufficient, much P is delivered from the land to surface waters in runoff.

Phosphorus loss from cropland on a per-acre basis may often be small compared to the amounts of P applied and to the amounts removed in harvest. However, when the P loss from a large land area funnels into the relatively small water body, the P concentration levels in the water can increase dramatically. An important consideration is that, in many watersheds, most of the P loss from cropland occurs from a small part of the watershed.  Significant reduction in P loss to surface waters can then be achieved by changing management practices on a relatively small part of the land area.

The factors contributing to P loss from agricultural land to surface waters are commonly grouped as source (site and management) factors (Manure Phosphorus and Surface Water Protection II: Field and Management Factors ), and transport factors (Manure Phosphorus and Surface Water Protection III: Transport Factors) (Table 1). The site and management factors include: soil test P; and time, method and rate of manure and fertilizer P application. Tillage systems, use of cover crops, and use of residues for ground cover may also be included in the site and management factors. Transport factors include:  erosion, runoff, subsurface and surface drainage, percolation and underground movement of P to seepage areas, distance from P source to concentrated water flow or a water body, and atmospheric deposition. The interaction of source and transport factors leads to P runoff.