United States agriculture has been touted as a model for production and a leader of innovation. The nation's intense agricultural production, however, can lead to adverse impacts to the surrounding environment. In some cases, it is the excess of basic nutrients required for plant and animal growth that can cause this degradation. Ongoing research aims to improve understanding of nutrients used and released in agricultural production, including how they interact in the environment, the damages the y can cause, and ways to prevent or correct the damage. Federal policies concerning agricultural nutrients have changed over time. Currently, few federal regulations govern the environmental impacts of nutrients from agriculture. Some environmental laws specifically exempt agriculture from regulatory requirements, and others are structured so that agriculture is not addressed by most, if not all, of the regulations. The major federal response continues to be through research, education, outreach, and voluntary technical and financial incentives to producers. Recent events involving degraded water quality have raised questions as to whether the current federal response to agricultural nutrients is adequate or should be altered. This has prompted both administrative and congressional action. This report discusses the types and sources of nutrient pollution from agricultural production; possible environmental effects of nutrient pollution; examples of current control measures; the federal response to excess nutrients, including regulatory and incentive-based programs; and future considerations for nutrient management policy at the federal level. Agriculture is one of a number of industries that produce, use, or release nutrients that may adversely affect the environment. The lack of discussion in this report of other industries that might release excess nutrients does not imply that one industry is more or less to blame for environmental harms. The science and methods of pinpointing the exact source of excess nutrients causing environmental harm are still evolving. Regardless, whether released by agriculture, lawn care companies, or sewage treatment plants, the environmental harms of excess nutrients are much the same. A large body of research, including numerous technical publications, explores the relationship between agriculture and nutrients. This report gives an overview of current knowledge, while further complexities of individual issues are discussed in many of the sources cited throughout the report. Similarly, the examples used to describe best management practices and federal response should not be considered exhaustive lists. In many cases, the success or failure of a particular nutrient management practice will vary greatly by location. This report highlights examples of these practices but is not meant to imply or suggest that they can be universally applied. In addition to the federal response, the private sector, nonprofit groups, and state and local governments play active roles in the management of nutrients from agriculture, either through regulation or technical support. These nonfederal efforts are outside the scope of this report. Nutrients are elements essential to plant growth. Plant roots absorb nutrients––including water, oxygen, and others––from the soil. As crops grow and are harvested, they gradually remove the existing nutrients from the soil. Over time, most soils will require additional nutrients to maintain or increase crop yield. When nutrients are added in excess of the plants' ability to utilize them, there is an increased risk that the nutrients will enter the surrounding environment (water or air) and create problems such as algal blooms (discussed below). Plants utilize nutrients in different ways, and each plant has a different set of nutrient requirements. How, where, and when plants utilize nutrients can greatly affect the overall yield and plant production. For a farmer seeking to maximize crop yields and lower input costs, it can be critical to understand a crop's nutrient requirements. Basic nutrients––carbon, hydrogen, and oxygen––are the most abundant elements in plants. In addition, plants utilize other nutrients commonly referred to as macronutrients and micronutrients (see Figure 1 ). In agricultural production, the focus generally rests on the three primary macronutrients––nitrogen (N), phosphorus (P), and potassium (K)—because of their relative abundance in plants. Micronutrients, while not commonly discussed, may have just as much of an effect on plant growth as macronutrients when levels are too high (toxic) or too low (deficient). This report focuses on two of the three primary macronutrients (nitrogen and phosphorus), because of the volume used in agricultural production and the relative potential for environmental harm if they are overused. This is not meant to indicate that other nutrients do not also pose an environmental harm if overapplied. Plants use nutrients in an ionic form rather than as raw elements (see Figure 2 ). Nutrients are taken up by plants in three forms: interception—by direct contact with the nutrient; mass flow—when nutrients move with water as the plant transpires; and diffusion—when nutrients move from high to low concentration. Interception accounts for a very low percentage of nutrients taken up by plants. Mass flow, on the other hand, is the most substantial method of nutrient movement toward a plant's roots. This is particularly important for more "mobile" nutrients (e.g., nitrogen) and less important for relatively "immobile" nutrients (e.g., phosphorus). Diffusion is most important for nutrients that are relatively immobile, have low solution concentrations, and are needed in large amounts (e.g., phosphorus and potassium). Plants use nutrients at different times and at different rates throughout the growing cycle. For example, for corn, phosphorus is important early in the growing season, but not necessarily later in the season. Nitrogen is important in both the beginning and later in the growing season, before the plant begins the stem-extension phase. Various nutrients are also applied differently by farmers, based on knowledge about nutrient mobility in soils. For example, nitrogen can be applied using a broadcast method (discussed further under " Best Management Practices ") because of its mobility in the soil. Phosphorus, on the other hand, is most effective when placed below the soil surface near the root system because it is generally immobile. Fertilizers are either organic or inorganic materials applied to the soil to promote plant growth. They may either contain a specific nutrient or may be used to increase the availability of other plant nutrients. Organic fertilizers are those derived from living matter (e.g., manure) whereas inorganic fertilizers are synthetic or mined (e.g., urea as a nitrogen source or potash as a potassium source). The primary organic fertilizer is animal manure. Manure can be a good source of plant-available nutrients (including but not limited to nitrogen and phosphorus), as well as providing increased soil organic matter, increased water and nutrient retention in soil, and decreased soil density. The nutrient content of manure can vary greatly depending on animal type, diet, bedding, moisture content, and storage method. Prior to the development of inorganic fertilizers, organic "wastes" were a major source of nutrients for crop production. According to the U.S. Department of Agriculture (USDA), 15.8 million acres, or about 5% of all U.S. cropland, use manure as fertilizer ( Table 1 ). A number of factors prevent a wider use of manure fertilization. Primarily, the cost of transporting manure can be prohibitive. The nutrient content of the manure, the type of crop grown and nutrients required (i.e., the nutrient needs of plants being grown may not match nutrients in readily available manure), compaction from manure application equipment, and the relative cost and availability of inorganic fertilizers, among other factors, also affect the use of organic wastes as fertilizers. From a livestock perspective, animal waste is a by-product of production that must be managed to avoid environmental harms and for the health of the animals themselves if raised in captivity. For a confined production system, manure must be utilized—typically by being collected, stored, and distributed elsewhere, likely on available crop- or pastureland. In the case of grazing livestock, the animals deposit manure directly onto pastureland, thus fertilizing the associated grassland. High stocking rates, however, may lead to an imbalance, which in turn could create an excess supply of nutrients. If not properly managed, manure can adversely impact water quality through surface runoff and erosion, direct discharges to surface waters, spills and other dry-weather discharges, and leaching into soil and groundwater. It can also result in emission to the air of particles and gases such as ammonia, hydrogen sulfide, and volatile organic chemicals. Theoretically, of the total manure produced, more could be utilized on additional cropland than what is currently utilized in order to potentially reduce the risk of environmental harms from excessive concentrations. Regions with a higher capacity for manure are largely due to the type of soils and crops grown in the area. For example, the soils (mollisols) and crops (corn) grown in the Midwest are able to utilize additional nitrogen from manure ( Figure 3 ), whereas the soils (ultisols) and crops (cotton and rice) in the Southeast are able to handle more phosphorus from manure ( Figure 4 ). In some cases the ability to utilize additional manure application corresponds to areas with large livestock populations. In other cases, livestock production occurs in areas with low or modest capacity to assimilate additional manure, thereby increasing the risk of excessive concentrations. When the amount of manure produced by an operation is more than the assimilative capacity of the land, an excess of manure is created ( Figure 5 and Figure 6 ). This can create a greater risk for potential runoff and leaching of manure nutrients and subsequent water quality issues. Geographically, areas with excess farm-level nutrients correspond to areas with increasing numbers of confined animals. Inorganic fertilizers consist of nutrients that are mined or created synthetically. In most cases, compared to organic fertilizers, inorganic fertilizers are more concentrated, their nutrient content is easily identifiable, and in some cases, they are more cost effective to use.