Balance of phosphorus (P) inputs and outputs from all sources in a specific component of an agricultural system, such as a field, an animal facility or a farm.
Accounting methods using amounts of each material and P concentrations can make simple estimates of the difference between P in the inputs such as fertilizer, feed and manure and P in the crops harvested, animals sold, manure exported or other outputs.
A positive balance indicates an accumulation of P, while a negative balance indicates a depletion of P. The “sufficiency” approach to P fertilizer application, 4R management principles, and current university P fertility guidelines are all based on the P balance concept.
To balance the P inputs and outputs so that P on the farm or in a field remains adequate to meet crop and animal requirements but does not represent an excess that could be a potential source of P loss to the environment. After P inputs and outputs have been balanced, edge-of-field practices can then be implemented in fields to minimize event-based P loss.
How Does This Practice Work?
Phosphorus pollution requires a source of the nutrient and a mechanism to transport it to a water resource. This practice aims to limit the potential for soils to be a P source. Phosphorus balance is determined by the managed material/nutrient transfers to, from and within a field, an animal facility or farm. If the flow of P in exceeds the flow of P out, a positive P balance will occur and nutrients will accumulate in that component of the system, contributing to the source of P. This accumulation will often be indicated by excessive soil test levels in the farm fields.
For a farm that specializes in animal production, overall farm balance can be roughly estimated based on animal density or external feed sources, as illustrated in the table below. This simple classification can be used as a starting point for assessing the nutrient balance for an operation and for helping to determine the need for a more detailed P balance assessment.
Phosphorus can be lost from fields to surrounding water resources through erosion, runoff and leaching. When P accumulates in an animal facility, it may be a source of P loss in runoff discharges from the facilities, or it may be part of residual “sludge” in manure storage structures that must be accounted for at some time in the future.
If the difference between P inputs and P outputs is negative, P is being depleted from that component of the agricultural system. Depletions will be beneficial where large amounts of P have accumulated. However, if low soil test levels are present possible crop P deficiencies could result from a negative P balance.
The matrix that follows can be used to assess the potential agronomic or environmental impact of nutrient balance for individual fields, groups of fields or a farm, depending on the different soil test conditions on a farm.
The actual loss of P from farms to water resources will depend on the transport mechanisms operating on the source. By managing the flow of nutrients to maintain a balance between inputs and outputs, the contribution of the source can be minimized and the risk of P loss reduced.
Nutrient balance at the farm level is usually determined by a farmer’s strategic decisions. These decisions are based on a wide variety of factors, especially those external to the farm, that are quite different from the factors influencing day-to-day farm activities.
The outcomes of these strategic decisions may not be influenced by their consequences for the balance of nutrients. An example of the lack of connection between these decisions and their environmental consequences would be intensifying animal production by increasing reliance on feed from off the farm to support more animal production with little concern for the fate of the additional nutrients.
Strategic decisions not only impact the farm level balance, but also constrain the management options available to achieve balance on the fields within the farm. Unlike most traditional Best Management Practices, implementing a nutrient balance practice will usually require strategic changes in the farm operation particularly in the case of animal operations.
Examples of strategic farm changes might include:
- reducing the animal density on the farm to reduce the inputs of nutrients
- securing more land for manure application, thus increasing the outputs in the form of crop removal
- or moving manure off the farm as an additional output of nutrients.
Where This Practice Applies and Its Limitations
This practice applies to all types of agricultural production, but it may be more easily implemented in crop-based operations than animal operations. Where the P inputs are closely related to crop production, economic factors often directly encourage nutrient balance.
Controlling P accumulation in excess of crop utilization potential is primarily seen in situations where there are significant sources of nutrient inputs to farms that are not directly related to crop requirements. For example, this would include inputs of nutrients in feeds for intensive livestock and poultry enterprises.
In cases where P imports to a region (e.g., animal feed) greatly exceed capacity to land apply manure, higher level policy intervention and assistance may be required to make P balance feasible.
Balancing phosphorus inputs and outputs at the farm and field scales is a critical step for controlling the P loss process. For fields with soil P fertility near the agronomic optimum, P balance combined with adequate transport controls should be effective to maintain P loss below levels of environmental concern.
Practice effectiveness will be limited by the presence of “Legacy P.” Legacy P is the buildup of P stores in the soil due to long-term, historical management decisions (e.g., decades of land application of manure; long-term use of the build-and-maintain soil fertility philosophy). In cases where Legacy P is present, hydrologic losses may exceed water quality goals even under a negative P balance. Additional implementation of transport controls may be required in these cases to meet both agronomic requirements and environmental goals.
Soil Test P can be a useful tool to screen fields for the presence of Legacy P. While it is not the only factor influencing P loss, it provides a gauge for where to target further efforts or to place transport controls for P loss mitigation.
The costs to remove P from the agricultural system to achieve the P balance associated with this practice may seriously inhibit the feasibility of implementation (see below). The physical accounting for P may not be as difficult to implement as corrective management responses.
Cost of Implementing the Practice
For most field crop operations, establishing nutrient balance is compatible both with the goals of 4R nutrient management and university-evaluated soil fertility recommendations. Balancing P fertilizer application with crop uptake will likely result in net savings on fertilizer input costs. Costs associated with maintaining a phosphorus balance will be primarily related to the costs of annual or rotational soil testing (every 1-2 years).
For animal operations the costs associated with implementing this practice can be very high and possibly prohibitive. Maintaining a phosphorus balance for animal operations will likely require alternatives to land-spreading of animal waste such as increased manure storage and off-site transport of manure.
Farms that are in a start-up phase or about to expand are best able to cover the sunk costs in their business plan, so complying with new strategic requirements, such as planning for P balance, should be an important consideration in these situations. However, these additional costs may encourage other farmers to quit farming, and encourage the remaining businesses to get larger to cover the costs.
Operation and Maintenance
When P is accumulating in excess of crop utilization potential, this practice will require intentional, ongoing effort to maintain nutrient balance at the field and farm levels. Detailed records of inputs and outputs along with soil test phosphorus levels will be helpful at the field or farm level, depending on the outcomes of a preliminary P balance evaluation. These can be used to identify the opportunities for improving the balance and ensuring that goals are routinely achieved.
New skills and management capacity may be required for this accounting and evaluation for animal operations. For example, if additional cropland is acquired to provide additional capacity for crop nutrient utilization, animal-oriented producers may have to take on greater crop production responsibilities. Additionally implementing this practice will likely require a much greater off-farm focus for these operations; for example, in developing and servicing off-farm markets for manure or dealing with manure importers, brokers and haulers, than the historic focus on field and farm management practices.
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Lanyon, L. E. and D. B. Beegle. 1993. A nutrient management approach for Pennsylvania: Plant nutrient stocks and flows. Agronomy Facts 38-B. Department of Agronomy, The Pennsylvania State University, University Park, PA. 8 pp.
Pearce, A., and R. Maguire. 2020. The state of phosphorus balance on 58 Virginia dairy farms. Journal of Environmental Quality 49(2): 324–334. doi: 10.1002/jeq2.20054.
Saporito, L.S. and L.E. Lanyon. 2004. Evaluating the spatial and temporal dynamics of farm and field phosphorus and potassium balances on a mixed crop and livestock farm. Nutrient Cycing in Agroecosystems 69: 85-94.
Sharpley, A.N., H.P. Jarvie, A. Buda, L. May, B. Spears, et al. 2013. Phosphorus Legacy: Overcoming the Effects of Past Management Practices to Mitigate Future Water Quality Impairment. Journal of Environmental Quality 42(5): 1308–1326.
Wironen, M.B., E.M. Bennett, and J.D. Erickson. 2018. Phosphorus flows and legacy accumulation in an animal-dominated agricultural region from 1925 to 2012. Global Environ. Change. 50: 88–99.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
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Pease, L. 2023. Phosphorus Balance. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/phosphorus-balance/