Reducing Phosphorus Impact On Water

Best management practices address fertilizer, manure, irrigation, and soil.

Reducing P losses from agriculture ultimately requires balancing P inputs with crop P removal, coupled with soil management to retain nutrients. Simply implementing BMPs that reduce soil erosion is not adequate. At high soil concentrations, P will become mobile and can be transported in solution when runoff from rainfall or irrigation occurs. Both source and transport control strategies must be implemented on fields that drain to sensitive surface water. Fortunately, all fields do not contribute equally to P export from watersheds. Most P export comes from only a small portion of the watershed as a result of relatively few storms or irrigation events.

Managing fertilizer
Phosphorus fertilizer applications at recommended rates can minimize P loss in agricultural runoff via increased crop uptake and more residue cover. Fertilizer application to build soil test levels is no longer a desirable practice. Rather, producers should attempt to optimize crop response by applying fertilizer at agronomic rates.

Placement of P fertilizer will influence the amount of P available for transport to surface water. Correct placement of fertilizers in the plant root zone will improve fertilizer use efficiency and seedling vigor, and reduce the amount of P in runoff. Phosphorus fertilizer should not be broadcast on the soil surface without incorporation, except on perennial forages. In established alfalfa stands, P fertilizer should normally not be applied in the late fall or winter when growth is minimal and runoff potential is high. Broadcast applications generally are less efficient and leave more P at the soil surface than banding. Band application at planting is considered the most efficient method for row crops. Subsurface placement is especially important under reduced tillage cropping systems to achieve maximum crop yields.

Variable fertilizer rate management can improve fertilizer use efficiency and economic returns in some cases. While this strategy can be adopted for any fertilized field, it makes the most sense in relatively large fields where the producer has knowledge of how crop yields and soil type vary across the field. To use a variable fertilizer rate strategy:

1) Divide the field into different management units based upon a map of yields and soil types.

2) Soil sample the management units separately.

3) Fertilize each unit according to P soil test level and yield capability.

Managing manure
Manure is an excellent source of P for crop production. However, if manure is not managed properly, runoff may carry both soluble and sediment-associated nutrients to surface waters. Producers need soil and manure analyses to determine the correct application rate based upon crop uptake of N or P. Approximate table values are available for various manure sources, but manure sampling and analysis are the most accurate way to calculate nutrient credit.

As with commercial P fertilizers, manure should be incorporated immediately after application. Injection of liquid manure beneath the soil surface with specialized equipment is also a recommended practice. Unlike commercial fertilizer, the P content of manure can vary significantly. Additionally, N and P contents are not equal. On sensitive fields, manure rates may need to be based upon crop P need rather than N. In these situations, supplemental fertilizer N will be required to meet yield goals. In many cases, the best program is to rotate fields receiving manures to avoid salt and nutrient buildup.

Site characteristics such as land slope, surface residue, and proximity to surface water must be used to determine which management measures are needed to protect surface water from P enrichment from manure. In some cases, sites with excessive slope or highly erodible soils are not suitable for manure application, even with careful management. Application of manure on frozen or wet soils subject to runoff is not recommended.

Runoff from feedlots and manure stockpile sites also can contribute nutrients to nearby surface waters. Best management practices to reduce runoff impacts include yard shaping, settling basins, diversions, and filter strips.

Managing soil
Since the majority of P leaving a field is bound to sediment, practices that reduce sediment movement offsite greatly reduce P losses. A number of management practices and structures for controlling runoff and erosion are currently available for use. In some cases, there is a trade-off between reducing runoff and increasing deep percolation to groundwater. BMPs for managing surface runoff and soil erosion are listed in Table 4 below.

Buffer strips between erosive fields and surface water can help settle soil sediment and thereby reduce sediment borne P loads.

Managing irrigation
Managing irrigation to reduce P transport is similar to managing irrigation to reduce erosion. Water applications should be applied at a velocity appropriate to the slope and length of run that minimizes down cutting. In some cases, it is necessary to split fields into shorter runs to achieve the optimum uniformity without excess velocity. Use of polyacrylimides (PAM) has been shown to allow producers to increase flow rate while reducing sediment detachment and transport.

Reducing irrigation runoff altogether is the most effective way to reduce or eliminate P movement. Surface irrigation system upgrades such as surge valves and tailwater pits or switching to a properly nozzled sprinkler system will largely eliminate runoff to natural water bodies.

Summary

• Good P management should balance P inputs and outputs.
• Fields that are highly erosive and/or are near surface water need more management to reduce P losses.
• Manure rates should be based upon crop renewal for fields with high to very high soil P test levels that are near surface water bodies.

by Reagan Waskom

Table 4. Erosion control BMPs for reducing surface losses from crop fields.

Best Management Practice	Description
Conservation tillage	Cropping system that maintains at least 30% of the soil surface covered with residues after planting.
Conservation cover	Perennial vegetative cover established and maintained on highly erodible lands where other BMPs are insufficient to reduce adverse water quality impacts.
Conservation cropping sequence	Crop rotation sequence designed to increase crop residues on the soil surface to reduce erosion.
Delayed seed bed preparation	Cropping system in which all crop residues are maintained on the soil surface until 3-4 weeks prior to planting the succeeding crop.
Grass filter strip	Permanent sod strip planted at the base of sloping fields or between the field and surface water bodies.
Grassed waterway	Sodded channel that provides a non-erosive outlet for runoff.
Contour farming	Crops are planted on the natural contour of the land to reduce erosion.
Strip cropping	Alternating strips of row crops and solid seeded crops planted on the contour.
Terrace	Earthen embankment constructed across the slope to reduce slope length and runoff velocity.
Diversion	Grassed channel constructed across the slope, uphill of a tilled field, to divert excess water to areas where it can be managed properly.
Sediment control basin	Basins constructed to collect runoff and trap sediments.
Constructed wetland	Artificial wetland created downhill from feedlots and irrigated crop fields where sediment and runoff are collected and assimilated by growing vegetation.
Polyacrylimides (PAM)	Irrigation water amendment that flocculates suspended sediment.