Management Practices to Reduce P Transport Factors
Management strategies that are effective in reducing the potential for P transport have been developed and verified through research and practical application on farms. Some of these strategies have a long history of use, but were targeted at soil and water conservation, rather than reducing P delivery to surface waters. Terraces are physical earthen barriers to runoff and can be very effective in reducing P losses to erosion and runoff (Fig 11). Erosion losses are often reduced by 90%, compared to a non-terraced location. However, terrace systems are costly to install and maintain, and interference with field operations further increases the cost of using terraces.
Figure 11. Physical barriers, such as terraces, that intercept or slow water flow reduce erosive capacity and increase sedimentation and P adsorption. (Photo by Bahman Eghball)
Figure 12. Vegetative filter strips, or buffer strips, are barriers that intercept or slow water flow, thereby reducing erosive capacity and increasing sedimentation and P adsorption. (Photo by Bahman Eghball)
Buffer strips, also known as filter strips, to protect streams and lakes are effective in reducing runoff P concentration (Fig. 12). The effectiveness of buffers has varied in different studies, but the mean and median reduction in total P in runoff was estimated to be 2.9% per foot of buffer strip width. Buffer strip effectiveness is reduced if water flow through buffers is concentrated through small parts of the strip (Fig. 13). The strips will be most effective if overland flow is dispersed along the length of the buffer strip as it passes through.
Figure 13. Buffer strips are most effective in reducing sediment and nutrients in runoff water if flow is dispersed along the length of the strip. When flow through the buffer is concentrated, the buffer is less effective. (Graphics by Charles Wortmann)
Narrow buffer strips, about 4 feet wide and composed of switchgrass, can intercept overland runoff flow in the field and reduce runoff P loss by more than 50% following P application (Figs. 12 and 14, Table 2). The thick arrangement of switchgrass stems in a well-established filter strip can be expected to significantly reduce the rate of flow, leading to increased sedimentation and additional sites for P adsorption.
| Table 2. The quantity of P loss was reduced when the runoff passed through narrow (40', 1.2m) hedges of switchgrass. | |
|---|---|
| ----% reduction ---- | |
| Total P | 49 |
| Sediment P | 53 |
| Dissolved P | 50 |
| Bio-available P | 47 |
Figure 14. Narrow vegetative filter strips are an alternative to terraces in creating barriers that intercept or slow water flow, thereby reducing erosive capacity and increasing sedimentation and P adsorption. (Photo by Charles Wortmann)
Grassed waterways are used to deliver runoff water from fields to drainages without causing gully erosion (Fig. 15).
The prevention of gully formation is very important because of the large amount of P loss that can occur. Grassed waterways may be effective in reducing the amount of P in runoff; but if designed for rapid movement of water, P reduction may be small. Depending on the reduction in the peak and average runoff flow rates, significant sedimentation and P adsorption can occur with waterways. Waterways need management to maintain their effectiveness if large amounts of sediment are trapped.
Wetlands allow for much sedimentation of sediment P and adsorption of dissolved P, as well as P-uptake by plants (Fig. 16).
Figure 15. Grass waterways allow delivery of runoff water from fields without causing gully erosion. The waterways may be effective in reducing P in runoff if properly designed and managed.
Figure 16. Wetlands may be constructed for the sake of reducing sediment and nutrient delivery to surface waters.
Such wetlands may occur naturally or may be constructed for the sake of reducing sediment and nutrient delivery to surface waters. Management of wetlands may be needed to maintain their effectiveness. Management practices may include occasional burning, harvesting vegetation or cutting larger trees.