When excess phosphorus from the farm reaches downstream water bodies, it causes eutrophication and harmful algal blooms like the ones in Lake Erie and Saginaw Bay. Much of this phosphorus is soluble and readily available for algae growth. This phosphorus originates from mineralization of soil organic matter and application of commercial fertilizer and manure.
Williams et al. compared two fields in Ohio on drained silt loam soils with similar soil test phosphorus. The two fields were under no-till for over 30 years prior. Two days after surface application of fertilizer in spring, one field was tilled with a disk and the other field was undisturbed. Following subsequent rainfall, the no-till field lost considerably more phosphorus through the drains than the disc-tilled field.
Macropores in the no-till field created pathways that quickly moved phosphorus from the surface to the drains. Incorporating fertilizer after a surface application in fine-textured macropore-prone soils can be a good practice to reduce the risk of phosphorus loss through subsurface drains.
No-till fields can retain more moisture due to increased organic matter and can reduce soil erosion because of improved soil structure. However, there is risk of phosphorus transport through macropores with no-till, according to Baker et al. No-till can support the development of macropores in fine-textured soils, which allows phosphorus to quickly move from the soil surface to the drains without having to pass through the soil to reach the drains.
The extent of macropore development will depend on the soil. Some clay soils have greater potential for macropore development because of their swell and shrink characteristics. When no-till is an in-field conservation practice, the trade-off between the benefits and unintended consequences of no-till should be considered.
Jarvie et al. reported that most of the dissolved reactive phosphorus (phosphorus that is readily available for algae) entering Western Lake Erie basin was due to:
- An increase in soil phosphorus, which means elevated soil test phosphorus. Baker et al. reported increased soil test phosphorus in the top 1-inch of the soil that often have no-till and reduced-tillage practices.
- Rapid water transport, which is because of more land being subsurface drained over time and drainage systems designed with narrower drain spacing.
No single conservation practice will retain all excess phosphorus in the soil. A combination of conservation practices in the field and at the edge of the field must be used to meet water quality and crop production goals. At Michigan State University’s Department of Biosystems and Agricultural Engineering, we are evaluating drainage conservation practices including controlled drainage and saturated buffers to reduce nutrient loss in drainage water.