Learn strategies for growing feedstocks while reducing erosion.
|Erosion near the edge of a freshly planted field after heavy spring rain. Bare soil is particularly susceptible to erosion. Crop residues can help hold soil in place. Photo: Michael Bomford.|
There is strong interest in production and use of agricultural crops for biofuels. However, these crops, like all others, need to be grown in a manner that sustains the natural resources necessary for the long-term health of agriculture. One of these natural resources is soil and soil erosion may be of significant concern with production of selected crops if they are not properly managed.
Soil erosion occurs when soil particles detach from the soil surface by rain or flowing water and are then transported by water or wind. Living vegetation or crop residues protect the soil surface from erosion, but when the soil surface is not covered by plant materials, water dislodges soil particles from aggregates. These detached particles bounce around on the soil surface and can plug large pores, which can lead to lower water infiltration, increased water runoff, and in turn, more soil erosion.
Runoff’s capacity to transport sediment, or soil particles, depends on both water flow velocity and sediment characteristics. When flow velocity slows, sediment may be deposited. This usually happens when the slope of a channel is reduced, perhaps at the base of a slope or because of an obstruction -- for example, vegetative filter strip, fence line, terrace -- in the flow path. Within agricultural fields, soil removed from one place may be deposited in another, creating a long-term productivity loss on the upslope areas from where soil was removed and a short-term productivity loss downslope, where crops have been buried by sediment.
As a guide, to reduce soil erosion by about 50 percent of that occurring for a bare soil surface, 30 percent of the soil surface needs to be covered by crop residues following planting (Laflen and Colvin, 1981). A generic relationship between crop residue coverage and soil erosion compared to that occurring on a bare soil is shown in Figure 1.
Many crop residues, including corn stover and straw from wheat and other small grains currently left on fields after harvest to minimize erosion are being considered as primary feedstocks for cellulosic biofuels. Removal of these residues will reduce surface cover and increasingly leave surface vulnerable to erosion.
Perennial crops are much more forgiving relative to above-ground biomass harvest impacts on soil erosion. After establishment, harvesting methods for perennials such as switchgrass or miscanthus leave sufficient surface cover to limit soil loss to a tolerable level (van Ouwerkerk et. al., 2007). However, establishment of crops such as miscanthus and/or switchgrass, which require multiple years for development, may require surface residue cover during their sensitive establishment period to limit soil loss. If these crops are established on marginal sloping lands, additional measures such as terracing will likely be required to avoid soil loss both during and after the establishment period.
When farmers start growing crops for biofuel production, they should carefully consider strategies such as cover cropping and crop rotation to minimize soil erosion, especially if they are adopting new production practices that could increase the duration or area of bare soil.