Soil Erosion in Croplands

The first person to create a continuous furrow in the soil surface for planting seeds increased the potential of soil erosion from that site by leaving loose, unattached soil peds and particles lying on the surface exposed to the raindrop impact, flowing water, or wind. We know now that the immediate loosening of soil particles was only the first step in the process of cropland deterioration that occurs when soil is disturbed, and that soil erosion initiates a sequence of events that influences soil ecology. Physically, loose soil moved and deposited by water fills micro- and macropores in the surrounding soil. Once the pores are plugged, infiltration is reduced, increasing the amount of water that is on the surface and capable of doing work, that is, moving soil. Additionally, water that does not infiltrate is water that is not available to soil organisms - which physically and chemically (through excretion of sugars and proteins) bind soil particles together. Depriving organisms of fundamental resources results in changes in the microbiotic community structure, and the biologic activity that makes the soil productive. Disturbing the soil also results in the loss of carbon from the soil in the form of CO2, generally believed to result from a flush of aerobic microbacterial activity as bacteria in a relatively oxygen-deprived system are exposed to the atmosphere and abundant oxygen. This process is especially troublesome in semiarid grasslands where biological activity is suppressed or prevented through a year of fallow and the soil organic carbon might never have been greater than 8% of the surface layer of soil. The consequences of this process are trivial when and where it happens irregularly and infrequently, but when it occurs regularly on a biannual basis the finite pool of organic carbon in a soil system can become depleted, leading to further declines in microbiotic activity and biological binding properties. The consequence is that, over time, the soil cannot sustain abundant vegetation, whether crop, native, or alien plants and the soil is left bare and easily detached by raindrop impact and concentrated flow in rills and gullies.

Improperly managed cropland is the source of most sediment and may be the place where most economic damage occurs, depending on the relative value of productive soil versus downstream water quality. In the United States in the early 1990s, emphasis changed from protecting the soil resource to protecting downstream water quality for fish habitat and water recreation. Currently, with increased emphasis on production of crops for both food and energy production, erosion damage to cropland is receiving increased attention.

Hillslope erosion on cropland can result in rills and channels that uproot or damage small seedlings, create gaps in the crop, and affect yield. This is especially problematic for small grain crops. Material from hillslope erosion can also deposit locally and smother small seedlings. On steep cropland of the Pacific Northwestern United States (Palouse River Basin), a 1978 study estimated that in little more than 100 years of cultivation, all original productive topsoil had been lost from 10% of the cropland, and 25-75% lost from another 60% of the cropland by water and tillage erosion. In this region, soil losses from winter effects range from negligible to greater than 200tha-1 on individual fields. On steep slopes of fields in summer fallow with dust mulch created by a rod weeder, infrequent high-intensity summer storms can erode as much as 50% of the dust mulch layer of 15 cm. Potential yields have been affected. Increased variability across the fields makes management more difficult and decreases the efficiency of nitrogen and other applied fertilizers.

Soil loss from cropland is highly dependent on the amount of cover provided by residue, predominately stems, from the previous crop, and cover from the current growing crop. Direct seeding into high quantities of residue is highly effective in preventing rill and inter-rill erosion. In areas with a soil-restrictive layer, increased infiltration can lead to increased gully erosion.

There are a number of farming practices that can reduce soil erosion in croplands. Where crops are irrigated, careful management of soil water levels and the use of polymers mixed into furrows can nearly eliminate soil lost in the tail water in furrow irrigation. Runoff and erosion from crops that are sprinkler irrigated benefit greatly from careful monitoring of soil water levels, application rates, low-impact sprinkler heads, and maintenance of soil surface cover. Recent advances in remote sensing and sprinkler system control technologies have improved producers' ability to apply water only to areas in fields where the crops need water, thus reducing or eliminating water loss from runoff.

Soil loss from cropland is highly dependent on the amount of cover provided by residue, predominately stems, from the previous crop, and cover from the current growing crop. Tillage practices, used with irrigated or rain-fed crops, play an important role in the reduction of soil erosion. Traditionally, inversion tillage has been used to control weeds and prepare for seeding. Inversion tillage cuts into the soil surface and churns or turns the soil surface over, burying any residue or seeds. This practice leaves the soil surface essentially bare and exposed to rainfall. An alternative to inversion tillage is the use of no-till or direct seeding technology, where the soil surface is only disturbed enough for fertilizer and seed to be placed in the ground. The disturbance is a fraction of the depth found in inversion tillage, although surface disturbance varies depending on the type of no-till drill used. No-till farming is very effective for erosion control, but it typically makes weed control more difficult and produces lower yields than traditional inversion farming. In areas with a soil-restrictive layer, increased infiltration from no-till can lead to increased gully erosion.

Depending on the climate conditions where crops are produced, soil erosion can be controlled by cropping frequency and intensity. In the higher-precipitation zones of semiarid regions where crops are planted in the fall, the production of a crop every year can maintain levels of soil organic matter and produce sufficient above-ground biomass to protect the soil surface from wind and rain. In humid regions, intercropping - the planting of an early ripening crop in a later maturing crop - provides additional aboveground biomass for soil surface protection, as does planting two or more crops in serial fashion through the year.

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