Maintaining Crop Residue Cover
Protecting soil aggregates from raindrop impact is another key to maintaining water capture. The crop residue maintained on the soil surface by no-till absorbs raindrop energy and protects soil aggregates during rainfall events. The amount of crop residue maintained on the soil surface is directly related to cropping intensity (Table 4.1). In a low evapotranspiration (ET) environment (i.e., Sterling, CO), a WF system had an average of only 2,680 pounds per acre of crop residue on the soil surface over a 12-year period as contrasted to 4,690 pounds per acre in the wheat-corn-millet-fallow (WCMF) diversified cropping system. The same relationship for cropping systems existed across the climate gradient, but the quantities of residue were less at locations in higher ET environments (i.e., Walsh, CO). The 2,480 pounds per acre of residue on the soil surface in the WCMF cropping system at the high ET location will protect the soil better against raindrop impact and wind erosion than will the 1,570 pounds per acre of residue in the WF cropping system.
Table 4.1 The average annual crop residue on the soil surface at winter wheat planting in diversified cropping systems as affected by climate (evapotranspiration) over a 12-year period.
(Lb residue on soil surface per ac)
*WF = wheat fallow, WCF = wheat-corn-fallow, WCMF = wheat-corn-millet-fallow, CC = continuous cropping
**ET = evapotranspiration
Crop residue is lost during both fallow and cropping periods. During the traditional summer fallow period, about 30 to 50 percent of the crop residue is lost (Figure 4.1) by microbial decomposition and physical destruction by the wind. Climate has a great impact on residue loss. In a low ET location, the loss during summer fallow is about 30 percent, while at a high ET location, the loss may be as much as 50 percent. Residue loss is much lower during the winter fallow period, when temperatures are lower and there is potential snow cover-ranging from 22 to 28 percent (Figure 4.1). Crop residue loss during the cropping period is greater than during either fallow period (Figure 4.2), irrespective of evapotranspiration rate. About 45 to 52 percent of the crop residue is lost during the winter wheat growing period and from 42 to 54 percent during a spring crop (e.g., corn or proso millet) growing period.
Residue loss is a constant process driven by climate and microbial activity, which interact to result in large losses over time. Crop diversification results in increased residue production and greater residue retention (Table 4.1). The crop canopy also absorbs raindrop energy and thus preserves soil structure. Summer crops are present when about 77 percent (Figure 4.3) of the annual precipitation is received, providing excellent soil protection.
Figure 4.3 Long-term precipitation distribution in eastern Colorado.
Figure 4.4 Soil water evaporation from a bare and residue covered soil.
Crop residues greatly slow the rate of soil water evaporation. Each tillage operation results in the loss of residue, with the amount of loss dependent on the amount of soil disturbance (Table 4.2). In addition to protecting soil structure, residues also retard soil surface evaporation. However, if enough time passes without a rainfall event, soil under the residue will lose as much water as bare soil (Figure 4.4). Nonetheless, residue covered soil loses water over a longer period, thus allowing greater opportunity for infiltration and use by a growing crop. These water savings will not be as great if a summer crop is not grown.