Converting CRP Land to Corn: Impacts and Mitigation

Laura Ward Good, Dept. of Soil Science

Currently in Wisconsin there are more than 600,000 acres enrolled in the USDA’s Conservation Reserve Program (CRP). The contracts for approximately 44 percent may expire in 2007 and 2008. Given current rising demand for corn, it is likely that at least a portion of these acres will go into a corn-based row crop rotation. If these highly erodible lands are converted to corn, will this lead to disastrous soil loss?  Are there ways to manage corn on former CRP lands that will keep the soil quality and conservation gains from the Conservation Reserve Program from being totally lost?

To answer these questions, I used Snap-Plus software to evaluate soil loss with different corn rotation and tillage combinations on eleven representative steep (soil mapping unit D) fields from counties with significant CRP acreages (Table 1). Estimated soil loss for established perennial grass hay is compared with ten different corn rotation and tillage combinations in Table 2 and Figure 1.

Table 1. Location and site characteristics of fields used for soil loss estimations.

Table 2.  Average, maximum and minimum estimated soil loss for example CRP fields by rotation and tillage in tons per acre per year.

Abbreviations: Cg = Corn for grain, Cg-baled = Corn for grain with half stalks baled,  S= Soybeans, Cs = Corn silage, Cs w/cvr = Corn silage followed by a small grain cover crop, NT= No-till, ST=Strip-till, CP= Chisel plow, Fcult = Field cultivation

Figure 1. Average estimated rotational erosion for example CRP fields by rotation and tillage.

            Estimated soil loss for grass hay was minimal for all sites (0.1 ton per acre).  Soil loss for the corn rotations ranged from two to hundreds of times greater than for grass hay. No-till and strip-tilled corn for grain, however, had estimated soil loss values below the Natural Resource Conservation Service (NRCS) standard for tolerable soil loss (T, shown in Table 1) at all sites.  Most of the fields could not meet T with even a one-pass tillage system (Cg: Fcult) and none could meet T if corn silage was grown without a cover crop. Fields with gentler slopes are expected to lose less soil under the same rotations, but when the Snap-Plus analysis was run with the same soils with “C” slopes (averaging 9%), soil losses were still well above T for the corn silage rotations.

            Snap-Plus also contains a field runoff phosphorus (P) loss indicator, the P Index, and a soil quality indicator, the Soil Conditioning Index (SCI).  P Index trends closely mirrored those for soil loss, as most of the runoff phosphorus lost from these fields (in the absence of unincorporated fertilizer and manure P applications) is expected to be attached to eroded sediment.  The SCI is a comparatively new index used by the NRCS to indicate the effect of a management system on soil organic matter. It takes into account crop biomass additions and removals, field operations, and erosion.  If the calculated SCI value is positive, organic matter will be increasing with the rotation, and the reverse is true if the SCI is negative. Grass hay had the highest SCI values, followed by no-till and strip-till corn for grain, indicating that corn for grain systems with minimal tillage and residue removal will lead to continued accumulation of soil organic matter in a field, but not at the same rate as for CRP.  Chisel-plowed and corn silage rotations without a cover crop had negative SCI values – these rotations will deplete existing soil organic matter.

Converting CRP from permanent grass lands to corn will certainly increase sediment and phosphorus loads in runoff from these areas.  However, implementing management practices that minimize tillage and retain a significant amount of crop residue on the surface will reduce adverse impacts to soil and water resources.  Harvesting the entire corn plant, as is done for corn silage, will lead to soil losses that are orders of magnitude higher than tolerable soil loss.

Snap-Plus is public domain software designed for use by growers, agronomists and other agricultural professionals in Wisconsin. It can be obtained from this website:   www.snapplus.net. It includes the RUSLE2 soil loss calculation software. The field level information it requires is readily available to growers.  With it, growers can assess potential sediment and phosphorus losses resulting from management changes on their own land and alternative practices to minimize those losses.