In addition to the quantity of residues remaining following harvest, a variety of types of residues is also important. The goal should be a minimum of three different species in a rotation, more if possible.
The percent of the time that living roots are present during a rotation is also important. The period that active roots are present varies considerably, ranging from 32% of the time for a corn-soybeans rotation to 57% for a beans-wheat rotation to 76% for a three-year beans-wheat-corn rotation (table 11.1).
As mentioned above, when soils are covered with living vegetation for a longer period of time, there tends to be decreased erosion as well as a decreased loss of nitrate and less groundwater contamination.
| Table 11.1: Comparison of Rotations: Percent of Time Active Roots Are Present and Number of Species | |||
|---|---|---|---|
| Rotation | Years | Active Rooting Period (%) | Number of Species |
| Corn-soybeans | 2 | 32 | 2 |
| Dry beans-winter wheat | 2 | 57 | 2 |
| Dry beans-winter wheat/cover | 2 | 92 | 3 |
| Dry beans-winter wheat-corn | 3 | 72 | 3 |
| Corn-dry beans-winter wheat/cover | 3 | 76 | 4 |
| Sugar beets-beans-wheat/cover-corn | 4 | 65 | 5 |