Species Richness and Active Rooting Periods

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