The purpose of providing a previous crop is to provide the model with information about remaining crop residues and their influence on N availability (immobilization in high carbon residue, or additional N in legume residue). We are working on incorporating cover crops and other crops into the Adapt-N rotation options. In the meantime, if the crop planted in the previous year is not listed as an option under Previous Crop, a crop with a similar growth pattern (planting density, crop residue) should be selected. Here are a few suggestions about how to generate the most accurate results:
- Small grains, straw harvested: Substitute silage corn for small grains. The reason for this is that, like silage, small grains after straw harvest leave a minimal amount of surface residue, so that little immobilization of nitrogen occurs. The amount of root residue left by densely planted grains is likely similar to more broadly spaced, but larger, corn root residue.
- Small grains, straw left on surface: When following small grains where straw was not harvested (if residue is still fairly thick by corn planting time) use “grain corn” as the previous crop, so that some immobilization of N in residue will be accounted for, although imperfectly.
- Vegetable crops/potatoes: Substitute soybean for vegetable crops/potatoes, but be sure to add the soybean N credit listed on your results page to your Adapt-N recommendation (15 lbs or 25 lbs, depending on location and soil type) for non-legume vegetable crops. Like soybeans, most vegetable crops leave minimal residue, but most do not fix nitrogen.
- Cover Crops: Currently your best option is to use a cover crop credit (if applicable) as suggested by your local extension system. We plan to have a beta-module for cover crops available for testing for the 2016 season.