Crimson clover, used as a winter cover crop, was tested as a source of nitrogen fertilizer to grow sweetpotatoes. Winter green manures offer a more stable soil nitrogen supply due to the time involved in decomposition to release nutrients, and have the potential to decrease nitrate leaching. Clover resulted in yields equivalent to the commercially recommended rate of nitrogen fertilizer. Measures of sweetpotato quality, including the storability and sprouting of roots the following spring for plant production, were the same for clover and inorganic fertilizer. In one location root protein levels were lower resulting from the clover treatment than the fertilizer treatment, but since yields were the same, and there is no premium for protein , the value of the sweetpotatoes was the same. In other tests crimson clover supplied as much or more N than the commercial fertilizer recommendation with no differences in root protein.
Crimson clover was also tested as a nitrogen source in two year rotational systems of corn followed by sweetpotatoes and sweetpotatoes followed by corn. It was compared to no nitrogen fertilizer, half the recommended rate and the recommended rate for both crops and analyzed to see if there was any nitrogen carryover from one crop to the next that resulted in yield differences. High indigenous nitrogen fertility of the soils resulted in yield decreases due to the addition of either source of nitrogen in the first year for the sweetpotatoes. In the second year there were no differences in sweetpotato yield for any of the nitrogen levels. Corn fertility in the first year did not affect the performance of the sweetpotatoes in the second year. Corn yield resulting from crimson clover incorporated just prior to corn planting was the same as the commercial nitrogen fertilizer rate in both years. However, there was a statistically significant reduction of corn yield following sweetpotatoes grown using crimson clover in the first year of the rotation. Nitrogen levels in the grain were the same as in the fertilizer treatment, indicating that nitrogen was present during grain maturation, but the corn matured slightly earlier, suggesting a delay in nitrogen availability, and hence a lower grain yield. This effect could not be separated out in the second test, and requires further investigation to see if it is consistent. Excluding this possible second year effect, crimson clover was sufficient as the sole source of nitrogen fertilizer for both corn and sweetpotato production.
An economic analysis on the sweetpotato-corn and corn-sweetpotato rotations revealed inconsistent results. This was due in a large part to the high nitrogen fertility of the soils, such that adding nitrogen fertilizer caused sweetpotato yield reductions during the first year. The value and costs involved in sweetpotato production were much greater than those of the corn such that the profitability (or lack of) of the sweetpotatoes dominated the economic analysis. The only two year treatments that were better than using the commercial levels of nitrogen fertilizer were where sweetpotatoes were grown in the first year with no nitrogen fertilizer. In the second year, where sweetpotatoes followed corn, there were positive responses to nitrogen fertilizer. Adding no fertilizer was less profitable than adding some. The most profitable treatment was using crimson clover in the first year for corn followed by half the commercial rate of inorganic fertilizer for sweetpotatoes the second year. This resulted in a premium of $1751 over the commercial practice of using nitrogen fertilizer for both crops. The best rate of return per dollar was for the no nitrogen for corn – half nitrogen for sweetpotato treatment, which returned $1214 per hectare. Net economic gains of $1,000 or more per hectare were realized through nine of the twenty-four alternative nitrogen treatment-rotation strategies evaluated. Fifteen of the twenty-four treatments resulted in a positive economic change compared with the current practice. A majority of the fifteen positive net economic value changes (nine of fifteen) involved use of a cover crop as part of the alternative treatment. Economic analysis of treatment data indicates that the use of crimson clover as a cover crop would benefit a large number of local growers who rotate corn and sweetpotato in a planting sequence.
Another study was aimed at investigating the physiological mechanisms involved in nitrogen use by sweetpotatoes. The response of sweetpotatoes to nitrogen fertilizer was split into two components: how well the plant can recover nitrogen from the soil and how well it can convert that nitrogen into storage roots. Clones high in one component are not necessarily high in the other. The concept behind this split is to pick the clones highest in each component and cross them to see if they can be combined into a higher yielding clone. When selection is based on yield only, some of the clones highest for one component are missed, and then so are some of the potentially best breeding combinations. This study measured the nitrogen uptake and nitrogen utilization ability of several sweetpotato varieties and found that there was variation among them for both components. An additional analysis was used to say which component was responsible for most of the variation, allowing breeders to focus resources on the component that will make the most difference. This was dependant on how yield was measured and on the level of nitrogen fertility. Results suggest it is worth making crosses based on the nitrogen components to see if yield ability can be increased and the inheritance of these traits be determined.