A study was done in Niger to see how growing pearl millet and cowpea together or separately affects soil and crop productivity. They found that planting cowpea after millet led to better crop yields. The soil also had more organic matter and magnesium after using cowpea in previous years. Adding nitrogen to the soil helped the next millet crop grow better, especially after growing cowpea. Overall, the crops did better after growing cowpea and millet together compared to growing them alone.