Humic products from two companies promoted positive grain yield responses from rainfed corn and soybean in multiple field trials in central Iowa, although the degree of response varied. Yield responses were generally agronomically modest, due to the fertile soils and high yields that typify Iowa farming. Yield responses to the humic products were largest during droughtier conditions. Corn leaf area and root growth responded favorably to the humic products, and root growth benefits were clearly more consistent than for grain yield. The structural biochemistry of corn roots responded in patterns that suggest complex interactions of humic products with environmental conditions and fundamental plant processes. In a nitrogen (N) fertilizer response field study, corn grain yields increased with humic product application at higher N rates, but at inadequate N rates corn grain yields decreased with humic product application. These results demonstrate that humic products affect crop growth through biostimulatory mechanisms, not by increasing N availability.