However, the benefits derived from the cover crop appear to go beyond nitrogen substitution. We believe the use of the cover crop improves soil structure, increases water infiltration and enhances nitrogen uptake efficiency. Water Availability Conventional corn yields have been consistent and slightly higher than Yolo County averages throughout the experiment, but not as high as the low input system for the last five years. In 1994, infrared aerial photographs indicated water stress in the conventional plots during grain fill. In order to develop an explanation for these differences, intensive soil water monitoring was implemented and soil physical and chemical properties were measured in all three systems starting in 1995.
Data show that in the conventional system, water penetration decreased as the season progressed.
As a result, the irrigation schedule was modified so that the conventional system received water twice as frequently as the other two systems for the duration of the 1995 corn season. Despite this modification, the total amount of water applied over the season was still less, and the profile was never recharged past 18 inches of depth. Soil measurements of resistance, taken with a cone index penetrometer down to 15 inches, indicated a zone of compacted material in the subsoil of the conventional system. This compacted zone may have been caused by spring planting into higher moisture when the ground was wetter, even though ground preparation and beds were completed under dry conditions the previous fall. The low input and organic systems are considerably less moist during spring incorporation and bed preparation, because water has been extracted by the cover crop.