For very low-yielding, rain-fed cornfields, expect residual nitrate levels to be high. This will primarily be from unused fertilizer nitrogen (N). Soil mineralization of N will have been quite low in rain-fed fields due to low moisture. Limited crop N removal and little or no leaching means that unused fertilizer N remains in the soil. Unless we have an unusually wet winter and spring, this nitrate-N will be mostly available for crop use next year. The only way to accurately account for residual nitrate-N is through deep sampling, to a depth of at least 2 ft.; this may be difficult until soil moisture is restored. Unusually high residual nitrate levels following soybean harvest are not likely as soybean typically depletes soil nitrate-N to reduce energy needed for biological N fixation.
For irrigated fields, total irrigation amounts for 2012 were double or more compared to 2011 applications, with many more irrigation events for sprinkler-irrigated fields. Generally nitrate leaching with irrigation is not a concern with sprinkler-irrigated fields, due to irrigation amounts being 1 in. or less per event. Such frequent irrigation, combined with warm soil temperatures all summer, may have led to above-average mineralization of nitrogen from organic matter. Consequently, residual nitrate-N levels for sprinkler-irrigated fields may still be higher than normal, even if yields are not that much lower.
Furrow-irrigated fields generally will have larger amounts of water applied per irrigation event, which can result in nitrate leaching below the root zone, depending on how irrigation was managed. Expect residual nitrate-N levels to be quite low for furrow-irrigated fields. However, variability in residual nitrate can be very high with furrow irrigation. It will be critical to sample residual nitrate from the upper, middle, and lower third of the field relative to the location of irrigation pipe, since more leaching will occur in the upper third of the field.