Soil testing critical after a drought year
Most crop producers in Nebraska are in the midst of harvest now, and planning for next year is not on top of their “to-do” list. However, as harvest wraps up, soil sampling should be near the top of their list. This has been among the most challenging growing seasons in 50 years, with rainfall amounts far below average. Irrigation has moderated drought impacts for many Nebraska growers, but at a significant cost. The combination of drought conditions, heavy irrigation, and widely varying yields means that soil testing is more important than ever this year.
In a recent CropWatch article, Grassini et al. noted that irrigated corn yields across Nebraska are predicted to be moderately lower (3-8% below average), while dryland yields will be substantially reduced (32-67% below average). These estimates are based on weather data from 2012, and assume no yield-limiting factors other than temperature and solar radiation for irrigated production. Many areas of Nebraska had high temperatures during pollination, which may further reduce yield from modeled predictions. The big question is how drought will affect nutrient requirements for next year. The net effect of crop nutrient removal on soil nutrient availability will vary from field to field, and with locations within fields.
For very low-yielding, rainfed corn fields, expect residual nitrate levels to be high. This will primarily be from unused fertilizer nitrogen. Soil mineralization of N will have been quite low in rainfed 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 feet; 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 one inch 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.