Soil testing critical after a drought year

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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.

Residual Nitrate

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.

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.

Phosphorus, Potassium, and Other Nutrients

For irrigated fields with yields close to normal, expect little impact from drought on soil test levels of phosphorus (P), potassium (K), and other nutrients. For dryland fields, crop uptake of these nutrients will be less than normal. However, this may have little impact on soil test levels for P, K, and other nutrients. There may be some tendency toward higher soil test values, but soil tests for these nutrients are an estimate of plant availability over the growing season. There is evidence that soil test K can be influenced by soil drying, and some states have started analyzing soil K from field moist samples. However, most Nebraska soils are quite high in potassium, and thus slight differences in soil test K that can occur with the analytical process have no impact on the fertilizer recommendation.

Silage and Residue Harvest

In many cases, growers harvested fields intended for grain production as silage. In other cases, there will be opportunity to harvest crop residue as feed due to reduced pasture and hay production. Either way, growers should be aware of nutrient removal resulting from biomass harvest. Significant amounts of P, K, and micronutrients, as well as carbon, can be removed from the field with biomass harvest. For example, a 150 bu/ac corn crop will uptake approximately 64 lb P205 and 42 lb K2O in grain, but will have approximately 36 lb P205 and 144 lb K2O/acre in stover. The value of these nutrients should be considered when pricing baled stalks for livestock feed. More information on the impacts of harvesting crop residues is available in the Extension publications Baling Corn Residue (EC711)  and Harvesting Crop Residues (G1846). If stalks are grazed rather than baled, expect much of the nutrient content in grazed residues to return to the field, minus the harvested weight of cattle once they are taken off the field.

The Bottom Line

Expect soil nutrient values — particularly residual nitrate — to be more variable this fall than in recent years from field to field, and within fields. The only way to know the availability of soil nutrients is to soil test. Fertilizer costs can easily exceed $100/acre for irrigated corn. Nitrogen prices currently range from about $0.50 to $0.70/lb N, depending on the fertilizer source. Phosphorus prices range from $0.45 to $0.80/lb P205, depending on P source and the price of N fertilizer. Thus, an irrigated corn field requiring 180 lb N/acre at $0.60/lb, and 60 lb P205 at $0.60/lb, will have a fertilizer cost of $148/acre. Making accurate estimates of profitable fertilizer rates for next year will require accurate soil test information. Guidelines for soil testing are available in two Extension publications — Guidelines for Soil Sampling (G1740) and Soil Sampling for Precision Agriculture (EC154).

While there should be plenty of time for soil sampling this fall due to an early harvest, consider sampling next spring instead. Dry conditions this fall may make sampling difficult, and spring sampling may result in a more accurate prediction of nitrate-N availability, depending on weather conditions this winter. Nebraska is fortunate to have several good analytical labs where growers can have soil samples tested. Contact any of these Nebraska soil testing labs for information on current pricing and how to ship samples.

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