Because of the drought this growing season, soil sampling after harvest becomes very important when making nutrient management decisions says Ron Gelderman, South Dakota State University (SDSU) soils Extension specialist.
"Soil sampling should be part of any nutrient management program but is even more important after a dry year with limited yields," Gelderman explains. "For fields that were severely moisture stressed, available nitrogen (N) carryover may be higher than normal."
Gelderman says all growers should take 2-ft. soil samples throughout their fields and have them analyzed for nitrate-N, especially for those fields going into a non-legume crop.
"If the rotation hasn't yet been set, sample and analyze as if it will be a non-legume crop. As the old adage goes, 'It is better to have it and not need it, than to need it and not have it.' The additional sampling cost is inconsequential compared to potential fertilizer savings or yield gain," he states.
Gelderman says past drought years have shown higher than average nitrogen carryover levels. For example, the average carryover nitrate-N level following corn, is about 70 lb./acre. In a dry year, it would not be unusual to measure 100 to 120 lb./acre of carryover N after poor yields on some of these moisture-stressed fields.
"That is a difference of 30 to 50 lb. or about $18 to $30 an acre in savings with today's N prices. Some laboratories have been reporting average carryover nitrate-N values of 20 lb./acre higher than average for this fall," he says. However, that is not always the case, thus, the reason testing is a must this fall.
"One of our moisture-stressed nitrogen rate trials on corn in the state had near-average carryover levels. In that case, if the grower 'guessed' at carryover levels of 30 lb./acre more than average, yield might very well be 5 to 10 bu./acre lower than it would be with the proper test and recommendation. We cannot predict what the carryover levels will be, therefore, every field should be tested."
Because there could be variable nitrogen carryover within fields, high-yield variability may result due to differential soil moisture within that field, so Gelderman recommends zone sampling.
"A zone sampling program based on yield zones may show some large nitrogen fertilizer savings for next year and will put the nitrogen where it is needed and not oversupply other areas of the field where it is not," he states.