Time to think about fall soil sampling

Small grain harvest is wrapping up. I have heard wheat yields ranging from 40 to 80 bu/a and many fields in that 50 to 60 bu/a range. Fall soil sampling offers some advantages in that most fields are usually drier than spring which allows a better soil sample. In addition, sampling before any tillage also gives a better sample. For many spring crops planted on these acres, it gives the producer time to plan their soil fertility program for the coming year. If planning for a non-legume crop on these acres, a two foot (0-6 and 6-24 inch) sample should be taken for residual nitrate-N.

Each year we have questions on when is the best time to soil sample these fields for the next crop. Our long-term soil test results indicate by mid to late September the residual nitrate-N test has stabilized, on average, and is fairly constant until late April when it begins to increase again. So a soil test anywhere in this time period after small grains provides a good estimate of residual N and what is needed for fertilizer N for the next crop. However, soil sampling as early as mid-August shows an average residual nitrate-N level of only 10 lbs./a less than one month later. Therefore for those acres that will be planted to winter wheat this fall, early soil sampling will not significantly impact next year’s fertilizer recommendations.