The trend for “topsoil” samplers in our trade area is to move as much of their soil sampling from after harvest, to early summer (mid-May to early July) sampling. Nearly all of this early summer sampling is done in a standing crop such as soybeans. Sampling is done before soybean height becomes too tall to minimize any crop damage. This trend has worked hand-in-hand with the increase of 2.5 acre topsoil grid sampling as well. When all things are considered, everyone involved benefits from early summer sampling. The growers and agronomists benefit from having their results early, the applicator benefits because he has more time to plan for the VRT application and the laboratory benefits by having a few less samples to test during the fall rush. .
One major benefit in earlier summer sampling is soil core quality. Sampling conditions in early summer allow for better depth control and consistency compared to normal fall conditions. Early summer sampling allows you to avoid sampling dry, cloddy, tilled fields in the fall. It is well known that sampling fields after fall tillage makes it difficult to get a quality sample (i.e. disked, chisel, rip or plow). Tillage makes it difficult to get good quality soil cores that provide a consistently “true” and repeatable soil test result.
Another benefit to early summer sampling is in the consistency of soil test result values when compared to fall sampling. In a 2-year AGVISE demonstration project, we collected 73 sample points in 9 fields in central, west-central MN and southeast SD. We marked the points with GPS and sampled in June and then resampled after soybean harvest (before tillage). When averaged over all 73 points between fall and early summer sampling, the only minor difference found was in the potassium test levels. The fall 2011 samples tested slightly lower in potassium than the early summer samples due to the dry conditions last fall. Research has shown that very dry soil conditions can cause lower than normal K test values. When soil samples are taken in May through early July, the soil is much more likely to be moist, which will result in slightly higher K test values, which are more closely related to crop response in research. Early summer sampling would mean more consistent K soil test levels with less fluctuation caused by dry soil conditions in the fall.
Growers, retailers, agronomists and samplers benefit because much of this work gets done early in the summer where they can meet together, discuss the fertility plans and have plenty of time in the fall to make the applications without all the rush of fall season. For a more detailed look at this comparison and topic, you can go to our website (www.agvise.com) and choose the link for “2012 Seminar Presentations” and then choose the link for “Early Summer Sampling.”