Early summer topsoil sampling continues to grow more each year. Last year our Benson, MN lab set a record for early summer soil samples (January 1 to July each year) as shown in the chart.
While the amount of early summer soil samples have increased greatly, the number of fall samples is also increasing at a slower rate. If we look at the comparison of early summer vs fall samples, we see that the overall percentage of early summer samples is increasing to about 50% for the past 3 out of 4 years (see chart). The summer of 2013 was very wet which hindered summer sampling.
The vast majority of early summer soil samples are topsoil only samples from 2.5 acre grid in size. Most of these samples come from unfertilized soybean fields and are tested for non-mobile nutrients (P, K, Ca, Mg and Zn), pH, organic matter and soluble salts. In some cases, the carbonate (CCE) test is also included to determine the risk of IDC on soybeans. Mobile nutrients like nitrogen, sulfur and chloride are not tested on early summer samples.
Early summer sampling should not be done in soybean fields that have been fertilized or manured, the fall before or in the spring. If you do, the P and K test reports will be inflated. Fertilized soybean fields should be sampled after harvest to get the most accurate soil test levels.
There are many good agronomic and practical reasons to do more early summer sampling. These include the following:
- Early summer sampling provides good soil test information for P, K, Zn, pH,
- Early summer sampling provides excellent quality soil cores due to moist soil conditions.
- Variable rate application of nutrients/lime can be decided in the summer, when everyone is not so rushed compared to right after harvest in the fall.
- Fall application of fertilizer or lime can be applied right after harvest because soil testing is done and the VR maps are ready to go.
- It is easier to find labor to do summer grid sampling than to find labor in the fall after harvest when schools have started again.