It's that time of year - soil sampling season! These suggestions come from our friends at Midwest Laboratories, one of the largest soil labs in the U.S.:
Soil Sampling Suggestions
Sampling soils under very dry conditions may increase variability in soil test results. A couple of items to consider this fall:
1. Make sure you are able to obtain a good soil core for each sampling point. The soil probes may be difficult to get into the ground or difficult to get the complete depth desired. It is also possible that the core is powder dry and may not be the complete depth. Proper sample depth (6 inches minimum) is essential for calibrating fertilizer recommendations from laboratory results.
2. During dry conditions, exchangeable Potassium (K) levels can test lower than what is actually available. K fertilizer added is very soluble, as is the K in the plant residue. Without adequate rainfall, this K is not readily available to the plants and this is what the laboratory is testing for.
3. The soil pH can also be affected by dry field conditions. Soil pH could show slightly more acidic (.1 to .3 pH units) than expected/normal values. Even with these three potential issues, it is very important to analyze your soil in dry conditions to help determine the amount of nutrients left in the soil that the plants did not uptake because of poor growing conditions. A crop that is yielding half or less than expected will leave a significant amount of nutrients in the soil and residue, assuming the crop isn't being removed for forage. The organic matter, phosphorus, CEC and base saturations along with most of the micronutrients will not be affected by the dry conditions. Carry over nitrogen can also be a concern. The lack of yield combined with little moisture to leach nitrate may leave some pretty large amounts behind. If you are able to get a good core and a good representative sample, it is usually going to be okay to sample. You may also want to delay sampling until after a substantial rain event (@ 1 inch). Another option is to utilize proven historical yields to determine crop removal rates.