We frequently get the question at the lab; “When is the best time to take a soil sample?” Soil fertility is not static. Soil test levels fluctuate naturally through the year as nutrients are taken up by growing plants and returned from residue. Application of fertilizer, manure or other nutrient sources increases the amount of crop nutrients, causing soil test levels to increase immediately after application. During the growing season, soil test levels will decline as nutrients are taken up by plants. Overall, if the application rate approximates crop needs there will be a minimal effect after nutrients in crop residues or cover crop are returned to the soil through decomposition. To minimize the effects of these processes on soil test levels, it is commonly recommended that soil samples be collected at approximately the same time of year each time a field is sampled to reduce variability introduced by the normal crop growing cycle.
While the results of a single soil test can provide information needed to define the fertility program for a given area for the coming set number of years, the real value comes from looking at the data collected from several sampling events, taken at a similar point during the growing cycle, and identifying the trends in the soil test values. For example, if a soil sample is collected and the resulting data shows a lower than desired fertility level, we may need to apply more fertilizer for the next crop. If soil test values increase toward a target level over sampling cycles, it indicates the fertility program is working as intended. The fertility program would need to be adjusted if soil test levels are trending lower or higher than intended.
Traditionally, fall has been the most popular time within this region to soil sample, with nutrient application made soon after. Fall soil sampling and fertilizer application requires several steps:
With many steps condensed into a short time period and with often less than ideal weather forecasts, things must happen quickly. The soil fertility program is a critical investment that has a major impact on a grower's bottom line, and making decisions related to the program should be done with great care. It is hard to optimize a rushed management decision.More are switching to spring soil sampling to reduce this time crunch. Samples can be collected throughout the spring, even in the planted crop. Soil test data can be processed so that fertilizer and lime applications can occur immediately after harvest. Crop conditions and weather during the growing season can be also evaluated to further refine soil fertility programs. There is considerable value to sampling well ahead of the fertilizer application season, providing time and flexibility to make better fertilizer management and purchasing decisions.