Editors Note: During the 2023 fall sampling season we have noted a large number of small soil samples. Some samples represent less than one sample core of soil. While smaller samples might seem desirable to reduce shipping costs, the greater risk is the negative impact on data quality. Special handling of small samples may result in an additional prep fee.
Originally posted 1/29/21
Most commercial or university soil testing laboratories provide soil sample bags to you at little to no cost. Every lab designs its sample bags with its own logo and contact information in the hope that a sample will be collected, placed in that bag, and sent back to the lab for analysis. While each lab’s sample bags may look unique, most sample bags have one common feature, a line or some other indication of how much sample should be put in the bag. So why is sample volume so important?
The most obvious reason that a lab indicates this volume of soil, is that we need enough material to process and analyze. We also like to have some extra in case a component needs to be reanalyzed for quality control purposes or should the customer request additional testing.
The less obvious, but possibly more important reason to fill the sample bag to the indicated line is to obtain a valid representation of the area sampled. Soil nutrient levels can vary greatly even in a very small area. By collecting enough cores to fill the bag, your results are more apt to represent the true average of the area sampled. Research has shown that a minimum of 8 individual soil cores need to be collected to make a single sample. Collecting fewer than 8 cores increases the potential that a single unusually high or low testing core will skew the results. Taking a subsample of the collected soil in the field can bias soil test data, whenever possible send all of the collected soil providing it does not exceed the soil fill line on the bag. A lab dried and ground sample mixes and homogenizes much better than field moist samples. 8 soil cores from a 6" to 8" sampling depth fit in the soil bag.