The most important aspect of attaining soil data that is correct and usable is to first take quality soil samples. Soil sampling is a combination of art and science, and getting the best possible information requires attention to several factors. There are two primary methods of sampling: one is very straightforward and requires little thought; the other requires an understanding of soil characteristics and more extensive training.
Soil Sampling Method #1: Grid Sampling
The first method, called grid sampling, attempts to eliminate human error and the need for much training by placing arbitrary squares on field. The person taking samples merely goes to the center of the square and takes the sample. Generally, the protocol suggests taking four probes of soil at six inches in depth, constituting one sample.
Soil Sampling Method #2: Zone Sampling
The method known as zone sampling (sometimes called “smart sampling”) requires much more training. The person taking the samples needs an understanding of soil characteristics, what to watch for in the field, and some general protocols that ensure a more accurate sample.
When zone sampling, maps are drawn using the information that’s available. This can include topography, soil types, yield maps, past farming practices, and possibly other information deemed pertinent. Zones are then divided into irregular areas of likeness.
Standard Protocols for Zone Sampling
In zone sampling, protocols direct sample takers to stay 60 to 100 feet from any edge or side of a field, to take samples in diagonal patterns across spreader travel, to take 15 to 20 cores of soil for each sample, and to be mindful of cores that are unlike the majority of cores taken.
The purpose of staying away from sides or edges of fields is to avoid skewed data due to overlapping of past fertility applications. Sample takers must be mindful of odd corners or field characteristics that may change spreader travel. Because fertility spreading equipment typically moves in the same pattern, as the crop is planted, going in a diagonal pattern across spreader traffic can also help to ensure that previous applications are less likely to influence the data. Finally, the person taking the samples needs to carefully observe the sample cores and discard any that don’t resemble the majority of the sample because maps are often not precise enough to differentiate every inch of a field.
How Many Cores Are Needed for a Sample?
The number of cores taken for a sample is critical for accuracy. Statistically speaking, there comes a point at which no more cores are needed. When taking soil samples, each additional core has a decreasing influence on the integrity of the sample. For this reason, 15 to 20 cores are more than adequate for an accurate sample.
There is more to sampling than just putting soil in a bag. Sample takers must learn the art and science of soil sampling to generate accurate data that will provide important insight into the condition of your soil.