Soil Sampling Protocol

For each field, piece of land, area, or specific plant type in question:

If you have a specific crop or set of plants (annuals or perennials) that you are interested in, take soil cores from halfway between the base/stem/trunk of the plant and the drip line of the plant. For smaller annual plants, this location will be close to the base of the plant.
Make a grid of the land, either mentally or on a digital map. The maximum size of land that can be sampled in one “space” of that grid is 1 acre. In other words, each space of the grid can be a maximum of 1 acre in size.
Randomly select at least 40% of the grid spaces (using a random number generator) to sample from. If desired for future reference and/or testing, mark the areas you are sampling on a map or get GPS coordinates of each sampling area.
Take 3 soil cores (using a soil corer, apple corer, hand trowel, shovel, etc.) from around a random plant within each grid space chosen. If there are no plants present then take 3 soil cores from three different representative spots within each grid space. Make sure to scrape away any organic matter layer on the top of the soil before taking each soil core.
Take samples and hand over or mail priority (3 days or less) the same day that samples were taken, if possible.

Below are some more details about the sampling protocol:

Avoid going right to the boundary of the field and any areas that are not representative of the field (e.g. the ridge line or a depression or a patch of plants that is not representative of the whole). The exception to this is if you are specifically wanting to know about those unrepresentative areas. Then, you would only sample those areas for a given soil biology test.

Each soil core only needs to be 3” deep and only about ½” in diameter. If using a garden implement to take soil cores, you might be taking more soil per “core” than ½” diameter.

A maximum of 2 cups of soil is sufficient per soil test (but a lower volume is okay if you are taking fewer soil cores).

Place all of the soil cores for one soil test into the same bag. No need to mix the samples, as that will be done in the lab before samples are analyzed. (Note on the exception to this standard: Especially for larger areas/fields or more soil taken per core, to reduce the amount of sample material sent to the lab, you may combine and thoroughly mix the sample material separately. Do the mixing in a sterile container and then place a smaller amount of the homogenized mixture in a bag or other vessel that will go to the lab).

For any single sample, please ensure that you do not fill the bag more than 1/2 full with material. If dropping off locally, seal the bag and leave a small part of the edge of the top unsealed (open to the air) – do not expel the air from the bag, as this will limit the oxygen available to the biology in the sample, which may result in anaerobic conditions being formed. If mailing the sample, completely seal the bag but leave as much air in the bag as possible – do not expel the air from the bag.

All sample bags should be labeled with the name of the sample, date the sample was taken out of the ground, and plants present or desired in the area on the outside of the bag. Store the sample in conditions as close to those that it came from as possible before handing it over or mailing it (dark location, similar ambient temp., etc).

You must repeat the sampling protocol for each individual field, paddock, piece of land, or particular plant species that you want to know the biology of, if the fields have different conditions, different plants, different management practices, etc. The exception to this is a piece of land that is a polyculture that intentionally contains plants of different kinds (and that may be in different successional stage categories). A mixed polyculture landscape can likely be sampled as one single unit unless there are specific problem areas or plants in question. Use different sample bags for each soil test.

There is no theoretical maximum to the area of land that can be covered by 1 soil microbiology test.

Assuming that the plant succession is the same, the topography is the same, and the land is relatively homogenous, any sized land can be assessed, microbiologically speaking, using one soil microbiology assessment. More soil tests will give more confidence to the data, and it is never a bad idea to have more than one assessment done of a given piece of land. The more tests, the better, within reason and within what can be afforded, especially for larger pieces of land.