Although it is possible to dig into and analyze your own soil for basic nutrient amendment, it is recommended that those gardening in unknown soil (soil never used for the purpose) get their soil professionally analyzed. Most gardeners will also benefit from periodic sampling and analysis as well. You can have you soil analyzed at most county agricultural extensions, many colleges and universities, and by private companies specializing in agricultural analysis. The first step is taking the samples from your garden plot.
Choosing a Soil Sampling Area
The positions you should plot for soil sampling will depend largely on the type of terrain, the size of the area, and how many samples per hectare your analysis partner will require. Most county extensions and universities will specify about one pint of composite sample soil per acre and may have a one pint minimum no matter the size of your growing space. Be sure to have the forms and information before you begin.
Sampling should be done within all points of the area used to grow plants. You should take 15-20 samples to complete your composite and keep separate samples from areas of differing terrain. Avoid sampling from non-growth areas, dead zones you do not plan for remediation, back furrows, manure piles, waterways, and so on. Samples should be more or less random and should represent, as a whole, the entire growing area.
Step-by-Step Directions for Getting a Soil Sample
Before beginning, refer to the section above on choosing your sampling area. It may help to draw a rough map of the area to be sampled and then mark random points within it that cover most of the area to be sampled evenly. A zig-zag or randomly-spaced spiral pattern works best.
You should sample before any fertilization or other work has been done to the soil. The optimal times to take a soil sample are early spring after the winter thaw and before planting, or late fall after harvest and before cover crops have been sown. Some who grow plants in “two wave” operations (two crops per year) will sample in between, after harvest and before new planting.
Using an auger, soil tube, or spade, dig down to plow depth (roughly 12 inches) and pull the sample. Put it into a clean plastic bag or sampling bucket. Once all samples are taken, mix them thoroughly and put into your sample bag or container for the laboratory. Label it with your name, address, field ID, etc. – all of which should match your forms exactly. If sampling multiple fields, keep them separate and be sure the field IDs are different for each.
Before sealing the samples, air dry them (DO NOT use artificial heat) per the lab’s instructions. Some may not require this, but most will. This should be done in a relatively clean, dry area such as in a barn away from animals or garage away from exhaust fumes. A closet or pantry shelf is also ideal.
Tips for Taking a Soil Sample
Clean your sampling tool thoroughly with soap and water before taking samples. This keeps foreign matter from non-garden activities out of the sample.
Be aware of large elevation changes, drainage differences, etc. in your sampling space. A small drop-off in the middle of a field could create two separate growing zones which should be sampled separately as the nutrients and water likely run off from the higher to lower ground, thus creating two distinct microsystems.
What To Do With Results from a Soil Sample
Most standard test results will give the pH level, total soluble salts and sodium absorption ratio, organic matter percentages, water-extractable nitrate totals, phosphorous levels, and water-soluble potassium totals. You can usually add trace mineral tests for iron, zinc, manganese and copper, which are important if you are growing certain types of crops.
These results will usually include recommendations for remediation in terms of what should be added and in what ratios and quantities. A recommendation of 10-10-5, for example, would mean you should use a nitrogen-light mixture (10) with a heavier phosphorous and potash ratio (N-P-K or Nitrogen-Phosphorous-Potassium). If using chemical fertilizers, you need only purchase a mixture of the recommended type. If growing organically, you’ll need to tune your inputs (manure or compost mixtures) to roughly match the recommendation.
Want to learn more about how to take a soil sample?
Check out these helpful resources:
Soil Sampling for Home Lawns and Gardens, a PDF from North Carolina Department of Agriculture and Consumer Services
Taking a Soil Sample from University of Minnesota Extension