Illinois Soil Testing Association
How to Take a Soil Sample
A soil test is only as good as the sample provided to the
testing lab. Generally, a subsample should be taken from
the layer of the soil from the surface to 7 inches deep.
Three to five subsamples should be collected from 3 to
Because of soil variations, taking too few samples by
subsampling too large an area may not give an accurate
fertility map of the field. Remember, sampling is the most
critical part of soil testing.
A Guide to Plant Sampling
High crop yields and quality depend on many factors - adequate water and nutrients, the
correct plant population and variety, disease and insect resistance or control, etc. One of
the most important is nutrient status of plants, which reflects the nutrient supply of the
plant at the time of the sampling.
Nutrient status of plants is an unseen factor in
plant growth, except when deficiencies become
so acute as to appear visually as deficiency
symptoms. The nutrient status of plants can be
measured only by plant analysis in a laboratory.
Most ISTA members are equipped and qualified
to do plant sampling.
Usually, plant analysis is most useful as a diagnostic tool for future correction of
problems, although, in the case of young plant samples, there is often time for remedial
fertilizer application to the crop. Combined with soil test information, results of plant
analysis allow fertilizer practices to be fitted more closely to soil-plant needs.
What to Sample
Parts of plants to sample vary with the plant and stage of growth. Most laboratories
provide instruction sheets, which describe the part of the plants to sample and at what
growth stage samples should be taken. Also, these instructions indicate the number of
plant parts needed for a sample. Use a clean plastic pail or paper bag, not metal, to
collect the sample. A metal container may contaminate the sample.
If plant parts have soil, fertilizer, dust or spray residues on them, they should be cleaned.
- Cleaning with a dry brush is best
- If that isn't adequate, wipe sample with damp cloth
- For stubborn residues, wash with water. Do not prolong washing
- Air-dry samples in shade, not in sun
- Clean paper bags or envelopes are best for mailing tissue samples to avoid
contamination. Never place fresh tissue samples in plastic bags.
Get a mailing kit from your local extension office, fertilizer dealer or commercial plant
analysis laboratory. Read the instructions, fill out the questionnaire. The more
information, the better the interpretation of the plant analysis is likely to be.