Take several sub samples from through out the field and mix them together to obtain an average for the entire area. The number of sub samples is dependent on the size of the field; as a rule of thumb, take about 10 sub samples for every acre in the field.Thus, if you planned on a one acre food plot, plan on taking ten sub samples that you would combine into a single sample for the field. Sub samples should be collected from the top layer of the soil, which is 0 to 6 inches deep.Mix sub samples together to form a uniform sample and discard any plant material that could have been collected.
    Check your local county extention office for soil labs in your area. 

Note: this info was taken from Mississippi State University web site @ msucares.com under soil sampling for food plots.