To create this picture of nematode distribution, individual soil cores from which nematodes were extracted and counted were taken at intervals throughout the field. Each peak on the graph indicates a location where nematodes were detected. The higher the peak, the greater the number of nematodes present. Note that for graphs (C), (D), and (E), the particular genera were found throughout the field. For the genera in graphs (A) and (B), the nematodes had a "patchy", or nonrandom distribution.
For purposes of our discussion, lets assume that (A) represents stubby root nematode, (B) represents root-knot nematode, (C) represents spiral nematode, (D) represents cyst nematode, and (E) represents pin nematode. If, to sample the field, you simply walked to the center of it, took a shovel full of soil and placed it into a plastic bag, labeled the bag, and shipped it a diagnostic laboratory; the lab would likely tell you that you had spiral, cyst, and pin nematode present.
Based on this information, you would consult a listing of nematode-host associations and decide, for example, that you could safely plant carrots, because although plant parasitic nematodes were present, they were not species known to be harmful to carrots.
However, after planting, severe stunting occurs in portions of the field inhabited by root-knot or stubby root nematodes which were not detected in your initial sampling.
Because nematodes frequently have a spotty, patchy, or nonrandom distribution such as that shown in (A) and (B), a more accurate picture of which nematodes are present can usually be obtained by randomly taking a number of smaller subsamples and combining these into a single composite sample, than by taking a sample at a single location.