Make one composite sample that is representative of a uniform area for each unit sampled. For high value crops sample in 5 to 20 acre units. Up to 40 to 80 acres can be effective for uniform soils.
Confine problem samples to the specific area such as the one inch of soil next to a dying seedling.
With the slice on the blade of the shovel, rake the sides off with a knife, trowel or hands so a ribbon of the root zone soil 1 inch wide and 1 inch thick can be placed in a clean container, plastic is best. Repeat at random across area. Be sure the surface crusts are mixed in this sample as they can contain soluble fertilizer salts that can affect plants. Fill out information sheet. Do not place paper with wet soil - paper decomposes.