Sampling depths vary depending on the plant selection.
For turf: the critical depth for preplant consideration is 0 to 6 inches
Shrubs: 0 to 18 inches
Trees: 0 to 24 or 36 inches
For deep-rooted plants with salt intolerances, sample multiple depths separately (i.e. 1, 2, 3 feet).
Imported soil should be sampled separately. Many times, imported top soil is poor in nutrition and high in pH and salts. Know where imported soil, cuts and fills are located.
In a mixed landscape with problems, notice whether the affected plants are shallow, medium or deep rooted. This information will give you an idea of what depth to sample.