This method follows Lafitte (1994). The root system of a mature wheat plant can reach a depth of 1 to 1.2 m, but if some barrier exists in the soil above 1.2 m, the roots will stop at the barrier and be unable to tap deeper water and nutrients. Potential yields will be reduced. If there seems to be a problem check for barriers in two or three places in a field though one place may be adequate if the crop looks uniform and is on a level area. If the crop looks different in different areas of the field, you will need to measure soil depth in each area.

To measure soil depth

You will need a shovel. A soil corer can be used instead, but a corer can miss hardpans if the user is not familiar with the soils in an area. In addition, a corer can often hit stones in rocky soils, making the user think that the soil is shallower than it actually is.

Pick an area where the plants look uniform, and start digging with a spade, preferably when the soil is moist (near field capacity), as digging in dry soil can be difficult. Dig until you strike any barrier. This can be a plow pan, a hard pan, a rock layer, an impermeable clay layer, acid subsoil, a water table, or a salt band. If you are uncertain that what you find is a barrier, examine the roots of the crop to see if they penetrate the area. Scraping along the face of the hole with a pocket knife will often help in detecting a compacted layer. Record this depth.

Dig to about 40 cm, as plow pans usually form above this depth. Use a soil corer below that depth, taking several cores from the bottom of the hole you dug in order to avoid possible errors caused by striking rocks. Break the cores at several places to see if roots are still present at different depths.

Looking at road cuts or quarries in the area can also give you an idea of the soil profile, but it cannot fully substitute for soil depth measurement in the field, since soil depth will vary within a region.