The evaluation should be for the specific plants that are important to the use of the soil. These plants should be indicated. The root-restriction depth may differ depending on the plant considered. Root-depth observations preferably should be used to make the generalization. If these are not availableand often they are not because roots do not extend to the depth of concernthen inferences may be made from morphology.
Some guidelines follow for physical restriction.
These are generally not determinable by field examination alone. Physical root restriction is assumed at contact to rock, whether hard or soft. Further, certain pedogenic horizons, such as fragipans , infer root restriction. A change in particle size distribution alone, as for example loamy sand over gravel, is not a basis for physical root restriction. A common indication of physical root restriction is a combination of structure and consistence which together suggest that the resistance of the soil fabric to root entry is high and that vertical cracks and planes of weakness for root entry are absent or widely spaced.