The degree to which various tillage operations alter soil physical properties is poorly understood and, at present, cannot be adequately predicted. In fact, few reports exist which relate short or long-term effects of tillage upon bulk density (Db) or mechanical impedance (MI). The purposes of this review are (1) to indicate the order of magnitude of changes in Db and MI resulting from tillage, (2) to identify major problems associated with measuring Db and MI in field situations, and (3) to present recommendations to aid in the measurement and interpretation of Db and MI in future tillage studies.
In field studies, Db and MI measurements exhibit both spatial and temporal variability. The spatial variability results from vertical and lateral changes in soil properties such as texture, structure, and organic matter content and from the effects of past soil management practices. In general, each tillage operation produces non-uniform changes in soil physical properties. Hence, valid sampling to assess Db and MI associated with specific tillage operations requires sampling as functions of depth and position (distance normal to the direction of travel of tillage tines, blades, etc.). Temporal variation in Db and MI, if not recognized, creates problems in data interpretation.
Published Db values from tillage studies, based upon soil core samples, range from <1.0 to:1.7g/cm3.