| Summary: | Soil type and the previous history of cultivation can both affect soil strength. I characterized soil strength by measuring tensile strength and penetration resistance (PR) as a function of matric potential (the strength characteristics). It was expected that microaggregation should allow the soil to fail easily along failure planes in between the microaggregates. Five Tanzanian soil types were compared ranging from a hardsetting Paleustalf at one extreme of strength behaviour, through a Paledoll, Paleudalf and a Paleustult, to a strongly microaggregated Orthox expected to be at the weak end of the scale. At each site soils that had a history of 7 or more years of cultivation and cropping were compared with newly cultivated soils. The Orthox and Paleustult had the best PR characteristic for ease of root growth and the Paleustalf had the worst characteristic which suggested that it would not permit root growth at matric suctions exceeding 100 kPa. The Orthox also had much the most favourable tensile strength characteristic. At air dryness (100 MPa suction), the previously uncultivated and previously cultivated Orthox topsoils had 3 and 9 times less tensile (and compressive) strength, respectively, than the corresponding Paleustalf topsoils. For three of the soils (Orthox, Paleudoll and Paleustult) a previous history of cultivation was found to have significantly and substantially reduced the tensile strength of the topsoil at any given matrix suction compared to the newly cultivated soil. A study was made of techniques for quantifying microaggregation. The limitations of current techniques are discussed and a scheme for determining microaggregation <i>sensu stricto</i> is proposed. This scheme avoids the assumption that water-stable slaked soil fragments are necessarily microaggregates.
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