SummaryThe variation with water content is examined of soil cohesion, soil‐metal friction, susceptibility to soil compaction, implement draught and the slope and intercept of the virgin compression line of critical state soil mechanics theory. For a given soil, all these relations are shown to exhibit turning points at a similar water content which corresponds to the cone penetrometer plastic limit, as determined with a drop‐cone penetrometer, rather than the Casagrande plastic limit. The cone penetrometer plastic limit, which can also be more reliably determined than the Casagrande plastic limit, is the better indicator of soil behaviour in the field and of the water content at which the soil changes from the brittle to the plastic st