The relationship between modules of rupture and moisture content, of three clayey soils within a suction range of 1-15 bars was studied. The results showed that the turning points of moisture content for rupture modules of latosol, red earth amd yellow earth were 25.3%,15.3% and 15.7% respectively. According to the characteristic curves of these soils, taking the moisture contents of the soils at 8 bar as the critical values of the turning point, their errors were less than 1% (dry weight percent) as compared with the measurements. As compared with other 14 typical characteristic curves published in literature, it is suggested that this critical value would be suitable for most sails with the errors less than 2% (dry weight percent).Owing to the turning point of the moisture content far modules of rupture at which the pressure to break down the soil clad increases sngnificantly, this critical value of soil moisture content could be considered as a parameter for the lower limit of sail tillage.