USE OF A PENETRATION RESISTANCE CHARACTERISTIC TO PREDICT SOIL STRENGTH DEVELOPMENT DURING DRYING

Rapid drying of hardsetting soils frequently results in poor seedling emergence. Prediction of seedling response to seedbed strength requires an understanding of the manner in which soil strength develops during drying. Penetration resistance is an empirical estimate of the cavity expansion pressure required of the emerging seedling. However, use of penetration resistance to predict soil strength during drying is hampered by the greater error in penetration resistance vs. bulk soil strength measurements, uncertainty over the role of drying rate in strength development, and limitations in the theoretical basis of penetration resistance compared with other strength quantities. In this experiment we measured the penetration resistance, shear strength and tensile strength development during drying of a hardsetting red-brown earth following flood irrigation. The results showed first, that although penetration resistance had a slightly higher coefficient of variation, ease of replication resulted in penetration resistance having a substantially smaller 95% confidence interval than either shear or tensile strength. Second, soil strength developed more rapidly at high drying rates; however, variation in penetration resistance was accounted for by changes in soil water content (r2 = 96%). Therefore drying rate did not influence the soil strength characteristic. Third, penetration resistance was a linear function of effective stress as predicted by theory, and unlike the bulk strength measures did not deviate within the range of water contents measured. We conclude that the penetration resistance strength characteristic can be used to predict seedbed strength in hardsetting soils for a wide range of packing, wetting and drying conditions.