2001 Volume 88 Pages 53-60
Internal fissures in clayey soil are important for tillage efficiency. The roots reinforce the soil and absorb soil moisture, resulting in shrinkage and fractures in the massive soil structure. The objective of this study is to clarify the influence of roots on the prevention of wide cracks and the formation of small fissures during desiccation. Soil samples were taken from the surface layer of inter-row spaces of a clayey paddy three weeks after the release of ponding water in the summer 1998. In half of the sampling sites, the stems and leaves of the rice plants were cut away just before the release of the ponding water, in order to compare the effects of the two resulting water absorption conditions. After measurement of the volume fractions of the solid, liquid, and air phases, the oven-dried samples were soaked in water and then scoured with tap water in a sieve. The remaining roots were dried and weighed. The relationships among volumetric air ratio, moisture ratio, and root density were statistically analyzed, with the volumetric air ratio being regarded as the ratio of newly formed fissures due to desiccation. Positive direct correlations were observed between root density and air ratio for all plots. In he plots with transpiration, since the moisture ratio correlated with root density and air ratio, a large proportion of the variances in air ratio explained by moisture ratio and root density were shared. These results lead to the following conclusion : Rice plant roots inhibit the formation of wide cracks and induce small fissures in clayey well-puddled soil during desiccation. With transpiration, these effects are attributed to the compound functions of roots : absorption of water and soil reinforcement. However, even without transpiration, soil reinforcement by roots uniquely contributes to the development of small fissures.
