Effects of roots and salinity on law of development for farmland soil desiccation crack

Abstract: Soil shrinkage cracks which come into being with consumption and evaporation of water, cause changes in soil infiltration capacity and lead to the preferential migration of soil water and solute. To investigate the development law of desiccation cracks on the soil surface under the condition of different roots and salt contents, a set of method, based on digital image processing, was proposed to extract the crack geometry parameter, which greatly improved the efficiency and accuracy of image processing, and then researched the crack morphology variation with water content. In this paper, a laboratory dry test was carried out in the view of root content and soil content of soil samples. It was concluded that when the mass water content >30%, there was little difference among the crack area density of the soil samples with different root length densities. As the mass water content continued to reduce, the crack area density increased gradually, and reached the maximum and kept stable when the mass water content reduced to 10% or so. The influences of different root length densities on the crack area density were quite different (P<0.05), the greater the root length density was, the more obvious the role of root to anchor the soil was, and the smaller the soil crack area density was. When the mass water content >30%, the crack length densities of soil samples with different root length densities all grew fast with the reduction of mass water content, but with little difference. As the moisture content continued to reduce, the greater the root length density was, the greater the crack length density was, and significantly different (P<0.05). And the crack network connectivity index gradually dwindled with the root length density increased. The soil crack area density of the soil sample with a salt content of 2.0% was obviously greater than the soil crack density of the soil samples with other salt content in the whole drying process. When the moisture content was between 27% and 34%, there were little difference among the crack area density with a salt content of 0.5%, 1.0%, 1.5%, respectively, but were all greater than the crack area density of the control group, namely the soil sample with a salt content of 0. As the moisture content continued to reduce, the influences of different salt contents on the soil crack area density became more and more obvious. Soil crack area density and length densities of different salt contents were all significantly different (P<0.05), and it indicated that the greater the salt content was, the larger the soil crack area density was, when the crack network got stable, and the smaller the length density was. Also, the greater the salt content was, the greater the network connectivity index was, and when the salt content got 2.0%, the network connectivity index was 1, which is the maximum value in theory, namely, there was no single-linkage crack. The research results have important practical significance for guiding cracked soil irrigation.