Assessing the impact of soil shrinkage and pore size dynamics on rice crop yield in expansive clay soils

Soil shrinkage during the drying process (water stress) is one of the main issues in expansive soils of paddy fields. It occurs due to decrease in soil water content, resulting in changes in soil volume and the geometry of pores, leading to the formation of cracks and higher water loss. The aim of this study was to assess the shrinkage characteristic curve and pore size of paddy soils to determine the shrinkage -swelling behavior in Guilan province, Iran. 120 soil samples were collected from the study area. Pore size was determined using soil moisture retention curve (SMRC). It was established by plotting the soil water content (θ) versus the corresponding matric suction (h), and the shrinkage curve by plotting the void ratio (e) against the moisture ratio (υ). The suction-pore relationships were also determined. Furthermore, the geometric factors indicating the change in vertical (subsidence) and horizontal (crack) volume of the soils were determined and varied from 1.23 to 2.53, indicating that the vertical change in soil volume is predominant. The zero, residual and proportional shrinkage phases accounted for less than 2 %, 8–38 %, and 61–91 % of the total soil volume change, respectively. The shrinkage capacity of the soils ranged from 0.52 to 1.37. Cation exchange capacity and clay content were identified as the most important factors affecting soil shrinkage properties. In general, the studied paddy soils have great potential for swelling- shrinkage and cracking during the drying process due to the large percentage of expandable clays and the medium to fine pores. The resultant cracks negatively affect crop yield by damaging plant roots and increasing water losses through the soil profiles.