The role of stones in soil water retention

Do stones modify the pore space in soil directly, or do they interact with the fine earth fraction? The contribution of each of these phases to porosity and water retention was analysed in mixtures of glass fragments <6 mm with a silty-clay soil and a clay soil in the 10-80% glass range. Each mixture was prepared wet and air-dried before being saturated with water. Water contents were measured at matric suctions ranging from 0.05 to 1000 kPa, and the bulk densities were measured in air-dry mixtures and at a matric suction of 0.5 kPa. The volume balance calculated by subtracting the pore volume due to soil fabric from the total pore volume of the mixtures shows that a variable proportion of porosity corresponded to coarse lacunar pores. The volume of the lacunar pores was affected little by the water content and generally increased from 0.01 to 0.21 cm(3) g(1) with increasing glass content but remained less than 0.01 cm. 3 91 in silty-clay soil mixtures with less than 50% glass. These coarse lacunar pores retained water at suctions < 10 kPa. They were determined by two kinds of mechanisms, i.e. filling in the case of the silty-clay soil and shrinkage in the case of the clay soil. In these two cases there were two distinct types of pores in the mixtures: fine earth pores and coarse lacunar pores. A third mechanism, i.e. substitution, occurs in mixtures with large silty-clay soil content, and determines the predominance of the fine earth pores. A clay content of 30% in the fine earth fraction and 50% fine earth in the mixture are threshold values proposed to predict pore space modifications induced by stones in soils.