THE SOIL PARTICLE 91 



49. Volume weight of soils. — The actual weight of dry 

 soil in any given volume is generally expressed by volume 

 weight,, a figure indicating the number of times heavier the 

 dry soil is than the water that wiUoccupy the same soil vol- 

 ume. Thus, if the dry soil in a cubic foot of space weighs 

 99.8 pounds, the volume weight would be 99.8-^62.42 or 1.6. 

 The volume weight differs from specific gravity in that it 

 compares the weight of the dry soil to the weight of water 

 that will occupy the total soil volume — that is, the space 

 usually filled by soil particles, soil air, and soil water. Specific 

 gravity, however, compares the weight of the dry soil to that 

 of water that will occupy only the volume of the particles 

 alone, taking no consideration of the normal pore space. It 

 is consequently always the higher figure. 1 



This volume weight figure depends on the texture of the 

 soil, the structure and the amount and condition of the organic 

 matter. The particles of sandy soils always tend to lie in 

 close contact, thus increasing the weight of soil to a given 

 volume. The particles of the finer soils, such as silt loams, 

 clay loams, and clays, on the other hand, being smaller and 

 lighter, do not lie so closely together. A greater total pore 

 space is, therefore, usually present in the finer soils and the 

 volume weight is correspondingly lowered. Mineral soils may 

 range in volume weight from 1.10 to 1.35 for clay to 1.55 to 

 1 . 70 for sand. 2 The influence of texture on the volume weight 

 is thus evident. 



The structural and organic condition of soils often pro- 

 duces wide variation in volume weight. When a soil is loos- 



1 As a soil is compacted, its volume weight increases due to the increase 

 volume occupied by the soil particles and the corresponding decrease in 

 pore space. If it were possible to compact a soil to a completely solid 

 condition, its volume weight would approach its specific gravity as a 

 limit. Specific gravity represents, therefore, 100 per cent, soil particles. 

 Volume weight in comparison indicates the proportion of space occupied 

 by the soil particles. 



2 Sandy soils are commonly spoken of as light soils, while clays are 

 called heavy. Such usage refers to working properties and has no 

 reference to actual weights. 



