Clay, clay loam, and silt loam surface soils can have a high volume weight depending on their condition. These variations are to be expected even in soils of the same texture. There is a tendency for the volume weight to rise due to the organic matter , less aggregation, a compaction due to the weight of the overlying layers, and perhaps in certain cases to a downward movement of clay. The way the soil is managed can make a difference in the volume weight of the surface soil due to cultivation. The addition of farm manure in large amounts tends to lower the weight figure.
Intense cultivation tends to raise the weight figure.
The pore space of soil is occupied by air and water in constantly varying proportions. The amount of pore space is determined by structural conditions, such as texture, compaction, and aggregation. The finer soils are generally lighter due to the tendency of the small particles to resist compaction and the readiness with which granulation occurs, especially when sufficient decomposing organic matter is present. Surface sands and sandy loams usually contain somewhat less pore space than silt loam, clay loam, and clay surface soils. Two types of individual pore space in general occur in soils, macro and micro.