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Volume weight is the actual weight of dry soil in any given volume. This figure indicates the number of times heavier the dry soil is than the water that will occupy the same total soil volume.
Now this volume weight will differ from specific gravity in that it compares the weight of the dry soil to the weight of water. Soil volume is the space usually filled by soil particles, soil air, and soil water. As a soil is compacted, its volume weight increases due to the increased volume occupied by the soil particles and the corresponding decrease volume occupied by the soil particles and the corresponding decrease in pore space.
Structural condition is the specific gravity of the soil particles and their arrangement and this determines the volume weight. Particles of sandy soils generally tend to lie in close contact, thus increasing the weight of soil in a given volume.
Particles of finer soils, such as silt loams, clay loams, and clays, on the hand, being smaller and lighter, ordinarily do not exist so close together. 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. It seems to be the size of the individual pore spaces rather than their combined volume that, after al, is the important consideration. The loosening and granulating of heavy soils promote aeration, not so much by increasing the pore space in total but by influencing the proportion of the macro- and micro- interstices.
Volume weight, besides being an interesting and significant physical characteristic, is very important as a basis for certain computations. Weight figures, especially those for the furrow-slice, may be used to calculate the amounts of water, the amounts of organic matter, and the actual number of pounds of the mineral constituents that are present. Volume weight is necessary in the calculation of the total pore space present in soil. This gives us a means of judging and comparing soils.
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