e soil pulled from a test hole and evidence of them can be smeared by the sides of a backhoe bucket. To determine the extent of this structure, a trench sidewall should be scraped care- fully to remove the surface contacted by the shovel or bucket. Naturally, sand is structureless since it cannot form clusters. Clay soils may form blocks or long vertical columns (Figure 3.10). Silty loams form moderately blocky peds. Loams and sandy loams with some fine particles can form large blocks (Figure 3.11). SELECTION OF SITE AND SYSTEM Figure 3.9 Soil peds. Figure 3.10 Aggregated blocks of clay. ONSITE WASTEWATER DISPOSAL Figure 3.11 Clod of sandy loam. A well developed structure is associated with the rapid percola- tion of water through the soil. If the peds are flat and plate-like, considerable lateral flow might be expected. If the structure of fine- grained soils has been obliterated by tractor tires, buckets, or shoes, or if fine-grained soils exhibit no natural structure, slow percolation of water can be anticipated. Permeability We have been discussing the ability of soils to transmit water. This property of soil is called permeability. When water moves from one point to another through the soil, it does SO in response to two major forces: gravity and capillarity. Gravity, as we all know, is the force that makes water flow downhill. But capillarity, the attraction of water to soil particles (and to itself), can move water up against the force of gravity (Figure 3.12). With differing relative amounts of these forces acting in diverse soils, water moves at different rates and in different directions. Sand has very large pore spaces between the particles, but few in number compared to silt or clay. The volume of the total pore space is smaller in a cubic foot of sand than it is in a cubic foot of silt or SELECTION OF SITE AND SYSTEM Air Spaces Capillary Water Water Level Figure 3.12 Movement of water by capillarity. clay, SO sand holds less water per unit of volume. However, the fact that the pore spaces are large i