e decrease and increase in the amount of oxygen available to the soil bacteria living in this zone. When the zone is saturated, oxygen becomes depleted, anaerobic bacteria predominate, and their activities result in the formation of soluble iron and manganese compounds. The soil loses color. When the zone is exposed to air, these iron and manganese compounds oxidize (rust) to colored deposits. Soil mottles can be formed by processes other than fluctuating water tables and fluctuating water tables do not always leave soil mottles. Even so, their presence in high water table areas is a rea- sonable sign that a fluctuating water table is present and should be considered when designing a drain field. Soil colors other than mottles can be informative in that certain characteristics of soil drainage often correspond to certain soil col- SELECTION OF SITE AND SYSTEM SILTY SANDY SILTY CLAY CLAY LOAM SANDY SANDY SILT LOAM LOAMY Percent sand Figure 3.7 Soil texture triangle. ors. In a soil that is saturated most of the year, iron oxides are reduced and the soil becomes bluish-grey in color. Where free water is commonly at or near the surface, the surface may be dark to black. Plants may have difficulty growing under these conditions, and plant root depth may supplement soil color in assessing water table depth (Figure 3.8). Well drained soils show no mottling due to periodic saturation. The surface of these soils is generally less dark than that of poorly drained soils and the subsoils may be more brightly colored. The texture of well drained soils is medium sandy to stony. ONSITE WASTEWATER DISPOSAL MODERATE DRAINED Depth inches Brownish Rusty Stain Brown Rusty Stain Rusty Stain Figure 3.8 Root depth and soil drainage capacity. Soil Structure Soil particles are often associated with each other, forming clus- ters called peds. Between these peds is space through which water and air can pass much more readily than they pass through the peds (Figure 3.9). Often the existence of these peds cannot be detected from observing th