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Custom Soil Resource Report <br /> map for soil map units can be rendered.Aggregation must be done because, on <br /> any soil map, map units are delineated but components are not. <br /> For each of a map unit's components, a corresponding percent composition is <br /> recorded.A percent composition of 60 indicates that the corresponding component <br /> typically makes up approximately 60% of the map unit. Percent composition is a <br /> critical factor in some, but not all, aggregation methods. <br /> The aggregation method "Dominant Condition"first groups like attribute values for <br /> the components in a map unit. For each group, percent composition is set to the <br /> sum of the percent composition of all components participating in that group. These <br /> groups now represent"conditions" rather than components. The attribute value <br /> associated with the group with the highest cumulative percent composition is <br /> returned. If more than one group shares the highest cumulative percent <br /> composition, the corresponding "tie-break"rule determines which value should be <br /> returned. The"tie-break" rule indicates whether the lower or higher group value <br /> should be returned in the case of a percent composition tie. The result returned by <br /> this aggregation method represents the dominant condition throughout the map unit <br /> only when no tie has occurred. <br /> Component Percent Cutoff:None Specified <br /> Components whose percent composition is below the cutoff value will not be <br /> considered. If no cutoff value is specified, all components in the database will be <br /> considered. The data for some contrasting soils of minor extent may not be in the <br /> database, and therefore are not considered. <br /> Tie-break Rule:Lower <br /> The tie-break rule indicates which value should be selected from a set of multiple <br /> candidate values, or which value should be selected in the event of a percent <br /> composition tie. <br /> Topsoil Source <br /> Topsoil is used to cover an area so that vegetation can be established and <br /> maintained. The surface layer of most soils is generally preferred for topsoil <br /> because of its content of organic matter.Organic matter greatly increases the <br /> absorption and retention of moisture and nutrients for plant growth. <br /> The upper 40 inches of a soil is evaluated for use as topsoil. Also evaluated is the <br /> reclamation potential of the borrow area. Normal compaction, minor processing, and <br /> other standard construction practices are assumed. <br /> The soils are rated "good," "fair," or"poor"as potential sources of topsoil. The <br /> ratings are based on the soil properties that affect plant growth; the ease of <br /> excavating, loading, and spreading the material; and reclamation of the borrow <br /> area. Toxic substances, soil reaction, and the properties that are inferred from soil <br /> texture, such as available water capacity and fertility, affect plant growth.The ease <br /> of excavating, loading, and spreading is affected by rock fragments, slope, depth to <br /> a water table, soil texture, and thickness of suitable material. Reclamation of the <br /> borrow area is affected by slope, depth to a water table, rock fragments, depth to <br /> bedrock or a cemented pan, and toxic material. <br /> 26 <br />