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U. S. CEPARTMENT OF AGRICULTURE NATURAL RESOURCES CONSERVATION SERVICE PHYSICAL PROPERTIES OF SOILS EndnoCe -- PHYSICAL PROPERTIES OF SOILS PAGE 2 OF 4 08/19/00 This report shows estimates of some characteristics and features that affect soil behavior. These estimates aze given for [he major layers of each eoil in the survey area. The estimates are based on field observations and on test data for these and similar soils. CLAY as a soil separate cone is ea of mineral soil pareiclea that are less than 0.002 millimeter In diameter. In this report, Che estimated clay content of each major soil layer is given as a percentage, by weight, of the soil material Chat is less than 2 millimeee re in diameter. The amount and kind of clay greatly affece the fertility and physical condition of the soil. They determine the ability of Che soil to adsorb cations and co retain moisture. They influence shrink-swell potential, permeability, pl ascicity, she ease of sail dispersion, and other soil properties. The amount and kind of clay in a soil also affect tillage and earchmovang operations. MOIST BULK GENSI TY ie [he weight of soil (ovendryl per unit volume. Volume is measured when the soil is ac field moisture capacity, the moisture content a[ i/3 bar moisture tension. Weight is determined after tlryi ng the soil ac 105 degrees C. In this report, [he ea [imated moist bulk density of each major soil horizon is expressed in grams per cubic centimeter of soil material that is leas than 2 millimeters in diame Cez. Bulk dens icy data are used to compute shrink-swell potential, available water capacity, total pore apace, and other soil properties. The moist bulk density of a soil indicates the pore space available for water and roots. A bulk density of more than 1.6 can restrict water storage and root penetration. Moie[ bu" density is influenced by texture, kind of clay, content of organic matter, and soil structure. PERMEABILITY refers to the ability of a soil to transmit water or air. The estimates indicate Che rate of downward movemene of water when the soil is saturated. They are based on soil charactetia tics ODeerved in the field, particularly structure, porosity, and texture. Permeability is consideretl in the design of soil drainage systems, septic tank absorption fields, and construction where [he rate of water movement under saturated conditions affects behavior. AVAILABLE WATER CAPACITY refers to the quantity of water that the soil is capable of storing for use by plants. The capacity Eor water storage is given in inches Of water per inch of soil for each major soil layer. The capacity varies, depending on soil properties chat affect the retention of water and the depth of the root zone. The most important properties are the content of organic matter, soil texture, bulk density, and soil -~ structure. Available water capacity ie an important factor in the choice of plants or trope to be grown and in the design and management of irrigation systems. Available water capacity is not an estimate of the quantity of water actually available to plants at any given time. SHRINK-SWELL POTENTIAL is the potential for volume change in a soil with a loss or gain of mo is CUre. Volume change occurs mainly because of Che interaction of clay minerals with water and va ties with the amount and type of clay ma ne rals in the soil. The size of the load on the soil and she magnitude of [he change in soil moisture content influence Che amount of swelling of voila in place. Laboratory measurements of swelling of undisturbed clotls were made Eor many voila. For others, swell ins was estimated on the basis of the kind and amount of clay minerals in the soil and on measurements of similar soils. If the shrink-swell potential is ratetl moderate to very high, shrinking and swelling can cause damage to buildings, roads, and other structures. Special design ie often needed. Shrink-swell potential classes are based on the change in length of an unconfined clod as moisture content is increased Erom air-dry to field capacity. The change is based on the soil fraction less Chan 2 millimeters in diameter. The classes are "Low," a change of less than 3 percent; "Moderate," 3 [O 6 pI t; and "High," more Chan 6 percent. "Very high," Brea ter than 9 pe rc enc, is sometimes used.