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V. a. vcrxrti ncni ur nunaVUU unc N;gTUkAL RESOURCES CONSERVATION SERVICE PHYSICAL PROPERTIES' OF SOILS Endnote -- PHYSICAL PROPERTIES OF SOILS 06I~ 21/v8 This report shoos estimates of some characteristics end features that affect soil behavior. These estimates are given for the major layers of each soil 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 consists of mineral soil Darticies that are less than 0.002 millimeter in diameter. In this report, the estimated clay content of each major soil lever is given as a Dercentage, by veight, of the soil material that is less than 2 millimeters in diameter. The amount and kind at clay greatly affect the fertility and physical condition of the soil. They determine the ability of the soil to adsorb cations and to retain moisture. They influence shrink-svell potential, permeability, plasticity, the ease of soil dispersion, and other soil properties. The amount and kind of clay in a soil also affect tillage and esrthmoving operations. MOIST BULK DENSITY is the veight of soil (ovendryl per unit volume. Volume is measured when the sail is at field moisture capacity, the moisture content at 113 bar moisture tension. Height is determined after drying the soil at 105 degrees C. In this report, the estimated moist bulk density of each major soil horizon is expressed in grams per cubic centimeter of soil materiel that is less then 2 millimeters in diameter. Bulk density data ere used to compute shrink-swell potential, available eater capacity, total pore space, and other soil properties. The moist bulk density of a soil indicates the pore space available for eater and roots. A bulk density of more than 1.6 can restrict eater storage and root penetration. Moist bulk 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 the rate of dovnvard movement of aster when the soil is saturated. They are based on soil characteristics observed in the field, particularly structure, porosity, and texture. Permeability is considered in the design of soil drainage systems, septic tank absorption fields, and construction vhere the rate of water movement under saturated conditions affects behavior. AVAILABLE WATER CAPACITY refers to the quantity of eater that the soil is capable of storing for use by giants. The capacity for vfter storage is given in inches of eater per inch of soil for each major soil layer. The capacity varies, depending on soil Draperties that affect the retention of eater and the depth of the root zone. The most important properties ere the content of organic matter, soil tezture, bulk density, and soil structure. Available eater capacity is an important factor in the choice of Dlants or crops to be groan and in the design and management of irrigation systems. Available voter capacity is not en estimate of the quantity of water actually available to plants at any given time. SHRINK-SWELL POTENTIAL is the Dotential for volume change in a soli with a loss or gain of moisture. Volume change occurs mainly because of the interaction of tier minerals with eater and varies with the amount and type of clay minerals in the soil. The size of the load on the soil and the magnitude of the change in soil moisture content influence the amount of swelling of soils in place. Laboratory measurements of swelling of undisturbed clods were made for many soils. For others, swelling vas estimated on the basis of the kind and amount of clay minerals in the sail and on measurements of similar soils. If the shrink-swell potential is rated moderate to very high, shrinking and suellin9 can cause damage to buildings, roads, and other structures. Special design is often needed. Shrink-swell potential classes are based on the change in length of an unconfined clod as moisture content is increased from air-dry to field capacity. The change is based on the soil fraction less than 2 millimeters in diameter. The classes are 'Lou,' a change of less than 3 percent; 'Moderate,' 3 to 6 percent; and 'High,' more than 6 percent. 'Very high,' greater than g Dercent, is sometimes used.