Water properties of caliche.

Water absorption and retention by hard caliche nodules (rocks) collected from soils in southern New Mexico were determined. The rate of water uptake by the criiche rocks was rapid and water content at saturation was 13.0% by weighht (24.7% by volume). At a matrix potential of -0.7 MPa, the rocks retained 10.6% water by weight, an 18% loss from saturation. Water loss from saturated rocks to a dry atmosphere was slow, but most of the absorbed water was released. The rocks contained only 0.6% water by weight (1.1% by volume) after 34 days in a desiccator. Both iaboratory and field trials indicated that, although indurated caqche iayers will absorb iarge amounts of water, the water does not pass through the layers to the soil below. Caliche is commonly found in soils in the arid and semiarid southwestern United States. Although the chemical composition of caliche varies spatially, calcium carbonate (CaCOs) is always the major constituent. Deposits of caliche often limit the downward extension of plant roots and the volume of soil from which plants can extract water. Thus, an understanding of plant distributions on arid rangelands is often dependent upon a knowledge of how caliche deposits influence the availability of soil water. The dissolving and leaching of CaCOs by rainwater, followed by the evaporation and rapid removal of soil water by plants, leads to precipitation of CaCOsand the development of caliche deposits in soils (Gile et al. 1966, Shreve and Mallery 1932, Stuart and Dixon 1973, Stuart et al. 1961). Deposits of caliche often form along and below contacts between coarse-textured soil layers or between coarseand fine-textured interfaces (Stuart and Dixon 1973). These carbonate deposits may be in the form of indurated or “hard”caliche-which does not slake when an air-dried portion is placed in water-or in the form of nonindurated “soft” calichewhich does slake when an air-dried portion is placed in water (Gile 1961). Both types of caliche often exist together. Caliche deposits may be in the form of either nodules or layers. In either form, the material is usually parallel to the soil surface in either continuous or discontinuous layers. Depth of the deposits varies from near the surface to a depth of a meter or more. With maturity, whole deposits may become hardened and strongly indurated, especially if they contain high amounts of calcium or aluminum silicates (Stuart et al. 1961). Where soil horizons are so strongly impregnated with carbonate that their morphology is determined by the carbonate, a petrocalcic or Bkm horizon may be designated (USDA Soil Conserv. Serv. 1981). Since hardened deposits are not easily weathered, the layer, upon exposure, can form a caprock (Lattman 1977). The development of carbonate horizons is a useful indicator in determining soil age (Gile 1970). In calcareous soils, those with caliche layers and those with enough CaC03 to potentially form caliche, water penetrability and soil sorptivity decrease as the CaCOs content in the sand fraction increases. Precipitates in such soils can block pore spaces and increase the length of passages available for soil water movement. As a result, both the water storage-capacity and the rate of water Authors are former graduate student, Animal and Range Science Department, New Mexico State University, Las Cruces, now range research officer, Gaborone, Botswana, Africa; range scientist and hydrologist, U.S. Department of Agriculture, Agricultural Research Service, Las Cruces, N. Mex. 88003; and associate professor, Department of Experimental Statistics, New Mexico State University, Las Cruces, respectively. This article is published as journal article 960, Agricultural Experiment Station, New Mexico State Univ., Las Cruces. Manuscript received November 1, 1982. JOURNAL OF RANGE MANAGEMENT 36(6), November 1983 advancement (hydraulic conductivity) in the soil are decreased (Tayel 1975, Verplancke et al. 1976). Removal of CaCOs from a soil increased soil porosity and soil water retention at all suction levels tested by Stakman and Bishay (1976). Calcareous soils were also found to be more susceptible to compaction damage and clogging of micropores by cementation (Talha et al. 1978). Gile (1961) found that infiltration rates of carbonate horizons ranged from 0.13 to 14.99 cm per hour and that infiltration rates decreased exponentially as carbonate content increased. Caliche is highly insoluble in soil water except when the soil water contains abundant CO2 (Shreve and Mallery 1932). The insolubility of caliche in water has lead to the assumption that most caliche is highly impermeable to water (Lattman 1977). However, Shreve and Mallery (1932) found that “hard”caliche would absorb small amounts of water (3-6% by dry weight) whereas “soft” caliche absorbed up to 17% water. Shreve and Mallory (1932) also found that water transferred slowly through thin (l-cm thick) caliche layers. They concluded that caliche was a deterrent to the penetration of water from the surface to lower depths, and that, once water reached lower depths, caliche effectively retained it. Caliche often occurs within the rooting zone of plants on arid rangelands (Gile and Grossman 1979). Thus, water properties of caliche may influence the amount of soil water available to plants. The objective of this study was to determine the water absorption, retention, and transfer characteristics of indurated caliche. Materials and Methods Caliche samples were obtained from a mesquite (Prosopis glandulosa Torr.) duneland site on the Jornada Experimental Range (administered by the U.S. Department of Agriculture, Agricultural Research Service) in Dona Ana County, N. Mex. On-site examination was made by Soil Conservation Service personnel. Interdunal soils were identified as coarse-loamy, mixed, thermic Typic Haplargids of the Onite series and as coarse-loamy, mixed, thermic Typic Calciorthids of the Wink series. Dunes tall enough to qualify as pedons were classified as mixed Typic Torripsamments of the Pintura series. All of the soils contain petrocalcic layers, generally horizontally discontinuous and ranging from the surface to a meter or more in depth. These soils are in an arid area where mean annual rainfall is 230 mm. Mean annual temperature is 15OC. The average temperatures are maximum in June (36“C) and lowest in January (13.3OC). Hard caliche nodules (rocks) were gathered from the field site. A layer of caliche rocks is typically found just above the solidly indurated caliche layers. Four weight classes, with 5 rocks in each class, were chosen, based on oven-dry weight as follows: (1) 40-70 g; (2) 70-100 g; (3) 100-150 g; and (4) 150-300 g. Water Absorption To determine rate of water uptake and water content at saturation, we oven-dried (105OC) the caliche rocks to a constant weight (48 hr) and submerged them in distilled water. The rocks were removed from the water and excess water removed with towels, then immediately weighed and replaced in water. This procedure was performed after 1, 5, and 15 min; 1 hr; 5 hr; and 24 hr of submergence. Weight determinations at 36 hr showed that a constant weight had been reached at 24 hr. Water content (percentage of dry weight) was calculated for each rock at each time interval.