Relation of Percolation Rates through Saturated Soil Cores to Volume of Pores Drained in 15 and 30 Minutes Under 60 Centimeters Tension

THE permeability of different horizons of soil profiles as a factor in crop adaptation, runoff and erosion, drainage, and irrigation has received considerable emphasis in recent years. The Soil Conservation Service has recognized the need for quantitative laboratory measurements of soil permeability on certain soils to serve as an aid to conservation surveyors in properly evaluating this factor and relating it to soil properties observed in the field. The principal methods of measuring the permeability of soils to water are by determining percolation rates under standard conditions and by measuring the amount of water removed from soil pores by applied tensions. When percolation rates are measured through short saturated cores of soils having their natural field structure, fortuitous channels and worm holes may give higher percolation rates than would occur in the field if the channels happened to end a short distance below the point at which a core was taken. Under saturated conditions a single worm hole through a soil core might give a very high percolation rate to a soil which without the worm hole would have a very low percolation rate. There is considerable evidence indicating that worm holes in soils generally proceed in an approximately vertical direction and that they have only a limited influence on the horizontal movement of water in soils. As water drains from a soil, the tension with which the water remaining in the soil is held, increases and tends to remove water from the worm holes or any other large openings. Tension methods for evaluating pore size distribution in soils are based on the principle that water can be removed from large pores by low tensions without removing it from the smaller pores. It would appear that the large channels in a soil would not be effective in draining the soil except during the times when free water is present and since the channels are emptied at a low tension the soil would remain in a wet condition unless water moved out of it through the smaller pores. The pore size distribution of a soil may be characterized by determining the pore volume drained at equilibrium with successively increasing tension values, or as shown by Bendixon and Slater (1) and Bendixon, Hershberger, and Slater (2), the volume of water removed in a given length of time by a tension of 60 centimeters or some other selected value may be used for evaluating porosity as related to percolation rates. The latter procedure has the advantage of requiring less time while the pore volume drained at equilibrium with various tensions has the advantage of giving a more complete picture of the pore size distibution in the soil. The pore drainage values for 15 minutes and 15 hours under 60 centimeters tension have been used extensively in the Southeastern states in conjunction with percolation rates through saturated cores under unit head for characterizing soil permeability. In most of this work, small tension plates for individual soil cores were used employing blotters as a porous medium. These plates are essentially similar to those of Learner and Shaw (3) except that each plate holds only one soil core. The individual plates permit measuring the water drained from each core without weighing the cores. When large tension plates of the type reported by Learner and Shaw (3) are used, the amount of water removed from each core is usually determined by the difference in weight of the core before and after draining on the tension plate. Where this method is used and a large number of cores are on the plate it is inconvenient to determine the pore volume drained in less than 30 minutes due to the time consumed in weighing the cores. It is the purpose of this paper to show the relation of pore volume drained in 15 minutes under 60 centimeters tension to percolation rates under unit head. Determinations of the relation of pore volume drained in 15 minutes to pore volume drained in 30 minutes under 60 centimeters tension were made on a number of cores to provide a basis for converting the 15-minute values to 30-minute values and for estimating percolation rates from 30-minute pore drainage values.