Field Percolation Rates of Four Wisconsin Soils Having Different Drainage Characteristics 1

HE rate of intake of water into the soil, or the infiltration capaT city, is one of the primary factors affecting the amount of surface run-off in the field. The infiltration rate is ordinarily at its maximum when water is first applied to the soil, and then as the pore spaces become filled and swelling occurs, it decreases until a more or less stable minimum is reached. This minimum is the field percolation rate, that is, the rate at which water moves through the saturated soil profile, and it is governed largely by the permeability of the least pervious horizon. This minimum or stable infiltration rate or percolation rate, being a quantitative determination of the rapidity with which water can move through a saturated profile, is a measure of the internal drainage. This rate controls soil water movement when water is abundant, as during periods of prolonged precipitation, or during the early spring when the soil is saturated as a result of thawing, spring rains, and low evaporation. This rate is not necessarily important in establishing the rate of run-off and erosion during the drier summer period when only the upper portion of the soil is likely to be saturated by any one rain. In Wisconsin, as in other mid-western glaciated states, the internal drainage of several soil types is a major factor controlling the kind of crops adapted to these types. Perhaps the outstanding examples in Wisconsin are the Spencer (Colby) and Marathon silt loams in the north-central portion of the state (9).3 These two soils occur side by side and are often intermingled, yet, because of the effect of different substrata and degrees of post-glacial erosion upon their profile development, the former has poor while the latter has good internal drainage. On the poorly drained Spencer, such crops as alfalfa and potatoes usually fail, but they are well adapted to Marathon soils. The Spencer is subject to frost heaving, is cold and wet, and is late in reaching a tillable condition in the spring, while the Marathon in the same neighborhood does not show these unfavorable characteristics. Since the crop adaptations of these soils were correlated with internal drainage conditions, quantitative measurements of percolation rates were desired. These measurements, made by a. buffer compartment method, were compared with the porosity and percolation rate of cores from the various horizons of these soil profiles, and similar studies were made of two other widely different soils, the Miami silt loam and the Superior clay loam.