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This article in SSSAJ

  1. Vol. 57 No. 3, p. 809-816
     
    Received: July 23, 1992
    Published: May, 1993


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doi:10.2136/sssaj1993.03615995005700030030x

An Improved Method for Quantifying Soil Macroporosity

  1. V. R. Vermeul,
  2. J. D. Istok ,
  3. A. L. Flint and
  4. J. L. Pikul
  1. Geosciences Dep., Pacific Northwest Laboratories, Richland, WA 99352
    Dep. of Civil Engineering, Oregon State Univ., Corvallis, OR 97331
    U.S. Geological Survey, P.O. Box 327, Mercury, NV 89023
    USDA-ARS, Northern Plains Soil and Water Research Center, Sidney, MT 59270

Abstract

Abstract

Quantitative information on macroporosity is needed to predict water flow and solute transport in field soils. A method was developed for determining the number, shape, and size distribution of soil macropores. Horizontal serial sections sawed from paraffin-impregnated soil cores were photographed under ultraviolet (UV) light. Anthracene, mixed with the paraffin, fluoresces a bright bluish white under UV light and provides a sharp contrast between the soil matrix and the paraffin-filled pore space. Section photographs were converted to 256 level, grey-scale digital images using a flat-bed scanner. Image processing was used to classify each pixel in a digital image as pore space or soil matrix, to group the pore space pixels into pores, and to measure the area and perimeter of each pore. The method was able to measure pores with an equivalent radius ≥85 µm. Macroporosity in soil cores sampled form a tillage path and from an adjacent, undisturbed (notill) region was quantified. Tillage sections contained, on the average, 9.4 macropores/cm2 with an equivalent macroporosity of 8%, while no-till sections contained 0.8 macropores/cm2 with an equivalent macroporosity of 0.3%. Computed intrinsic permeabilities for tillage sections that included macropore information were significantly larger than values computed using micropore information alone, suggesting that macropores must be included in permeability calculations when the number of macropores is large. The developed method should be useful for quantifying macroporosity in nonskeletal soils.

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