Graduate Thesis Or Dissertation
 

Soil physical properties and available water capacity of southwest Oregon forest soils

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  • Measurements for water retention calculations and physical characterization were made on skeletal and non-skeletal soils in southwest Oregon. A new bulk density sampler was designed for the physical characterization of the steep, skeletal soils commonly encountered in this area. The new sampler and the techniques required for the measurement and calculation of physical properties and available water are presented. The bead cone bulk density sampler is a modification of the sand cone excavation technique. The design allows a sample volume (1.5 x 10⁷ mm³) large enough to include rock fragments up to 160 mm in diameter which is important due to the large spatial variability of rock fragments. The bead cone is also designed to be easily portable and operate effectively on slopes up to 100 percent. A field test was conducted to compare bead cone and sand cone sampling techniques and results. Total soil density, fine soil density, rock fragment content and volumetric water content were measured or calculated with no statistical difference observed between the results obtained with the two samplers. On soils where slope or rock fragment size are not limiting for the sand cone both samplers work well: beyond these limits the bead cone is the preferred technique. Measurements of total soil density, fine soil density and rock fragment content using the bead cone and a 76 mm diameter, 76 mm length corer were compared on 30 soils in southwest Oregon. The core method compared well for rock fragment contents below 15 percent. There was, however, little agreement in soils with higher rock fragment content due to physical impedance and the small volume of the corer. The bead cone sampler was used to aid in the characterization of the physical properties of southwest Oregon. Soils were assessed at forty sites covering nine parent materials. The sites were sampled for total bulk density, particle size distribution, dry season water content and field capacity water content. The physical properties of the rock fragments significantly influence the calculation of seasonal water content and total available water. The porosity of the rock fragments ranges from 10 to 50 percent and contributed an average 15 percent of the total available water. The porosity of rock fragments must also be included in the calculation of rock fragment volume and fine soil density. The fine soil averaged over 12 percent organic matter with 50 percent of mineral soil being sand. Linear models using these soil physical properties were then used to predict water content at field capacity, seasonal low and total available water. The use of probablistic prediction models to estimate available water content may be valuable for long range management planning but appear to be of little value in management of specific sites. It was concluded that, using the methods presented, direct measurements of available water would be easier and more accurate than the models used for prediction.
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