Water potential is one of the most important physical properties of soil but remains among the most difficult to measure reliably. We describe a water activity meter that can be conveniently used to measure water potentials in soils ranging in wetness from near saturation to air dry (water potentials from −0.4 to < −100 MPa). The meter senses the dew point of water vapor with a miniature cooled mirror located above a soil sample in a closed chamber. Readings for individual samples are obtained rapidly (2–5 min). Accuracy of the measurement is checked periodically using saturated soil solutions. Repeatability of measurements was found to be equal or better than the rated precision (± 0.003 activity units). Water potentials (calculated from water activities using the Kelvin equation) were obtained for soils ranging in texture from sand to clay. The meter was used on soil samples taken from sites near Richland, WA, and summertime water potentials were found to be as low as −264 MPa.
Abstract. Use of either three or six occasions to spread similar total amounts of slurry was investigated over the late winter/early spring period in two years, on plots with underdrainage. Slurry was spread on four 7 m by 25 m plots which were instrumented to allow collection of drainflow and runoff. The ‘mobile water’ pore space allowed transport of a small proportion of applied slurry through the 0.75 m soil profile. Evidence of a polluted ‘mobile water’ pulse occurred after periods of up to 48 hours following spreading, suggesting subsurface rather than overland flow. Timing of the pulse depended on whether subsequent rainfall accelerated the flow The contaminated flow persisted for 6–12 hours. The worst contamination of 39 mg/l NH4+ ‐N occurred under ‘bypass flow’ conditions, when 52 mm of rain fell during the 48 hours after a 5.2 mm application. Two contributory factors appeared important. The first was the long duration of the high intensity storm and the second was an extended period of severe frost believed to have created fissures associated with frost heave. Making several applications of slurry in amounts less than 35 m3/ha is preferable to fewer larger applications during spring, for cereals on medium to heavy soils. This recognises the likelihood of there being underdrainage on arable land and the optimal use of well‐drained, partially frozen ground to avoid compaction by slurry tankers.
Wormsloe State Historic Site near Savannah, GA contains nearly three centuries of recorded land use activities, from 1736 to the present. It provides a unique setting to investigate the long‐term influence of historic land use on sandy soils of the Coastal Plain. We used a Dualem 2S (Dualem, Inc.) electromagnetic induction (EMI) meter and an AGI SuperSting R8 (Advanced Geosciences, Inc.) electrical resistivity (ER) meter in combination with traditional soil sampling to investigate soils and soil disturbance at Wormsloe. Three sites (Cabin, Dairy, and Forest Charcoal) were chosen to survey using both instruments. EMI surveys were made at each site on a 14 × 12 m grid, and ER surveys were made on a 7 × 6 m grid nested within the larger EMI grid. Electromagnetic induction was measured to depths of 10 and 210 cm, and ER was measured to a depth of 100 cm. Geophysical data was converted to a visual format using ordinary kriging in ArcGIS (ESRI) for EMI data and inversion software EarthImager 3D (Advanced Geosciences, Inc.) for resistivity data. These visual maps were used to direct traditional soil sampling. We found EMI was useful for detecting broad soil patterns originating from natural soil variation or large features, such as foundations, while ER more accurately identified artifacts of human origin, as verified by soil sampling.
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