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doi: 10.2136/sssaj1962.03615995002600040005xpmid: N/A
Using needle penetration and bulk density as measures of mechanical impedance, the effects of mechanical impedance on corn seedling root growth were studied in the laboratory. The corn seedlings were grown in artificially compacted samples of a Colo clay where the moisture was at a tension of 10 cm. in some samples and 100 cm. in others. The rate of corn seedling root elongation decreased linearly as bulk ensity of Colo clay increased from 0.94 g. per cc. to 1.30 g. per cc., or as needle penetration into the Colo clay decreased. In ohter laboratory experiments, the depth of corn seedling root penetration into sand contained in glass tubes of differing diameters was found to be linearly related to the depth of penetrometer penetration.
doi: 10.2136/sssaj1962.03615995002600040006xpmid: N/A
Aggregate and structural stabilities were determined on samples taken under 10 cropping systems on Beltsville silt loam soil. Water stability of aggregates from 2 mm. to 4.76 mm. in diameter was measured by the wet‐sieving method of Bryant, Bendixen, and Slater. Structural stability was determined on samples < 2 mm. and < 1 mm. by the air‐to‐water permeability ratio method of Reeve. The measurement of soil aggregate stability by wet‐sieving distinguished six different groups of cropping systems according to Duncan's Range Test (4). The order of placement of the cropping systems was in agreement with other experimental results of Wilson et al. concerning the effect of cultivation and forage crops on aggregate stability. The measurement of structural stability by the air‐to‐water permeability ratio method did not distinguish soil structural differences associated with the cropping systems. Attempts to increase the sensitivity of the air‐to‐water permeability ratio failed. Water permeability offers some promise as a measurement of structural stability differences associated with cropping systems.
doi: 10.2136/sssaj1962.03615995002600040007xpmid: N/A
Water held in porous media is under the influence of a number of forces. The energy of the water in these force fields may be described with the aid of the potential theory of classical physics or with the aid of classical thermodynamics. Desorption curves were determined for two sand fractions and a silt fraction at 48° and 44°C. Temperature coefficients of the potential calculated on the basis that surface tension‐radius of curvature was the principle water‐holding mechanism were compared with the experimental observations. The results indicated that in the fine sand (53–74µ) and silt (13.5–18.0µ) other mechanisms besides surface tension‐radius of curvature may be involved in holding the water in the porous medium.
doi: 10.2136/sssaj1962.03615995002600040008xpmid: N/A
Theory and principles of the double‐tube method, as presented in a previous paper, were applied and tested in laboratory and field studies. The former were carried out in a sand box, where the hydraulic conductivity of 3.87 cm. per min. obtained with the double‐tube principle agreed closely with the known conductivity of 4.10 cm. per min.
Harward, M. E.; Theisen, A. A.
doi: 10.2136/sssaj1962.03615995002600040009xpmid: N/A
X‐ray diffraction studies of Oregon soil clays during the past few years have resulted in the observance of a number of phenomena that were somewhat disconcerting. Results of a number of different studies have indicated that the clay minerals identified in a given sample are dependent upon specimen carriers, method of Fe removal, dispersion reagents, cation saturation of clay, and in some cases, peeling of specimens. Further, the situation is complicated by the fact that the effects vary not only with different soils, but also to some extent with different size fractions of the same soil.
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