Cotton root growth and activity was determined at different levels of subsoil bulk density and O2 concentration. Subsoil cores of desired bulk density were aerated leaving both the soil moisture content intact and the surface soil exposed to diffusion of gases from the atmosphere. Measurements of root growth and activity in the subsoils included depth of root penetration, root yield, water uptake, and carbon dioxide production in the subsoil.
Mechanical impedance was more detrimental than low O2 for root growth in the subsoils at bulk densities above 1.5 g. cm.‐3. At lower densities root growth was depressed at O2 levels below 10%, and there was a strong interaction of O2 and bulk density.
The level of CO2 and of subsoil density were varied in a growth chamber study of the interaction of these two factors on root growth of cotton seedlings at constant O2 concentration. At very low density root elongation rate decreased progressively with increasing CO2, although moderate to good growth occurred even at 24% CO2, the highest concentration used. At higher subsoil level densities, CO2 had little effect on root penetration. Retardation by CO2 was somewhat more severe for roots growing into solution culture than for those growing into loose subsoil, indicating that application of results of aeration experiments in nutrient solution to soils should be made with caution. Results suggest that in well‐drained, medium‐to‐coarse‐textured soils such as the Norfolk, CO2 will seldom be a limiting factor for root growth even in compacted zones near the surface. The mechanical impedance of such zones, will definitely restrict root penetration and proliferation in subsoils.
A method is described for rapidly obtaining large, undisturbed soil cores ranging from 16 to 38 inches in diameter and to depths of up to 9 feet. Such cores, in some cases containing several thousand pounds of soil, may be used in-place or moved to a controlled environment for a number of purposes; e.g., studies of transpiration, evapotranspiration, moisture flow, and others. In such cases, disturbed soil materials often are not satisfactory because of the influence of disturbing the soil on such properties as moisture transmission, moisture retention, and root distribution.
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