A bio-assay by means of the tube technique indicated whether an inadequate aeration or mechanical impedance excludes roots from the soil beneath the plow pan in a sandy loam. The soil profiles assayed had normal ABC horizonation or inverted CBA horizonation, with the top soil, horizon A, lying beneath the plow pan, horizon B. Half of the buried and half of the normal A horizons were compacted somewhat. A mulch made the temperature and moisture regimes in the unburied topsoil nearly equal to those in the buried. Prior fertilization tended to equalize the available nutrients. Tomato roots were inserted into the normal and the buried A horizons by means of plastic tubes. Subsequent observation showed that the roots were in fact restricted to the intended horizons. Since the tomato plants elongated as rapidly, flowered as soon, and weighed as much when their roots were grown in soil beneath the plow pan as when their roots roots were grown in soil above the pan, the plow pan is evidently not a significant barrier to soil aeration beneath it. Therefore, the blocking of the roots by the plow pan in this soil must be caused by the mechanical inpedance and not lack of aeration.