Quantitative data are given for the effect of artificial compression of the soil on water capacity, resistance to flow of water, absorption of water, pore space, and penetrability. Exps. with beans and oats show that soil density has little effect on height of tops and area of assimilatory organs, but that underground organs are markedly modified, resulting in smaller and stronger root systems in denser soils. While the dry wts. of tops show no marked differences, the dry wt. of roots in denser soils suggests that, though the root systems are smaller, the soil is more completely used. Exps. with beans in waterlogged cultures failed; with oats similar results were obtained with compressed soils and with waterlogging. The differences in growth between compressed and uncompressed soils can be attributed, therefore, to soil texture, and not to diminished aeration. Presence of root hairs and absence of aerating tissue in the root further suggest that no important part is played by differential aeration consequent on compressing the soil. An attempt to evaluate the relation between rhizome internodes of Mercurialis perennis and soil texture showed that internodes in clay were shorter and thicker than those in chalk. Penetrability in the soil at successive levels in natural plant communities tends to decrease with increase in density, a usual concomitant of increase in depth from the surface. Penetrability also varies directly with moisture content.