AN IMPORTANT EFFECT OF SOIL COLLOIDS ON PLANT GROWTH

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Abstract
The first of 2 expts. concerning living root toxins showed that solns. in which roots have grown are deleterious to other plants; the 2d, that increasing the number of plants grown in a soln. reduces the yield proportionally. In both expts. control cultures made under a different level of nutrients showed that these effects are not attributable to an exhaustion of available nutrients. Two factorial expts., one with wheat and one with corn, also were made in pots with mixtures of fine gravel and fine soil in different proportions and in combination with different levels of nutrients and of soil moisture. They showed that plant yields increase considerably when the proportion of fine materials increases, the increment being greater with high levels of nutrients or moisture than with low levels. Consequently, the increased yield cannot be attributed to the nutrients which the fine materials contain or to their greater water-holding capacity, but must be due to another factor. The factor concerned, in the author''s opinion, is space. The space factor presents the peculiarity that its influence increases as growth advances, whereas for almost all soil factors the contrary is true. In fact, the effect of increasing the proportion of fine soil, though slight at the beginning, became more and more pronounced as growth advanced. In the wheat expt., the density at heading was much less affected than the grain yield. The effect was the same as that obtained in another expt. by increasing the depth of soil. The explanation is that soil colloids absorb the living root toxins and aid in their oxidation. The effect is analogous to that of charcoal. which is also known to increase the yield of water or soil cultures. Consequently, in addition to their influence on the chemical and physical properties of the soil, soil colloids have another effect on plant growth; they increase the available space. This effect is considerable. With the highest levels of N, P. and water, the yield increased in the corn expt. from 3.78 to 27.26 g. per pot when the proportion of fine soil increased from 10 to 40%.