THE COLLOIDAL CLAY FRACTION OF SOIL AS A CULTURAL MEDIUM

Abstract

A BETTER understanding of the properties and behavior of colloidal clay makes it now possible to use this fraction of the soil in plant nutrition studies as a means of obtaining a closer approach to complete chemical control of the soil factor in plant growth. Though the soil may be more than a mixture of mineral particles of sand, silt, and clay sizes as a consequence of their possible chemical linkage (Sideri, 1937), yet the sand and silt separates are too large to offer much active surface. The surface activity for the adsorption of mineral nutrients and their delivery to the plant by ion exchange (Jenny and Cowan, 1933; McCalla, 1937) must then rest with the colloidal clay fraction. Such activity in the clay fraction of the Putnam silt loam, for example, suggests a supply of nutrients one hundred times as large as that in the leachings from the lysimeters at Cornell (Lyon and Bizzell, 1921). Recent experimental use of colloidal clav as a growth medium for soil microorganisms and for leguminous plants suggests a wide range in its adaptability to well controlled nutritional experiments (Albrecht, 1932, 1933); Albrecht and Jenny, 1931; Albrecht and McCalla, 1937), aiming at a nearer duplication of soil conditions than has previously been possible. The low concentration of soil solutions and irregularities in plant growth in aqueous media have suggested that the soil provides the plant from sources ot,her than simple solution conditions. Hoagland and M\artin (1923) suiggested that the soil imiedlium was more productive than aqueous solutions when they said, In several experiments we have found the average weight of the kernels produced in solution cultures about 80 per cent of the weight of the kernels produced in a good soil. Jones and Pember (1925) observed less plump kernels of grain for the solution cultures than for the soil, and also a wider ratio of straw to grain. Some workers (McCall, 1916; Breazeale, 1906) ventured the idea that the addition of solid material to a solution culture may result in the adsorption of toxic substances by the solid and thus increase plant growth. Jennings (1919) found that the total plant growth was increased by the addition of colloidal silica to the cultural solution. The effect was attributed to the silica taken by the plant. ITaley's (1923) data, showing increased plant growth from the addition of 75 grams of finely ground orthoclase to the solution, could not be attributed to this as a supply of potassium since the plants in the aqueous solution alone contained the more potassium. Maclntire, Shaw, and Young (1925) found silica lessening the injurious effects by magnesium on plant growth with increasing benefits as more silica was used. Shive (1919, 1920) using fine sand, concluded that this was without effect on the nutrient solution