There still remains much to be clarified in the basic process of nutrient uptake and in accounting for root behavior in the nutritional environment provided by soil. There seems to be some doubt among some plant physiologists as to whether the cation exchange capacity of roots is of any consequence, and whether exchange to such sites constitutes any essential part of the ion absorption process. Because they lack specificity, the cation retaining sites of plant roots are not believed to be those involved in active uptake; moreover, some are external and some are internal. Most experimentation on nutrient uptake is carried out using very young seedlings immersed in simple salt solutions containing a large excess of the ion under study. By diffusion the free space rapidly comes into equilibrium, and by exchange the cation retention sites become extensively filled with the cations supplied. The situation in soil may be very different because there the external sites at least will directly reflect the cation array on the soil colloids, through equilibration this will result in a modification of the composition of the soil solution in immediate contact with the external surface of the roots and therefore by diffusion that of the solution in the free space. Ultimately it should be found possible to reconcile and incorporate in a unified scheme the soil solution theory of plant nutrition in soils and the contact exchange theory. As with the soil population where that in the immediate vicinity of the roots is recognizably different from that at a distance, so it is likely that the soil solution in the immediate vicinity of roots may be substantially modified by the presence of ions on the surfaces of the root and the micro-organisms thereon. The equilibrium reached is likely to be a complex one between the surfaces of clay and organic colloids on the one hand, and actively metabolizing root tissues and micro-organisms on the other.