II. CHLORIDE UPTAKE AND CONCENTRATION GRADIENTS IN SOIL

The first paper of this series described experiments in which slices of soil were taken at successive distances from a plane of onion seedling roots. Water fluxes across the root plane and water content gradients in the adjacent soil were reported and discussed. This paper reports the uptake of chloride by the seedlings in the same experiments and the measurements of the consequent gradients of chloride in the adjacent soil. A theoretical model of ion transport to roots is described, and used to try to account for the chloride uptake and to explain the chloride gradients observed. Chloride was studied first because the transport of non-adsorbed ions is easier to treat theoretically, and because chloride is similar to nitrate in that neither is usually adsorbed in soil and both are taken up readily by plants. Furthermore, small amounts of chloride can be easily measured by using 36chloride as a tracer. The labelling with 36chloride of the soil blocks used in the root plane experiments was described in Part I of this series (Dunham & Nye 1973). The 36chloride analysis of the onion seedlings and soil blocks is described below. Definitions of the symbols used are given in the Appendix.