Estimation of the distribution of living roots of plants under field conditions

Summary A method is described for estimating the relative distribution of living roots of graminaceous plants growing under normal field conditions and which does not involve separation of roots from soil.It has been shown that 24 hours after rubidium-86 has been injected into the base of shoots its concentration is sufficiently uniform throughout the root system for the volume of roots in different zones of soil to be inferred from their content of the tracer. During this period outward diffusion from the root system is negligible and no rubidium-86 enters dead roots.The energy of the gamma radiation from rubidium-86 enables it to be measured accurately in soil samples of appreciable size (e.g. 3kg). Consequently it is possible, using suitable analytical equipment, to obtain representative information on root distribution at different depths in soil by combining the appropriate sections of numerous cores which have been withdrawn from the soil in the neighbourhood of injected plants. The advantages of this procedure lie in the fact that losses of root in the separation of the soil are avoided and that living roots of the species under investigation are alone measured, the time taken in this operation compares favourably with that occupied in the manual separation of roots from soil provided suitable analytical equipment is available.