Estimation of the Distribution of Plant Roots in Soil

Abstract

THE difficulty of measuring the distribution of living roots in soil is an obstacle to quantitative studies of the performance of root systems in natural conditions. Even the most careful procedures for separating roots from soil are subject to limitations¹. Errors occur especially in estimates of the distribution of the finer lateral roots, which may be of major interest because they can account for a considerable fraction of the total nutrient uptake². It is, moreover, often impossible to distinguish living from dead roots. An alternative approach to this problem is the injection into plant shoots of radioactive tracers which become reasonably uniformly distributed throughout the root systems. Racz et al.³ and Halstead and Rennie⁴ injected phosphorus-32 into plant shoots and estimated the distribution of living roots from measurements of phosphorus-32 in soil samples. The low penetrating power, however, of the beta radiation from phosphorus-32 makes it possible to assay only small soil samples, and the quantities of tracer (for example, 300 µc./plant) which are convenient for measurement⁴ can cause the concentration in roots to be within the range which may induce radiation injury⁵. A tracer which emits gamma radiation of reasonable energy would have obvious advantages and the use of rubidium-86 was therefore investigated.