AN EVALUATION OF THE MINIRHIZOTRON TECHNIQUE FOR ESTIMATING ROOT DISTRIBUTION IN POTATOES

The minirhizotron technique was evaluated for estimating root length density in potatoes (Solanum tuberosum 'Record') by comparing observations in angled 45-degrees glass tubes with corresponding root length densities obtained by (a) destructive core sampling, and from (b) counts of root tips on the soil face of excavated trenches. Measurements were made in a field experiment in Bedfordshire, UK, in 1985, with shallow and deep cultivation as variables, and in a glasshouse trial. Only at depths below 0.3 m did root lengths observed with minirhizotrons reflect at all closely those estimated from core sampling and even then there was a tendency to overestimate root density. In the surface cultivated layers, where 80-90% of the total root length was present, results from minirhizotrons were unreliable, probably because of poor soil-tube contact and, in soils which shrink on drying, preferential root growth at the interface between the soil and the glass. Minirhizotrons can provide a realistic estimate of the rate of root growth of potatoes with depth over time when compared with maximum depths of water extraction, but appear to be unreliable for providing a quantitative measure of total root length density.