Most studies of plant roots growing in soil involve destructive sampling or involve estimates of root growth only at a soil‐viewing surface interface where growth conditions are different from those in bulk soil. This research tested the feasibility of using preferential attenuation of thermal neutrons by roots to overcome these two disadvantages of current methods. Radiographs were obtained of soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) roots growing through bulk samples of unsaturated soil. Plants were grown at 22 C in loamy sand soil wetted to 9% volumetric water content. The soil was contained in either 2.5‐ or 5.0‐cm thick aluminum boxes. At irregular intervals, the sample containers were transferred into a 6° diverging 0.04eV (thermal) energy neutron beam obtained from the Ames Laboratory Research Reactor, U. S. Energy Research and Development Administration, Iowa State University, Ames, Iowa. The samples were exposed to the neutron beam for 8 to 10 min. This exposure allowed about 5 × 109 neutrons/cm2 to strike an indium collector plate attached to the rear of the container. After the collector plate was removed from the neutron beam, photographic film was attached to the collector plate for 1 ½ hours; then the film was processed. The preferential neutron scattering by the roots allowed elongation rates of soybean radicles or seminal roots of corn to be determined easily through either 2.5‐ or 5‐cm thick soil samples. Small lateral roots of either species were less clearly distinguishable and resolution must be improved before elongation rates can be measured for laterals with a diameter smaller than 0.33 mm. Plants were not harmed visibly by the thermal neutron fluxes used in these experiments.

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