Root-soil contact of maize, as measured by a thin-section technique

Abstract

In models of oxygen, water and nutrient uptake by plant roots, the degree of root-soil contact is an important parameter. An observation technique is required to evaluate to what extent root-soil contact depends on plant species, soil texture and structure. Thin sections for studying soil structure may be used for this purpose, provided that roots do not shrink during section preparation, and that all root cross sections are recognized. Maize was grown in pots with soil aggregates obtained by sieving and compacting to three bulk densities. Thin sections were made by freeze-drying samples before impregnating the soil with resin. Two checks were made on the validity of the method. Firstly, visual appearance of roots with intact epidermis, cortex and other tissues did not show signs of shrinkage. Secondly, the agreement was checked between root lengths obtained by washing duplicate soil samples and the number of root cross sections counted on horizonal and vertical thin sections. For the latter, the angle at which roots intersected the thin-section plane was determined from the shape of the cross sections. The frequency distribution of calculated angles was in agreement with the frequency distribution expected for a randomly oriented set of cylinders when an error term was included in the simulated measurements. Some results are presented for a field test of the thin-section method with barley on a calcareous marine sandy loam. Root hairs, apparently undamaged by sample preparation, are important for bridging the gap between roots and soil in this situation. According to the experience presented, the thin-section technique is suitable to derive the degree of root-soil contact, as influenced by species, soil texture and structure, in samples obtained from pot or field experiments.