PREEXISTING CHANNELS AND SOYBEAN ROOTING PATTERNS

Abstract

We used a trench-profile method to study soybean (Glycine max (L.) Merr.) root distribution and vertical channel patterns in Flanagan (Aquic Argiudoll) silt loam. Root distribution was quantified as the number of first-order lateral roots, the depths at which tap and first-order lateral roots encountered burrows, and the maximum rooting depth where white, young roots with growing tips occurred. In this work, we called all round vertical channels in the soil burrows, not attempting to determine whether they had been made by worms, insects, or roots. Numbers per unit area and diameters of burrows were measured at 20-cm-depth increments from 10 to 130 cm. The first-order lateral roots occurred in the top 30 cm of the soil, and most roots encountered burrows at 30 to 45 cm below the surface. If a taproot failed to encounter a burrow at this level, the root tip died. Once in a burrow, the root grew downward within the channel to the point at which the burrow ended. The number of burrows with diameters greater than 1.5 mm increased with increasing depth from the surface to about 40 cm, then decreased, with very few burrows below a depth of 130 cm. Under sod, the same trend in distribution was observed, but with more burrows below 50 cm than were seen under soybeans. The number of burrows at a depth of 30 cm ranged from 500 to 1400 m−2. The maximum total cross-sectional area of burrows at the 30-cm depth was about 0.007 m2 m−2 of soil surface. The numbers of burrows, with the tendency of roots to grow in these burrows, suggest that burrows have a profound effect upon the shape of the root system, as well as upon rapid gas and water movement.