The Effects of Soil Bulk Density on the Morphology and Anchorage Mechanics of the Root Systems of Sunflower and Maize

Abstract

The effects of soil bulk density and hence strength on two contrasting species of herbaceous annuals, the dicot sunflower (Helianthus annuusL.) and the monocot maize (Zea maysL.), were investigated by comparing the morphology and mechanics of field-grown plants in soil with a low and high bulk density. Soil with a low bulk density had a significantly lower penetration resistance (118±4.4 kPa) than the high bulk density soil (325±12.2 kPa;P<0.0001). Soil strength affected shoot and root systems of both species but had no significant effect on shoot height. In both species roots were thicker closer to the stem base in strong soil compared to those in weaker soil. Sunflower tap-roots growing in strong soil tapered more rapidly than those in weak soil. Only in maize, however, were roots growing in weak soil stiffer than those in strong soil. Despite only small absolute differences in the penetration resistance of the soil both species growing in strong soil had greater anchorage strength than those in weak soil. As a consequence more plants in weak soil lodged compared with those growing in strong soil. This study shows that plants can, to a small extent, respond to changes in soil strength, but that changes do not appear to compensate fully for alterations in soil conditions. Furthermore it may be possible, by manipulating soil strength, to control lodging.