Grass barriers in cassava hillside cultivation: Rooting patterns and root growth dynamics

Abstract

Living grass barriers can effectively reduce soil erosion on hillsides under cassava [ Manihot esculenta Crantz]. Competition below ground in barrier-crop systems is, however, poorly understood. The objectives of this study were, therefore, (i) to describe rooting patterns and spatial root distribution of cassava and of grasses commonly used as live barriers in soil conservation, and (ii) to determine root growth dynamics over time, using direct observation and quantitative methods. Field research was carried out near Santander de Quilichao, Colombia, on a Typic Dystropept soil where average annual precipitation is 1799 mm. Single 20-m rows of vetiver grass [ Vetiveria zizanioides (L.) Nash], lemon grass [ Cymbopogon citratus (DC. ex Nees) Stapf] and guatemala grass [ Tripsacum andersonii J.R. Gray] were grown on a 13% slope. Cassava was planted in rows on either side of the grass barriers. Observation pits were dug in the cassava-grass plots and perspex sheets fitted closely to exposed walls. These pits allowed periodic tracing of roots. Root length was recorded in metres either per total of an exposed surface measuring 0.8 (depth) and 1.2 m (width) or measuring 0.2 (depth) × 1.2 m (width). Because of their vertical growth and their weak tendency to branch, the roots of vetiver grass rarely mixed with cassava roots, signifying a largely separate soil exploration by the two plants. In contrast, the profuse vertical and horizontal spread of guatemala grass and cassava roots indicated a joint exploration of soil volume. The cassava-lemon grass system revealed an intermediate pattern. The longest roots per exposed area of 0.2 × 1.2 m were observed for cassava growing on both sides of the vetiver grass barrier, and increasing from 6.0 m in the 0–20 cm profile segment to 7.3 m in the 20–40 cm segment. In contrast, roots of cassava with guatemala grass shortened from 5.4 m in the uppermost profile segment to 3.6 m in the 20–40 cm segment. Vetiver grass exhibited the slowest and steadiest root growth and guatemala grass the fastest. Final total root lengths obtained from the entire exposed soil profile were 7 m for vetiver grass, 16 m for guatemala grass and 17 m for lemon grass. Cassava root growth was very slow initially, but increased 25–28 weeks after grass planting when grass root growth temporarily slowed down as a result of cutback. Final total cassava root lengths obtained from the exposed soil profile were 17, 10 and 10 m in association with the three grasses, respectively. The differences in rooting pattern, root growth dynamics and final root lengths suggest that different degrees of root interaction may have occurred, with possible consequences for soil exploration, and nutrient and water uptake.