Effect of Roots and Tillage on Soil Erosion on a Weathered Hawaiian Soil With Low Erodibility

Sustainable farming of marginal lands in the tropics is partially constrained by destructive effects of soil erosion. One farming system proposed for controlling erosion on steep lands is alleycropping, in which crops are grown between parallel hedgerows of trees or shrubs. This study was originally designed to evaluate the mechanisms by which alleycropping could reduce erosion on a steep slope. The site was located on Kauai, Hawaii, on a steep slope (40%) of a soil classified as the Halii series, an Anionic Acrudox. A preexisting thick vegetation of ferns and shrubs was cleared by bulldozer and subsequently disked twice and hand-raked. Twelve plots, 3 replications of 4 treatments (bare, monocrop, and 2 variations of an alleycrop) were installed on 16 by 4 m plots surrounded by steel sheet metal. Runoff and sediment was collected and measured from each plot for 1 year. However, soil loss and runoff were not related to treatments, occurring only on 2 plots (Plots #11 and #12) despite several heavy storms. Total runoff and soil loss were 1.1% of rainfall and 0.7 T/ha and 1.9% and 27 T/ha for Plot 11 and 12, respectively. Fine root content, tillage and exposure, and mineralogy of the soil were investigated to explain these results. Disking and raking had created a well aggregated highly porous, and friable soil structure with high infiltration rates. Exposure and drying of the soil apparently allowed its structure to remain very stable and resistant to structural and aggregate disintegration under the force of raindrops. The two plots with runoff were apparently disked to a shallower depth, thus limiting these effects of tillage and exposure on the soil structure. Very high fine root content in Plot 11 bound soil aggregates together and reduced soil loss in this plot despite significant runoff. A sharp increase of halloysite content correlated with an increase of runoff and soil loss, but was believed to have affected chiefly only runoff. This highly weathered tropical soil was resistant to erosion, but the demonstrated high spatial variability in prior vegetation, clearing, tillage, and mineralogy, must be taken into account in designing future research in tropical farming systems