EFFECTS OF STRAW MULCH ON RUNOFF AND EROSION FROM SMALL AGRICULTURAL PLOTS IN NORTHEASTERN OREGON

An experiment was conducted to evaluate the effects of four rates of s traw mulch on runoff, infiltration, and erosion at a site in northeast ern Oregon. Straw mulch densities of 0, 25, 50, 75, and 100 percent co ver were tested using simulated rainfall applied to 3.33 m2 plots loca ted on a 16 percent north facing slope. Soil type is a Thatuna silt lo am (fine-silty, mixed mesic Xeric Argialboll). The zero cover plots we re replicated four times. The mean rainfall input rate of 28 mm/h did not vary significantly over the four replications. Median drop diamete r of 1.3 mm at the center of the spray pattern is a nominal design cha racteristic of the rainfall simulator and is a representative size for natural rainfall in this area. Percent straw cover and normalized soi l loss were highly correlated (r = 0.99), while no significant correla tion between runoff volume or infiltration volume and soil loss was ob served. This can be explained by the presence of a slowly permeable la yer below the tillage depth and by the high organic matter content of this soil. The data suggest that smaller amounts of mulch (<25%) are o f little value in erosion control and that a threshold value below whi ch surface cover is ineffective may exist. This is a significant depar ture from the exponential relationship used to determine straw mulch e ffects on erosion. The data from this and earlier, similar experiments exhibit a large variability, especially in the erosion component, and emphasize a need for more experimental data to define the cover-soil loss relationship. There is almost no experimental field data for mulc h covers less than about 30 percent, so further research is urgently n eeded to define the relationship for these lower cover conditions.