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Soil Wet Aggregate Stability in Dryland Pacific Northwest Intensified Crop Rotations

John D. Williams, Catherine L. Reardon, Stewart B. Wuest, Dan S. Long
Soil Science Society of America journal 2018 v.82 no.2 pp. 455-462
Brassica carinata, Triticum aestivum, aggregate stability, arid lands, atmospheric precipitation, conventional tillage, crop rotation, crops, erodibility, fallow, minimum tillage, oilseeds, reduced tillage, silt loam soils, soil aggregation, soil structure, spring, water solubility, water stable soil aggregates, winter wheat, Pacific States
Core Ideas Soil attributes can be improved by intensifying crop rotations in combination with reduced tillage. There was weak evidence that intensifying wheat systems with oilseed crops improves aggregate stability. Reduced tillage produces fewer small class size aggregates that impede infiltration. Improving soil aggregation in the semiarid inland Pacific Northwest (PNW) cropping region can improve water infiltration of its erodible silt loam soils. We compared the individual crop phases of six different crop rotations in plots located in a 200‐ to 300‐mm mean annual precipitation area of the PNW to determine if any crop, sequence of crops, tillage, or combination thereof would result in differences in mean weight diameter or size class distribution (1 mm, 250 μm, 125 μm, 53 μm, <53 μm) of water soluble aggregates. Diversification and intensification of crop rotations under minimum tillage showed statistically significant and positive changes to soil aggregation compared to the winter wheat (Triticum aestivum L.)–fallow rotation typical of this region; however, the effect was crop‐specific and of minor practical consequence. Ethiopian mustard (Brassica carinata) produced soil aggregates with a larger mean weight diameter and more 1‐mm class aggregates than winter wheat. The latter effect was the result of two highly aggregated samples collected in separate years. In the wheat‐fallow rotation, there were significantly (P < 0.05) more 250‐μm sized aggregates and fewer small class aggregates (53 μm and <53 μm) under minimum tillage than under conventional tillage. While crop diversification may not substantially improve soil structure as measured by water‐stable aggregates, the reduction of tillage can reduce the amount of small size aggregates which are known to impede water infiltration and increase erodibility.