Soil wet aggregate stability in dryland Pacific Northwest intensified crop rotations

Authors: Williams, John; Reardon, Catherine - Kate; Wuest, Stewart; Long, Daniel

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2017
Publication Date: 3/22/2018
Publication URL: https://handle.nal.usda.gov/10113/6472321
Citation: Williams, J.D., Reardon, C.L., Wuest, S.B., Long, D.S. 2018. Soil wet aggregate stability in dryland Pacific Northwest intensified crop rotations. Soil Science Society of America Journal. 82:455-462. doi:10.2136/sssaj2017.08.0302.
DOI: https://doi.org/10.2136/sssaj2017.08.0302

Interpretive Summary: Improving soil quality the semiarid inland Pacific Northwest is important for the continued productivity of rainfed crops. Good soil quality is important to prevent soil and nutrient loss, and provide a rich environment with adequate water for crop growth. Increasing the number and kind of crops grown in a field provides these conditions in many different regions of the world, as does a reduction in tillage. We looked at two different tillage types and a several crops to see if these ideas would carry over to this region that only receives 10 to 14 inches of rainfall each year by measuring the size of soil aggregates. Of the crops we looked at, only a spring oilseed, Ethiopian mustard, increased the amount of large soil aggregates compared to the traditionally grown winter wheat but the improvement was not consistent and not likely to have an observable impact on the soil. However, the amount of small aggregates that are easily erodible and impede water infiltration were reduced under tillage compared to traditional inversion 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 class size aggregates which are known to impeded water infiltration and increase erodibility.

Technical Abstract: Improving soil aggregation in the semiarid inland Pacific Northwest cropping region can reduce the erodibility and improve water infiltration in the silt loam soils. We compared the individual crop phases of six different crop rotations in plots located in 300 mm mean annual precipitation area of the inland Pacific Northwest 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 under minimum tillage showed significant and positive changes to soil aggregation compared to the winter wheat fallow rotation typical of this region; however the effect was crop specific and of minor, practical consequence. The largest difference soil aggregation was observed in comparisons of spring oilseed Ethiopian mustard (Brassica carinata) and winter wheat (Triticum aestivum) where the oilseed produced aggregates with a larger mean weight diameter and more 1 mm class aggregates than the wheat. This latter effect, however, was the result of two highly aggregated samples collected in separate years. In the wheat fallow rotation, there were significantly more 250 µm sized aggregates and fewer small class aggregates (53 µm and <53 µm) under reduced tillage compared to 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 class size aggregates which are known to impeded water infiltration and increase erodibility.