Near-surface soil aggregate structural properties, such as aggregate size distribution, stability, strength and wettability, determine the extent to which a soil will erode under water or wind erosive forces.
Knowledge of aggregate structural properties is especially important in semiarid regions, such as the Great Plains, where low precipitation, high evaporation and variable biomass production — coupled with intensive tillage and fallow — can accelerate soil’s susceptibility to wind and water erosion.
By leaving crop residues on the soil surface and minimizing soil disturbance, no-till practices often increase soil-organic carbon concentration. This increase in soil-organic carbon may lead to improved stability of near-surface aggregates over plowed systems because carbon-rich materials provide organic binding agents to soil.
A comprehensive assessment of soil structural properties and their relationships with soil-organic carbon across a range of soils under different scenarios of tillage and crop management have provided a better understanding of no-till effects.
In the July–August 2009 issue of Soil Science Society of America Journal, Humberto Blanco and colleagues document changes in aggregate resistance to raindrops, dry aggregate wettability and dry aggregate stability, as well as their relationships with changes in soil-organic carbon content in a regional study across four soils in the central Great Plains.
Long-term tillage (more than 19 years) experiments, including moldboard plow, conventional till, reduced till and no-till were studied at Hays and Tribune, Kan., Akron, Colo., and Sidney, Neb. The crop rotations included winter wheat–grain sorghum–fallow at Hays and Tribune and winter wheat–fallow at Akron and Sidney.
The results of this study revealed that no-till farming increased soil aggregate resistance against raindrops and water repellency over plowed systems, particularly at the soil surface (0- to 2.5-cm depth).
The kinetic energy of raindrops required to disintegrate 4.75- to 8-mm aggregates from no-till soils equilibrated at –0.03, and –155 MPa matric potential was between two and seven times greater than that required for plowed soils.
The water-drop penetration time in aggregates from no-till soils was between four and seven times greater compared with that in plowed soils. Reduced till was less beneficial than no-till, but was more beneficial than conventionally tilled soils.
A no-till-induced increase in soil-organic carbon concentration partly explained the improvement in aggregate properties. The soil-organic carbon concentration was greater in no-till than in conventionally tilled soils near the surface.
Kinetic energy of raindrops required for aggregate disintegration increased positively (r = 0.60), while aggregate wettability (r = –0.53) decreased with the increase in soil-organic carbon concentration.
Soils rich in organic carbon most likely provided organic binding agents to stabilize aggregates. Soil-organic carbon compounds also imparted slight hydrophobic properties, reducing aggregate slaking and the amount of soil that will be eroded. Aggregate wettability was positively correlated (r = 0.70) with wet aggregate stability.
This regional study showed, however, that no-till management may not improve dry-aggregate size distribution and stability, which are sensitive parameters of wind erosion. Aggregates in no-till soils were equally strong or slightly weaker when dry compared with those in plowed soils.
“Little or no improvement in dry-aggregate stability indicates that crop residues must be maintained on the surface of no-till soils to reduce wind erosion,” Blanco says. “No-till soils with limited residue cover may be even more vulnerable to wind erosion than plowed soils, for which the transient roughness created by tillage may reduce wind erosion.”
Under typical conditions, wind erosion rates are, however, expected to be lower in no-till soils with high levels of residue on the surface. According to Blanco, gains in soil-organic carbon concentration under no-till are responsible for improvement in wet-aggregate stability and water repellency.
Blanco and his coauthors concluded that aggregates from no-till soils were more water-stable under rain, less wettable and had greater organic carbon concentration than soils under conventional tillage.
Blanco says this research is continuing and expanding across the central Great Plains to comprehensively evaluate tillage and cropping-system impacts on soil structure, hydrology, compaction and their relationships with tillage-induced changes in soil-organic carbon.