By leaving crop residues on the soil surface and minimizing soil disturbance, no-till practices often increase soil organic carbon (C) concentration. This increase in soil organic C may lead to improved stability of near-surface aggregates over plowed systems because C-rich materials provide organic binding agents to soil.
A comprehensive assessment of soil structural properties and their relationships with soil organic C across a range of soils under different scenarios of tillage and crop management would provide 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 C content in a regional study across four soils in the central Great Plains. Long-term (>19 years) tillage experiments including moldboard plow, conventional till, reduced till, and no-till were studied at Hays and Tribune, KS; Akron, CO; and Sidney, NE. 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 C concentration partly explained the improvement in aggregate properties. The soil organic C 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 C concentration. Soils rich in organic C most likely provided organic binding agents to stabilize aggregates. Soil organic C 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 C 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 C 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 C.