Soil Quality for Environmental Health

Slaking

What it is: Slaking is the breakdown of large, air-dry soil aggregates (>2-5 mm) into smaller sized microaggregates (<0.25 mm) when they are suddenly immersed in water. Slaking occurs when aggregates are not strong enough to withstand internal stresses caused by rapid water uptake. Internal stresses result from differential swelling of clay particles, trapped and escaping air in soil pores, rapid release of heat during wetting, and the mechanical action of moving water.

In contrast to slaking, tests for aggregate stability measure how well soil withstands external destructive forces, such as the splashing impact of raindrops. Both poor aggregate stability and slaking result in detached soil particles that settle into pores, and cause surface sealing, reduced infiltration and plant available water, and increased runoff and erosion.

Why it is important: Slaking indicates the stability of soil aggregates, resistance to erosion and suggests how well soil can maintain its structure to provide water and air for plants and soil biota when it is rapidly wetted. Limited slaking suggests that organic matter is present in soil to help bind soil particles and microaggregates into larger, stable aggregates.

USDA NRCS Conservation Agronomist, Ray Archuleta, uses soil slaking and miniature rainfall simulator demonstrations to show how management affects soil quality on a farm located near Reidsville, North Carolina in the Piedmont MLRA. (video provided by the University of South Carolina - Earth Sciences and Resources Institute):

Specific problems that might be caused by poor function: Slaked soil particles block soil pores, form a soil crust, reduce infiltration and water movement through soil, and increase runoff and erosion. Small aggregates produced by slaking settle together resulting in smaller pore spaces than where present with larger aggregates. Pore volume may be reduced and the ability of plants to use water stored in pore spaces may be altered.

Conservation practices that lead to slaking include:

Conventional tillage methods that disturb soil and accelerate organic matter decomposition,
Burning, harvesting or otherwise removing crop residues, and
Using pesticides harmful to soil organisms that cycle organic matter and promote aggregation.

What you can do: Conservation tillage systems, such as no-till, reduce slaking by reducing soil disturbing activities that break aggregates apart and accelerate decomposition of organic matter. No-till and residue management lead to increased soil organic matter and improved aggregate stability and soil structure, particularly when cover crops or sod-based rotations provide an additional source of residue.

Conservation practices that minimize slaking include:

Conservation Crop Rotation
Cover Crops
Prescribed Grazing
Residue and Tillage management

For more information go to Soil Management Practices.

Measuring slaking:

The Slake or Soil Stability Test is described in the Soil Quality Test Kit Guide, Section I, Chapter 9, pp. 20 - 21. See Section II, Chapter 8, p. 72 for interpretation of results.

Herrick J.E., W.G. Whitford, A.G. de Soyza, J.W. Van Zee, K.M. Havstad, C.A. Seybold, and M. Walton. 2001. Field soil aggregate stability kit for soil quality and rangeland health evaluations. Catena 44:27-35.