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Chapter 4. Soil Architecture and Physical Properties Part 2. Factors Promoting Aggregation. Polyvalent (e.g. Al 3+ or Ca 2+ ) rather than monovalent exchangeable cations like Na. Shrink-swell from wet-dry and freeze-thaw cycles
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Chapter 4 Soil Architecture and Physical Properties Part 2
Factors Promoting Aggregation • Polyvalent (e.g. Al3+ or Ca2+) rather than monovalent exchangeable cations like Na. • Shrink-swell from wet-dry and freeze-thaw cycles • Active microbial biomass generating humic substances and “glues”. • Fe and Al-oxides stabilize many aggregates
Figure 4.16. Aggregated soil is flocculated by Ca while the dispersed soil below is dominated by Na, which has a large hydrated radius and does not effectively flocculate soils.
Effects of lime and appropriate tillage in a garden soil. Lime + tillage when soil was moist, not wet.
Tillage and Tilth • Tilth is an old generic term used to describe the physical condition of the soil in relation to plant growth • “Good tilth” means the soil is loose, non-plastic, and not dense. • We use consistence (Table 4.5) to estimate and “semi-quantify” tilth.
Tillage has mixed effects on aggregation. In general, it decreases macropores. However, if we add lime plus organic matter as we till, the overall effects can be beneficial.
Engineering Properties • Cohesion and strength: Can a soil hold a give slope? • Compression and compactibility: Will a soil material readily compact to serve as a roadbed or foundation? • Expansion or shrink-swell potential: Will a soil change in volume with normal wetting and drying cycles to a point of foundation damage?
Soil materials are routinely compacted into subgrades for roads and parking lots. This must be done at appropriate moisture contents.
Figure 4.53: Atterburg Limits
Landslide at I-81 at Hollins due to shrink-swell clays with high water holding and low strength.
Overview looking west across Wise County, VA, in 1982. All open light brown and white areas in middle ground are coal surface mines. E. Kentucky Powell River Project
Rocky acidic mine soil in 1980 formed in oxidized mixed overburden
Very shallow (<50 cm) mine soil formed in mixed oxidized and acidic (pH 5.0) mine spoil over intact siltstone bedrock. Approximately 1/3 of the 1980 soils were shallow.
Twenty year-old forested mine soil described in 1980 with well developed litter layer and weakly expressed, thin, Bw horizon.In many instances, these layers were not thick or well-expressed enough to be “cambic”.
Older mine soil (20 yrs old) described in 2002 that had been re-graded in 1989 and capped with a lift of local “topsoil”. This soil was very acidic (pH 4.0) at depth, but had been surface limed to pH 6.0 for pasture production. A Cd
Within months of being successfully revegetated, these mine soils (note two words used here) develop readily discernible A horizons characterized by browner colors and aggregation. As discussed later, these horizons are also significantly different in physical and chemical properties.
Weak to moderate medium and coarse angular blocks broken out of Bw horizon in 12 year-old mine soil formed from siltstones.
Sandy tailings and dispersed humates (Bhs horizons) being returned to mineral sands (rutile and ilmenite) mine in central Florida.
Three year-old mine soil with distinct precipitated/flocculate humate plus Al band (horizon?) in reclaimed mining pit. Of 15 pits observed, 13 had a feature like this within 1 m of the surface that appeared to correlate with the water table high. Daniels et al. (1992; ASMR Proceedings)
Humate layer and faint lamellae like deeper banding in 10 year-old mine soil in central Florida.
Twenty year-old mine soil in excessively well drained position on reclaimed ridge. Texture here is too coarse to call these cambic horizons, but I’m certain that they are pedogenic. They are higher in C and Al than deeper sands; but usually not in Fe.
Oxidized, pH 5.5 overburden over reduced carbonate (2%) containing overburden at depth.
Throughout the 1980’s and 1990’s, reduced, carbonate containing overburden such as that shown here was preferentially selected and placed as topsoil substitute materials due to high pH and extractable cations. This practice led to adverse properties for native hardwoods and introduced conifers, but generated great mine soils for forages.
Mixed Topsoil + Weathered Overburden (A+B+C+R) Rocky (15% fines), High pH (7.5) Sandstone Spoil
Just when you think you’ve seen it all, you end up at a site like this one, Tavistock Farms in Leesburg. Fill area comprised of Triassic shrink-swell clay materials (fat clays to the geotechs) treated with CaO for “stability”.
At this site, the developer’s plan calls for the upper 0.5 m plus the soil excavated from footer excavations to be used on site for turf and some very high value landscape woody material plantings (e.g. large hardwood trees and tranplanted shrubs.
Here, the soil pH varies from 6.9 to > 9.5 in areas where the free CaO is unreacted or only partially carbonated. The challenge here will be to figure out how much S needs to be added and what physical processing will be necessary to get the cemented soil broken down to a fine enough size to hold water and support plant growth.
In summary, the soil will vary significantly in color, texture, structure and aggregation, and density by horizon. All these factors will affect plant growth, water movement and soil engineering properties!
