TITLE 5 SOIL EVALUATOR CERTIFICATION TRAINING SOIL STRUCTURE Prepared for: Commonwealth of Massachusetts Department of Environmental Protection By L.A. Spokas, PhD, Stockbridge School of Agriculture University of Massachusetts Amherst Presented by: New England Interstate Water Pollution Control Commission STRUCTURE OF MINERAL SOILS Soil structure refers to the aggregation of individual soil particles into larger units with planes of weakness between them 1 Particles are held together (by organic matter, iron, and clay) to form distinct units called peds. Peds are individual, natural soil aggregates, which have units of mm to m in size. An example of a ped is a granule, a prism, or a block. Peds are NOT clods. 1. Buol, Southard, Graham, and McDaniel, Soil Genesis and Classification, 5 th Ed. 2003 http://homeguides.sfgate.com/break-up-plowed-soil Clods are chunks of soil formed artificially, often compressed due to plowing or other disturbance, or excavation.
STRUCTURE RANGES FROM STRUCTURE LESS TO HIGHLY DEVELOPED. Structureless Soils Single Grain Massive Soil Structures Granular Blocky Angular Subangular Prismatic Columnar Platy Soil structure forms by wetting and drying cycles. During periods of desiccation soil compression occurs due to capillary tension, causing cracks to form. When this soil becomes wet again, it swells. The protruding points on adjacent ped faces touch, causing pressure or shearing forces at these points. The shear forces cause coarse particles (sand) to move away from the pressure, leaving the platy clay particles behind. The clay particles are forced into parallel orientation along the ped face by the pressure of swelling. Clay Skins (clay coatings along a natural plane of weakness)
This clay skin on the ped face causes the soil to crack in the same place when it next dries. The formation of structure requires TIME and the presence of shrinking and swelling clays New England Soils are very young, and There is very little clay in our soils, and no shrink swell clays GRANULAR STRUCTURE 2 cm
talk.newagtalk.com Granular Structure Notice the small peds, held together by organic matter. Granular structure is most prevalent in O, A and Ap horizons a granule BLOCKY STRUCTURE a block Horizontal and Vertical extents of structure are equal.
An example of good blocky structure academic.emporia.edu Prismatic Structure Vertical extent of structure is greater than the horizontal extent. a prism Prismatic Structure colbydigssoil.com
Columnar Structure www.geog.ucsb.edu passel.unl.edu Structure should be easily visible on the pit wall. PLATY STRUCTURE Horizontal extent of the structure is greater than the vertical extent. a plate 2 cm PLATY STRUCTURE www.omafra.gov.on.ca
Typical platy structure on a dense till C horizon STRUCTURELESS SOILS Single Grain Massive
STRUCTURE SIZE Platy Shape of Structure Prismatic/ Columnar Blocky Granular Size ------------------ size (mm) -------------------- Very fine <1 <10 <5 <1 Fine 1-2 10-20 5-10 1-2 Medium 2-5 20-50 10-20 2-5 Coarse 5-10 50-100 20-50 5-10 Very Coarse >10 >100 >50 >10 size refers to the smallest dimension of plates, prisms, and columns. Substitute thin for fine and thick for coarse for platy sizes. STRUCTURE GRADE Grade refers to the strength of firmness of the peds. Weak Peds are observable in place, but cannot be removed. Moderate Peds can be removed from the profile for hand examination. Strong Removed peds are rigid and durable in hand. Most structure in MA Soils is weak! STRUCTURE DESIGNATION GRADE, SIZE, SHAPE moderate, thick, platy structure strong, medium, prismatic structure
CONSISTENCE A MEASURE OF THE SOIL MATERIAL S RESPONSE TO PRESSURE Class Loose Very friable Friable Firm Very firm Extremely firm Slightly rigid Rigid Very rigid Test Clod cannot be obtained (e.g. sands) Clod fails under very slight pressure between thumb and forefinger Clod fails under slight pressure between thumb and forefinger Clod fails under moderate pressure between thumb and forefinger Clod fails under strong pressure between thumb and forefinger Two hands are required to rupture clod Ruptures when placed underfoot with full body weight Ruptures when struck with 1 kg hammer Cannot be ruptured with hammer Single Grain, Loose (Structure, consistence) STRUCTURE & CONSISTENCE Weak fine granular, friable (grade, size, shape; consistence) Massive, friable Note: Structureless soils (single grain and massive) have no size or grade.