Soil Evaluator Day 2, Presentation 4-3/27/2018

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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 SOIL STRUCTURE Io9.

1 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 SOIL STRUCTURE Io9.com 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 Soil Structure, Page 1

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.

Structureless Soils Single Grain Massive Soil Structures Granular Blocky Angular Subangular Prismatic Columnar Platy Soil structure forms by wetting and drying cycles.

2 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. Soil Structure, Page 2

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

3 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 Soil Structure, Page 3

4 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 Soil Structure, Page 4

5 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 Soil Structure, Page 5

6 Prismatic Structure colbydigssoil.com Columnar Structure 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 Soil Structure, Page 6

7 PLATY STRUCTURE Typical platy structure on a dense till C horizon Soil Structure, Page 7

STRUCTURELESS SOILS Single Grain Massive STRUCTURE GRADE

Weak Peds are observable in place, but cannot be removed.

8 STRUCTURELESS SOILS Single Grain Massive 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 Soil Structure, Page 8

9 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. Soil Structure, Page 9

STRUCTURE OF MINERAL SOILS

STRUCTURE OF MINERAL SOILS 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