Lecture 4B. Soil Structure.pdf

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil Physical Properties SOIL STRUCTURE 2 SOIL PHYSICAL PROPERTIES Soil Structure Structure refers to the way soil particles clump together into large units The arrangement of primary particles into secondary particles, units or peds Descr...

AGRI 51 PRINCIPLES OF SOIL SCIENCE  Soil Physical Properties SOIL STRUCTURE 2 SOIL PHYSICAL PROPERTIES Soil Structure Structure refers to the way soil particles clump together into large units The arrangement of primary particles into secondary particles, units or peds Describes the clustering of soil particles into aggregates of certain size, shape and stability. Particle arrangement 3 SOIL PHYSICAL PROPERTIES Soil Structure Peds - Aggregates that occur naturally in the soil Clods - Clumps of soil caused by tillage 4 SOIL PHYSICAL PROPERTIES Soil Structure 5 SOIL PHYSICAL PROPERTIES Soil Structure Soil particles are aggregated by binding agents such as organic matter, lime and other binding agents like Fe/Al oxides, clays and silica Can modify the effect of texture on air-water relations in soils. Types can be classified on the basis of the shape of the peds 6 SOIL PHYSICAL PROPERTIES Soil Structure 1. Spheroidal (Granular and Crumb) Aggregates not over ½ inch in diameter; Lie loosely and are readily shaken apart Characteristics of many surface soils Could either be: a. Granular - Porous, may be separated from each other in a loosely packed arrangement b. Crumb - Very porous 7 SOIL PHYSICAL PROPERTIES Soil Structure 1. Spheroidal 8 SOIL PHYSICAL PROPERTIES Soil Structure 2. Plate-like (Platy, Lenticular) Characterized by relatively thin horizontal peds or plates. Often inherited from the parent materials, especially those laid down by water or ice Most noticeable in the surface layers of virgin soils but may characterize the subsoil horizons as well 9 SOIL PHYSICAL PROPERTIES Soil Structure 2. Platy Lenticular - a subdivision of platy. The units are overlapping lenses parallel to the soil surface. They are thickest in the middle and thin towards the edges. Lenticular structure is commonly associated with moist soils, texture classes high in silt or very fine sand (e.g., silt loam), and high potential for frost action A. microlenticular B. lenticular 10 SOIL PHYSICAL PROPERTIES Soil Structure 3. Block-like (Angular blocky, Sub-angular blocky) Fragments range from a fraction of an inch to 3 or 4 inches in thickness Usually confined to the subsoil and their state of development and other characteristics have much to do with soil drainage, aeration and root penetration 11 SOIL PHYSICAL PROPERTIES Soil Structure 3. Block-like (Angular blocky, Sub-angular blocky) Angular blocky – edges of the blocks are sharp and the rectangular faces distinct. Sub-angular blocky – the corners are mostly rounded. 12 Angular blocky Sub-angular Blocky SOIL PHYSICAL PROPERTIES Soil Structure 4. Prims-like (Prismatic, Columnar) Characterized by vertically-oriented aggregates or pillars which vary in length and may reach a diameter of Prismatic 6 or more inches Commonly occur in subsoil horizons in arid and semi-arid regions When well developed are a very striking feature of the profile Columnar 13 SOIL PHYSICAL PROPERTIES Soil Structure 5. Wedge The units are approximately elliptical with interlocking lenses that terminate in acute angles. They are commonly bounded by small slickensides. 14 Soil Physical Properties 15 Soil Physical Properties 16 Soil Physical Properties 17 SOIL PHYSICAL PROPERTIES Soil Structure Structureless (Single Grained, Massive) Single grained - Sand behaves as individual grains, so very sandy soils seldom have much structure. Single grained (Sand) Massive - Finer soils lacking structure are a solid mass stuck together like molding clay. Massive soil is typical of some C horizons. Tillage or compaction of wet clayey soil may result in massive soil in the A horizon. Massive 18 Soil Physical Properties a. single grain structure, coarse sand b. massive structure,loess; c. massive structure, mainly coarse sand coated with Fe oxides; d. prismatic structure, loess; e. blocky structure, Bt horizon of a loess Luvisol; f. subangular blocky structure, Bv horizon, loess; g. crumb structure, Ap horizon, loess; h. columnar structure i. platy structure, plow pan in loess 19 SOIL PHYSICAL PROPERTIES Soil Structure Soil Structure is classified based on: 1. Type 2. Size 3. Grade 20 SOIL PHYSICAL PROPERTIES Soil Structure Types: 1. Granular/Crumb 2. Angular/Sub-angular Blocky 3. Platy/Lenticular 4. Wedge 5. Prismatic/Columnar Structureless: 1. Single Grained 2. Massive 21 SOIL PHYSICAL PROPERTIES Soil Structure Size - The six size classes are very fine, fine, medium, coarse, very coarse, and extremely coarse. 22 SOIL PHYSICAL PROPERTIES Soil Structure 23 SOIL PHYSICAL PROPERTIES Soil Structure Grade - Grade indicates the distinctness of units. Criteria for grade classes are the ease of separation into discrete units and the proportion of units that hold together when the soil is handled. 24 SOIL PHYSICAL PROPERTIES Soil Structure Water movement through different soil structure shapes. Rapid flow Moderate flow Slow flow 25 https://www.nature.com/scitable/content/water-movement-through-different-soil-structure-shapes-59719714/ SOIL PHYSICAL PROPERTIES Soil Structure Common Structure Along a Profile 26 SOIL PHYSICAL PROPERTIES Soil Structure Formation of Soil Aggregates Several biological, physical, and chemical processes interact to form aggregates. Chemical: A. Flocculation - The mutual attraction clay and organic molecules. Flocculation of clay particles results into microscopic clumps or floccules. B. Adsorption of cations - Polyvalent cations – Ca2+, Fe3+, Al3+ Can form bridges that bring clay particles together, neutralize the negative surface charges on clay particles 27 SOIL PHYSICAL PROPERTIES Soil Structure Formation of Soil Aggregates Several biological, physical, and chemical processes interact to form aggregates. Physical: A. Volume changes in clayey materials Swelling and shrinking actions that accompany freeze-thaw and wet- dry cycles in soils create fissures and pressures breaking apart large soil masses and compressing soil particles into defined structural peds 28 SOIL PHYSICAL PROPERTIES Soil Structure Formation of Soil Aggregates Several biological, physical, and chemical processes interact to form aggregates. Biological: A. Action by plants Plant roots move particles as they push their way through the soil. Plant roots and fungal hyphae exude sugar-like polysaccharides and other organic compounds forming sticky networks that bind together individual soil particles and micro-aggregates into macro- aggregates. 29 SOIL PHYSICAL PROPERTIES Soil Structure Formation of Soil Aggregates Several biological, physical, and chemical processes interact to form aggregates. Biological: B. Action by animals Burrowing and molding activities of soil animals Earthworms and termites move and mix soil particles 30  Thanks! ANY QUESTIONS? Message me. 31