Principles of Soil Science_Soil Physics_Soil Structure.pdf

Principles of Soil Science AGRI 16 Module 3 Soil Physical Properties Course Description This course is designed to provide students the...

Principles of Soil Science AGRI 16 Module 3 Soil Physical Properties Course Description This course is designed to provide students the basic principles of soil science: the nature of soil, properties of soils. This course will introduce the fundamental, even theoretical, principles of soil science, necessary as foundations in understanding the practical management options of soils. The field applications of principles are also emphasized. The companion laboratory course exposes the students to determination of basic soil properties, soil sampling methods and other methods to determine the productivity of soils. Learning Objectives 1. describe soil structure, its formation and importance to land management 2. identify the different classes of structure and their influences on soil Topic: Soil Structure The pattern of arrangement of individual soil particles into aggregate and the aggregate into soil mass The arrangement of soil particles into certain sizes and shapes Arrangement and organization of primary and secondary particles in a soil mass Possible choices of soil structure are: Granular Blocky Prismatic Columnar Platy Single-grained Under natural conditions in the field, soil particles in the presence of cementing agents are capable of binding themselves into units known as peds or aggregates. Formation of soil structure Soil particles may be present either as single individual grains or as aggregate (group of particles bound together into granules or compound particles A ped a natural soil aggregate or cluster of particles differentiated from a clod, which results from disturbance such as plowing. Such unit constitutes building blocks of the resulting soil structure. Soils possess a structure if they satisfy the following two conditions: ▪ There is aggregation ▪ There is a definite pattern of arrangement o If any of the two conditions is not met, the soil has no structure or is structureless. Soils which predominate with single grains are said to be structureless while those possess majority of secondary particles are said to be aggregate, granulated or crumb structure. o Sandy soils o aggregation seldom occurs o particles exists as separate entities o is referred to as “single-grained” type of structureless category. o Another type under this category is the "massive" ▪ the particles are held together equally in the soil mass. ▪ When broken, the mass shatters into various sizes and shapes. ▪ The massive type is common in compacted and in most dried paddy soils For soil with structure, field description normally 'includes the following information. o Shape of peds (type) o Distinctness or durability of peds (grade) o Size of structural units (class) Formation of Soil Structure The bonding of soil particles intro structural unit is the genesis of soil structure. In aggregate formation, a number of primary particles such as sand, silt and clay are brought together by the cementing or binding effect of soil colloids. o Organic matter – binding agent o Colloidal clay, iron and aluminum hyrdoxides  o Products of microbial decomposition (gums, polysaccharides) o Some fungi and bacteria have cementing effect probably due to the presence of slimes and gums on the surface of the living organisms produced as a result of the microbial activity o Adsorbed cations o Na-causes dispersion o Ca-encourages flocculation o Fine clay (including sesquioxides) Types of Structure Spheroidal (granular and crumb subtype) o Resembles cookie crumbs, includes rounded peds or aggregates whose diameter is not more than 1 cm. o Ordinarily, the aggregates are called granules and the pattern of arrangement as granular. o when the granules are especially porous, the term crumb is applied. Platelike (platy) o thin, flat plates of soil that le horizontally o usually found in compacted soil o When the units/layers are thick they are called platy while when they are thin it is laminar Prismlike (prismatic and columnar subtype) ▪ this type is characterized by vertically oriented aggregates or pillars which vary in length with different soils and may reach a diameter of 6 or more inches. ▪ Prismatic - vertical columns of soil that might be a number of cm long. It can be found in lower horizons. Tops of the prisms are still plane level and clean cut ▪ Columnar –. vertical columns of soil that have a salt cap at the top. Usually found in soils of arid climates Blocklike (blocky and sub-angular blocky subtypes) Cube-like; edges are sharp and rectangular faces are distinct; some are more or less rounded Commonly in clayey subsoils particularly in humid regions Has considered effect on drainage, aeration and root penetration When the faces are flat and distinct and the edges are sharp angular, the structure is named as angular blocky. When the faces and edged are mainly rounded it is called sub angular blocky.  These types usually are confined to the subsoils, commonly of the humid regions, and their stage of development and other characteristics have much to do with soil drainage, aeration and root respiration. Grades of structure Grades indicate the degree of distinctness of the individual peds. Determined by the stability of aggregates Terms to describe the grade of soil structure Table 1 Classification of structure of soil materials no aggregation or orderly arrangement 0 structureless no noticeable aggregation such as conditions exhibited by loose sand poorly formed, non-durable, weak arrangement of natural surfaces of weakness, when disturbed, soil material breaks into a mixture if few 1 weak entire aggregates poorly formed, indistinct formation of peds which are not durable and much unaggregated material well-formed, indistinct peds, distinct arrangement of natural surfaces of weakness, when disturbed, soil material breaks into a mixture of entire 2 moderate aggregates moderately well-developed peds, which are fairly durable and distinct durable distinct peds, when disturbed, the soil material separates mainly into entire aggregates, weakly attached to each other that breaks almost 3 strong completely into entire peds. very well formed peds which are quite durable and distinct Table 2 Size classes of soil structural units Type Size class Platy Prismatic/Columnar Blocklike Spheroidal Very fine (thin) 10 mm Extremely coarse - >500 - - Soil Management related to soil structure POOR SOIL STRUCTURE (Massive or GOOD SOIL STRUCTURE ( Well- compacted soil) aggregated crumb) Dominantly micropores Balanced macro and micro-pores ▪ Poor drainage ▪ Micropores for water storage ▪ Poor aeration ▪ Macropores for drainage, aeration and root growth ▪ Poor root growth Hard to till Easier to till Low porosity High total porosity High bulk density Low bulk density Factors affecting Soil Structure 1. climate – influence on the degree of aggregation 2. Organic matter – improves structure 3. Tillage – cultivation implements break down the large clods into smaller fragments and aggregates 4. Plants, roots and residues – excretion of gelatinous compounds and exudates from roots 5. Animals 6. Microbes 7. Fertilizers 8. Wetting and drying 9. Exchangeable cations 10. Inorganic increments 11. Clay 12. Water References: Aggie Board Review (UPLB-MADECOR) 2013 compilation of lecture materials for licensure examination in agriculture. Baver, L.D., W.H. Gardner and W.R. Gardner.1972. Soil Physics. 4th ed. Wiley & Sons, Inc., New York. Brady, N.C. and R.R. Weil. 2001 The Nature and Properties of Soils. 13 th ed. Prentice-Hall Inc. New Jersey. Department of Soil Science. 2005. Lecture Handbook in Soil Science 21 (Principles of Soil Science). Department of Soil Science. College of Agriculture. Central Mindanao University. CMU, Musuan, Bukidnon. FAO. 2006. Guidelines for Soil Description. Food and Agriculture Organizations.4 th ed. Rome, Italy. Topp, C.G., G.W. Parkin and P.A. Ferre. 2008. Soil Water Content. p. 939-962. In Carter, M.R., and E.G. Gregorich (eds). Soil sampling and methods of analysis. 2nd ed. Canadian Society of Soil Science, CRC Press and Taylor & Francis Group. Oxford, UK Prepared by: PRECIOUS DANE P. TAGAS FACULTY, AGRICULTURAL SCIENCES