Chapter 2 : Principles of Soil Science

Topics covered in this snack-sized chapter:



Introduction to Soil Processes arrow_upward


  • Soils are complex and dynamic systems, in which many processes are taking place.

    • Soil Processes

  • Weathering
    • This refers to the breakdown and decomposition of rocks and minerals by factors including air, water, sun and frost.
    • Physical weathering involves continual breakdown of rocks into smaller and smaller particles.
    • Chemical weathering involves alteration of the chemical composition of rock minerals.
  • Decomposition and Humification
    • Decomposition is the breakdown of plant derived material into its simpler organic constituents. This is accomplished by enzymes, earthworms, mites and other organisms.
    • Humification is the breakdown of plant remains leading to the formation of different types of humus. It is probably the most important biological process taking place in soils.
  • Capillary action
    • Where evaporation exceeds precipitation, moisture moves upwards within the soil profile by capillary action. It is therefore in the reverse direction to leaching.
  • Leaching
    • Wherever rainfall exceeds evaporation and there is free downward movement of water through the soil pore system, soluble minerals are leached or removed from the soil profile.
    • Continual leaching tends to impoverish the upper mineral horizons by removal of basic cations (cations are ions having a positive electrical charge e.g. Ca2+).
    • Leaching is most active in sandy soils with high porosity and is least in fine-textured soils such as clays which have restricted pore spaces.
  • Translocation
    • The movement of material in solution or suspension from one horizon to another is referred to as translocation.

    Soil Formation arrow_upward


  • Soil formation, or pedogenesis, is the combined effect of physical, chemical, biological and anthropogenic processes on soil parent material.
  • Soil is said to be formed when organic matter has accumulated and colloids washed downward, leaving deposits of clay, humus, iron oxide, carbonate, and gypsum. As a result, horizons form in the soil profile.
  • How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil.
  • They are: parent material, climate, topography (relief), organisms and time.
  • Soil Color
    • Generally soil color is determined by the amount of organic matter and the state of the iron.
    • Soil color is also related to soil drainage, with free draining, well AERATED soils (with pore space dominated by oxygen) having rich brown colors.
    • In contrast, poorly draining soils, often referred to as gleys, develop under Anaerobic conditions (the pore space dominated by water) and have grey or blue-grey colors.
    • Soils with periodic waterlogging are imperfectly drained and are often highly mottled with blotches of contrasting color. Mottles are often rusty in color and are due to iron concentration.
    • Such colors are the result of oxidation-reduction, iron is the main substance affected by these processes. If the iron is released in an anaerobic environment, then it stays in the reduced state giving it the grey blue color of water logged soils.
  • Soil Texture
    • Soil texture is a term used to describe the distribution of the different sizes of mineral particles in a soil.
    • Textures range from clay, sand, and silt at the extremes, to a loam which has all three sized fractions present .The main influence of texture is on permeability which generally decreases with decreasing particle size.
  • Soil Structure
    • The clumping of the soil textural components of sand, silt and clay forms aggregates and the further association of those aggregates into larger units forms soil structures called peds.
    • These peds evolve into units which may have various shapes, sizes and degrees of development.

    Soil Classification arrow_upward


  • Soil classification deals with the systematic categorization of soils based on distinguishing characteristics as well as criteria that dictate choices in use.
  • Soil classification is a dynamic subject, from the structure of the system itself, to the definitions of classes, and finally in the application in the field.
  • Two Systems of Classification
  • Pedological Classifications
  • (soil weathering, texture, chemistry, profile thickness, etc.).
  • Engineering Classifications soil texture and degree of plasticity (Atterberg Limits).

    • Soil colloids arrow_upward


  • Soil colloids are organic and inorganic matter with very small particle size and a correspondingly large surface area per unit of mass.
  • Important Properties of Soil colloids:
    • Size
    • Surface Area
    • Surface Charge
    • Cation/Anion Exchange
  • Types of Soil Colloids
    • Crystalline silicate clays (Phylosilicates) -tetrahedral and octahedral crystal sheets.
    • Non-crystalline silicate clays (Andisols) –Dominatelyamorphous clays (allophane and imogolite).
    • Iron and aluminum oxides (Oxisols)- Dominatelygibbsite (Al-oxide) and goethite (Fe-oxide).
    • Organic (humus) colloids (Histosols)- Non-crystalline colloids dominated by long C-chain molecules.

    Soil Reaction arrow_upward


  • Soil reactivity refers to the degree of soil acidity or alkalinity.
  • Soil reaction is important because it affects nutrient availability, microbial activity and plant growth.
  • It is measured in units of pH, which is approximately defined as
  • pH = -log [H+]
  • More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic).
  • Practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms.
  • The effect of pH is to remove from the soil or to make available certain ions. Soils with high acidity tend to have toxic amounts of aluminium and manganese.

    • Water, Aeration and Soil Density arrow_upward


  • Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth.
  • Soil is capable of being a storehouse of water and becoming the main source of water for land plants.
  • Soil water plays a significant role in several natural processes- evaporation, infiltration and drainage of water, diffusion of gases, conduction of heat, and movement of salts and nutrients are all dependent upon the amount of water present in soil.
  • Soil moisture can be improved with aeration.
  • Aeration of Soil refers to the extent of air gaps in soil.
  • Aeration commonly refers to the process of using mechanized equipment to either puncture the soil with spikes (spike aeration) or remove approximately  cores of soil from the ground (core aeration).
  • Soil Density
    • Particle Density (PD) - weight per unit volume of individual soil particles (Mg/m 3 = g/cm3 ), calculated as PD = Ws / Vs
    • Bulk Density (BD) - weight of soil per unit volume in its natural state (Mg/m3 = g/cm3 ), calculated as
    • BD = Ws / (Vs + Vp ),
    • Where
    • Ws = Weight of soil solids
    • Vs = Volume of soil solids
    • Vp = Volume of soil pores

    Soil Organisms and Ecology arrow_upward


  • Soil ecology is the study of the interactions among soil organisms, and between biotic and abiotic aspects of the soil environment.
  • It is particularly concerned with:
    • The cycling of nutrients.
    • Formation and stabilization of the pore structure.
    • The spread and vitality of pathogens
    •  The biodiversity of this rich biological community.
  • Soil organisms classification on basis of size
    • Macrofauna – Moles, Prairie dogs, earthworms, millipeds .
    • Mesofauna – Springtails and Mites.
    • Microfauna – Nematodes and single-celled protozoans.
    • Flora – Plants Roots.
    • Microorganisms – Fungi, Bacteria, and Actinomycetes.

    Soil Organic Matter arrow_upward


  • The organic soil matter includes all the dead plant material and all creatures, live and dead.
  • Most living things in soils, including plants, insects, bacteria and fungi, are dependent on organic matter for nutrients and energy.
  • Soils have varying organic compounds in varying degrees of decomposition.
  • Organic matter holds soils open, allowing the infiltration of air and water, and may hold as much as twice its weight in water.

    • Soil Nutrients Management arrow_upward


  • Sixteen nutrients are essential for plant growth and reproduction.
  • They are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, boron, manganese, copper, zinc, molybdenum, and chlorine.
  • All the nutrients with the exception of carbon are taken up by the plant through its roots.
  • All those brought through the roots, with the exception of hydrogen, which is derived from water, are taken up in the form of ions.
  • Plants may have their nutrient needs supplemented by spraying a water solution of nutrients on their leaves, but nutrients are typically received through the roots by:
    • Mass flow
    • Diffusion
    • Root interception


    Thank You from Kimavi arrow_upward


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