This document provides an introduction to pedology and soil physics. It discusses key concepts in soil physics including soil texture, structure, physical properties, productivity, and soil as a dispersed system consisting of solid, liquid, and gas phases. It also summarizes methods for determining soil texture in the field or laboratory, including mechanical analysis and use of the soil textural triangle. Factors that influence soil productivity like climate and physical fertility are also addressed.
This document provides an introduction to pedology and soil physics. It discusses key concepts in soil physics including soil texture, structure, physical properties, productivity, and soil as a dispersed system consisting of solid, liquid, and gas phases. It also summarizes methods for determining soil texture in the field or laboratory, including mechanical analysis and use of the soil textural triangle. Factors that influence soil productivity like climate and physical fertility are also addressed.
This document provides an introduction to pedology and soil physics. It discusses key concepts in soil physics including soil texture, structure, physical properties, productivity, and soil as a dispersed system consisting of solid, liquid, and gas phases. It also summarizes methods for determining soil texture in the field or laboratory, including mechanical analysis and use of the soil textural triangle. Factors that influence soil productivity like climate and physical fertility are also addressed.
This document provides an introduction to pedology and soil physics. It discusses key concepts in soil physics including soil texture, structure, physical properties, productivity, and soil as a dispersed system consisting of solid, liquid, and gas phases. It also summarizes methods for determining soil texture in the field or laboratory, including mechanical analysis and use of the soil textural triangle. Factors that influence soil productivity like climate and physical fertility are also addressed.
Soil Physics is a branch of soil science that deals with physical
properties of soil as well as measurement, prediction and control of physical processes taking place in and through the soil. Soil physical properties include soil texture, soil structure, soil colour, consistency, density, thermal regime, soil water, porosity, infiltration, hydraulic conductivity etc. Soil Physics could fundamentally be regarded as both basic and applied sciences. This is because, Soil Physics involves application of the principles of Physics to the characterization of soil properties and understanding of soil processes involving transport of matter or energy. SOIL PRODUCTIVITY
Soil productivity is an economic concept and signifies the capability of the soil to produce specified plant or sequence of plants under well defined specified systems of management and environmental conditions. This suggests that productivity is not soil fertility alone but a function of several factors (e.g. climatic condition and soil factors). Soil productivity is measured in terms of output or harvest.
On the other hand, soil fertility refers to the inherent capacity of the soil to provide adequate amount and proper balance of nutrient for the growth of specified plant when other growth factors (e.g. light, water, temperature and favourable soil physical environment) are favourable.
In addition to chemical fertility i.e. presence of adequate nutrient in the soil and absence of toxic agents, the soil should also, be physically fertile. That is, the soil must be loose, soft and friable, possesses no mechanical impedance to root development, has pore volume and size distribution that allow entering, movement and retention of water and air to meet plant needs and has optimal thermal regime. SOIL AS A DISPERSED SYSTEM Soil is made up of 4 basic components: mineral matter; organic matter, soil water and soil air. On the basis of these, there are three phases in the soil. These are solid phase, liquid phase and gaseous phase.
The Solid Phase: The solid phase is broadly composed of inorganic
(mineral) and organic constituents. The mineral constituents form the bulk of the soil solid phase and consist of particles of various sizes, shapes and chemical composition. The mineral constituent specifically composed of primary and secondary minerals. The primary minerals are the quartz and feldspars (most abundant) with relatively small amount of pyroxenes, amphiboles, olivine, micas etc. The secondary minerals originates from the break down of primary minerals and examples are silica, alumina, iron oxides etc. They constitute the most active site in the soil. The organic fraction constitutes a small fraction of the soil solid phase and it includes residues at various stages of decomposition as well as life organisms.
The Liquid Phase: About 40 to 50% of the bulk volume of soil
body is generally occupied by soil pores or voids which may be partially or completely filled with water. The liquid phase is an aqueous solution of salts because the soil water keeps salts in solution which act as plant nutrients. When all the soil pores are completely filed with water, the soil is said to be saturated.
The Gaseous Phase: This is the soil air/atmosphere. It is made up
of mixture of gases. It composed mainly of nitrogenous gas, oxygen, carbon dioxide and water vapour. The volume of the gaseous phase is dependent on that of the liquid phase. The pore space not filled by water is automatically occupied by air. The dispersed nature of the soil and its constituent inter-phasal activities give rise to such phenomena as: (i) adsorption of water and chemicals; (ii) ion exchange; (iii) adhesion and cohesion; (iv) dispersion and flocculation; (v) swelling and shrinking and (vi) capillarity.
The three phases of the soil play definite roles. The solid phase provides mechanical support for and nutrients to the plants. The liquid phase supplies water and along with it dissolved nutrients to plant root. The gaseous phase satisfied the aeration need of the plant. Thus, the 3 phases complimentarily shared the soil’s function to sustain plant growth. SOIL TEXTURE
Soil texture is the relative proportion of various soil separates in
a soil. It is usually expressed on percentage basis. Soil separates are group of soil particles of given size range i.e. different size of particles which together make up a given soil.
The main textural classes are sand, silt and clay. These textural classes may be modified by addition of suitable adjective based on relative amount of each separate that make up the soil e.g. Loam: Soil material with clay, silt and sand in close proportion (e.g. 7-27% clay; 28-50% silt and <50% sand). Loamy sand: Materials with about 80-90% sand. Sandy loam: <7% clay; <50% silt; about 52% sand.
Other modifications include silty loam, sandy clay loam, clay
loam, gravelly loamy sand etc. Determination of Soil Texture
Soil texture may be determined on the field by textural feel
and in the laboratory by soil mechanical analysis or soil particle size distribution. The mechanical analysis in the laboratory may be carried out either by Pipette or hydrometer method.
After the proportion of each of the soil separates are
determination, the textural class of the soil is identified using a USDA Soil Textural Triangle. The sides of the soil texture triangle are scaled for the percentages of sand, silt, and clay. Soil texture triangle Systems of soil particle size classification There are two widely used systems of soil classification. These are: United State Department of Agriculture (USDA) and International Soil Science Society (ISSS)
USDA Classification system
Fraction Diameter (mm)
Very coarse sand 2.00 – 1.00 Coarse sand 1.00 – 0.50 Medium sand 0.50 – 0.25 Fine sand 0.25 – 0.10 Very fine sand 0.10 – 0.05 Silt 0.05 – 0.002 Clay <0.002 ISSS Classification system
Generally Materials: >20 mm diameter – stone 20-2 mm diameter – gravel <2mm diameter – Fine earth (soil)
Importance of soil texture
•It affects water and nutrient holding capacity of the soil •It influences the type of crop to be grown •It indicates type of management needed for crop growth and for engineering purposes. Mechanical composition of soil The mechanical composition of soil is a basic requirement in the soil physical investigation useful for land capability classification and in the study of soil morphology, genesis, classification and mapping. Soil mechanical analysis is the procedure for determining the particle size distribution of a soil sample. Steps in soil mechanical analysis
Sample collection Air dry the sample at room temperature If the sample contains high amount of organic matter remove the organic matter using H2O2 Dispersion of the sample in an aqueous solution using Calgon solution (Sodium hexametaphosphate). . Carry out mechanical agitation by shaking or using ultrasonic vibration Determination/quantification of size fraction (1) Sieving (for coarse fraction) – use nest of sieve corresponding to the desired particle size (2) Sedimentation (fine fraction) The principles of sedimentation are that the velocity of fall of particle in a viscous medium is influenced by (i) the viscosity of the medium (ii) density difference between the medium and the falling particle (iii) the size and shape of the material.
The law which govern sedimentation of particles is called
Stoke’s Law which states that resistance offered by liquid to the fall of a rigid spherical particles vary with the circumference of the sphere (and not its surface) OR the terminal velocity of a spherical particle settling under the influence of gravity in a fluid of a given density and viscosity is proportional to the square of the particle radius. The Stokes’ law consists of the factors contributing to the cause of settling and resistance to settling. Particle volume = 4/3πr3; density difference = d1–d2; g = gravity; Particle circumference = 2πr Viscosity = η; V = velocity of sedimentation Therefore, if Force of settling = resistant to settling Force = mass x acceleration i.e F = mg (where g is acceleration due to gravity) Note that density = mass/volume; therefore mass = ρv 4/3πr3 (d1 – d2)g = 2πr.η.3v V = 4/3πr3 (d1 – d2)g = 2r2(d1 – d2)g 2πr.η.3 9η V = settling velocity (cm s-1); d1 =particle density; d2 = density of fluid (g cm-3); r = particle radius (cm); η = viscosity of the medium The time (t) required for particle to settle through a given depth (h) in liquid may be obtained from the Stokes’ Law V = 2r2(d1 – d2)g 9η Re-write r2 in terms of particle diameter (D) i.e. r2 = ¼ D2 Therefore, V = 2D2(d1 – d2)g 36η V = (d1 – d2)g D2 18η Velocity = distance (h) / time (t); hence, t = h/V t = 18η.h (d1 – d2)g D2 Example: Calculate the velocity of sedimentation of silt particle 0.006 mm and clay particle 0.0002 mm diameter at 30oC. The viscosity of the water at 30oC was 0.00798 cm-1 s-1. Assuming the gravity to be 980 cms-2. SURFACE RELATIONSHIP
The extent of the surface of dispersed soil system is described in terms of the soil specific surface.
The soil specific surface is defined as the sum of the surfaces of constituent dispersed soil particles referred to unit mass or unit volume.
Specific surface of soil (Am) or (Av) = Total surface area of soil (As) Mass or volume of soil (Ms or Vs) That is, Am or Av = As Ms or Vs Therefore, on mass basis Am = As cm2 or m2 or m2 Ms g g kg
On volume basis, Av = As cm2 or m2
Vs cm3 m3 Many of the soil physical and chemical reactions are related to inter facial surface phenomena and are thus influenced by soil specific surface area.
Soil properties such as plasticity, swelling, soil strength, water
retention, CEC and nutrient availability are strongly affected by soil surface area.
Soil specific surface depends on:
i. Particle size ii. Particle shape iii. Mineralogy of the material VOLUME-MASS RELATIONSHIP
The volume-mass relationship among the three phases of soil
could be diagrammatically represented as:
Va AIR Ma ≈ 0 Va = volume of air Vf Vf = volume of void Vt = total volume Vt Water Mw Ma = mass of air Mw = mass of water Ms = mass of solid Ms Solid Based on the diagram above, a quantitative representation of the 3 phases can be expressed in terms: Particle density (ℓs) = Ms/Vs (g cm-3) Vs = volume of solid Bulk density (dry): (ℓb) = Ms/Vt (g cm-3) Total porosity (Pt) = Vf/Vt or (Va + Vw)/Vt Void ratio (e) = (Va + Vw)/Vs Soil wetness: This can be expressed relative to total mass or total volume In term of mass: mass wetness (w) = Mw/Ms. This is called gravimetric water content In terms of volume (Ө) = Vw/Vt. This is called volumetric water content. Degree of saturation (s) = Vw/Vf
Air filled porosity (Pa) = Va/Vt. This is the
fractional volume of air in the field. SOIL STRUCTURE
Soil structure is the arrangement of soil particles to form peds. Or,
the arrangement of primary particles into secondary particles (aggregate).
Soil structure is strongly affected by changes in climate, biological
activities and soil management practices.
Measurement of soil structure
There are direct and indirect methods of measuring soil structure.
The direct methods involve measuring the size and shape of aggregate and pores of the soil. That is, three dimensional study of soil. This is done by thin section analysis with aid of powerful microscope e.g. scanning electron microscope, transmission microscope, petrographic microscope etc. The indirect methods involve measuring soil properties that depend on soil structure. These properties include: aggregate size distribution, aggregate stability, bulk density, porosity, pore size distribution, permeability, infiltration etc.
Importance of soil structure
•It affects water and nutrient holding capacity of the soil
•It affects germination and root growth and development •It affects water retention and transmission of fluid in soil •It affects soil aeration •It influences soil thermal properties Measurement of soil structural stability
There are various methods: the two common methods that will be considered are sieving method and water drop impact technique
1. Sieving method: there are wet and dry sieving techniques. What is measured here is the Mean weight diameter (MWD). For wet soil it is called MWDW and for dry soil it is called MWDD
The formula is given as
Where X = mean of diameter of particles size range separated by
sieve Wi = weight (proportional) of the aggregates in each size range Example
Aggregate diameter % X Wi X Wi (cm) 10 – 5 5 7.5 0.05 0.375 5–2 5 3.5 0.05 0.175 2 – 1.0 15 1.5 0.15 0.225 1.0 – 0.5 25 0.75 0.25 0.1875 0.5 – 0.0 50 0.25 0.5 0.125 MWD 1.0875 mm 2. Water drop technique: this is done by studying the impact of rain drop on soil aggregate stability. This is done in the laboratory by using a rainfall simulator. SOIL WATER
Soil water is the amount of water present in the soil available to
crop. Water can enter into the soil by precipitation or by irrigation.
Forms in which water exist in the soil
Water exists in 3 forms in the soil (i.) Hygroscopic water (ii.) Capillary water (iii.) Gravitational water. Importance of water to crop i. It is important in the absorption of mineral salts from the soil by plant. ii. It helps in the transportation of plant nutrients from the root to other parts of the plant. iii. Water is an essential raw material needed during photosynthesis. Some terminologies in soil water include Field capacity, water table, water log, wilting point Ways by which soil loss water i.) evaporation from the soil surface ii.) Transpiration from plant leaves, stem and fruit surface. iii. drainage iv.) erosion
Sources of water in the soil
i.) Precipitation ii.) Irrigation iii.) High humidity Water content and Water retention Water content: this is the amount of water present in the soil. This is usually expressed as a percentage of oven dried weight of soil volume.
Water retention: The spaces that exist between soil
particles, called pores, provide for the passage and/or retention of gasses and moisture within the soil profile. The soil’s ability to retain water is strongly related to particle size; water molecules hold more tightly to the fine particles of a clay soil than to coarser particles of a sandy soil, so clays generally retain more water Pore space Methods of determining soil water content
A. Direct methods 1. Gravimetric method 2. Volumetric method
B. Indirect methods 1. Electric resistant method 2. Neutron scattering technique 3. Gamma ray attenuation technique 4. Time domain reflectomery 5. Tensiometry method Gypsum Block Sensor Gamma ray technique Tensimeter in operation
Tensiometers Method of expressing soil water content 1. On mass basis 2. On volume basis 3. On depth basis
Forces acting on soil water
1. Matric force 2. Osmotic force 3. Body force Movement of water in soil Flow of water in soil may take the following forms: 1. Saturated flow 2. Unsaturated flow 3. Vapour movement Saturated water flow
Soil pores filled with water
The flow is influenced by 3 factors: Soil – termed permeability Fluid – termed fluidity (forces of viscosity and gravity) Moving force – termed hydraulic gradient Unsaturated water flow As expected, unsaturated flow is slower than saturated flow
The driving force of water under unsaturated
flow is the suction gradient: Water flow from region of low suction to high suction
The two forces responsible for unsaturated flow
are adhesion and capillarity Water Vapour movement in Soils Root zone normally have large air filled porosity Vapour moves from warm to cold region Downward during the day and upward during the night
Vapour density gradient may be caused by:
Difference in soil water potential Temperature difference THE HYDROLOGIC CYCLE • The hydrologic cycle: Hydrologic cycle is the set of processes by which water moves through different reservoirs on earth. The hydrologic cycle can be thought of as a series of reservoirs, or storage areas, and a set of processes that cause water to move between those reservoirs. The largest reservoir by far is the oceans THE HYDROLOGIC CYCLE Water Balance Water balance is an accounting of all water volumes that enters and leave the soil over a period of time
Water Balance Equation
• ∆S = P/I – (R + D + Et). Where ∆S is change in soil water content between sampling; P is precipitation; I = Irrigation R is the runoff; D is deep drainage and Et is evapotranspiration. THE WATER BALANCE TRANSPIRATION PRECIPITATION / IRRIGATION
EVAPORATION
Infiltration RUNOFF
Soil Surface
SOIL WATER CONTENT
DEEP DRAINAGE Solar Radiation Solar Radiation is the major source of soil heat. Only a portion of the emitted solar radiation reaches the earth’s surface. Part of the solar radiation may be:
1. Reflected by the clouds
2. Scattered into the atmosphere by atmospheric gases. 3. Absorbed by the ozone and water vapour. Percentages of the Emitted Solar Radiation Percentages of the emitted solar radiation reaching the earth surface are as follows:
1. Cloud free regions 75 %
2. Cloudy, humid regions 30 – 40 %
3. Global radiation (average) 50 %
Management of Soil Heat These methods includes: 1. Surface Covers 2. Mechanical manipulation of soil surface 3. Others (indirect effects) i) Irrigation – reduces temperature ii) Drainage – increases temperature iii) Weed control iv) Plants/trees SOIL EROSION Definition Soil erosion can be simply defined as the wearing away of soil materials from place to place by the agents of erosion such as water, wind and ice. In general soil erosion is broadly divided into 1. Geologic Erosion Soil erosion that occurs naturally, without the influence of human activities. 2. Accelerated Erosion Soil erosion resulting from human interference with the natural environment. Mechanics of Soil Erosion a. Detachment of soil aggregates into particles b. Transportation of the detached particle by floating, rolling and dragging. c. Deposition of the transported materials where the energy or force dissipates. Types of Soil Erosion by water Splash erosion: Sheet Erosion: The removal of a fairly uniform layer of soil from the land surface by runoff water. Rill Erosion: As sheet flow is concentrated into tiny channels (called rills), rill erosion occurs. Gully Erosion: When the volume of runoff is concentrated, the rushing water cuts deeper into the soil, deepening and fusing the rills into larger channels called gullies. Sheet erosion Factors affecting soil erosion 1. Climatic factor 2. Soil factors 3. Topography 4. Vegetation cover 5. Human activities e.g., Tillage, Overgrazing, Fires, Lowering of the water table (water use in excess of replenishment rate) - these accentuates wind erosion Measurement of Soil Erosion by Water The Universal Soil Loss Equation (USLE), was designed by Wischmeier and Mannering (1969), to predict annual soil loss by water in the USA but has been adapted and modified in some cases for prediction around the world. The USLE equation is as follows: A=RKLSCP A - predicted soil loss (kg m-2 s-1); R - rainfall erosivity K - soil erodibility; L - slope length; S - slope gradient or steepness C - cover management P - erosion control practices Conservation tillage Conservation tillage – tillage method that leaves at least 30% of plant residue on the soil after planting. Tillage on a general term is classified into 3 (ZT, MT and CT). Conservation tillage includes • Mulch tillage, • Zero Tillage • Minimum Tillage ( leaves about 15-30% residue) • Contour tillage • Contour strip cropping • Strip cropping
