SOIL COMPOSITION AND PROPERTIES

CLASS : JSS2

 

SUBJECT : AGRICULTURAL SCIENCE

 

TERM : FIRST  TERM E-NOTE

 

WEEK : WEEK 2

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WEEK 2

DATE:

TOPIC: PHYSICAL AND CHEMICAL PROPERTIES OF SOIL.

CONTENT: 1. soil texture

2. Soil structure

3. Soil pH.

SUB TOPIC 1: SOIL TEXTURE.

This refers to the relative proportions of the various sized particles that make up a soil sample.( i.e. the percentage of sand, silt and clay present in a soil sample).

It can also be defined as the degree of the finess or coarseness of a soil when felt. It was based on the soil texture that the soil is grouped into the various types.

Type of soil particles Diameter of particles in mm
ClayLess than0.002
Silt0.02 – 0.002
Fine sand0.20 – 0.020
Coarse sand2.0 – 0.20
Gravel2.0 and above

Soil texture can be determined by:

i. feel

ii. wetting and moulding

iii. sieving

iv. sedimentation .

IMPORTANCE OF SOIL TEXTURE

  1. It determines the type of soil that is found in an area.
  2. It influences water and air movement in the soil.
  3. It influences root penetration and workability.

EVALUATION

1. Explain the term soil texture

ii. How can soil texture be determined?

iii. State three importance of soil texture.

SUB TOPIC 2: SOIL STRUCTURE

This is the arrangement of particles of soil into aggregates. It shows the physical appearance of the soil as the particles are arranged.

TYPES OF SOIL STRUCTURE

  1. Single-grained
  2. Massive
  3. Granular
  4. Crumb
  5. Coherent
  6. Block and
  7. Prismatic.

IMPORTANCE OF SOIL STRUCTURE

  1. It influences the characteristics of the soil in terms of heat transfer, aeration, water movement , root penetration and workability.
  2. It also affects the rate of soil erosion.

What is Soil?

Soil Composition

Importance of Soil

  1. What is Soil? 

Soil is the loose or thin layer on the surface of the earth on which the plants and animals lives. It is the loose combination of inorganic and organic materials. Soil is the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.

Soil is considered to be the “skin of the earth” with interfaces between the lithosphere, hydrosphere, atmosphere of Earth, and biosphere. Soil consists of a solid phase (minerals and organic matter) as well as a porous phase that holds gases and water. Accordingly, soils are often treated as a three-state system.

Soil is the end product of the influence of the climate, relief (elevation, orientation, and slope of terrain), organisms, and parent materials (original minerals) interacting over time. Soil continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion.

Physical Properties of the Soil

  1. Texture: Texture refers to the relative proportions of particles of various sizes such as sand, silt and clay in the soil. Properties that are influenced by soil texture, include porosity, permeability, infiltration, shrink-swell rate, water-holding capacity, and susceptibility to erosion.
  2. Structure: The term texture is used in reference to the size of individual soil particles but when the arrangement of the particles is considered the term structure is used. Structure refers to the aggregation of primary soil particles (sand, silt and clay) into compound particles or cluster of primary particles which are separated by the adjoining aggregates by surfaces of weakness. Structure modifies the effect of texture in regard to moisture and air relationships, availability of nutrients, action of microorganisms and root growth. The clumping of the soil textural components of sand, silt and clay causes aggregatesto form and the further association of those aggregates into larger units creates soil structures called pedoliths or peds. The adhesion of the soil textural components by organic substances, iron oxides, carbonates, clays, and silica, and the breakage of those aggregates from expansion-contraction, caused by freezing-thawing and wetting-drying cycles, shape soil into distinct geometric forms. Classes of Soil Structure includes:
    • TypesShapeand arrangement of peds
      1. Platy: Peds are flattened one atop the other 1–10 mm thick. Found in the A-horizon of forest soils and lake sedimentation.
      2. Prismatic and Columnar: Prism like peds are long in the vertical dimension, 10–100 mm wide. Prismatic peds have flat tops, columnar peds have rounded tops. Tend to form in the B-horizon in high sodium soil where clay has accumulated.
      3. Angular and Subangular: Blocky peds are imperfect cubes, 5–50 mm, angular have sharp edges, subangular have rounded edges. Tend to form in the B-horizon where clay has accumulated and indicate poor water penetration.
      4. Granular and Crumb: Spheroid peds of polyhedrons, 1–10 mm, often found in the A-horizon in the presence of organic material. Crumb peds are more porous and are considered ideal.
    • ClassesSizeof peds whose ranges depend upon the above type
      1. Very fine or Very thin: <1 mm platy and spherical; <5 mm blocky; <10 mm prism-like.
      2. Fine or thin: 1–2 mm platy, and spherical; 5–10 mm blocky; 10–20 mm prism-like.
      3. Medium: 2–5 mm platy, granular; 10–20 mm blocky; 20-50 prismlike.
      4. Coarse or Thick: 5–10 mm platy, granular; 20–50 mm blocky; 50–100 mm prism-like.
      5. Very coarse or Very thick: >10 mm platy, granular; >50 mm blocky; >100 mm prism-like.
    • Grades: Is a measure of the degree of developmentor cementation within the peds that results in their strength and stability.
      1. Weak: Weak cementation allows peds to fall apart into the three textural constituents, sand, silt and clay.
      2. Moderate: Peds are not distinct in undisturbed soil but when removed they break into aggregates, some broken aggregates and little un-aggregated material. This is considered ideal.
      3. Strong: Peds are distinct before removed from the profile and do not break apart easily.
      4. Structureless: Soil is entirely cemented together in one great mass such as slabs of clay or no cementation at all such as with sand.
  1. Density: Density is the weight per unit volume of an object. Particle density is equal to the mass of solid particles divided by the volume of solid particles – it is the density of only the mineral particles that make up a soil; i.e., it excludes pore space and organic material.
  2. Consistency: This is the resistance of a soil to deformation or rupture and is determined by the cohesive and adhesive properties of the soil mass. Consistency is the ability of soil to stick to itself or to other objects (cohesion and adhesion respectively) and its ability to resist deformation and rupture. This is a term used to designate the manifestation of the cohesive and adhesive properties of soil at various moisture contents. A knowledge of the consistence of the soil is important in tillage operations, traffic and pond constructions. Consistence gives also an indication of the soil texture.Consistence is described for three moisture levels:
    1. Wet soil– non sticky, slightly sticky, sticky, very sticky; non plastic, slightly plastic, plastic and very plastic.
    2. Moist soil– loose, very friable, friable, firm, very firm, extremely firm.
    3. Dry soil– loose, soft, slightly hard, hard, very hard, extremely hard.
  • Porosity: Particle density can be determined using specific gravity bottle technique and bulk density by taking soil core samples of known volume in the field and determining the even dry weight (Black et al., 1965a). Pore space is that part of the bulk volume of soil that is not occupied by either mineral or organic matter but is open space occupied by either gases or water. Ideally, the total pore space should be 50% of the soil volume. The gas space is needed to supply oxygen to organisms decomposing organic matter, humus, and plant roots. Pore space also allows the movement and storage of water and dissolved nutrients.There are four categories of pores:
    1. Very fine pores: < 2 µm
    2. Fine pores: 2-20 µm
    3. Medium pores: 20-200 µm
    4. Coarse pores: 200 µm-0.2 mm
  • Temperature: Soil temperature depends on the ratio of the energy absorbed to that lost. Soil has a temperature range between 20 to 60 °C. Soil temperature regulates seed germination, plant and root growth and the availability of nutrients.
  • Colour: Soil colour gives an indication of the various processes going-on in the soil as well as the type of minerals in the soil. For example the red colour in the soil is due to the abundance of iron oxide under oxidized conditions (well-drainage) in the soil; dark colour is generally due to the accumulation of highly decayed organic matter; yellow colour is due to hydrated iron oxides and hydroxide; black nodules are due to manganese oxides; mottling and gleying are associated with poor drainage and/or high water table
  1. Soil Types

Soil type usually refers to the different sizes of mineral particles in a particular sample. Soil is made up in part of finely ground rock particles, grouped according to size as sand and silt in addition to clay, organic material such as decomposed plant matter.

Each component, and their size, play an important role. For example, the largest particles, sand, determine aeration and drainage characteristics, while the tiniest, sub-microscopic clay particles, are chemically active, binding with water and plant nutrients. The ratio of these sizes determines soil type: clay, loam, clay-loam, silt-loam, and so on.

In addition to the mineral composition of soil, humus (organic material) also plays an important role in soil characteristics and fertility for plant life. Soil may be mixed with larger aggregate, such as pebbles or gravel. Not all types of soil are permeable, such as pure clay.

There are three major types of soil which are;

  1. Sandy soil
  2. Clay soil
  3. Loamy soil

Sandy Soil

Sandy soil is the result of the weathering and disintegration of a variety of rocks such as granite, limestone and quartz.This type of soil is easy to cultivate but since it allows for more drainage than needed, it is important to water it regularly, especially during summer days.

Sandy soils are dominated by sand particles, but contain enough clay and sediment to provide some structure and fertility. There are four different types of sandy loam soil that are classified based on the size of the sand particles in the soil. You can determine whether your yard has this kind of soil using a simple test.

Classification

Sandy loamy soils are broken down into four categories, including coarse sandy loam, fine sandy loam, sandy loam and very fine sandy loam. The size of the sand particles is measured in millimeters and their concentration in the soil is used to determine which category a soil falls under. Sandy loam soils are made of approximately 60 percent sand, 10 percent clay and 30 percent silt particles.

Characteristics

Sandy loam soils have visible particles of sand mixed into the soil. When sandy loams soils are compressed, they hold their shape but break apart easily. Sandy loam soils have a high concentration of sand that gives them a gritty feel. In gardens and lawns, sandy loam soils are capable of quickly draining excess water but can not hold significant amounts of water or nutrients for your plants. Plants grown in this type of soil will require more frequent irrigation and fertilization than soils with a higher concentration of clay and sediment. Sandy loam soils are often deficient in specific micronutrients and may require additional fertilization to support healthy plant growth.

Identification

You can quickly identify sandy loam soil based on its physical characteristics. Pick up a handful of dry soil and slowly dribble water onto it. Work the water into the soil with your hand until it has a smooth consistency similar to putty. Hold the soil in your hand as though you are holding a pipe straight up and down and squeeze it. Sandy loam soils have a very gritty texture. If your soil is a sandy loam, it will form a cohesive ribbon of soil as it squeezes out between your thumb and finger that will fall apart before it reaches one inch in length.

Considerations

Plants that are grown in a sandy loam soil need frequent irrigation and fertilization to maintain healthy growth. The best way to improve a sandy loam soil for gardening is to mix organic matter into the soil. Incorporating a 2- to 4-inch layer of compost or peat moss over the area can significantly improve the ability of your sandy loam soil to hold nutrients and water.

Characteristics of Sandy Soil

  • large sized particles from 2mm to 0.02mm
  • particles consist chiefly of quartz
  • particles are coarsed grained, loose and gritty to touch
  • well drained and aerated
  • poor water holding capacity
  • no structure therefore it is not suitable for growth
  • easily heated up in the day and cools faster at night
  • easy to cultivate
  • large pore spaces

Clay soil

Clay is a kind of material that occur naturally and consists of very fine grained material with very less air space. Clay soil has the smallest particles among the three so it has good water storage qualities. It’s sticky to the touch when wet, but smooth when dry.

Due to the tiny size of its particles and its tendency to settle together, little air passes through its spaces. Because it’s also slower to drain, it has a tighter hold on plant nutrients. Clay soil is thus rich in plant food for better growth.

Clay soil is cold and in the spring, takes time to warm since the water within also has to warm up. The downside is that clay soil could be very heavy to work with when it gets dry. Especially during the summer months, it could turn hard and compact, making it difficult to turn. (When clay soil is worked while it’s too wet though, it’s prone to damage).

If moistened soil feels sticky, rolls up easily, and forms into a ball or sausage-like shape, then you’ve got yourself clay.

Though different soils have a wide range of colors, textures and other distinguishing features, there are only three types of soil particles that geologists consider distinct. The quality of soil depends on the amount of sand, loam and clay that it contains, because soils with differing amounts of these particles often have very different characteristics. Soil with a large amount of clay is sometimes hard to work with, due to some of clay’s characteristics.

Particle Size

Clay has the smallest particle size of any soil type, with individual particles being so small that they can only be viewed by an electron microscope. This allows a large quantity of clay particles to exist in a relatively small space, without the gaps that would normally be present between larger soil particles. This feature plays a large part in clay’s smooth texture, because the individual particles are too small to create a rough surface in the clay.

Structure

Because of the small particle size of clay soils, the structure of clay-heavy soil tends to be very dense. The particles typically bond together, creating a mass of clay that can be hard for plant roots to penetrate. This density is responsible for clay-heavy soil being thicker and heavier than other soil types, and clay soil takes longer to warm up after periods of cold weather. This density also makes clay soils more resistant to erosion than sand or loam-based soils.

Organic Content

Clay contains very little organic material; you often need to add amendments if you wish to grow plants in clay-heavy soil. Without added organic material, clay-heavy soil typically lacks the nutrients and micronutrients essential for plant growth and photosynthesis. Mineral-heavy clay soils may be alkaline in nature, resulting in the need for additional amendments to balance the soil’s pH before planting anything that prefers a neutral pH. It’s important to test clay-heavy soil before planting to determine both the soil’s pH and whether it lacks important nutrients such as nitrogen, phosphorus and potassium.

Permeability and Water-Holding Capacity

One of the problems with clay soil is its slow permeability resulting in a very large water-holding capacity. Because the soil particles are small and close together, it takes water much longer to move through clay soil than it does with other soil types. Clay particles then absorb this water, expanding as they do so and further slowing the flow of water through the soil. This not only prevents water from penetrating deep into the soil but can also damage plant roots as the soil particles expand.

Identifying Clay

There are several tests you can use to identify clay soils. If rubbed between your fingers, a sample of clay soil often feels slick and may stick to your fingers or leave streaks on your skin. Rubbed clay soil often takes on a shiny appearance as well, as opposed to the rough texture you would see with other soils. Clay soils do not crumble well, and a sample of clay can typically be stretched slightly without breaking. When wet, clay soils become slick and sticky; the soil may also allow water to pool briefly before absorption due to the slow permeation. Visually, clay soils seem solid with no clear particles, and may have a distinct red or brown color when compared to the surrounding soil.

Characteristics of Clay Soil

  • very small particles with diameter less tha 0.002mm
  • many pore spaces
  • sticky to touch and swells when wet, shrinkc when dry
  • forms aggregates or lumps
  • high water holding capacity
  • high mineral holding capacity
  • large surface area because of its fine particles
  • called heavy soily because it is difficult to cultivate
  • particles are tightly parked hence there is poor drainage and circulation of air
  • cooler soil with more uniform moisture content

Silty Soil

 

SUB TOPIC 3:SOIL PH.

Soil PH is a measure of the degree of the acidity or alkalinity of a soil. It is usually measured with a pH metre. A soil can either be acidic or alkaline.

An acidic soil is usually corrected by liming. Liming materials includes: calcium carbonate, calcium hydroxide, wood ash, calcium oxide, calcium bicarbonate and magnesium oxide.

CAUSES OF SOIL ACIDITY.

1. Prolonged use of inorganic fertilizers.

2. Crop removal.

3. Leaching.

Soil acidity can be determined by using

i. Litmus paper

ii. BDH universal soil indicator method

iii. Colour metric method

iv. Electro metric method

EFFECTS OF SOIL ACIDITY.

1. It is toxic to plant roots.

2. It interferes with absorption of some plant nutrients

3. It reduces the activity of soil organisms.

EVALUATION

  1. What is pH scale?
  2. If a pH metre indicate 7 ,what does it signify?
  3. List three effects of soil acidity

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Chemical Properties of the Soil

Soil pH, Acidity and Alkalinity

pH can be defined as the measure of the concentration of Hydrogen and Hydroxyl ion in the soil. It can also be referred to as the degree of acidity and alkalinity of the soil. The hydrogn ion concentration is called the acidity while the hydroxyl ion concetration is called the alkalinity. Any soil pH below 7 is acidic while pH above pH above 7 is alkaline. Soil wih pH of 7 is neutral. pH 2 is strongly acidic, pH 6 is slightly acidic, pH is neutral, pH 8 is slightly alkaline and pH 13 is strongly alkaline.

DenominationpH range
Ultra acid< 3.5
Extreme acid3.5–4.4
Very strong acid4.5–5.0
Strong acid5.1–5.5
Moderate acid5.6–6.0
Slight acid6.1–6.5
Neutral6.6–7.3
Slightly alkaline7.4–7.8
Moderately alkaline7.9–8.4
Strongly alkaline8.5–9.0
Very strongly alkaline> 9.0

Soil Acidity

A Soil is acidic if it’s level of hydrogen ion concentration is below 7 when measure with PH scale i.e.

The causes of soil acidity are;

  1. Leaching: Excess rainfall leaches base cation from the soil, there by increasing the percentage of acidic ions in the soil.
  2. Use of acidic fertilizer: Ammonium (NH4+) fertilizers react in the soil in a process called to form nitrate (NO3), and in the process release H+
  3. Presence of acid parent materials.
  4. Plants nutrient update.
  5. Presence of sulphur in the soil.
  6. Removal of product from the farm or paddock
  7. Weathering of Minerals
  8. Inappropriate use of nitrogenous fertilizers
  9. Build up in organic matter

Soil acidity can be removed by using liming materials rich in calcium. Soil acidity can be corrected easily by liming the soil, or adding basic materials to neutralize the acid present.

It can be removed by the application of

  • Limestone
  • Slaked lime
  • Quick lime
  • Calcium Hydrogen Trioxocarbonate
  • Wood ash
  • Basic slag

Soil acidity can be determined by using

  • litmus paper
  • BDH universal soil indicator method
  • Colourimetric method
  • Electrometric method

Soil Alkalinity

   This is caused by excess quantities of soluble minerals in the soil. Alkaline soils have a high saturation of base cations (K+, Ca2+, Mg2+ and Na+). Soil alkalinity is also an accumulation of soluble salts which are classified as either:

  1. Using acidic fertilizer
  2. Application of Sulphur to the soil.
  3. Use of irrigation to dissolve some of the salts.
  4. Soil Composition

The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air.

  1. Mineral matter– obtained by the disintegration and decomposition of rocks;
  2. Organic matter– obtained by the decay of plant residues, animal remains and microbial tissues;
  3. Water– obtained from the atmosphere and the reactions in soil (chemical, physical and microbial);
  4. Air or gases– from atmosphere, reactions of roots, microbes and chemicals in the soil
  5. Living Organisms– both big (worms, insects) and small (microbes)
  6. Importance of the Soil
  7. Soil provides required nutrients for growth and development for plants.
  8. Man plants his crops on the soil.
  9. Man also builds his shelter on the soil.
  10. Soil also serves as habitat for some animals and useful micro organisms.
  11. Food and other biomass production
  12. Environmental Interaction: Storage, filtering, and transformation
  13. Biological habitat and gene pool
  14. Source of raw materials
  15. Physical and cultural heritage
  16. Platform for man-made structures: buildings, highways

 

REFERENCE TEXTS

  • Junior Secondary Agriculture For Nigerian Schools By Anthony Youdeowei et al
  • New Intensive Agricultural science For JSS 1-3 By E . U Okaro
  • Prescribed Agricultural science for JSS by S.A. Omoruyi et al
  • Essentials Of Agricultural Science For JSS And Colleges By Earnest Chukwudi