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What is Texture Class?

The term describing the proportions of large and small particles (for example, sand, silt and clay) is called ‘Texture Class’.


What are Texture Grades?

A range of different particle sizes (texture grades) is recognised in soils.


What is the order and size of Texture Grades?

Gravel >2000
Coarse Sand 200-2000
Fine sand 20-200
Silt 2-20


What are pores in soil?

Spaces into which clay particles,water and air are retained.


What does packing of large particles result in?

Clearly, packing of large particles (sand) results in large pores so there are plenty of air filled pores and few small pores holding water.
Conversely, when clay particles are packed many small pores are produced so that aeration is less satisfactory and much more water is held in the soil.


What is sand made up of?

Quartz Si02


What are feldspars

Silts are often made from feldspars.(K,Ca and Na silicates)


What main minerals do feldspars contain?

KAl Si3O8 orthoclase
NaAl Si3O8 albite
Ca Al2 Si2 O8 anorthite


What particle size is Sand and Silt?

2-2000 um


What do sand and silt comprise of?

These coarser fractions contain minerals largely derived by breaking up of the parent rock from which the soils were formed. Consequently they commonly contain minerals (minerals are chemical compounds in soils and rocks) that are resistant to chemical breakdown.


What is the particle size of Clay?


What is clay comprised of?

The clay particles are very small and therefore have a large surface area per unit weight on which reactions can occur. Thus, they are important for nutrient and water retention.


What are the minerals formed in clay fraction of soil?

• crystalline layer silicates;
• non crystalline Fe and Al oxides and hydroxides (sesquioxides), CaCO3, MnO2.


What are Crystalline Layer Silicates?

The layer silicates are made up of flat segments stacked one on top of each other like a stack of plates. Each plate is made up of layers of Al(OH)6 or SiO4 (these are subsequently abbreviated to Al and Si layers).


What is the principle mineral in Layer Minerals ?

The principal mineral is kaolinite. Here, each segment or plate is made up of one layer of Al and one layer of Si.


What is basal spacing?

The distance between the bottom of one plate and the bottom of the next is called the basal spacing. For kaolinite, the basal spacing is 0.7 ηm. A ηm is 0.000000001 of a metre (1 x 10-9m).


What are the segments or plate of layer minerals made up of?

Each segment or plate is made up of one Al layer sandwiched between two Si layers.


What are the other important mineral layers?

illite - the interlayer space contains K+ and the layers cannot swell when water enters the space. The basal spacing is 1.0 ηm.
vermiculite - the K+ is absent in the interlayer space and Ca2+ and Mg2+ are present in the interlayer. As these larger cations occur in the interlayer space, the basal spacing is about 1.4 ηm.
smectite (montmorillonite) - many cations are present in the interlayer space and considerable quantities of water can enter this interlayer space. This mineral swells causing considerable swelling and shrinking (with associated cracking) of soils containing this mineral as they wet and dry. The swelling and shrinking is associated with water entering and leaving the interlayer space respectively. Hence, the basal spacing is variable ranging from 1.5 - 4 ηm.


What is meant by Isomorphous Substitution?

Isomorphous substitution results in the minerals carrying negative charge on their surface.
If Al3+ with 3 positive charges is replaced by Mg2+ with only 2 positive charges the mineral has one less positive charge than negative charges so the particle becomes overall negatively charged. This happens without altering the physical structure of the mineral - therefore it has only one form (isomorphous) although elements within the structure have been substituted.


What are Non Crystalline Minerals?

As soils are formed from their parent material, various oxides of iron, aluminium (known as sesquioxides) and manganese are produced. In semi arid areas, concentrations of CaCO3 (lime) or CaSO4 (gypsum) may occur at any depth within the soil.


What is organic material made up of?

Derived from living organisms not from rocks. The organisms involved include plants, animals as well as macro (worms etc.) and micro (bacteria, fungi etc.) fauna;

Based on a carbon skeleton. Organic matter can be broken down into carbohydrates, proteins etc. but little is known about the structure of the original organic molecule as it occurred in the soil;

Normally represented as Organic Carbon and has concentrations of only 1-5% in most soils in Australia, that is, 1g of carbon in every 100g of soil. This is much lower than most overseas arable soil, where up to three times as much organic matter is present. Note: %OM ≈1.75 x OC%


What is the organic fraction made up of?

Fresh animal and plant remains;

Partly decomposed (slightly resistant to decomposition) plant and animal remains;

Residues resistant to decomposition known as humus;

The ‘biomass’ of the living macro- (worms etc.) and micro- (bacteria, fungi) organisms. These have different functions, not all are good (eg pathogens).


Why is organic matter important to soils?

While nutrients especially N, P, and S are bound up in the structure of skeleton of organic matter they are not immediately available for plant uptake but they are not prone to loss from soil by leaching. They can eventually be released for plant uptake.


What is nutrient retention?

Retention results from:

• the organic matter carrying negative charge like the clay minerals, as described in the previous Section. In organic matter the negative charge results mainly from the hydrolysis of organic acids. The equation below represents one such acid grouping (carboxyl: -COOH) on the edge of a piece of organic matter.


How does the organic fraction bind soil?

The organic fraction binds individual soil particles into stable structural aggregates and will influence the movement of air and water as well as the ease of root penetration through soils.


How doe organic matter provide energy in soil?

Organic matter provides ‘energy’ for soil microorganisms. Without these organisms, nutrients would not be released from organic matter into plant available forms.


How does soil protect?

The surface soil is protected from rain drop or machinery damage where mulches of organic residues are present. Organic mulches and stubble reduce the formation of crusts on the soil surface. These crusts impede germination and restrict entry of rainfall into soil.


How is organic matter broken down?

Organic matter provides the energy and nutritional base for soil microbial growth. The large organic molecules are enzymatically broken down and then oxidised to yield energy for the microorganisms, (the same way we use sugar for energy.) The enzymes are produced by the microbial population.


What does Net Immobilisation mean?

This means that any supply of nutrient in the plant available inorganic form in the soil will be used by the microorganisms (not available to the plants).


How can soil management be implemented?

i) Continuous cultivation or soil disturbance and cropping
ii) Continuous vegetative cover: (Example, pasture or grass cover)
iii) Green manure crops, lawn clippings, animal waste return


What factors influence the rate go organic matter breakdown?

• aeration (O2 supply). Microbial respiration rate is higher in well aerated soils. Under conditions of very low O2 concentrations in soil, for example in waterlogged soils, organic matter accumulates due, in part, to the slow respiration rate.
• temperature. Mineralisation is normally very slow when temperatures occur below 6 10oC.
• pH. The optimum pH for mineralisation is around neutral.
• moisture. Restricted breakdown occurs in both wet (near saturation) and dry (below wilting point). See Section 5.3 for definition of these terms.
• toxic compounds (e.g. polyphenyl) in organic matter.
• the presence of organic structures in plant tissues that are complex in structure and hence resident to decomposition, for example, lignin.
• and finally, the C/N ratio (C/P or C/S for P and S deficient soils respectively.)


What are soil aggregates?

The individual sand, silt and clay particles do not remain separate in the soil mass. They are joined to form larger units known scientifically as peds but are commonly called: aggregates.


What the organisation of soil particles called?

This organisation or arrangement of the soil particles is called soil structure.


How does structure have an influence on pore size distribution?

Structure has a marked influence on the pore size distribution and thus physical aspects of soil fertility, for example:

Ease of root development (the more compact the soil, the more difficult this is).

Entry, storage and drainage of water from soil (where aggregates are rounded, water can move rapidly through the large channels between the aggregates).

Exchange of CO2 from soil and O2 into the soil.

Ease of cultivation (less compaction, easier to cultivate).


What can bad management of soils do?

Because structure is the arrangement of particles, the particle arrangement can easily be altered by bad management practices such as:

Excessive cultivation (crushing aggregates).
trampling by stock or use of heavy machinery when the soil is wet.
irrigation with large water droplets (drops smashing aggregates).


How is the formation of soil structure formed?

As two clay particles come together to aid in structure formation, the cation swarm comes in contact first. There is a tendency for the particles to repel each other. (This is due to the electrostatic force of repulsion, ie. like charges repel).

However, IF the clay particles can get very close, there are a number of forces that are numerous but weak. (These are van der Waals, polar molecular forces.) These weak forces operate over short distances and can draw the clay particles together if they can get close enough. The movement of the clays together will occur IF the layer of adsorbed cations is thin.


What three things influence the cation layer?

Higher valency cations. i.e. cations carrying more than one charge, for example, Ca2+ or Al3+ versus Na+ (more charge per ion means fewer ions and thin layers result).
Lower hydrated ionic radius. i.e. cations that adsorb little water on their surface.
Higher salt concentration in the soil water. A high salt concentration in soil water (many dissolved ions) compresses the cations around the clay giving thin layers and enabling clay particles to attract each other.


What are the two groups of Aggregate Size?

The mechanism of stabilisation depends on the size of the aggregates. Aggregates can be divided into two size groups:

a. macroaggregates being those with a diameter greater than 250 um (0.25 mm), and
b. microaggregates being those with a diameter less than 250 um.


How are Microaggregates bonded together?

1. Cation Bridges between organic matter and inorganic clays.
2. Clay flocculation.
3. Attraction of inorganic clays carrying opposite electric charge
4. Organic matter quartz bonds.


What causes structural deterioration?

i. Cultivation
One of the functions of cultivation is to break up the soil structure to produce a seed-bed of fine aggregates that will enable good germination.
ii. Effects of water

Entry of water into soil can produce two types of structure breakdown:
a. slaking: Only a portion of the structural bonds are broken so that the aggregates are reduced in size.
b. dispersion or deflocculation:


How does water influence structural breakdown?

Raindrop or irrigation damage.
As water hits the ground, the force breaks the bonds holding soil aggregates together. Obviously the greater the intensity of the storm or application rate of irrigation water and greater the duration of that event, the greater the structure breakup.

Rapid entry of water into dry soil. This results in compression of air in pores which forces the aggregates apart. The effect of this ‘entrapped air’ normally results in slaking.

Swelling clay minerals. Aggregates in soils containing swelling clay minerals (for example, montmorillonite/smectite) expand when wet which again disrupts the bonding of the particles. Slaking is normally the outcome.

Water containing high Na+ levels. High Na+ levels adsorbed on the negatively charged clays often results in complete separation of the clay particles, that is, dispersion. Use of irrigation waters containing high Na+ levels can result in very severe structure breakdown.


What management techniques can be used to improve soil structure and stability?

Sowing down, for example permanent vegetative cover (pasture). Soil from under pasture is normally well structured and the aggregates stable due to the constant release of organic residues.
Incorporation of organic residues. These may be plant residues, a green manure cover crop, spreading of animal dung or adding compost.
Synthetic organic materials. The effects on structure of organic materials such as poly vinyl alcohol, dextrins etc. have been studied during research programs.


What is the description of Soil Structure?

It is common, especially during soil surveys, to describe soil structure according to the following:
Where no aggregation is apparent, the soils are considered structureless.
This may be seen where sand grains are loose and non aggregated (like a beach sand), that is, single grained structureless or where the soils are compact, that is, massive structureless (like a hard compacted layer).
Where aggregation is apparent they are described depending on:
- shape of aggregates (type of structure) where terms such as platy, granular, blocky etc. are used.
- How easy it is to see aggregates (degree of development);

- size of aggregates.