Soil Composition-physical properties

Soil Composition

Soils are made up with the following components.
 Mineral particle (Sand, Silt and clay)
 Dead organic matter (Humus)
 Living organic matter (Plant roots and bacteria)
 Water
 Air
 Mineral salts

• Ideal Composition of Soil
  The mineral matter comes from the parent material and the organic matter from the remains of vegetation growing on the soil.
  The jagged line means the percentage of water and air is always changing.
  Total pore space should remain constant at 50%.
• Mineral Matter
• Made up of mineral particles of different sizes.
  Classified based on size:-
  –Gravel = > 2 mm
  –Coarse Sand = 2 mm - 0.2 mm
  –Fine Sand = 0.2mm – 0.02mm
  –Silt = 0.02mm – 0.002mm
  –Clay = < 0.002mm
• Sand and Gravel
  Large particles
  Wide pores
  Free draining and therefore well aerated.
  Can suffer drought in dry periods.
  Do not add fertility to soil.
• Silt and Clay
  Chemical weathering creates new clay minerals.
  Important sources of plant nutrients e.g. K, P, Ca, Mg. This is called natural or inherent fertility because it occurs naturally.
• Large surface areas covered in negative charges. This allows them to hold onto nutrient cations (K+, Ca++) and stop them from being washed out of the soil (leached). They then release the ions for plant growth.
  This is known as ion exchange. Clay is better than silt at ion exchange and colloidal clay is best.
• Organic Matter
  Can vary from plant and animal debris to fully decomposed material (humus).
  As with mineral matter as organic particle size decreases they become a better source of nutrients and become more involved in ion exchange.
  Humus is in fact better at ion exchange than clay and colloidal humus is better than colloidal clay.
• Humus is the product of the breakdown of organic matter.
   It is the remains of dead animals and plants.
   Fully decayed material forms CO2, water and mineral salts. But the incomplete product is a dark sticky material called HUMUS.
   Normal sandy soils as mentioned before are free draining and soil water could leach out (including the minerals dissolved in it).
   Humus in the soil absorbs the water and can greatly improve the quality of otherwise poor (sandy) soils.
   Both clay and humus can hold minerals in the soil. The minerals are in the form of cations.
   This is known as cation exchange capacity.
   Humus is useful to the soil because:
  i. It contains minerals (the type and amount depend on the source of the humus).
  Leaving Certificate Ag. Science – Soil Science
  ii. It holds minerals in the soil due to its high cation exchange capacity.
  iii. It improves and strengthens the crumb structure of heavy soils.
  iv. It forms clay humus complexes, which improves soil stability.
  v. Its dark colour improves the warming capabilities of the soil (i.e. it is able to absorb more heat from the sun)
  vi. Can make the soil more acidic (an advantage and disadvantage depending on the use of the soil).
   There are two categories of humus: Fast decaying and slow decaying.
   Fast Decaying Humus:
   Usually formed from soft parts of plants (cellulose based material)
   Important as food source for earthworms and bacteria.
   The material decays in a matter of months.
   Slow decaying Humus: Usually formed from hard parts of plants (lignin based materials)
   It is important for soil improvement (See the points above)
   Decays over a period of several years and forms stable complexes with clay particles.
  
   Soils are grouped also by the amount of humus in the upper horizons.  < 10 % O.M. Mineral soil.
   10 – 17 % Humus soil
   17 – 35 % Slightly peaty soil
   35 – 50 % Peaty soil
   > 50 % Peat (Turf)
  
  
  

 

• Soil Air
  Found in soil pores.
  Slightly more carbon dioxide and less oxygen than atmospheric air due to plant roots breathing (respiration).
  If the carbon dioxide cannot diffuse up to the surface it builds up causing carbon dioxide poisoning of the plants.
• Soil Water
  Held in the soil by capillary forces.
  Large pores don’t hold onto water tightly but small pores do.
  Soil water contains plant nutrients in solution and so is sometimes called the soil solution.

 

• Soil Texture
  Defined as the percentage of sand, silt and clay in the soil.
  For example soil could be sandy, silty, clayey or a loam (equal amounts of sand, silt and clay).
  Most important soil property because it is fixed and it influences other soil properties.

 

• Soil Structure
  Defined as the coming together of the primary soil particles (sand, silt, clay) into larger, separable units called peds or aggregates or crumbs.
  Structure formation increases soil pore space and therefore drainage and aeration.
  The first step in structure formation is flocculation where colloidal particles flock together into clusters.

 

• Flocculation
  Occurs when the negative charges on the colloidal particles are satisfied by polyvalent cations.
  This causes the particles to flock together to form floccules that act as cement between the larger soil particles.
  Some cations are better than others at promoting flocculation but the ones that are best at it, Al and Fe, are abundant in Irish soils. Irish soils have a high degree of structural formation.
• Structure in the Field
  Structure in the field is formed by cementation and separation.
  The following encourage cementing and separation:-
  –Wetting and drying
  –Freezing and thawing
  –Activity of roots
  –Activity of earthworms
  –Tillage operations
  Good structure is usually found in topsoil and gets poorer the further you go down.
  Poor structure is also found in compacted areas e.g. in gateways and on tracks.
  The best structure is usually found under permanent grassland where there is high levels of organic matter, earthworms and roots.
• Soil Aeration
  Ideally 25%.
  Need large pores for air to get in and out of soil.
  Structure increases pore size and so also aeration.
  If there is soil compaction a soil ripper or subsoiler can help to aerate a soil.
• Soil Water
  Held by adsorption and capillary forces.
  Adsorbed water is so tightly held it is not available for plants.
  Water in the larger capillaries is available for plants but that in very small capillaries is not.
  Gravitational water in very large pores is available when it is present but it drains away quickly.
  When all the pores in a soil are full of water it is saturated.
  When the gravitational water drains away the soil is at field capacity.
  When the plants have extracted all the water that they can the soil is at permanent wilting point.
  The water that the plants extract between field capacity and permanent wilting point is called the available water capacity of the soil.
  Moisture stress is when plants have to exert a lot of pressure to remove water from the soil as it reaches permanent wilting point.
• Soil Temperature
  Affects all physical, chemical and biological processes in the soil.
  To increase temperature we try to reduce water content and increase air content because air is 4 times easier to heat up than water.
  Well drained soils heat up earlier in spring and so crop growth commences earlier and higher yields are obtained.
• Soil Colour
  Somtimes soil colour is considered a physical property of the soil.
  If the colour is dark brown the soil tends to be high in humus and is therefore more fertile.
  Iron gives soil its red colour when combined with oxygen.
  If the soil is light in colour it is a sign that the soil has been leached of nutrients and iron.
  If a soil is dark it can absorb more sunlight and is therefore warmer.
• Soil pH
   pH is a measure of the acidity of a soil.
   It is important to consider because plants require certain pH’s to grow well.
   If pH changes the plant may not be able to produce as much, or may in fact die as a result.
   Earthworms and soil bacteria thrive in neutral or near neutral conditions.
   In soils, pH ranges mainly from 4 to 9.
   Soils can be classified by their pH values