Agric soil structure

• soil structure


  is the arrangement of soil particles
• the 4 principal types of soil structure

  1. spheroid structure(Granular)
  2. Platy
  3. blocky
  4. Prism-like
• Two types of Prism like structure

  1. prismatic - are vertically longer and their aggregates have flat tops
  2. columnar - are vertically longer and their aggregates have rounded tops
• Properties of platy structure

  1. occur in clay pan soil
  2. Are thin , flat plates of soil that lie horizontally
  3. are usually found in compacted soil
  4. permeability of such soil is limited
• Properties of prismatic

  1. are vertically longer and their aggregates have flat hops
  2. usually found in lower horizons
• properties of columnar

  1. are vertically longer and their aggregates have round tops "salt caps"
  2. found in soil of arid climates
• Properties of blocky

  1. are blocky structures that are usually 1.5 to 5.0 cm in diameter
  2. mostly found in B-horizons
• spheroid(granular)

  1. can be sun-divided into granular and crumb structure
  2. granular the aggregates are round and hard whereas with the crumb aggregates are softer and more porous
  3. the crumb is the most desirable structure for vegetation because of the presence of organic matter
• properties of granular

  1. found in the A-horizon (top soil)
  2. commonly found in surface horizons where roots has been growing
  3. resembles cookie crumbs and is usually less than 0.5 cm in diameter
• size classification

  1. thin or fine
  2. medium
  3. coarse or thick
  4. very fine
  5. very coarse
• Soil structure naming

  1. grade
  2. class
  3. type
• Factors influencing the development of soil structure

  1. soil texture
  2. organic matter content
  3. soil water content
  4. microorganisms
• Development of structures

  1. Amorphous soils ( massive and single grain) can be formed as the result of too low clay content which can swell
  2. In dry regions, soluble salts, play a role in the development of a prismatic structure
  3. A crumb structure in the top is closely allied with the organic content of the A-horizon. the organic content is dependent on vegetation which is a function of the climate of the region
• Factors which play a role in the development of soil structure

  1. The colloidal matter in the soil
  2. the type of mineral present
  3. Climate
  4. Alternating moisture and drought
  5. Plant roots
• The colloidal matter in the soil


  the cohesive forces between clay particles are very strong and these play an important role in the formation of aggregates. The humus colloid also play an important role where the clay content is very low
• The type of clay mineral present


  they type of clay mineral present will determine the type of soil structure that will be formed . Montmorillonite-clay is inclined to form blocky structure and kaolinite is inclined to form a platy structure.
• Climate


  Rainfall and temperature influence the erosion of primary minerals. From the erosion of primary minerals secondary minerals which determine the structure type will originate. Also the present of humus is determined by the vegetation which is directly dependent on the climate
• Alternating moisture and drought

  a hard compact clod will be broken up into smaller aggregates if it is alternately wet and dry. Drying out cause shrinking of soil masses which in turn lead to cementing of the clay particles . Tension occurs in the soil mass which causes it to break up into clods to form aggregates
• Formation of Peds

  1. when clay shrinks, cracks are formed
  2. organic matter and fine particles move into cracks
  3. when moist clay swells , the cracks close
  4. the material in the cracks is compressed
  5. lines of weakness are formed
• what separates aggregates?


  Lines of weakness
• influence of plant roots on the development of soil structure

  1. the penetration of roots into clods
  2. cause a compression of soil away from the roots
  3. lines of weakness are formed
  4. the clod will break up and form aggregates
  5. the dense root mass of sod crops dies off
  6. to form organic colloids which act as a stabilser
• Formation of aggregates


  1. A clod will break up

  2. Dense root mass of sod crops dies off

  3. Organic colloids act as stabilisers
• Flood irrigation


  Leads to rapid absorption, compresses air in soil pores, can break up aggregates
• Usage of heavy implements

  Leads to compaction of soil and weakens soil structure
• Tilling of dry soil

  Tends to powder aggregates
• Tilling of wet soil


  Leads to smearing of aggregates
• Continual cultivation


  Increases oxygen content, stimulates microbe activity, decomposes organic matter, deteriorates soil structure