chemical

• The smallest and most chemically active particles in soil are clay and humus particles
• Clay and humus particles attract ions due to the charges on their surfaces
• The attraction of ions by clay and humus particles affects plant nutrition and soil pH
• Chemical properties of soil include:

  • Flocculation
  • Cation exchange
  • pH
  • Plant-available nutrients
• Flocculation is the clustering together of soil particles to create larger structures called floccules
• Positively charged cations are attracted to negatively charged humus and clay particles
• Cation exchange is the ability of soil particles (clay and humus) to attract, retain and release cations
• Cations are positively charged ions, e.g. H+, K+, Ca2+
• Cation exchange capacity is the quantity of cations that a soil adsorbs
• Colloidal clay and colloidal humus have the highest rates of cation exchange in the soil
• pH is a measure of the concentration of the hydrogen ions in a solution
• Factors affecting cation exchange include:

  • Humus content
  • Clay content
  • Soil texture
  • pH
• Hydrogen and aluminium ions (H+ and Al3+) are acidic ions
• Calcium and magnesium (Ca2+ and Mg2+) are alkaline ions
• The optimum pH level for crop growth is between 6.5 and 7.5
• Outside these ranges, a very low or very high pH can reduce the availability of nutrients to plants
• The activity of some microorganisms may decrease because of unsuitable environmental conditions
• Soils derived from sandstone or granite are acidic because of their hydrogen and aluminium content
• Soils derived from limestone are alkaline because of their calcium and magnesium content
• Most crops will grow in a pH range of 5.5–8.5
• Optimum pH for crop growth is 6.5–7.5
• Liming involves spreading ground limestone on the soil
• Ground limestone contains calcium and magnesium which neutralise acidic ions in the soil
• Neutralisation is achieved through cation exchange
• Ground Limestone requirements:

  • Moisture content no greater than 2.5%
  • Total Neutralising Value (TNV) no less than 90%
  • All ground limestone should pass through a 3.35mm sieve and not less than 35% of it should pass through a 0.15mm sieve