Loam soil. Image credit betterground.org

Loam soil. Image credit betterground.org

ZIMSEC O Level Combined Science Notes: The physical components of the soil


  • Soil is made up of rock/soil particles
  • Some soils are easy to compress because they have air in them
  • Soils are usually moist
  • Soils also have dissolved minerals in them

Sand, Loam and Clay soils

  • Different types of soil have different characteristics
  • For example sandy soils are different from clay soils
  • Sand soil is light and composed loose particles
  • Sand soils also have larger particles and there is more air spaces between the particles
  • They thus drain more quickly and do not retain water
  • Clay soil is darker, heavier and more compact
  • Clay soils have small particles and there is little air between these particles
  • They retain water because they drain much more slowly
  • Loam soils have properties that are between those of sand and clay soils
  • Loam soils also contain rotted plant matter

Minerals in the soil

  • All soils contain mineral salts
  • These salts are dissolved in the soil water or they may be combined with the clay particles
  • When soil water drains through the soil it carries the minerals with it
  • This is called leaching
  • Leaching-is when water dissolves and carries minerals through the soil.
  • Sand soil leaches easily as it drains much more easily
  • Leaching takes away all the vital elements that plants need for growth
  • Farmers need to reduce leaching by maintaining a healthy soil structure
  • This can be done by adding manure and compost to the soil on a regular basis
  • Whenever rain occurs there is run off and infiltration
  • Run off-this is when water flows away
  • Infiltration-this is when water filters into the soil
  • Infiltration is reduced when the soil is hardened for example as a result of animal hooves beating the soil or as result of erosion
  • Water must penetrate the ground to maintain underground reserves
  • This can be done by ploughing and
  • ity/alkalinity of the soil
  • If soils are too acidic lime can be added to reduce its acidity
  • digging the soil
  • This ensures not too much water is lost via run off and more of it penetrates the soil

Soil PH

  • Plant roots and organisms are also affected by the acid
  • If the soil is too alkaline manure/compost can be added
  • Farmers find it beneficial to know the PH of the soil and to keep it at optimum levels
  • Most soils have a PH of around 6-7
  • They should have their soils tested regularly at soil laboratories or use portable test kits
  • They can therefore find out which crops best grow in their types of soil
  • Different crops favour different types of soil
  • For example cotton prefers dark clay soils
  • Maize grows in a wide variety of soils
  • Tobacco grows in light sandy soils with good drainage
  • Climatic conditions also need to be taken into consideration when choosing the crops to grow

To access more topics go to the Combined Science Notes page.