This is the degree of acidity or alkalinity f a soil. This depends on hydrogenous concentration.

The pH range is experienced using a number line.



In the soil solution with greater H+, it will be acidic and the smaller the number, the more acidic soil is e.g. 5.8 is a weak cid and 3 is strong acid.

Alkalinity increases with increasing numbers e.g. 8 is a weak alkaline 12.5 is strong.

Importance of soil ph

  • Determines the presence and activity of desirable soil organisms e.g. in strongly acidic soils, the nitrogen fixing bacteria will not be active.
  • Determines the type of crop that can be grown in an area since each crop has a pH range in which it grows best e.g. highly acidic soil will have toxic levels of aluminum and manganese which most plants can’t tolerate.
  • Affects availability of nutrients and their up take by plant roots.
  • Acidity causes disintegration of clay minerals and encourages leaching i.e. so much nutrients like iron, calcium, magnesium will be lost. Phosphate becomes and unavailable copper, zinc and Aluminium are also reduced.
  • When soils are very alkaline, root growth is restricted. Clay particles move downwards and form an important meable clay pan.
  • Affects prevalent of certain plant pathogens in acidic soils, fungal disease are prevalent.

Determining Soil pH

There are two methods:

Use of a pH meter

This is an expensive equipment do its not used in schools.

Colour dye indicators


  • Barium sulphate –        25 cm3 measuring cylinders
  • Universal indicator
  • Different dry soil samples


  • Measure 30cm3 of each soil into measuring cylinders (250cm3)
  • Add 20 cm3 of Barium Sulphate. This is chemically inactive but but will break down soil and help particles to settle at the bottom of the cylinder.
  • Distilled H20 is added till the cylinders are
  • A few drops of universal indicators are added.
  • The mouth of the cylinder is covered shaken vigorously then left to settle for a about 45 minutes.


A  Clear area will be seen in the middle of the cylinder.  It will be placed against the coloured chart where different colours represent pH ranges and results filled in the table.

Soil  Sample Colour of Clear Area pH Range
Sandy 6.5 Slightly acidic
Loam 7.5 Slightly Alkaline
Clay 5.5 Strongly Acidic

Causes of soil Acidity

Parent Rock: from which soil has been formed granite soils from which sandy soils have been formed, have excess silica which can – with water to form a silicic acid making sandy soil acidic.

Rain water: Carbon dioxide in the atmosphere can be dissolved in rainwater to form a weak acid i.e. carbonic acid which flows into the soil.

Application of acid forming fertilizers: Every ammonium sulphate, sulphuric acid are used by plants i.e. CNH4 SO4 + HO2 = 2HNO3 + H2SO4 + H2O

Removal of Soil Acidity

Addition of agricultural lime e.g calcium carbonates, calcium hydroxide.

Effects of Agricultural Lime:

  • Neutral acidic affects.
  • Improves physical condition of soil, clay soil becomes easier to cultivate + aggregates sandy soil particles improving water holding capacities.
  • Prevents plant diseases which flourish in acidic soils.
  • Converts some soil compounds e.g phosphates into form unusable by plants.
  • Provides nutrients essential for crop growth e.g calcium and magnesium.


  • Addition of other materials rich in calcium e.g paper mill, refuse wood arsh, sugar factory lime
  • Drawing land to control water logging/lodging.
  •  Addition of organic matter to prevent excessive leaching
  • Irrigating sandy soil to dilute acidity.

Over liming Injury

It will take soil pH to high reducing availability of nearly all plant nutrients especially iron, magnesium, zinc, baron, phosphorus.

Correction of excessively alkaline soils

Use of adequate amounts of acid forming fertilizers e.g. sulphate of ammonia, ammonium nitrate.

Experiment G

Experiment to Complete the Rate of Which Water Moves Through Different soils (Drainage)


  • Three 250ml measuring cylinders
  • Three 100ml measuring cylinders
  • Three filter funnels
  • Filter paper/cotton wool
  • Three dry soil samples:- sand, clay, and loam
  • Water
  • Stop clock


  1. Plug the two filter funnels with either cotton wool or filter paper.
  2. Put equal volumes of soils in each if the funnels (about 30ml).
  3. Put each of the funnels over the 250cc measuring cylinders and mark them.
  4. Use the 100cm3 measuring cylinder to measure 50cm3 of H2O (water)
  5. Pour the H2O over the three (3) soils at the same time.
  6. Use the stop clock to note the starting time.
  7. Note which soil releases, the time taken to drain 2cc, 5cc, 10cc and all the water out of soil and complete the table below.
Soil Sample Time for 1st drop to drain Time to drain 5cm3 ml Time to drain 10cm3 Time for all the water to drop
Sand    7 sec    50 sec    120    360
Loam    62 sec    72 sec    200    400
Clay    122 sec    190 sec    220   480

Qn 1.  Which soil drains fastest and why?

Qn 2.  Which soil drains slowest?

Qn 3.  Plot a graph of the amount of water drained against time.





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