Soil Chemical Properties

Soil Chemical Properties
Soil Chemical Properties

Minerals originate from slow breakdown of parent
material
Mineral salts from soil particles dissolve in soil water
Inorganic salts required by plants for nutrition
Hence study of Soil Chemical Properties

Chemical Properties
The main properties of concern to us are:
1. Soil Organic matter
2. Soil pH
3. Soil particle – soil nutrient relationships

Soil Organic Matter
Living organisms together with decaying residues
Decaying residues known as Humus
Humus
is dead organic matter in state of continuous decomposition
promotes activities of soil organisms (as host and food source)
adds nutrients to the soil
improves structure e.g. binds sandy soils
conserves soil moisture
prevents erosion
Soil pH
Term used to express of degree of acidity and
alkalinity
Measured on a scale from o-14
Neutral on scale = 7
< 7 is acidic
> 7 is alkaline
Testing done to measure soil acidity or alkalinity
Soil pH Testing
Use of a soil indicator to test acidity or alkalinity
An indicator is a substance that allows changes in
colour
Colour changes compare to chart that allows you to
measure soil acidity
Effect of pH on plant growth
Incorrect pH prevents soil from taking up nutrients
Crops sensitive to pH changes
Each crop has a minimum, maximum and optimum
pH
Minimum – pH below which crop would not grow
Maximum – pH above which crop will not grow
Optimum – middle of minimum and maximum and
ideal for crop growth
Most crops have a ideal range of pH 5.5 - 7
Correcting soil acidity
Soil acidity caused by:
 Leaching – excess water washing away ions
 Nutrient uptake – plant growth and removal
 Fertilizers – excessive use of acid fertilizers
 Sulphur – soils with high S resulting in sulphuric acid
and can be corrected by:
 Application of lime (limestone)
 Proper drainage
 Use of appropriate fertilizers
Soil particle – soil nutrient relationship
Soils can retain nutrients due to the presence of:
 Clay
 Organic matter (OM)
 Aluminum and iron compounds
These substance are responsible for the ‘adsorption

complex’
Adsorption is the attraction to a solid surface
Clay and OM have a layer of negative charges
Some soil nutrients have positive charges resulting in
attraction
Nutrient content of the soil –
There are 16 essential elements needed for proper plant growth, Hydrogen, Carbon
and Oxygen come from the air
MACRO PRIMARY SECONDARY MICRO OR TRACE

NUTRIENTS NUTRIENTS NUTRIENTS NUTRIENTS
Nitrogen N Nitrogen Calcium Iron Fe
Phosphorous P Phosphorous Magnesium Manganese Mn
Potassium K Potassium Sulphur Copper Cu
Calcium Ca Boron B
Magnesium Mg Molybdenum Mo
Sulphur S Zinc Zn
Chlorine Cl
Macro nutrients
NITROGEN (N) PHOSPHOROUS (P) POTASSIUM (K)
Absorbed as nitrate or Absorbed as phosphate ions Absorbed as potassium

ammonium ions ions
Promotes vegetative Stimulates root growth Stimulates formation

growth and transfer of
carbohydrates
Lack of nitrogen Lack of phosphorous causes; Lack of potassium

causes; stunted growth, purple green leaf color, poor causes drying of leaves
yellowing of leaves and root development, stunting of from the tip backward,
leaf drop green parts mottling of leaves and
premature falling of
buds
Soil Chemical Properties - Review
Minerals originate from slow breakdown of parent
material and dissolve in soil water
Inorganic salts required by plants for nutrition
Hence study of Soil Chemical Properties

1. Organic matter
2. Soil pH
3. Soil particle – soil nutrient relationship
Assessment
1. What is soil pH?
2. Jim is a vegetable farmer who received a soil pH

tester, however is unsure of its worth to his farm.
Briefly provide advice to Jim as his Extension Officer.
3. Explain the causes of soil acidity and how it can be

corrected in agricultural soils.
Answer 1
A measure on a scale of 0-14, the acidity or alkalinity
of a soil
Back
Answer 2
Used to determine the pH of the soil.
plant vegetables suitable to their pH range thereby

facilitating plant uptake of up nutrients.
maintain the soil pH so that the vegetables would grow in

optimum pH conditions.
Efficient use of appropriate fertilizers to minimize soil acidity

Back
Answer 3
Soil acidity caused by:
 Leaching – excess water washing away ions
 Nutrient uptake – plant growth and removal
 Fertilizers – excessive use of acid fertilizers
 Sulphur – soils with high S resulting in sulphuric acid
and can be corrected by:
 Application of lime (limestone)
 Proper drainage
 Use of appropriate fertilizers
Keep on learning!

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Soil Chemical Properties

Soil Chemical Properties

Mineral salts from soil particles dissolve in soil water

Inorganic salts required by plants for nutrition

Hence study of Soil Chemical Properties

1. Soil Organic matter

3. Soil particle – soil nutrient relationships

These substance are responsible for the ‘adsorption

MACRO PRIMARY SECONDARY MICRO OR TRACE

Absorbed as nitrate or Absorbed as phosphate ions Absorbed as potassium

Promotes vegetative Stimulates root growth Stimulates formation

Lack of nitrogen Lack of phosphorous causes; Lack of potassium

Inorganic salts required by plants for nutrition

Hence study of Soil Chemical Properties

2. Jim is a vegetable farmer who received a soil pH

3. Explain the causes of soil acidity and how it can be

plant vegetables suitable to their pH range thereby

maintain the soil pH so that the vegetables would grow in

Efficient use of appropriate fertilizers to minimize soil acidity

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