after successful completion of this lesson, you have learned to: list the 3 indicators of soil...

After successful completion of this Lesson, you have learned to:

• List the 3 indicators of soil health.

• List the physical indicators that help to know a healthy soil.

• Explain how different physical indicators reveal the soil health status.

• List chemical conditions in soils that influence soil health.

Virtual Academy for the Semi Arid Tropics

Lesson 2: How to know that a soil is healthy?

Course on Soil and Soil Health

Module II: Soil Health

After successful completion of this Lesson, you have learned to:

•Describe different soil chemical conditions which can provide information on soil health.

• List the biological indicators that control soil health.

•Understand various biological activities which can improve or affect soil health.





Soil health is evaluated in terms of indicators that measure specific physical, chemical and biological properties.

Although this separation is useful, it is

important to note that many of these properties are interrelated.

Some physical properties are shaped by biological activity, biological activity can be enhanced or limited by physical and chemical conditions, and chemical properties are altered by microorganisms.





In this Lesson there are 3 Units:

Unit 1: Physical Indicators

Unit 2: Chemical Indicators

Unit 3: Biological Indicators










After successful completion of this Unit, you will be able to answer:

1. How increased organic matter content in soils leads to better soil health?

2. How soil plant nutrients influence soil health?

3. What is soil reaction?4. What are the 3 measures of soil

reaction?5. Explain what soil pH 7 indicates?






After successful completion of this Unit, you will be able to answer:

6. What happens if a soil pH decreases from a pH 7?

7. What happens if a soil pH increases from a pH 7?

8. At what range of soil pH most of the plant nutrients will be available?

9. What a higher CEC value of a soil indicates?

10. At what soil pH range the base saturation will be 100%?






Chemical indicators

• Soil organic matter

• Plant nutrients

• Soil reaction

• Soil acidity

• Soil salinity

• Accumulation of toxic substances






Added organic matter Increased biological activity

May reduce soil-borne diseases Decomposition

Pore structure Aggregation improved increased Nutrients

released Humus formed Improved tilth

Healthy Soils






Higher organic matter content in soils leads to better soil health through:

• Improved soil structure and aggregation;

• Releasing and recycling of plant nutrients;

• Increased infiltration and percolation rates;

• Higher water holding capacity;• Suppression of pests and disease by

increased biological activity and competition.






Plant nutrients are present in a soil both in available and unavailable forms.

It is important that plant nutrients should be present in a soil in required amounts and at the time plants mostly need them.

Hence, the reserve of available plant nutrients in a soil indicate how healthy a soil is.

The total and available form of nutrients in the soil can be known through soil testing.






The mineral matter, organic matter and gases present in the soil react with each other resulting in soil reactions leading to both good soil health and also bad soil health.

The important measures of soil reactions are:

•Soil pH

• Cation Exchange Capacity (CEC)

• Percent Base saturation (BS%)






Soil pH is like the chemical pulse of a soil and indicates soil health and nutrient availability.

Soil pH is measured in a scale of 0 to 14.

A soil pH value of 7 indicates that the soil is neutral in reaction and most of the plant nutrients will be available to the plants at this pH level.






So, a soil around pH of 7 will be in good soil health.

Soil pH values below and above 7 indicate unhealthy soil.

As the value of pH either increases or decreases

than 7, the severity of soil problem increases for crop growth.






Decreasing soil pH from 7 results in

• Reduced microbial activity,

• Low organic matter decomposition,

• Fixation of phosphates,

• Deficiencies of potassium, calcium, magnesium, aluminum toxicity.






Increasing soil pH from 7 results in

• Phosphate deficiency

• Iron, manganese, and zinc deficiencies

• Sodium hazard

• Reduced water movement






The relative availability of plant nutrients at different pH values are shown in the figure.

In this figure, the width of a strip shows the relative availability of a plant nutrient.

Please observe that most of the nutrients are available between 6.5 to 7.5 pH range.






In a simple sense, every soil has the ability to hold nutrients in certain sites. The total of such sites in a soil is known as its Cation Exchange Capacity (CEC).

A soil with moderate to high CEC will be in good health in terms of having good capacity to hold and supply nutrients for crop growth.

Soils with low CEC can only hold smaller amounts of nutrients.






The CEC value of a soil can be known from soil testing.






Percent Base Saturation (BS%) refers to the proportion of the cation exchange sites in the soil that are occupied by the various basic nutrient elements such as calcium, magnesium, and potassium.






As the soil pH increases, BS% also increases

and vice-versa.






Soils with a pH range of 6.8 to 7.2 will

have a BS of 100% and such soils are healthy ones.

Low pH acidic soils will have low BS and are problem (unhealthy) soils. Such soils need to be limed to increase the pH and consequent BS%.






On soils that are properly limed, BS% may range from 70 to 90. On soils that are extremely acid (low pH), BS% ranges from 50 to 60.

The ideal BS% is given in the Table.

Base Ideal %

Calcium 65-80 %

Magnesium 10- 20%

Potaium 2 - 6%

Sodium < 5 %






Acidic soils and alkaline soils are problem (unhealthy) soils in the sense that many crops either cannot grow or give very poor yields.

These soils result due to chemical reactions that occur in soils leading to accumulation of certain elements in excess amounts replacing the other elements. The pH of soils indicate the nature of such soils.

Acidic soils Less than 6.5 pH Alkaline soils More than 7.5 pH






Certain heavy metals, pesticide residues, and organic compounds accumulate over time in soils due to improper agricultural practices.

Presence of such chemicals in soils lead to poor health of soils.

Soil analyses will help to identify the presence of such toxic substances in soils.

With this the Unit 2 on Chemical Indicators in this Lesson concludes.

The last Unit in this Lesson explains biological indicators of soil health.

Select Unit 3 in Lesson 2 from Module II contents.





after successful completion of this lesson, you have learned to: list the 3 indicators of soil...

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