After completing this lesson you have learned to:

1
Soil Health
Virtual Academy for the Semi Arid Tropics
Module II

Lesson 2 How to know that a soil is healthy?

• After completing 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.

Course on Soil and Soil Health
2

• After completing 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.

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• 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.

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• In this Lesson there are 3 Units
• Unit 1 Physical Indicators
• Unit 2 Chemical Indicators
• Unit 3 Biological Indicators

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• Unit 3 Biological indicators
• After completing this Unit, you will be able to
  answer
• List out various microorganisms present in a
  soil?
• What soil conditions influence the microbial
  population in a soil?
• How soil microorganisms are beneficial?
• What is humus?
• How can you know the organic matter content in a
  soil?

6

• Unit 3 Biological indicators
• After completing this Unit, you will be able to
  answer
• What is nitrogen mineralization?
• How do you assess increased nitrogen
  mineralization?
• What soil respiration indicates?
• How soil respiration is measured?
• How soil respiration is related to soil health?

7

• Unit 3 Biological Indicators
• Microbial population
• Organic matter content
• Nitrogen mineralization
• Soil respiration

8

• Unit 3 Biological Indicators
• Microbial population
• Many soil microorganisms live in the soil. These
  microorganisms include
• bacteria,
• Fungi,
• algae,
• actinomycetes, etc.

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• Unit 3 Biological Indicators
• Microbial population
• A single gram of soil (about 1/5 tea spoon)
  contain 100 million (10 crores) bacteria, 10
  million (one crore) actinomycetes, 100,000 (1
  lakh) fungi etc.
• The exact proportions of each of these
  organisms will depend on soil conditions such as
  available moisture, aeration, organic matter
  levels and the type of plants present. Chemical
  conditions such as acidity and alkalinity will
  greatly affect microorganism populations.

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• Unit 3 Biological Indicators
• Microbial population
• Many soil organisms are beneficial but some
  others are devastating to crops, humans and
  animals.
• Beneficial organisms take part in many
  physical and chemical reactions that improve soil
  structure, break down organic matter and recycle
  nutrients.

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• Unit 3 Biological Indicators
• Microbial population
• Others, however such as the tiny nematode, a
  worm, less than 0.1 mm long, and disease causing
  fungi and bacteria can cost farmers enormous
  money, every year, in loss of crop production.
• So, soil should contain more population of
  beneficial microorganisms for it to be in good
  health.

12

• Unit 3 Biological Indicators
• Organic Matter
• You have already learnt from the previous
  lessons about the importance of organic matter in
  soils.
• The organic matter content in soils ranges from
  almost nil (in sandy soils) to 15 to 20 (in
  forest areas).
• Most of the cultivated soils contain 0.5 to 5
  of organic matter.

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• Unit 3 Biological Indicators
• Organic Matter
• The organic matter content in a soil indicate
  the health of a soil. More the amount of organic
  matter better will be its health.
• It should be remembered that the organic matter
  in the soil is not stable and decomposed by many
  microorganisms over time and converted into
  humus.
• So, ultimately it is humus content that keeps
  going many activities in a soil.

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• Unit 3 Biological Indicators
• A soil analysis will provide the information on
  the organic matter content in a soil.
• One can assess the organic matter content in
  a soil by observing, and feeling with fingers a
  sample of top soil.
• Soil rich in organic matter will be dark in
  color (compare right side soil sample with the
  left side one), mellow, and a good amount of
  decomposing roots and plant residues.

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Observing a soil profile also gives you an idea
about organic matter content in a soil. Look at
the dark top layer in the left side soil profile
and compare with the right side one.
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• Unit 3 Biological Indicators
• Nitrogen Mineralization
• Nitrogen mineralization refers to the conversion
  of organic (unavailable) nitrogen in the soil to
  a form of nitrogen the plants can utilize for
  their growth and development.
  
  
  
  
  

Organic Nitrogen
Mineralization
Inorganic Nitrogen
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• Unit 3 Biological Indicators
• Nitrogen Mineralization
• This mineralization is carried out by certain
  microorganisms (bacteria etc.).

Organic Nitrogen
Mineralization
Microorganisms
Inorganic Nitrogen
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• Unit 3 Biological Indicators
• Nitrogen Mineralization
• So, higher nitrogen mineralization, due to
  increased activity microorganisms, results in
  more availability of nitrogen in soils for crop
  use.
• Thus, better crop growth with the increased
  available nitrogen content in soil indicates a
  healthy soil.
• A soil test will reveal the amount of
  available nitrogen content in soils.
• 
  

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• Unit 3 Biological Indicators
• Soil Respiration
• Soil respiration is one of the most frequently
  used indicator for measuring microbial activity
  in the soil.
• You have already learnt that microorganisms
  are the driving force in many biochemical
  processes (decomposition, mineralization etc.)
  taking place in soil.

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• Unit 3 Biological Indicators
• Soil Respiration
• Soil respiration is measured by the production
  of net carbon-di-oxide and consumption of oxygen
  in a soil due to the microbial activities.
• A higher rate of soil respiration is
  indicative of intense microbial activities in a
  soil and consequent benefits for better crop
  growth.

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Soil Health
Virtual Academy for the Semi Arid Tropics
Module II

• With this Unit 3 on biological indicators in
  the Lesson 2 on how to know that a soil is
  healthy and also this Module II concludes.
• Next, Module III is about managing and
  improving soil health.
• Select Module III from the list of course
  contents

Course on Soil and Soil Health