Nutrient cycles refer to the circulation of certain nutrients like nitrogen, carbon and water in nature.


Nitrogen Cycle

Nitrogen cycle involves the complex process by which nitrogen is naturally added and removed from the soil. It is a sequence of reaction, indicating the various means by which nitrogen is added to and removed from the atmosphere and the soil.


Importance of Nitrogen Cycle

Nitrogen cycle is nature’s way of regulating the amount of nitrogen in the soil and air.

Nitrogen fixation process involves soil organisms which add reasonable amount of nitrogen to the soil.


Ways By Which Nitrogen Is Added To The Soil in the nitrogen Cycle

These are the ways by which nitrogen is added to the soil in the Nitrogen cycle:

  1. Direct fixation by lightning during rainfall (Electrical discharge).
  2. Incorporation into the soil by free living bacteria or non-symbiotic bacteria.
  3. Nitrogen fixing bacteria in the root modules.
  4. Decomposition of organic matter.
  5. Application of nitrogenous fertilizers.
  6. Ammonification.
  7. Nitrification.

Soil can gain nitrogen through the following ways:

  1. Symbiotic Nitrogen Fixation: Some bacteria, such as Rhizobium leguminosarum, which live in the root nodules of leguminous plants can fix atmospheric nitrogen directly into the plant. The plant supplies carbohydrate for use by the bacteria while the bacteria supply the plant with combined nitrogen.
  2. Electrical Discharge: Nitrogen can also be fixed into the soil during lightning. Nitrogen in the air combines with oxygen to form nitric oxide which further undergoes oxidation to form nitrogen dioxide. The nitrogen dioxide will dissolve in rain water to form nitrous and nitric acid which later dissociates to form nitrate in the soil.
  3. Non-Symbiotic Nitrogen Fixation: Some bacteria such as azotobacter and clostridium also live freely in the soil and can fix atmospheric nitrogen into the soil either aerobically or anaerobically.
  4. Ammonification and Nitrification: The process involving the formation of ammonium compounds from dead and decaying plants and animals and their waste products like urine and feaces is called ammonification. A further reaction known as nitrification involves the conversion of ammonium compounds first to nitrite by nitrifying bacteria called nitrosomonas. These nitrites are converted by oxidation to nitrates by another bacterium called nitrobacter. Plants can only absorb nitrates from the soil.
  5. Application of Organic Manure and Nitrogen Fertilizers: These also add or supply nitrogen to the soil.


Ways by which Soil Nitrogen is lost From the Soil

  1. By denitrification: This process involves the conversion of nitrate to nitrogen gas by certain bacteria. The nitrogen gas escapes into the air.
  2. Soil erosion.
  3. Leaching.
  4. Soil pH.
  5. Bush Burning.
  6. Crop removal.
  7. Volatilisation.
  8. Oxidation reaction.
  9. Reduction reaction.


Processes that lead to the formation of Nitrate from Organic matter in the Nitrogen Cycle

  • Putrefaction.
  • Amminization.
  • Ammonification/mineralisation.
  • Nitrification.


  1. Putrefaction: Decay of plant and animal remains into simpler nitrogenous compounds by microorganisms.
  2. Amminization: Products from putrefactions reaction converted to simple amino compounds and amines.
  3. Ammonification: Conversion of amino compounds and amines into ammonium (NH4+) compounds.
  4. Nitrification: Oxidation of ammonium compounds into nitrites and nitrates.


Carbon Cycle

Carbon cycle involves the series of processes which contribute to the circulation of carbon in nature.

  1. Carbon dioxide is removed from the air mainly by photosynthesis during which plants use it to manufacture their own food.
  2. Carbon is lost in form of carbonates of calcium and magnesium through leaching and drainage.


The atmosphere gains carbon dioxide through:

  1. burning of fuel like coal and wood,
  2. the action of volcanoes which releases carbon dioxide,
  3. the respiration by plants and animals,
  4. the death, decay and putrefaction of plants and animals,
  5. diffusion of carbon dioxide from seas and other bodies of water, acting as reservoir of carbon dioxide.


Importance of Carbon Cycle

  1. Plants use carbon dioxide obtained from the air to manufacture their food during photosynthesis.
  2. It provides carbon which is the major building block of all organic matters.
  3. It helps to purify the atmosphere and also to maintain atmospheric level of carbon dioxide.
  4. Organic matter which is made from carbon helps to replenish soil nutrients.


Water Cycle

Water Cycle is defined as the continuous movement of water from the atmosphere to the earth and from the earth to the atmosphere.

The atmosphere receives water through:

  1. evaporation from oceans and land,
  2. transpiration from plants,
  3. breathing or respiration by plants and animals.


The land receives water through:

  1. rainfall or precipitation,
  2. infiltration and percolation.


Importance of water to crops

  1. Water provides the medium for absorption of mineral salts.
  2. It facilitates the transfer of nutrients to other parts of plants where they are used in various metabolic processes.
  3. Water is an essential raw material in the process of photosynthesis.
  4. It is used for hydrolizing food substances such as starch, proteins, fats and oil for easy transmission to other parts of plants.
  5. Water facilitates enzymatic activities occurring in crop plants.
  6. It is a constituent of plant protoplasm.
  7. The loss of water through transpiration is important for keeping the stomata open in gaseous exchange for respiration and photosynthesis.
  8. It has a cooling effect on crops.
  9. It helps to maintain plant turgor or turgidity.
  10. It helps to sustain life.
  11. It helps in seed germination.


Forms in which water exists in the soil

The forms in which water exists in the soil include:

  1. hygroscopic water,
  2. capillary water,
  3. gravitational water.


Ways of conserving water in the soil

  1. Stoppage or reduction of surface water run-off.
  2. Addition Of humus or organic manure
  3. Removal of weeds to reduce transpiration and water loss.
  4. Mulching.
  5. Cover cropping.
  6. Contour ridging.
  7. Appropriate tillage.
  8. Strip cropping.

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