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Natural method of nitrogen fixation : When lightening occurs, atmospheric nitrogen and oxygen combine to form NO. The nitric oxide [NO] combines with oxygen to from NO₂. This nitrogen dioxide combines with rain water to form HNO (Hyponitrous acid) and HNO₂ or HNO₂ and HNO₃. These nitrous acids and nitric acid are washed into the soil along with rain water and cause Acid Rain. This acidic rain water combines with other mineral salts to form nitrities and nitrates. The nitrites and nitrates are absorbed by the plants.

 

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(1) Iron : It leads to intervenial chlorosis and necrosis. 

(2) Zinc : Due to the deficiency of zinc, young leaves and internodes become extermely small, resulting in little leaf rosette and mottled leaf condition .

Calcium : It's deficiency produces necrosis in the leaf and stem. Plants have a stunted growth with suppressed flowering. Flowers fall immaturely and this process is called premature fall of flower.

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Potassium and magnesium both are essential elements as

Potassium -  is involved in maintaining the turgidity of cells and in the activation of enzymes. It helps in the opening and closing of stomata, to determine anion-cation balance in the cells, formation of cell membrane and in translocation of organic solutes in the phloem.

Magnesium-  is a constituent of chlorophyll. It is required in fat synthesis and nucleic acid synthesis. It activates enzyme in phosphate metabolism.

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Soil fertility. It is the ability of soil to provide good growth of plants. It is dependent upon pH value, size of soil particles, presence of minerals, presence of  air , optimum temperature etc.

Depletion of soil fertility is due to the following reasons :

1. Non-rotation of crops causes deficiency of minerals.

2. Precipitation of salts due to change of pH value.

3. Leaching of salts.

4. Over use of the land for cropping.

5. Over grazing.

7. Deforestation.

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Deficiency symptoms of Nitrogen:

1. Deficiency of nitrogen interferes with protein synthesis and hence all forms of growth are affected.

2. An early symptom is yellowing of leaves and development of chlorosis.

3. Older leaves turn light brown in colour and gradually dry.

4. Flowering is delayed or completely suppressed.

Deficiency symptoms of Phosphorous :
The deficiency of phosphorus causes :

1. disruption of general metabolism

2. abnormalities in the shape and size of chloroplast.

Deficiency symptoms of Sulphur :

1. Deficiency symptoms first appear in young leaves.

2. They become reduced, chlorotic followed by anthocyanin pigmentation in certain species.

3. Root system becomes extensive and stems become hard and woody.

Deficiency symptoms of Potassiam :

1. Deficiency symptoms are first observed on the older, lower leaves. Spots of dead tissue on older leaves are seen. 

2. Potassium deficiency in cereal crops develop weak stalk. These plants easily bent to the ground by wind or rain.

Deficiency symptoms of Calcium :

1. Calcium deficiency brings about decomposition of root epidermis, roots become mucous, the growing zone and lateral roots rapidly die off.

2. Its deficiency terminates growth of meristematic regions.

3. Margins of younger leaves show chlorosis which become nacrotic.

4. Twig or stalk just below the tip and seed head is often unable to stand erect.

Deficiency symptoms of Magnesium :

1. Extensive interveinal chlorosis of the leaves resulting in defoliation;

2. Yellowing of the leaves start from basal to younger ones;