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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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(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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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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Arnon and Stout gave the following criteria for the essentiality of elements :

1. The essential elements should be necessary for growth and reproduction of the plant. It should be indispensable. The plant should not be able to complete its lifecycle in the deficiency of the element. 

2. The element should not be replaceable and its requirement should be specific. The deficiency of the element should not be met by supplying some other element. 

3. There should be direct involvement of the element in the metabolism of the plant. 

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Artificial methods That restore the fertility of soil are :

(1) By adding farm yard manure or animal excreta to the soil.

(2) By adding fertilizers, manure.

(3) By green manuring.

(4) By crop rotation.

(5) By controlling over-grazing.