To efficiently provide the ideal amount of nitrogen fertilizer, some farmers make use of a "chlorophyll meter" that measures the amount of green light reflected off a leaf. why does this provide an indication of whether the plant may need more or less nitrogen?

Chlorophyll is a green pigment found in chloroplast in plants. It absorbs sunlight during the process of photosynthesis. It is a chelate molecule. In a chelate molecule a central metal ion forms bonds with a large organic molecule made of carbon, hydrogen and other elements like nitrogen or oxygen.

In chlorophyll, magnesium is the central metal ion and it is bonded to a large organic molecule called as porphyrin. Porphyrin constitutes four nitrogen atoms to which the central magnesium is bonded in square planar arrangement. Since chlorophyll is responsible for the green color of the leaves, more the green color more the chlorophyll content and more is the nitrogen content. Hence, chlorophyll meter measures this green light and finds out if the plant needs more or less nitrogen.

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