Sulphur $(\mathrm{S})$ is an important macronutrient in plants that is absorbed by the plants as $\mathrm{SO}_4^{2-}$ ion. It mainly functions as a component of vitamins (biotin, thiamine), proteins, coenzyme-A, amino acid (cystein and methionine) etc. It is also an essential component of ally sulphide (onion, garlic) and sinigrin (mustard). Deficiency of sulphur can lead to chlorosis in young leaves, extensive root growth, formation of hard and woody stem. It also causes the reduction in juice content of citrus fruit and tea yellow disease of tea. Sulphur is found in amino acids systeine, methionione, etc.

In biological nitrogen fixation, the atmospheric $\mathrm{N}_2$ gets reduced to $\mathrm{NH}_3$ by nitrogenase reductase present in some prokaryotes. $\mathrm{NH}_3$ is then oxidises to $\mathrm{NO}_2$ and $\mathrm{NO}_3$ by some other bacteria (Nitrosomonas and Nitrobacter). Various steps involved in nitrogenfixation are as follows

Pseudomonas and Thiobacillus are involved in the process of denitrification. They convert nitrate $\left(\mathrm{NO}_3^{-}\right)$and nitrite $\left(\mathrm{NO}_2^{-}\right)$into free nitrogen $\left(\mathrm{N}_2\right)$, which is released into the atmosphere.

(a) Hydroponics, Julius Von Sachs (1860)

(b) (i) Solanum lycopersicum (tomato)

(ii) Hibiscus asculentus (ladiesfinger)

(iii) Solanum melongena (brinjal)

(c) Aerating tube Provides oxygen for the normal growth and development of the roots growing in the liquid solution. Feeding funnel is used to add water and nutrients in the hydroponic system when required.

Nitrogenase is the most crucial enzyme found in the root nodules for $\mathrm{N}_2$-fixation. It is a Mo- Fe protein that catalyses the conversion of atomospheric nitrogen to ammonia. Pink coloured pigment called leghaemoglobin creates anaerobic conditions for the functioning of nitrogenase enzyme.

Critical Concentration	Deficient
A concentration of a nutrient measured in tissue, just below the level that gives maximum growth is defined as critical concentration. e.g., N, P and K are known as critical elements.	The concentration that is below the critical concentration and the deficiency of nutrient element can lead to a gradual fall in the plant growth is called deficient.

	Critical concentration	Deficient
Zn	0.5-1%	less than 0.5%
Fe	3.5-5%	less than 3.5%