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Ask where the nitrogen in soil comes from and your biology book will point to bacteria such as Rhizobium and Azotobacter. Nil Ratan Dhar, who led chemistry at Allahabad for decades, argued that sunlight plays a part too. He built a large research school at Allahabad and gave much of his money to science. This post covers his life, his idea of photochemical nitrogen fixation, a worked example on photon energies, and what a student can learn from how he worked.
Who was Nil Ratan Dhar?
The Indian National Science Academy's biographical memoir records that he was born on 2 January 1892 in Jessore, now in Bangladesh, and passed the Calcutta University entrance examination in 1907 at 15. A tribute in Science and Culture by his former student Smita Chowdhry describes how, at Presidency College in Calcutta, the chemist P. C. Ray shaped both his science and his values.
He went to London in 1915 and worked in the physical chemistry department of the Imperial College of Science, where, according to the tribute, he submitted his D.Sc. thesis in April 1917, about eighteen months after starting research. He then went to Paris, where he earned a second doctorate in early 1919; the tribute names Jean Perrin among his examiners. In July 1919 he joined Muir Central College in Allahabad as professor, and he stayed linked to Allahabad for the rest of his life. The memoir records his death in December 1986.
What did Dhar work on?
His early work was on the speed of chemical reactions and how light drives them. The memoir lists papers with titles such as "Einstein's law of photochemical equivalence", the rule that each absorbed photon can activate one molecule, and many studies of temperature coefficients and quantum efficiency. His 1923 paper on temperature coefficients of reactions in tropical sunlight in the Journal of the Chemical Society shows how early he connected Indian sunlight to his chemistry.
What was his idea about nitrogen in soil?
The memoir says that in presidential addresses to the National Academy of Sciences in 1935 and 1937 he described "photochemical nitrogen fixation". His argument ran like this:
Organic matter such as straw, leaves or dung slowly oxidises in soil and releases energy.
That energy, "aided by sunlight", fixes atmospheric nitrogen at the soil surface.
In his experiments, more nitrogen was fixed in light than in the dark, even though light reduced the numbers of bacteria and fungi.
Adding calcium phosphates increased fixation and helped hold the nitrogen in the soil.
He published on this in journals such as Nature, for example Nitrogen fixation with cow-dung in 1936, and he urged farmers to combine organic matter with phosphates. The Science and Culture tribute adds that he showed how molasses could reclaim alkaline soils in western Uttar Pradesh.
Keep the textbook picture alongside this. The NCERT Class 12 chapter Microbes in Human Welfare describes biofertilisers: Rhizobium fixing nitrogen in root nodules, and free-living bacteria such as Azospirillum and Azotobacter. Dhar's claim was that a non-biological, light-assisted route adds to these.
Worked example: can one photon of sunlight break N≡N?
Why would sunlight need help from oxidising organic matter? Compare energies.
Energy of a visible photon. Take green light, λ = 500 nm. With Planck's constant h = 6.626 × 10^-34 J s and c = 3.00 × 10^8 m/s, E = hc/λ = (6.626 × 10^-34 × 3.00 × 10^8) ÷ (500 × 10^-9) = 3.98 × 10^-19 J.
Per mole of photons. Multiply by Avogadro's number, 6.022 × 10^23: E ≈ 2.39 × 10^5 J, or about 239 kJ per mole.
Energy to break N≡N. The NCERT Class 11 chapter on chemical bonding gives the bond enthalpy of N2 as 946.0 kJ per mole.
Compare. 946 ÷ 239 ≈ 4. One green photon carries only about a quarter of the energy needed.
What wavelength would do it? λ = hcNA ÷ 946,000 J = 1.26 × 10^-7 m, about 126 nm. The NCERT electromagnetic waves chapter puts visible light at 400 to 700 nm, so this is far ultraviolet.
So a single photon of visible light cannot split nitrogen on its own. Dhar's scheme added chemical energy from oxidation. The memoir quotes 676 kilocalories released per mole of glucose oxidised; at 4.184 kJ per kcal, that is about 2,830 kJ, roughly three times the N≡N bond energy. This is bookkeeping, not a mechanism.
What mistakes do students make with photon energy?
Forgetting nanometres. 500 nm is 500 × 10^-9 m. Using 500 m gives an absurdly tiny energy.
Comparing per photon with per mole. Bond enthalpies are per mole. Multiply a photon's energy by Avogadro's number before comparing.
Mixing kcal and kJ. Older papers use kilocalories. Multiply by 4.184 to get kJ.
What can a student take from how Dhar worked?
Work on local problems. Alkaline soils, molasses waste and strong sunlight were all on his doorstep.
Work hard and finish. The tribute describes 9 a.m. to 9 p.m. days in London and a thesis in eighteen months.
Build institutions. The memoir says he built a research building in Allahabad, completed in 1934, named it the Indian Institute of Soil Science, and after the death of his wife, the physical chemist Sheila Dhar, in January 1949, renamed it in her memory and gave it to the university.
Teach generously. The tribute names R. C. Mehrotra and Atma Ram among the many students he trained.
A practice routine: repeat the worked example for 400 nm and 700 nm light, then explain in two sentences why plants and bacteria need special machinery to fix nitrogen. For more on light as photons, see our guide to photoelectric effect problems. If the set-up of photon-energy problems trips you up, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than just printing an answer.
Next step: calculate the energy per mole of red photons at 700 nm and compare it with the N≡N bond.
