A recent report entitled India: Research and Collaboration in the New Geography of Science has some info on India's relative strength in different fields of science.
Overall, India has a 3 % share in scientific publications -- its researchers published 30,000 papers in 2008.
The top five areas in which India's share is high (during the period 2004-08) are:
Agricultural Sciences: 5.65%
Materials Science: 4.81%
Pharmacology and Toxicology: 4.25%
Plant & Animal Science 3.77%
Interestingly, India's share is over 8 % in four subfields: Agricultural Engineering, Tropical Medicine, Organic Chemistry, Dairy and Animal Science.