You may have to pay some tax depending upon following facts:
1. Did you cross age of 65 years at any point of time during financial year 2009-10?
2. Do you have any other income from other source?
3. Actual date of purchase of plot in 2006, was it prior to 31st March 2006 or later?
4. Any investment in LIC, medical insurance, NSC, PPF etc during 09-10?
Depending upon the date of purchase, the indexed cost of purchase of plot will be calculated which will be deducted from the sale value to caculate long term capital gain on sale of this plot.
If you had crossed age of 65 years during any point in FY 2009-10, your income upto Rs 240000/- is exempt from tax. So if sum of your pension income, other income and long term capital gain as computed above minus the investments in LIC NSC etc is less than Rs 2.40 Lakhs, no tax shall be payable by you. Otherwise tax@ 20% on srplus will be levied.
If you are not a senior citizen, please replace figure of Rs 2.40 Lakhs above by Rs 1.60 Lakhs.