If you are harvesting on the whole land and the whole of the production will be considered your produce solely, then you are for all intent and purposes, owner of the land for the purposes of calculation of Zakat. In that case, Zakat becomes due immediately on the gross production as soon as the production occurs. The gross production would be subject to 5% of Zakat, if both labor/maintenance expenses and capital are being expended; but if either labor/maintenance expenses or capital are expended then 10% Zakat would be applicable.
This Zakat is payable only if gross production is equal to or above Nisab Threshold which in case of production/earnings is the minimum amount below which your state exempts tax or zakat on production/earnings. If your state does not provide a nisab then you can use the standard nisab of 653 kg of Dates or equivalent value.
The only expenses that can be deducted from gross production/gross receipt are those which are incidental in acquiring ownership of that particular form of means of production and/or are substantial expenses in maintaining/sustaining that particular form of means of production. Therefore, amount paid for acquiring ownership of the land can be deducted from gross production value of the period when it is paid.
Detailed Law of Zakat can be read in the following response:
Discussion 58129 • Reply 58137