 Also, the actual tax deduction might be less than what the analysis determines as the value of the excess amount. For example, if the land's value and that of the excess fertility together exceed the purchase price of the land, you must allocate each one based on the respective fair market shares. Don't depreciate the excess amount. It's a depletion deduction. There's no IRS form for depletion, so just add the deduction to the fertilizer line and separately track it.