You may file the Innocent Spouse form at any time. It does not need to be filed with your current return, and can be filed as you suggest, if and when you feel the need arises.
However, any year that you file a joint return
with your husband, if a tax liability
arises from the farm or from any other source related to income
for you or your husband, you can both be held jointly liable for those taxes. Filing an Innocent Spouse form would not necessarily relieve you of those responsibilities.
Innocent spouse relief is typically granted in a case where a married couple has filed a joint return, and one spouse has reported fraudulent information, without the knowledge of the other spouse. If your spouse is reporting legitimate information, then you would have no grounds to file for innocent spouse relief, simply based on the fact that the additional taxes were due relating to the farm.
That is why I recommend that you file a separate return if you feel this might be an issue.
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