Praise be to Allah.
It is proven that the Prophet (peace and blessings of Allaah be upon him) said concerning zakaah on produce of the earth: “For that which is watered by the sky or a spring, or by its roots, (the rate of zakaah) is one-tenth, and for that which is watered by a camel, (the rate of zakaah) is half of one-tenth.”
The Muslim must give this amount of zakaah on that which is produced by the earth, if it reached the nisaab (minimum threshold).
If it so happens that those who are in charge of zakaah – namely the committee that estimates the crops and takes zakaah from them – underestimated it, then the owner must give zakaah on the additional amount of crops, whether this extra amount reaches the nisaab or not. That is because the Prophet (peace and blessings of Allaah be upon him) enjoined a specific share as stated above, one tenth or half of one tenth, so this must be paid.
Majmoo’ Fataawa Ibn ‘Uthaymeen (18/64).
And Allaah knows best.