It definitely was not an income tax. It was the tenth animal or measure of grain that was produced on the land in a given year. If only 9 animals were born, no tithe was paid because it was only the 10th animal that passed under the counting rod that was holy to God. Citizens who didn't raise animals or crops didn't pay tithes.
Every tithe of the herd and flock—every tenth animal that passes under the shepherd’s rod—will be holy to the Lord. No one may pick out the good from the bad or make any substitution. If anyone does make a substitution, both the animal and its substitute become holy and cannot be redeemed.’” Leviticus 17:32-33