Deuteronomy 14:22 says, 'You shall truly tithe all the increase of your grain that the field produces year by year.' The Hebrew word translated 'increase' can also mean income or revenue. Obviously in an agricultural society, the increase comes primarily from the field. Today, most of us receive our income in the form of money or material things. I don't believe we need a Bible verse to tell us that if our increase is money, that's what we should tithe on. I have found over the years that those who faithfully tithe are greatly blessed, both spiritually and materially. I don't mean they get rich, only that they prosper. I can point to myself as an example. I have also observed that those who give 10% but don't tithe do not experience the same level of blessing. The tithe is the first ten percent. It is the ultimate way of telling God, I trust you to provide for me. And God never let's us down. I recommend you put Him to the test in this.