Abstract
In ancient Israel agriculture was a sacred activity. The land is God's property for ever.
In fact, the mundane proprietor is just a leaseholder. The harvest does not belong to the tenant at all.
He must separate a part as a gift for the poor. The rules are specified by Halachah.
It follows that the gift is not voluntary but mandatory. The poor does not get the gift without work.
He has to get the crops from the field edge in himself. The same is true for certain grape berries or olives left by the wine grower or farmer.
The rules regulating the separation of a part of the harvest for the poor are extraordinarily minute and sophisticated and opinions of individual rabbis may differ in details. Decisive is always the sense and purpose - therefore the amount of the gifts is not defined exactly.
The rules must guarantee to earn their living to both: the rich and the poor, because they are equal before God.