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You shall not sow your vineyard with a mixture of diverse kinds of seed; lest the fullness of the seed which you have sown be forfeited together with the increase of the vineyard.

Rashi

A mixture of diverse kinds of seed

Wheat and barley and grapeseed dropped by the hand [together].

Lest it be forfeited

As its Targum: contaminated. Any thing which is abominable upon the person, both for praise, such as sanctification, and for shame, such as prohibition, comes under the language of קדש; such as (Isaiah 65:5) "do not come near to me, for I have declared you unclean".

The fullness

This is the filling and supplement that the seed supplements.