Adam Clarke on Deuteronomy 28:23

Thy heaven - shall be brass, and the earth - iron - The atmosphere should not be replenished with aqueous vapours, in consequence of which they should have neither the early nor the latter rain; hence the earth - the ground, must be wholly intractable, and, through its hardness, incapable of cultivation. God shows them by this that he is Lord of nature; and that drought and sterility are not casualties, but proceed from the immediate appointment of the Lord.