Midrash su Levitico 26:20

3) "and I shall make your sky as iron and your earth as brass": This is worse for them than the later (imprecation). For in the later what is written? (Devarim 28:23) "And your skies over your heads shall be as brass," that the skies will "sweat," as brass "sweats," and the earth will not "sweat," as iron does not sweat — and this will guard its fruit (against rotting.) But here what is written? "and I shall make your sky as iron and your earth as brass," that the skies will not "sweat," as iron does not "sweat," and the earth will "sweat" as brass "sweats," and it will lose its fruit. (Vayikra 26:20) ("And your power shall be spent in vain, and your land will not yield its produce, and the tree of the field will not yield its fruit.") "And your power shall be spent in vain": Rebbi says: This is the vineyard, (which requires much labor). Others say: This is flax. Others say: This is their strength (itself). Variantly: This refers to one who married off his daughter with a great dowry, but before the seven days of rejoicing are completed his daughter dies — so that he buries his daughter and loses his wealth.

(Devarim, Ibid.) "and the ground will not yield its yevul": not even what you bring (movil) to it. — But perhaps a tree will yield fruit. It is, therefore, written (Vayikra 26:20) "and the tree of the land will not yield its fruit." — But perhaps he can go abroad and be steeped in good; it is, therefore, written (Devarim 28:23) "And your skies over your head shall be as brass, and your earth under you as iron."