Why does Moshe present the K'lalos here in the singular, whereas the K'lalos in Bechukosai are written in the plural? And why does he ascribe copper to Shamayim and iron, to Aretz, reversing the order from the Pasuk in Vayikra 26:19?
Rashi and Targum Yonasan: Since the current K'lalos are said by Moshe himself (Refer to 28:15:1:1), Moshe presented them in a more lenient form than Hakadosh-Baruch-Hu, who a. presented them in the (harsher) plural, and b. ascribed the iron, which does not sweat, to the heaven, and the copper, which does, to the earth. 1 Whereas, based on Moshe's curse, the heavens will produce a little rain, and the earth will not sweat to spoil the few fruits that grow. 2
Rashbam (unconnected to the Pasuk on Bechukosai): Instead of the wind blowing down rain from the sky, it will blow down on you dust and earth 3 from the mountaintops, which are dry. And the dust and the earth will pierce your eyes and blind you, and will force you to flee before your enemies (as Pasuk 25 continues).
Rashi: Implying that the heaven will produce not a drop of rain, whereas the earth will produce moist, thereby wetting and ruining whatever fruit does manage to grow.
See Targum Yonasan.