John Gill Commentary Leviticus 26:20

"and your strength shall be spent in vain; for your land shall not yield its increase, neither shall the trees of the land yield their fruit." — Leviticus 26:20 (ASV)

And your strength shall be spent in vain In endeavouring to till the ground, to plough, or sow, or to dig about the vines or olives, and prune them;

for your land shall not yield its increase produce corn, and bring forth grass, the one for the use of men, the other for the use of the cattle, and therefore both must starve:

neither shall the trees of the land yield their fruits such as vines, olives, figs, pomegranates which were very plentiful in the land of Judea, and on which they much lived, and on which their more comfortable subsistence at least depended, see (Habakkuk 3:17) ; all this is the reverse of (Leviticus 26:4Leviticus 26:5Leviticus 26:10) .