Tafsiran Imamat 26:20: Rashi, Or HaChaim, Rashbam & Siftei Chakhamim

ותם לריק כחכם AND YOUR STRENGTH SHALL BE SPENT IN VAIN — Behold, if a man does not toil in his field — neither tills, nor sows, nor weeds it, nor clears away the thorns, nor hoes it, and then at harvest time blight comes and strikes it (the field, i. e. destroys that which sprung up of itself), surely it does not matter much (he does not take it to heart). But if a man has toiled — he has ploughed, sown, weeded, cleared away the thorns and hoed it, and then blight comes and strikes it, surely then the teeth of that man become blunt! (a figurative expression for becoming speechless, terror-stricken) (Sifra, Bechukotai, Chapter 5 4).

ותם לריק כחכם, "and your strength will be spent in vain." The meaning is the same as if the Torah had written ותם כחכם לריק, meaning that your entire efforts will be to no avail. The Torah had to tell us this as the power of destructive forces often is only partially effective.

כחכם, for the earth will no longer lend you its strength.

ולא תתן ארצכם את יבולה — This means, the earth shall not yield even as much as you brought to it at sowing-time (יבולה as passive participle with suffix of יבל “to bring”, may denote: that which is brought to it) (Sifra, Bechukotai, Chapter 5 4).

ועץ הארץ AND THE TREE OF THE GROUND [SHALL NOT YIELD] — This implies that even from the very earth a curse will rest upon it — that it will not bring its fruits to development at the time when they should develop (Sifra, Bechukotai, Chapter 5 4).