As our habit, we now quote the Hebrew text.
כי תעבד את־האדמה לא־תסף תת־כחה לך נע ונד תהיה בארץ
As regards the Hebrew expression לא־תסף תת־כחה (that I translate ‘[the soil] will not multiply its power’) some scholars commented (bold is mine):
Albert Barnes: “The particulars of it are the withdrawal of the full strength or fruitfulness of the soil from him”.
E. W Bullinger: “strength = Metonymy (of the Cause). ‘Strength’ put for that which it produces.” (The Companion Bible)”
It is very interesting the Cambridge Bible’s comment, too:
“her strength] That is, ‘her fruits’. So the Vulg[ate] ‘fructus suos’. The word ‘strength’ is used in this sense for the produce of the soil in Job 31:39, “If I have eaten the fruits [אכלתי כחה, literally ‘(the) strenght I ate’] […] thereof (i.e. of the land) without money.”
The Septuagint (LXX) version, with its Greek expression προσθησει την ισχυν, seems to confirm the translation I’ve presented above.
In particular, the choice of LXX to translate the Hebrew verb יסף – which has the basic sense of ‘to add > to multiply’ - with the Greek verb προστιθημι. In fact, defining this Greek verb, from which the verbal form in Gen 4:12 came from (προσθησει), the Strong’s Lexicon wrote: “to place additionally, that is, lay beside, annex, repeat: - add, again, give more, increase, lay unto, proceed further […].” (also, similarly, see Thayer's Lexicon).
Moulton&Milligan: “προστίθημι [page 551] ‘add’. With Heb 12:19 we may compare Chrest. II. 372V. 11 (ii/A.D.) ‘καὶ προσέθηκεν· ‘Ἐχθὲς ἔφης ἄλλους ἐσχηκέναι παῖδας’ and P Strass I. 41.21 (A.D. 250) ‘Σαραπίων ῥ]η̣τωρ̣ πρ̣ο̣σ̣έθηκεν’ (his λόγος follows). Other ex[amples] of the verb are P Oxy III. 471.2 (ii/A.D.) ‘π]ροσθήσω τι κύριε περ[ὶ οὗ] θαυμάσεις οἶμαι’, [that is,] ‘I will add a fact, my lord, which will, I expect, excite your wonder’ (Edd.), ib. VII, 1062.4 (ii/A.D.) ‘προσθεὶς ὅτι τὰ θέρε̣ι̣ά ἐστιν τὰ κρείσσονα’ [that is,] ‘adding that the summer ones (sc. fleeces) were the best’ (Ed.), P Ryl II. 153.27 (A.D. 138–161) ‘ταῦτα οἱ προγεγραμμέ[νοι ἐπίτροποί] μου κα[ὶ κ]ληρονόμ[οι] μετελεύσονται καὶ προσθήσουσιν τῷ υἱῷ μου’ [that is,] ‘these (sc. certain allowances) shall be claimed by my aforesaid guardians and heirs and delivered to my son’ (Edd.), BGU I. 8ii. 15 (A.D. 248) ‘ἐδηλώ]θη προστεθεῖσθαι εἰ[ς ἀρίθμ]ησιν μην[ὸς Πα]ῦνι’, and Ostr 1159.3 (ii/iii A.D.) ‘πρόσθες εἰς ὄνομ(α) Ἐπων(ύχου)’ [that is,] “put down to the account of Eponychus.” With the use of the verb in Act 2:41 Preuschen (HZNT ad l.) compares Demosth. xviii. 39 ‘ὅσα ἑκουσίως προσετίθετο τῶν πολισμάτων’.”
We must not jump to the conclusion that the soil were not able to produce any fruit/crop, as some translations suppose:
“If you try to farm the land, it won’t produce anything for you.” (CEV)
“When thou shalt till it, it shall not yield to thee its fruit” (Douay-Rheims)
“If you try to grow crops, the soil will not produce anything” (Good News Bible)
In fact, the following dilemma would rise: ‘How did Cain survive if the earth produced no fruit/crop, at all?’ It would be just as well as he was immediately executed by God!
It won’t stand close scrutiny the other hypothesis, too (e.g. Gill and others), according which the God’s curse against the soil was localized on the sole agricultural works of Cain. In fact, the Bible itself help us to understand that that curse did last several centuries, affecting all the agricultural works of men in that span of time. Read, please, Gen 5:28-29: “And Lamech lived a hundred eighty and two years, and begot a son. And he called his name Noah, saying: 'This same shall comfort us in our work and in the toil of our hands, which cometh from the ground which the LORD [Jehovah] hath cursed.” (JPS)
Another clue that help us to well understand the Hebrew expression at issue, we can find it in some discourses of Lord Jesus – in Mat 13:8, 23; Mar 4:8, 20. There, he presents a kind of math multiplication coefficient that any ancient Hebrew farmer was able to expect from his cultivated field (bold is mine):
“But other seed fell on the good soil and produced grain, this one a hundred times as much and this one sixty and this one thirty. […] But what was sown on the good soil—this is the one who hears the word and understands it, who indeed bears fruit and produces, this one a hundred times as much, and this one sixty, and this one thirty."” (Mat 13:8, 23, Lexham)
So, from a single ‘unity’ of wheat (maybe, a ‘single grain’, remember the other words of Jesus (bold is mine): ”I most solemnly say to you, unless a grain of wheat falls into the ground and dies, it remains a single grain. But if it does die, it yields a great harvest.” (Joh 12:24, Williams’ New Testament), a Hebrew farmer was able to expect some of the following math multiplication coefficients: ‘x30’, ‘x60’, or, ‘x100’.
Then, the Hebrew expression used in Gen 4:12, makes reference to the amazing abilities (or, capacities, 'strengths') of the earthly soil to multiply what we sow in it.
What that the Creator said to Cain can be summarized so: “From this time on the soil will not produce – for you men – with the original lush vegetation, the luxuriance you are accustomed. You will reap – through your hard labour - only the necessary amount for your survival.”
Therefore, the correct translation of this verse must reflect the just-exposed idea.
Some samples of correct translations (bold is mine):
“When you try to cultivate the ground it will no longer yield its best for you” (NET Bible)
“When you cultivate the ground, it shall no longer yield its strength [it will resist producing good crops] for you” (The Amplified Study Bible)
“No longer will the ground yield good crops for you, no matter how hard you work!” (New Living Translation).
Only a final, short digression related to the soil’s amazing capacity to ‘multiply’ what man sow in it. I will report only a brief summary of what I’ve find in a file (in Italian language) called ‘Coltivazioni sperimentali per una valutazione della produttività agricola dell'età del bronzo nell’area padana’, di Marialetizia Carra, Maurizio Cattani, e Florencia Debandi (in ‘Ipotesi di Preistoria’, Vol. 5, 2012,1, pp. 79-100 [ISSN1974-7985]). This text remember us that – for some examples – Pliny the Elder wrote that in Northern Africa that from a bushel of sowed wheat the farmers were able to obtain 150 bushels of wheat [“In Italia in Subaritano dicunt etiam cum centesimo redire solitum, in Syria ad Gadara et in Africa ad Byzacium item ex modio nasci centum.” (Varrone, Res rusticae, I, 44, 2)].
Another Pliny's news was that that an African Proconsul sent to Emperor August a wheat branched plant with 400 spikes come from a single wheat grain! That historian reported also the news about a wheat branched plant with 360 spikes (Pliny the Elder. Naturalis Historiae, XVIII.21) “There is no grain more prolific than wheat, Nature having bestowed upon it this quality, as being the substance which she destined for the principal nutriment of man. A modius of wheat, if the soil is favourable, as at Byzacium, a champaign district of Africa, will yield as much as one hundred and fifty modii of grain. The procurator of the late Emperor Augustus sent him from that place—a fact almost beyond belief—little short of four hundred shoots all springing from a single grain; and we have still in existence his letters on the subject. In a similar manner, too, the procurator of Nero sent him three hundred and sixty stalks all issuing from a single grain. The plains of Leontium in Sicily, and other places in that island, as well as the whole of Bætica, and Egypt more particularly, yield produce a hundred-fold. The most prolific kinds of wheat are the ramose wheat, and that known as the ‘hundred-grain’ wheat. Before now, as many as one hundred beans, too, have been found on a single stalk.” [“Tritico nihil est fertilius. Hoc ei natura tribuit, quoniam eo maxime alebat hominem, utpote cum e modio, si sit aptum solum, quale in Byzacio africae. Campo, centeniquinquageni modii reddantur. misit ex eo loco Divo Augusto procurator eius ex unograno, vix credibile dictu, cccc paucis minus germina, exstantque de ea re epistulae. Misit et Neroni similiter CCCLX stipulas ex uno grano. Cumcentesimo quidem et leontini Siciliae campi fundunt aliique et tota Baetica et in primis Aegyptus. Fertilissima tritici genera ramosum ac quod centigranium vocant". (Pliny, Naturalis Historiae, XVIII, 21)].
"O give praise to the Lord of lords: for his mercy is unchanging for ever.
To him who only does great wonders: for his mercy is unchanging for ever." (Psa 136:3-4, BBE).
I hope these short notes will be useful to you.