I've seen some DMs treat Plant Growth like a shot of fertilizer. Others treat it like a dose of Speed.
That is, it might enrich the soil and thus promote healthier plants, or it might cause the plants to grow madly, depleting the soil of minerals and nutrients.
The Overgrowth version could be considered a good way to "let fields lay fallow", an old style approach to land management before commercial fertilizers were known. You'd let a field grow weeds, grass, whatever, for one year in four, then plow that growth back into the soil to act as organic fertilizer. This approach gives the soil time to convert raw mineral content into organic mineral content, something that helps make it easier for plants to take root and prosper in the following years.
Also, the Overgrowth approach could be very productive, depending on what crops you're trying to produce.
For example, it's a terrible thing to do to a wheat or barley field. It's a wonderful thing to do to grapes, berries or clover. (i.e. vine or ground cover crops.)
But also consider the spell's use away from formally planted fields. If a few acres of woodlands were to receive the "blessing", the available food for wildlife would increase. That means more game to hunt, more timber to harvest, etc.
And, consequently, more animals than the local food supply can support the following year, when the spell wears off. (The reason why people in many areas are discouraged from leaving corn feed out for deer in the winter.)
In an older version of the Tome of Magic there was a Druid spell, 2nd level if I recall correctly, that grew seeds to full grown trees instantly. One tree per two caster levels, no two seeds more than 60 feet apart.
We used knowledge of that obscure spell in a game to thwart the plans of some greedy merchants to take over an island controlled by the Elves. The Compact with the kingdom required the Elves to provide enough timber to build one ship each year, and enough of their people to man it for that year. That was their annual tribute/tax base, and over all it wasn't an unfair deal. The Elves were living up to the deal too, though no Elves ever served more than their compulsory year in the navy or merchant marine, while the merchant lords saw an entire island full of trees that could be clear cut, an island they couldn't touch unless the Elves broke the Compact.
We were sent in to "investigate" (read "instigate") problems there, and find an excuse to declare them in violation. (If we didn't take the job, the merchant lord would have sent someone else who wasn't as ethical, and the Elves would have been screwed.) Our solution was to show their Druid the spell, thus giving them a way to preserve their forest, and still increase their timber production enough to negotiate away the compulsory service part of the Compact. (Elves really don't like taking orders from people. The whole Chaotic Good thing.)
Of course spells like these don't make matter magically appear. Plants convert soil, water, air and sunlight into wood. The Elves were, in essence, selling water and dirt from their island to the kingdom in exchange for the right to be left alone. But since dirt was, and is "dirt cheap" they considered it a bargain.
So, while magic can address and even solve some problems, pretending to be wiser than Mother Nature is a risky business, and can have disastrous consequences, long term.