From what I remember, volcanic ash tend to create good soil. So we can have magma forge that generate an, effectively, infinite source of fertilizer (thou for balancing issue, probably takes a while to do so or only supply a small amount). In short, each application slowly reset the soil back to norm (natural erosion takes away old soil, new magma soil has the right balance of nutrient for most plant growth).
It tends to create good soil... for surface crops. Dropping volcanic ash into a farm is just like other forms of rock or gravel - it has nutrients that simply haven't been leeched out of it yet. What depletes the soil is not replinishing those nutrients by replacing the minerals that are taken out of the soil to produce fruits and vegetables, leaving behind the minerals that are useless to a plant.
For underground crops, what you need is chemical energy. This means either the carbohydrates of dead stuff or waste materials like manure generally, but vulcanism can provide us with chemosynthesis in the form of sulfur-based energy. The problem becomes making sure this is not simply a "Get Free Stuff Button".
I recently posted this as part of the ongoing forums-wide argument that seems to be going on about Improved Farming for weeks now, and I'm thinking I'm distilling this argument down to a final reduced form at this point:
The problem with farming isn't so much that there's "too much free stuff", it's that, as Footkerchief put it, it's "a free stuff button". You get something for nothing. All the food you ever need for free. So long as it's free, the problem doesn't change, you're just kicking the can down the road a little. Make dwarves eat twice as much? Just build twice as many farms. You're still getting everything for free, it doesn't really change the game in any meaningful way.
If it's "trivially easy" to designate 50 tiles of farm and make one dwarf have a farming labor enabled, how much more difficult is it to designate 100 tiles of farm, and enable farming labor for two dwarves?
If it's "trivially easy" to designate 100 tiles, and enable the farming labor of 2 dwarves, how much more difficult is it to designate 200 tiles, and enable farming for 4 dwarves?
This is exactly the problem with plenty of these "stopgap" solutions, like with making "time slow down" or "adjusting the value of quality modifiers" - when you start arguing over changing the arbitrary value of a hardcoded variable for "balance" purposes, nobody is going to ever agree on what random, arbitrary value you pull out of thin air - some people want it easier, others want it harder, and you're talking about a completley arbitrary value, so it's purely a matter of opinion, and nobody's right, so the winner is the one who shouts most stubbornly.
Just look at the real basic materials in this game: Food, wood, stone, glass, and metal. There's a few other raw materials, but these are the ones that really matter. With sand on the map, and magma kilns, glass is free - make anything and everything you can out of it. Stone is free - make anything and everything you can out of it. Food (including pig tails, bone, and leather) is free - make anything you can out of it. Wood is limited (if renewable) in supply, so you don't make anything out of wood you can make out of something you have for free, but you still make plenty of barrels and beds and bins of wood. Metal is the only thing you really have to think about conserving on, especially steel or bluemetal.
Farming needs to become more complex because the problem will never be solved until farming becomes something more than just a "free stuff button". If you have to WORK for your food, then it becomes something far more meaningful. Even if wood is common, as long as its a finite resource that is difficult to scale, you have to at least use SOME care as to what you use it for. That is, of course, unless you go the extra mile and create the (hard and complex to build) tree farm to specifically expand your wood production capabilities.
Keep in mind that in this game, there are no real challenges (aside from rare FBs that have broken breath attacks) to a fort beyond the first couple years. (Although I certainly have my own arguments for how to fix THAT problem.) Survival IS the only challenge in this game, and if the game currently makes survival too assured, too easy, then it's not a bad thing to suggest that we make it a little more difficult to just scratch out a homestead and defend it from the wilds. (And that's ALSO not to say that I haven't thought exactly about how to present this to players in the most meaningful way possible, but rather that making the system complex, meaningful, interesting, and an important, attention-consuming part of a player's fortress design took a higher priority.)
If we are using magma for fertilizer, there has to be some meaningful reason why you can't obsidian cast magma infinitely to get past any and all problems, or farming becomes a get free stuff button again.
Using magma as a source of biomass in a scalable way only makes sense if we make it hard to set up and scale. Just being magma, setup is already not terribly easy, but scalability is a problem. There needs to be a reason you can't just make 100 of whatever workshops that send out infinite resources for the same prices as 1, and since magma itself is always infinite, this becomes problematic if it involves anything but making the process of turning the magma into something useful is difficult to scale.
The obvious first answer is just making it take plenty of labor, but then it just becomes the new glass - it's still free, it just happens to be a sinkhole to stick all your excess labor.
Another answer is the arbitrary limit on how much fertilizer can be applied - maybe magma-based fertilizer is sufficient, but not wonderful, and you can't just stick more and more and more rocks onto the soil, there is a finite number of additional stones you can place in the soil at any given point in time. This, however, encourages players to just constantly be dumping more and more stones into every farm plot they have, which may be weird (but hey, at least it gets rid of stone!) Using magma farms as a source for biomass in this way would also need some similar limit.
Ideally, magma farms might require some sort of complex balancing act, the way that farms are now being handled, so that they require player thought and work to set up and balance properly, although that's both unrealistic, and opening up a whole new argument I don't really need right now.
Or, you know, we could just make roving magma monsters pop up out of your magma farms, and make it so magma gets depleted, so you can't seal that magma flow off. The more magme farms you have, the more likely Magma Forgotten Beasts become...