Sockless, could you spoiler that chart? It's kind of huge.
Anyway, yes, that chart and the wikipedia article that refernces it has been linked in the last thread several times. Again, the real point of that is that it doesn't really matter much, because the granularity of the soil is not a really relevant aspect of soil fertility. If anything, it will be compressed into a single "drainage" stat so that players aren't too handicapped by something that is really complexity beyond anything they can really control.
As for the alkaline soil, yes, I have seen that, as well. That will come in the "Advanced NPK and Specific Fertilizers" section.
We've also had discussions on what sort of land area you need to "realistically" farm for the numbers of people we are talking about, and this will be mentioned in one of the closing sections on the thread. I should also point out that the neutrality of that article you referenced (which I've seen before in the thread) is very much disputable, as it is basically a pro-veganism screed. Veganism wouldn't even be possible in medieval times thanks to Vitamin B
12 deficiency, as was raised back then.
For "hydroponics" in gravel... that's not really hydroponics except in some very technical definitions, and it's really beyond medieval farming methods, anyway.
For Aquaculture, that isn't farming by raft, that's fishing. Or rather, it's fisheries, which would certainly be an option available to ancient, much less medieval fishermen. Ironically, the article actually talks about how the Ancient Chinese first practiced this by trapping carp from their rivers. There's also the interesting Algaculture article, where algae is purposefully grown in shallow vats so as to reduce it to fuel or fertilizer.
Aquaponics, however, was what I was thinking of, and it entails using fish and plants floating on rafts above them in a symbiotic relationship to work properly. This was performed by the Aztecs, in what were called
Chinampas within the dwarven limits on technology, as opposed to true (water) hydroponics, which was not really done until the 17th century. They are basically "rafts" that are built into shallow lakes (which dwarves can create artificially), which let the plants to clean the sediments out of the water (as in hydroponics), but instead of manually resupplying the nutrients, the way that hydroponics has to be carefully monitored for proper nutrient balance, you just let the wild fish excrete fertilizer into the water. The plants clean the water of excrement for the fish to have clean water, and the fish go about excreting more.
I have already mentioned soil salinity in this thread. Likewise, I have mentioned NPK in water. Both are in the "Water Management" section.
Similar methods were used in some forms of rice paddy farming, apparently, letting small forms of carp or eels into the rice paddies to help fertilize the water, and hence soil.
I should probably get around to that "Alternative Farming" section soon.
But it also seems like any nutrients that are brought into the system would stay in it too. Like, new immigrants, wandering animals, or several goblin sieges.
Yes, this is ultimately not going to be a perfect model of balance. Dwarves will probably have some excess material to spend upon expanding soils, provided they conserve their materials well. I can just produce leaks in the system - humanoid operations are just never perfectly efficient, after all. There are also going to be some additional methods of producing some more fertilizers to throw into the farms, including through mining (which is non-renewable, hence short-term), and with breaking apart stone, including volcanic stone through the use of pest-vulnerable lichens and microorganisms, which can help lead to an overall throttle on the expansion of materials.
This, and all other matters that really come down to "making the numbers look right" are really what should be left for the last steps of the construction of the system. What you really need is to have the model constructed in theory before you can worry about making the constants fit the equation.
Secondly, as a minor point, and I'm almost completely certain that it's been mentioned, the spoiler rocks could probably function as a magic source.
I'm not sure whether or not that would be a good idea off the top of my head. On the one hand, it does give a good reason to why some areas of the game may have more magical flora and fauna, and it would punish players with less useful caverns if they plundered the cotton candy stalls of all their gooey goodness. On the other hand, I'm not sure I want to punish the players for that, necessarily. There's already a punishment for eating too much of the cotton candy. Plus, it's what's under the big top that holds the serious magical sources. Colonizing the circus could be a really fun place if we have actual circus-raised crops, much less if we could do the "Total Victory Condition" of altering the circus to Dwarftopia, where every eerie corner turns into an overfull ocean of fermented ambrosia after shooing off the last of the clowns.