NPK?
North Popular Korea?
The NPK model is covered in
this post.
Also,
super-concise TL;DR version is found here.
On the first page, I tried to create a set of hypertext links as a "Table of Contents" to keep things as easy to read as anything of this size can be.
The only problem I have with a system like that is that there's no inherent mechanic that encourages crop rotation. Seems to me that every crop depletes one or more aspects of the soil and the only way to raise it again is to use fertilizer.
Now, don't get me wrong, I think fertilizer is a great thing to have and use, but I like the "gamey" feel that crop rotation gives to things.
Well, it's not that I am proposing copying Rune Factory 4's model, I'm just saying it's a model that actually exists in a game, and one made mostly for "casual gamers", which directly goes at the heart of one of the most significant arguments against the NPK model I've been pushing from the start of this thread. People have been saying that anything more than two or three soil variables would be too much complexity, but in Rune Factory, you have a 7-variable soil quality system that is only a moderate impediment on players planting whatever they want. Seeing an actual example of a similar system in action is a valuable thing, since it gives you some concrete feedback as to how the player will respond to the impact of certain design choices.
It's not something that I have to go really far out of my way to micromanage (at least, no more than I already was in a game where I plant and harvest all the crops, myself), but it does have at least some impact on my choices, since I've had situations where I slammed head-first into 0 soil HP after planting a lot of strawberries in a single patch, and had to fallow that square for a few days before I could even grow corn I could plow in to recover the soil completely, and put back into service.
In fact, outside of trying to get the giant crops (which requires planting four of the same crops in that 2x2 area at the same time so that they "merge" into a multi-tile crop) it means that you pretty much just have one tile for corn/clover, one tile for heavy-feed a repeat harvest crop like strawberries or pumpkins, and two tiles for single-harvest crops (at least one of which you might want to make a light feeder). It's not exactly a "crop rotation", but it does encourage the player to at least spread out their use of the soil unless they're capable and willing to carpet-bomb the soil with fertilizers.
A huge difference between what I've been arguing for here, and what's in Rune Factory 4, however, is that the soil variables in Rune Factory 4 are far more segregated and distinct. A Formula A will add 0.5 to the growth speed multiplier, and nothing else. The "size" variable does absolutely nothing if you aren't going for a giant vegetable. You can have crops hypothetically capable of growing at quintuple speed, but nothing grows because soil is at 0 HP. My proposed system has fertilizers that impact multiple variables, and variables that are more inter-dependent. If things work as I want them to, that means that the player will have a less concrete idea of what is going on in the soil (especially since Toady isn't going to just put overtly gamey things like "the soil has 105/255 HP left", and instead have something like a "the soil is chalky and pale" to denote levels of biomass) but at the same time, won't necessarily need such a concrete idea in their day-to-day operation of farms. (Especially since farm overseers apply the fertilizers, themselves, meaning that players are mostly concerned with not running out of fertilizers or ordering crop schedules that consume inordinate amounts of fertilizers.)