You seem to be using a fertility modal that I'm not familiar with.
The official fertility modal as I understand it is
- Green manure and other raw biological material: poisonous to plants if there is more than a certain amount. Gets converted to organic matter over several months. When you plow a crop in, this is what is added.
- Organic matter: keeps nutrients from washing away in the water (not necessary for growth)
- Nitrogen: necessary for growth, washes away easily, can be created by growing legumes
- Phosphorus: necessary for growth. Closed on map nutrient cycle
- Potassium: just like phosphorus
None of the composts or other supplements are pure biological material or organic matter, they all contain plenty of nutrients to do the fertilizing for them.
I think NW_Kohaku is arguing that we don't need to bother modeling organic matter because different soil textures already have different amounts of nutrient leaching so organic matter would be redundant.
I want to include organic matter because I'm really attracted to the idea of allowing a badly maintained forest to erode into a sterile wasteland that can no longer hold any nutrients.
That's not actually the model...
The
specific post is here.
But basically, the model goes like this:
Water, Biomass, Nitrogen, Phosphorus, and Potassium are all soil nutrients that get tracked as part of the soil and changed in the soil constantly.
Acidity and CEC/Drainage are also tracked in the soil as basic soil stats that aren't likely to change much. (Putting humus depth here as a separate variable is also floated as an idea if keeping biomass for decomposers is desirable, but it's possible to consolidate it with biomass in general.)
Biological toxicity (this means things like hot fertilizer or leaving corpses on a plant to decompose), Heavy Metal toxicity (magma or other problem materials), and Salinity (irrigating with ocean water or adding ammonia to irrigation) are tracked as pollutants which are basically like bad nutrients.
Beyond that, there's a few stats that are also important, but not actually part of the soil, including energy source (I.E. sunlight available, but could also be if there is the right kind of magic field if we're talking the xenosynthesis stuff, ilke needing to be in a good biome if you're raising sun berries), temperature/climate, pest populations, plant crowding/weeds, and possibly also air quality. (Air quality being part of an even more advanced concept where caverns need oxygen replenishment so you can't completely seal them off unless you have (magic) oxygen-producing plants underground to scrub your air.)
All of the nutrient-type variables are needed within certain bounds for growth. (Plants don't grow without water or nitrogen or biomass, but there's an upper bound where you can over-water it, and that hurts growth, too. There's an inner bound with ideal growth, and an outer bound with growth slowed, and outside those bounds, growth stunts.)
Some plants don't need much biomass/humus depth, but those are mostly grasses that aren't going to produce anything edible to a humanoid. Most need at least some, and it's treated like a nutrient.
Of them, specifically, water is obvious - keep throwing more in, but ground covers, low drainage, and deep humus/biomass levels slow its rate of decline. Biomass needs some sort of compost added to it to keep up or else have plants tilled back in or soil-building plants. Nitrogen can have nitrogen fixators help build up levels, or compost, or lant/ammonia. P and K both just need something externally added, like bonemeal or potash.
Drainage and acidity don't change much unless you do very intensive work on it, like replacing the soil or stirring ground sulfur into the soil.
Pollutants shouldn't go up much besides maybe bio-toxicity unless you're doing something wrong, and salinity because you're adding in urea/ammonia when irrigating it as additional nitrogen (that's the "manure tea" part wierd was talking about), and they go down slowly as plants can leech things out. Salty soil needs things like saltgrasses (which are cattle fodder) to leech salinity out, for example.