I didn't pay much attention to it at the time, since at the time of the formation of this thread, the Adventurer Mode was pretty abandoned, however, with recent work done making it clear that anything that happens in Fort Mode should happen in Adventurer Mode, I'll try to work out some of the problems that would be unique to Adventurer Mode.
Specifically, there are two main problems: One is that the farming interface I described would need to have an Adventurer Mode equivalent; The other is that soil quality needs to be visible to an adventurer.
For adventurers, it would be easier to break it down into two different "modes" of farming, with one being similar to Fortress Mode, and the other being a manual mode.
If the player wants to set up a farm or ranch where hired laborers work for them, or manage farmlands as a mayor or the like, then it would make the most sense to make it so the player has some method of delegating authority to a farm overseer. A text option while on the Adventurer Site or wherever the farm is located could then be brought up that replicates as many of the scheduling-related functions as possible. In fact, it could just straight-up be the same UI that just exits out to Adventurer Mode instead of Fortress Mode, or at least reuse much of the same code, provided that could be accomplished smoothly.
This idea is predicated upon farming being done only on adventurer sites or areas tracked like it, which could restrain the bounds of the interface in the same way that Fortress Mode is restrained to the embark boundaries. I.E. you have to "own" a defined piece of real estate to start ordering it farmed, and have to be standing in it to open the menus.
The use of the same sets of interface should help keep things relatively easy to code, since it basically involves reuse of the same scheduling mechanics, although obviously, there would be the nature of the timescale to contend with.
Essentially, there would be no method of ordering farming of an underling (other than maybe a "go get that plant" order) other than using the farming interface, so designation of farm overseer would be a rather fast-and-loose designation. A player might designate a professional overseer over a giant plantation, or simply designate their spouse to take care of the family garden on their own while they go off adventuring. The idea being that there need be only one main farming-scheduling interface. (Or at least, two that share most of the same code and functions.)
For small-scale subsistence farming where only the player is farming, or for when the player just wants to pitch in, there also needs to be a way for the player to directly see the soil condition, and assess what can grow there, at least to a limited degree.
Manually growing plants, of course, throws away much of the attempts at automation I've spent a large amount of time explaining, but it's also mechanically simpler, so long as intelligible information can be sent to the player.
I think the best way to handle this situation is to let the player have a "look" function at the soil, which can add greater amounts of detail, given the adventurer's grower (farming) skill. Although a numeric display wouldn't be used, in going with the game's style, a color-coded system for soil (I.E. red -> brown -> yellow -> green -> cyan) similar to previously mentioned in the Interface section of the thread could talk about specific elements of soil quality.
Carbon could be described with a soil darkness, with carbon-heavy soil being a coal-like black soil, and depleted soils being pale. Water is simple to describe simply through dampness of soil. NPK macroelements could be represented by different elements of what plants exist in the soil. (Barring total desolation or utterly eroded soil, there should be at least some grass and weeds even in a mostly empty patch of soil. For ease of player use, assume players are looking at a universal weed that reacts to the same nutrient levels everywhere.)
The color, number or broadness of leaves, and the growths of fruit or flowers can indicate N, P, and K. Even if players don't understand exactly what each one means on its own, keeping each in order and having them color-coded is a strong hint.
Each item related to farming, such as seeds, fertilizers, and the like, could all have additional information on their item status pages. Players looking at a seed might see, again, color-coded ideal amounts of each nutrient for the current seed. Each potential fertilizer could also have a "helps ___ much if you have ___ problem" with a given nutrient. For example, rotting animal corpse might have a "helps MODERATELY if you have PALE SOIL" line of description.
Adventurers with a decent grower skill could also have a "is this seed good for this soil?" function in the info page on a seed, which could lead to comments on discrepancies between ideal seed conditions and current soil conditions, along with suggestions on what amendments would be ideal to a higher-ranking grower.
A no-skill farmer might be able to distinguish topsoil color, but have no idea what conditions a plant might grow in, or how to distinguish when soil is healthy for a plant, except from what lives and what dies. Each level up to a certain point could add a different layer of information, such as a direct gauge of when a plant needs more water on the first rank of skill, or needed nitrogen levels on the next rank of skill, and so on.
Toxins and environmental effects (including magic/xenosynthesis) add another layer to the mix. Environmental factors, like whether the climate is appropriate should be visible through other means, although seed descriptions should indicate what climate they prefer. Detecting local magical conditions should also be up to the senses of an adventurer more generally than specifically with farming, although again, a trained grower should be able to look at a seed and determine when it needs certain types of magic to grow, and a player can just look around for other things that rely upon that type of magic to assess for themselves. (I.E. look for bubble grass before trying to plant a sun berry.)
Soil toxins may be visible through similar means, by simply adding weeds that enjoy certain toxins, and having other plants die off because of those toxins. For example, if nothing but salt marsh grasses are growing in an area, it should be easy for someone looking at the local plants to figure out that the soil is salty and marshy.
