We haven't made any final decisions. I think a NPK+pH model does give you something back, because you'd get some really great varied local landscapes and it would take care of crop rotation, composting, naturally poor soil, or whatever else, but it introduces a farming interface problem to dwarf mode in terms of conveying the information in wholesome terms and allowing you to solve problems that come up.
http://www.bay12forums.com/smf/index.php?topic=60554.375the quote above is from Toady. I know farmingi improvements already have some quite lengthy threads. Ive opened this merely to discuss how Toady's statement above might work.
for those of you that arnt soil scientists NPK is a standard fertilser mix used by farmers in the real world. N being nitrogen (or Nitrate in most cases) P being potassium (Phosphate) and K being potassium. pH is parts of hydrogen but to most people who have been to school its how acidic something is.
a simple way of using this sytem in df would be to have a screen when you look at a farm (q on the farm, or k on the soil). The farm screen would give you info on NPK pH
pH: to have three classes for all soil layers/ mud low ph (acidic) neutral, and high ph.
different plants would have different preferences or tollerance ranges (to use the technical term) in pH. I suggest that plump hlmets be tollerant of any ph simply because they are a survival basic, but other plants could be specific to one or two pH classes.
in the real world acidic soils often form in water logged conditions, on less permable (more water proof) rocks, and at higher altitudes or in wetter places. Alkaline soils tend to form on well drained alkaline rocks such as chalk or limestone.
In you farm screen you should have the option to apply either lime, or ash.
lime an umbrella term for substances containing calcium carbonate (i think) can be made from chalk or limestone. This could be simplified in df to any flux material. It would need to be ground down tho, probs in the masons workshop.
Ash (other alternatives would be anything containing sulphur - coal but in the real world this would be quite dangerous) can be created already i think.
Applying one or the other would shift the pH class by 1 (so adding lime one season to a low pH soil would raise it neutral) . the effects should last 4 seasons (can be made longer). So you would need to add lime two sfor two seasons a year to maintain a natural low pH soil at high pH.
NPK are typically added as one in the real world either from an inorganic industrially produced fertilser (these fertilisers use crude oil (the ammonia process) and mined phosphorus, this probably is not an option for DF. Or real world farmers use organic fertiliser - usually manure from various animals. Since manure currently doenst excist in DF debris from the refuse pile should suffice. (or pretty much anything thats half decomposed. A job could be asssigned from the farmers workshop to create feriliser. The farmers would then collected food remains and animal remains and processes them into a fertiliser.
How often you add fertiliser depends on how nutrient poor your soil is. A possibility would be to have another low mediaum high system for NPK (as a group doing them individually would probs be too complex for df). 4 seasons of farming would decrease the nutrient value of the soil by 1 class with low being the baseline.) adding fertiliser for a season would raise the class by 1. So if you wanted to grow plants in a high nutrient soil you would have to fertilise two seasons of every year. In the real world you can fertilise whilst seeds are in the ground and plants are growing. So i suggest that the same applies in df, you can plant and fertilise simulantiously.
As with pH some plants will only grow in high nutrient soils, others will grow faster in hiogh nutrient soils, others wil not grow notably faster and are happy in low nutrient soils. again i suggest plump helments be fine in low nutrient soils.
in effect this sytem will mean that a fortress can subsist of plump helmets much as it can now, or it can invest labour in fertilising and grow more desirable crops.
side note - salt peter already exists in the game world (potassium nitrate) and could be used as a super fertiliser (it and phosphorus are also ingredients in some explosive recipes) but phosphate would need to be gained from another source. Possibly this needs to be included as a new rock type
http://en.wikipedia.org/wiki/Phosphorite