Basically, the root of the system, this is the most basic method by which we can make crops more interesting and dynamic.  To sum the progress of the thread up, while I was initially in favor of a very simple system, I was successfully convinced of the need for a more robust system to track soil nutrients.  Generally, I'm working with a 6-variable model right now, although there's things I am wavering on expanding into making more variables to include.  These variables are Water, N, P, K, pH, and biomass.

NPK stands for Nitrogen, Phosphorous, and Potassium, which are the three "Macronutrients", the most important atomic components in the soil.  Photosynthesis converts CO₂ and H₂O into simple sugars, like C₆H₁₂O₆.  Most of the building blocks of life are made of some combination of CHON or CHONP, so NPK are important components, but air and water are far more heavily used, while NPK are used in that specific order of prevalance.  With that said, they are all critical, it just requires lesser and lesser amounts by mass.  Air is not being measured in any of these systems so far, although it is certainly an exciting future pathway for the game to explore, especially if we are to have oxygen-producing subterranean plants.  Water is functionally the biggest "super nutrient" in the game, then, followed by NPK.

As they are arranged in order of how much is used by volume, they are also arranged in terms of how water-soluable they are, and by extention, how easily plants can gain access to them.  Water obviously flows easily through soil, nitrogen is very soluable, while potassium is very likely to bind to the clays in the soil, and is not very water soluable.  Water soluability affects how much runoff occurs IRL, although that may or may not need to be modeled.  Water soluability also affects how available a nutrient is to the plant's roots.  If something is water soluable, it flows through the soil, and the roots can easily be there to suck up the nutrients, while less soluable K requires the roots to dig right next to where the K ions are to suck them up.

This ultimately means that Water and Nitrogen are values that will fluctuate more wildly, while Potassium is very stable.  Water is something you will constantly need to replinish (although, honestly thinking about it, in a sealed cavern, you could probably find ways to reclaim the water).  Replacing nitrogen is the main point of crop rotations, and the soil can be depleted of nitrogen within just a year or two of planting "heavy feeder" plants.  Phosphorous takes a middle road on this, while Potassium is something that you can probably go for a few years without worrying about. 

Something about nitrogen, however, is that while you need plenty of it, it is also the easiest nutrient to replace, whether by planting nitrogen-fixing crops, like peanuts, or just by using urea, the primary component of urine, to make ammonia, which is how the overwhelming of nitrogen fertilizer is applied to the world's crops. 

Nitrogen fixation occurs because certain root-dwelling microorganisms of plants like clover, peanuts, and beans are capable of converting the stable N₂ in the atmosphere into a usable form for living beings.  Crop rotation is based around the planting of nitrogen fixating plants to restore nitrogen to the soil between the growth of nitrogen-heavy plants like wheat, rice, corn, or other major starch foods.

Phosphorous and potassium, however, are not so easily replaced from the functionally infinite well of atmospheric gasses, and generally, the best, most renewable means of replacing them is to just use dead or decaying materials to replace what you have taken out of the soil.

It is, however, entirely possible to mine for mineral fertilizers, such as apatite stones for use as a phosphorous fertilizer.

This would mean that expanding farms beyond the "easy soil layers", and "making your own soil" by muddying the stone would probably become a long-term project.  You would need to start with crushing some mineral fertilizers down to gravel, keeping them muddy, and trying to grow "rockbreaker" plants (or rather, moss and lichen) to break the fertilizer stones down into a useful soil.  (Of course, letting logs rot into soil by growing mushrooms also works to a degree.)

To a certain degree, it's not really worth retyping all this text, and reposting it is just a waste of character limits, so I'm just going to punch in a link to the post on how we could represent NPK to the player, this post on fertilizers (in fact, page 32 in general outlines the basis of this), and this link to a series of posts modeling how this could be raw-ified. 

There are still two more variables that I have to really go into, however, and those are soil pH and biomass.

Soil pH (acidity) is another very long-term variable.  In fact, it should take about a year to adjust a soil's pH a single whole number one way or the other.  Soil acidity is altered by the process of decomposition, so places like forests where plenty of rotting leaves fall will have more acidic soil, while deserts will have more basic soil, and different plants are adapted to different soil types.  Generally, you want very slightly acidic soil, however. 

Fertilizers, especially ammonia and urea, can alter soil acidity, while plants themselves don't generally do much to do so.  The general point of soil acidity, however, is that it's very slow and labor-intensive to change it, so it's probably better to just try to avoid having a soil acidity problem in the first place, or grow your plants to just suit local soil acidity rather than alter soil to be perfect for the crops you want. 

Biomass, meanwhile, exists largely for the purposes of growing funguses, which cannot photosynthesize, and as such, need something to decay to grow and gain chemical energy from.  Biomass is a measure of the available carbohydrates in the soil, generally from "dead stuff" in the soil.  At low values, it represents depleted/eroded soil, where few plants besides ground cover.  At medium values, it represents increasingly rich humus, suitable for sustaining most crop plants.  At very high values, it can represent an outright sludge of decaying material that may invite bacteria causing soil toxicity, but which is perfectly suitable for "crop" fungus. In cases of "mundane" fungus, biomass may be essentially the only nutrient worth tracking besides water.

Speaking of extra stats that may be trackable, soil drainage is another potential variable that I didn't talk about in a serious manner, but could potentially be a useful means of differentiating certain plants from other ones.  Plants such as grapes, for example, require plenty of drainage, and drainage is very difficult and expensive to build into your soil.  It basically requires manually creating raised planting beds, or artificially adding a slope to the landscape to create drainage.  Both of these are extremely labor-intensive operations, and represent massive allocations of time and effort to make certain types of crops more viable for farming.  (That is, if you want the best wines, you have to put in the effort to manually terraform your landscape for that purpose, and that purpose alone.)

Now then, as I say in one of those posts, I think that what we need to do is track all these numbers in terms of 0 to 255, at a minimum, as a means of having a serious way of nuancing the differences between one season's soil and another.  Potassium, for example, only goes up or down two or three points in some of the proposed plants I made up, so that there is only a very subtle change in soil fertility unless you ignore it for long periods.

Ultimately, if we are going to be serious about farming, water has to be a major part of this.

There are a few separate issues, all of which fall generally under "water", which range from watering the crops and irrigation to water availability to the mineral content of bodies of water and their effects on soil erosion, as well as general weather for modeling droughts or other significant hazards to farming.

Irrigation Methods:

Before I start, I should reiterate something from the NPK system, since I believe I glossed over this point, and let it be covered by a post I linked back to (which not all of you will necessarily follow): Water is a separate integer value that represents the moistness of the soil.  Actually flooding the tile with water (as in having a 1/7 water depth or more) will cause the water value of that tile to instantly max out, which may be harmful to many crops which are not suited for actual flooding.  Part of the system I have outlined is that you can kill plants with too much of something, including making their soil too swampy, although there are some exceptions that cannot be drowned.

First, and most simply, you can just plain put your fields outside.  Above-ground farms with no ceiling above them should be capable of just soaking up rain in any sufficiently humid climate.  A slightly more elegant weather system will probably be required, but I'll focus on that in another sub-section.  For now, I believe that the best way to model keeping crops watered through rainfall would be to declare the rain that falls when the game declares "it is raining" to be the especially heavy rains, while some unannounced invisible light showers can be simulated behind the scenes to keep the overall rainfall suitable for simply having open-air fields with no irrigation methods.  Basically, while showers occur, the game invisibly waters the fields for you.

Next, and least efficiently, we should be able to "bucket brigade" our water.  To a certain extent, this will be the "default" watering method of any underground farming method, and would heavily encourage building your reservoirs as close to the farm plots as possible.  This should obviously become very inefficient as farms grow larger, and would probably require multiple reservoirs near every set of farm plots to compensate, but by that time, you should really have moved on to better irrigation methods.

Beyond that, there is the "controlled flood" method.  Suitable for things like initial muddying or just before planting, flooding the soil obviously gets things wet in a hurry, but as I said earlier, it should be deadly to most plants that are already planted.  Some crops (like something modeled upon celery or rice) may be able to survive simply being planted in standing water.  (In fact, the reason why you grow rice in standing water is not because rice needs it, but because rice is one of only a few plants that survive that sort of treatment, and it makes an effective weedkiller.)  Generally, though, this should be discouraged, even if it is the most familiar method, currently.

Finally, we get to methods of true irrigation, and there was some contention regarding how best to do this.  In the interest of giving the idea a fair hearing, Silverionmox was a proponent of simply having visible water trenches dug alongside farm plots filled with certain levels of water that simply supplied water through the sides of the trench.  His main argument for this was that it would involve systems already part of how we deal with water, and he believed it was not a good idea to mix means of transporting water.

On the other hand, I was a proponent of a separate system of sending water to farm tiles.  There are two possible ways of doing this.  The more basic method would be to have farm tiles "improved" by having special furrows for letting water through, or a special "water trench" tile that only carries smaller amounts of water than are normally displayed in the game, aside from possibly in buckets. 

The "dwarfier" response to the need for irrigation, which I favor the most, would be to involve some sort of plumbing system with small pipes (not the kind you can walk through, which are slated to be introduced, but a smaller one just for water), which just have regularly punched holes in them to act as a "drip feed system". 

Gravity or pump-driven water pressure would allow either one of these irrigation systems to send water down its track, and we could abstract it to automatically evenly distribute the water.  The advantage of this sort of system is that if it is a system where the numbers are not based upon the already in-game physics system, it would be much easier for the dwarven AI to be able to understand, and hence control, and hence automate the entire irrigation process.  You can simply make each copper sprinkler pipe connect to a designated sluice or other control system, and dwarves could simply stand on top of that sluice's tile for as long as they need the sluice open, and it will close off the water supply as soon as they are done manually keeping it open, preventing any messy problems with irrigation systems being left open to flood your fort. 

Even more advanced irrigation systems may involve letting fertilizers become added into the reservoirs or sluices, so that fertilizer is distributed at the same time as water is distributed to the farm zones. Ammonia/nitrogen fertilizers are highly soluble, so this is an easy way to keep nitrogen levels at a constant rate throughout the year.  Simply mixing a small amount of ammonia into the water as it heads through the sluice to the plants creates the "manure tea" that allows for a trickle of fertilizer along with the stream of water.

To point something out, each farming zone would likely require their own independent irrigation systems, as different crops consume water at different rates, and you can kill plants by flooding them with too much water, meaning giving every crop equal amounts of water can be problematic.

Another thing that is useful about transporting water through "small pipe" methods is that modeling it should be significantly less of a burden on the CPU than the current random water sloshing of the physics engine - a small pipe that just has a "funnels x water per tick" function will not be as processor-intensive, which will be important, since we're going to be talking about using much more water, when water is already a significant drain on processor power.

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Water Availability:

As I have discussed at the beginning of this entire proposal, one of the ways in which I want to make players face a serious choice about how to manage their resources and food is to make the resources you can use to make growing food be finite, yet renewable.  While I have previously stated a goal of this to be trying to truly respect Conservation of Matter, water may well be a serious exception to this rule, as things like condensation rates and tracking of humidity levels may be too complex and finicky models even for Dwarf Fortress's standards.  Instead, water needs to be constantly introduced to an embark site, yet these rates of water input need to be better calibrated than they currently are. 

I want to also point out at this time, as I will probably have to point out in many areas, that the Volume and Mass suggestion contains a proposal to rework the way that objects, and especially water are tracked, which would make the tracking of water supplies easier overall than they would otherwise be.

Water currently is introduced to maps through what can only be called infinite water generation points.  Edges of maps where water starts out, whether it is a small brook, a tiny underground lake, an aquifer, or an entire ocean, they all produce water at the same rate (on a per generation tile basis), and they all have infinite quantities of water.  This is perhaps most amusing in cavern lakes, as if you embark on an entire 16x16 embark, the lake will be entirely self-contained, and no water will regenerate in the lakes in the caverns.  Embarking in the exact same location as a 15x16 or anything smaller, however, will mean that, if any part of the lake is cut off, the water flow into the rest of the cavern lake will be infinitely renewed, even if only a single tile was cut off, just as if it were an entire ocean.

What needs to happen is that this game needs to start tracking water input sources, and setting finite limits on how much water will be introduced per water source.  The ocean can pretty much remain the way it is, as there is no real need to quantify how much water is in the ocean, we can pretty much assume it's infinite, but everything else needs to be a depletable source.

*** LATE EDIT: As of 2015, California, for example, is going through consecutive years of extreme or "exceptional" drought.  Over 80% of California's water consumption is in agriculture.  Much of the blame is being placed upon the fact that Californians chose very water-intensive crops, such as alfalfa (used to feed cattle) or almonds (used for almond milk) when their water tables were high. 

Again, as a form of engineering challenge, working with limited amounts of available water per year can be a serious consideration for players, setting a "roof" on what sustainable crops they can produce, especially if they have to balance growing food crops, trees, cloth, and feed/pasture grass for livestock.  Dry areas may force players into hard choices about priorities, limiting population, trading for food and cloth, or encourage extreme engineering to manage their water supply.

Ironically, in grasslands too dry to be arable, grass-fed cattle are actually somewhat more suitable as a source of food compared to more water-intensive plants, although these tend to require wide-ranging semi-nomadic cattle drives to keep fed.  Embarks on tundra, at least, will no longer be a trivial event of just eating nothing but meat and eggs from poultry farms.

Periodic drought as a "disaster" (possibly with embarks having warnings of how severe a drought can be) where depletable water sources do not refill as normal for one or more years in a row (little/no rainfall and little/no cavern lake, brook, or aquifer refilling due to lack of rainfall/snowpack) to add severe challenges to farming management.

Similarly, with some work, a "flood" might occur. River tiles have a natural set of entry tiles that spawn water continuously.  Adding additional tiles a z-level above them that spawn water periodically during flood season or a flood event could create for a more dynamic water source, especially if the seasonal floods, like the Nile floods, (or old cave floods,) had different soil nutrients in its mud (contaminants). 

Something similar with magma could represent volcanic eruptions.  Raising a magma pipe up in terms of where magma will spawn to for a season can produce a Fun surprise for dwarves.  (Especially if potentially triggered by excessive magma dumping, like abyss creatures were in 2d...) If there were walls in the way, preventing full magma expansion (and it wasn't being drained off elsewhere...) it could create local pyroclastic events.  Receding magma flows could leave behind either new igneous walls (like mafic gabbro or basalt if there was plenty of expansion area, or felsic granite and andersite if there was pyroclastic activity) and leave magma-flooding contaminants/nutrients in the soil.

/EDIT ***

Brooks, especially, should have very finite amounts of water inflow.  We might actually want to go from the current method of having a 7/7 water tile covered by a "floor" that allows walking over a "shallow" brook to simply having a 1/7 or 2/7 stream inside of a riverbed, especially since most of these riverbeds are already recessed a single z level and have ramps on them, anyway.  All creatures can "wade" through a 1/7 or 2/7 water supply, anyway.  Water could then be added to brooks at a smaller rate.  Rivers or other larger water bodies can still remain the way they are, but have better throttling over their water inputs, although river width should make a fairly major impact, already.

Every source of water (besides the ocean) needs to have some sort of throttle on the amount of water it can provide - rivers and brooks need to have some sort of counter that tells the game how much water will be flowing through that body of water.  This should not be static.  There are two primary sources of water for rivers and brooks, and they are rainfall and snowpack. 

Rainfall should periodically flood brooks and rivers with excess water that exceeds the soil's ability to retain the soil, leading to brooks that occasionally fill up with greater amounts of water, while they may occasionally run dry at times in drier climates.  Rainfall will require a separate climate tracker for "upstream" weather patterns, although this may not necessarily need to be more complex than the on-map weather tracking, nor does it need to be checked as often.  An arbitrary upstream spot should be selected, and have its weather rolled to see when brooks will flood from upstream sources, provided the source of a brook is far enough away from the map (sometimes brooks start on the embark site, meaning the weather on-map is the source of the brook).  This can mean that it may be dry in the embark, but it rained on the mountain that feeds the brook that runs through your map, so you can at least draw water from the brooks.

Snowpack is a far more regular and predictable source of water, which is why it is relied upon by farmers to supply water to their fields far more than rainfall is.  Snowpack involves the snow that falls upon the windward sides of mountains gradually melting through the Spring and Summer months, and being replenished through the Winter.  This naturally leads to higher water levels in brooks during the warmer seasons, and less (possibly none) in the colder seasons.

Such periodic water supplies mean wise farmers ensure they build reservoirs or dams to ensure they have supplies of water even during the dry periods. 

Another common surface water source is the murky pool, which I will cover in conjunction with on-map rainfall.  Currently, rainfall is simulated in a fairly granular way by having "raindrops" that drop entire 1/7 water units.  If they fall in a murky pool, they become regular surface water supplies.  If they fall anywhere else, they basically just dissapear, although they "wash off" any contaminant they touch, and douse dwarves caught in it in water. 

Rather than this, I think a slightly better way of handling this concept would be to involve the use of the previously mentioned system for tracking "wild soil" zones, as well as farm zones.  Rainfall gradually just adds to the water variable of that soil zone any time it is raining.  If the water variable become maxed out, then an overflow occurs, and every chunk of overflow water will produce a 1/7 water unit to be produced.  If there is a murky pool in or adjacent to the wild soil zone, the 1/7 water unit will be spawned at one open murky pool tile (since it is designed to emulate the low-lying land where water pools in rainstorms).  If there are no murky pool tiles open (such as by being built over or already hitting 7/7), then the water "floods", and is produced at a random spot in the soil zone.  This means that completely saturated soil will start behaving similar to the way that rainfall behaves now, except with every tile possibly spawning 1/7 water units. 

This means that areas on the surface with no soil will probably have to get hit by 1/7 water units, as well, as paved stones and mountains with exposed stone and no soil will just immediately "flood".  This can, however, get very processor-taxing, so a method of abstracting this process in a less processor-intensive manner may be advisable. 

This may also open up a desire for storm grates or sloped roads or slanted roofing (although this may be simulated with better recognition of ramps) that you could use to somehow direct rainfall, not only to make sure it doesn't flood undesirable areas, but also so that you can divert more rainfall into water resevoirs.  Of course, this can depend slightly upon the sheer ability of the game to even have enough an ability to display some of these things to the player.

Speaking of which, rainbarrels should be buildable, which would use these same mechanics to gradually have water fill them.  Combined with the perhaps more industrial scale of a completely paved surface with a storm grate down to a resevoir should give enterprising dwarves the ability to ensure that water is not wasted on silly surface soil where it can evaporate without their aiblity to exploit it to its fullest potential.

A final method of water supply would be the underground water supplies and lakes.  Lakes, underground lakes, and aquifers are all bodies of water that do not rely so much upon a "flow rate" as it relies upon having a massive quantity of stored water, with a generally fairly slight refilling rate.  This would mean that these lake-type water supplies can be depleted much faster than they will be capable of naturally refilling, which can mean that aquifers or underground lakes can theoretically be drained completely, with only a tiny trickle of incoming water to replinish it.  Countering this, however, pumping water into an aquifer or lake should replinish it, so just pumping water from an aquifer into an aquifer using power supplied by the water flow of the water from the aquifer will still be a perpetual motion machine of sorts. 

The actual way this would appear to the player should actually be somewhat similar to how it appears now, with water continuously arriving from off-map, up until the point where you completely exhaust the reserved water of the lake or aquifer, at which point the z-level of the water table either drops, or if it is at the lowest possible point, it simply does not produce more water except at the rate at which the resevoir refills, which may be glacially slow.

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Soil Erosion:

This is one of those things where the concept is simple, but the implimentation may be extremely difficult, but it may be required as a means of achieving a truly excellent model for environmental factors, especially of the kind I was talking about back in the Forestry section. 

Water currently "muddies" stone or soil where water flows, allowing those areas to become fertile.  In a limited sense, this is relatively realistic, as water tends to dissolve soil nutrients, and then carry those nutrients with them as they flow, depositing some of them as they are carried by the water across a surface. 

To be a truly compelling model, however, water needs to be a bit more complex, and this may involve breaking the current water system to an extent to replace it with something a little more complex, more similar, again, to what I had suggested in Volume and Mass. 

Water, in order to act the way that I envision, needs to also have a soil nutrient tracking system.  This obviously shouldn't happen across every tile of water, but rather, across water bodies.  Contiguous water bodies need to have what amounts to their averaged-out dissolved soil nutrient content. 

When bodies of water meet, they need to average themselves out. 

When bodies of water flow over any surface, they create the "muddied" surface, or more accurately to the new method I am suggesting, create or add to a wild soil zone.  The newly created wild soil from mud should have soil nutrient levels matching that of the water, representing the deposited silt, while the water has its dissolved soil levels reduced to represent the loss of that silt.  The gain and loss of dissolved nutrient levels should, of course, be averaged by the number of units of water contained in the body of water, meaning large bodies of water will have barely noticeable changes in their nutrient content as they flow.

When bodies of water flow over already existing soil that does not have plants growing on top of the soil, soil erosion occurs, gradually taking nutrients from the soil, and putting it into the water body, increasing the water body's dissolved soil nutrient content, while decreasing the soil's retained soil nutrient levels.  This should occur much more quickly for the water-soluable nutrients of Nitrogen (10 times the rate of Potassium) and Phosphorous (3 times the rate of Potassium) more than it should for Potassium, the least soluable nutrient.  The amount of nutrients that water sucks up depends upon the volume of water compared to the surface area of soil it is leeching nutrients from.

Generally, different water sources should have different nutrient levels.  Murky pools should leech some nutrients from the surrounding land when it is being filled with rainwater, and should generally have fairly decent amounts of nutrients in them.  Rainfall itself, however, should have almost no nutrients, and only possible pollutants, although that is a matter for another section.  Brooks and rivers should contain nutrients brought by the gradual erosion of the mountains where they originate, but generally, brooks should be fairly pure, while only the widest, slowest rivers should carry massive amounts of silt from upstream, and the general length and area of the watershed of a river should be used to determine the dissolved mineral content of rivers.  Aquifers and lakes should generally pool a decent amount of dissolved nutrients, although water entering an aquifer is generally filtered by the soil, so that it has a lower saturation point.

Ultimately, this means that letting water flow over your fields is a bad idea, as you want them nutrient rich.  Having standing water that has time to settle, and which you can dump fertilizers into constantly, can allow farming of some types of crops like rice, since you can just bring the water itself up to saturation point, and make sure that there isn't too much water to dilute your overall amount of fertilizer.

Water added to soil that is less than the soil's saturation point does not produce soil erosion, so careful control of irrigation in underground chambers should not need to fear soil erosion, so long as nothing goes wrong... 

Soil erosion on the surface, however, after dwarves have properly trampled all the plants, can allow for the gradual change of an area of the map from forest or grassland into desert or muddy swamp where nothing lives.

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Reverse Soil Erosion (Floodplains)

As an extrapolation upon floodplains, if a river actually has more nutrients in it than the soil it runs over, then it will wind up adding to soil fertility.  This will actually create soil if there is none, which basically replaces the current "muddy" system of irrigation. 

The quality of nutrients that are deposited, however, is dependent upon the source of the water.  A clear mountain stream probably has few nutrients in it.  A broad river that has emptied a continent, like the Nile or the Amazon river, especially when close to its delta, will be a slowly drifting, nutrient-clogged body of more mud than water. 

These major riverways are then useful for floodplain agriculture, where "reverse soil erosion" lets you purposefully flood your fields every year to replenish your fields the easy way.

Just as a game balance technique, however, there needs to be some limit to how effective this is, or everyone will want to embark on the Nile for free farm goodies.  Either the reverse soil erosion technique doesn't produce very good soil, and hence crops, compared to a better-managed system, or the amount of water per year that you can get from such a river needs to be fairly small.  The idea of draining a river (even a fairly slow-moving one) like the Nile or even the Mississippi is pretty absurd, however, so the first option would have to be the best, and make reverse soil erosion just not particularly attractive, even in extreme cases like a Nile (which is very rare in-game, already).

Since rivers are based upon their upstream flow rates, very large rivers like a Nile should have the sort of massive changes in water level that wind up creating a seasonal flooding - the water flow may drop the river's depth to where some tiles are accessible when they weren't before, and you can farm the unflooded tiles, simulating the Nile river delta... but beware years that lack a flood, and even more, the years with too much of a flood.

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Magma Flooding

Magma, and the ashes and such that it leaves behind, are a tremendous source of nutrients, fresh from the core of the earth.  They can be fairly toxic and plagued with heavy metals, but magma should also be a prime means of very rapidly fertilizing a new soil, or creating one entirely.  Magma could potentially use methods similar or identical to the water flooding code to cause dramatic changes to the soil, or even leave a form of "magma soil" behind when it "evaporates" if there was no soil there before. 

Magma flooding can be a fast and easy way to replinish soil, although the heavy metal toxicity would be problematic for most plants if anything more than a light dusting took place, or water wasn't used afterwards to try to flush the heavy metals out.

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Salt

Salt is another major issue when dealing with water.  Salt water is currently handled by a simple flag that indicates salinity in the same basic method of map data storage that water already currently uses.

This might have to change somewhat, although to an extent, for the purposes of map data, the binary flag for whether or not water is salinated enough for the purposes of being "contaminated" for drinking might be just fine as-is.  What we will probably need, however, is a new variable for salinity of both water and of soil.  (This starts off my even more expanded set of variables to cover "pollution".) 

Salt water from places like oceans should obviously have a high starting salinity, while fresh water has a low to negligible starting salinity.  These waters, separate from their sources, might be mixed, with the salinity diluted, but this is mainly for the purposes of soil salinity.

While almost any living organism needs salt, almost any organism will die if exposed to too much salt, as osmosis (through control of salt) is the primary way in which cells maintain proper hydration.  This means plants in salty soil die of thirst, and salting the soil can ensure the death of most plantlife (and subsequently, animal life).  Only plants called halophytes are particularly adapted to salty water, and these are generally not crop plants, and would make poor ones, anyway, as they grow slower, but there's always making new plants up to fit the niche.

Irrigating soil with salty water, or some other means of accidental soil salinization (including even regular irrigation, see [url-http://en.wikipedia.org/wiki/Soil_salination]the Wikipedia article part on irrigation[/url]) will increase salt "pollution" of the soil, and make farming significantly more difficult until the soil can be leeched of the salt.

Some of the previously mentioned halophytes are actually best used for this - saltbushes will concentrate the salt in their leaves, so that if the saltbushes are harvested, the salt is removed from the soil, and can be taken elsewhere for disposal or even eating, as apparently, people do eat these things. 

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Weather:

Generally, the weather model currently in DF is fairly good for what we need it for.  It will need to be expanded to fit some of the changes I have talked about, however.

There should come wet and dry years, as well as merely wet or dry seasons.  Very stormy years, and possibly even things of rare, catastrophic nature, such as the eruption of the Krakatoa volcano, which caused "The Year Without a Summer" in North America and Europe, can liven up the battle against the elements themselves.  Droughts and floods, however, should generally be the main worries of farming dwarves.

Rain can also be more abstracted than it is currently.  Rain as it is now might be better made as slightly more rare than it currently is, and instead have "showers" that are more frequent, but with less obvious gameplay effects.  Showers produce small increases in wild soil water levels that allow wild plants or unirrigated farms to go the periods between the major rainstorms. 

The purpose of this thread is to solve several problems with the current game:

• Problem 1: Farming now is, generally, easy and boring.
  
  • Farming needs to be more interesting, with more crops that are different.
  • Current crops (especially above-ground) are nearly indistinguishable in how they are grown.
  • Farming should include more physical types of farms and farming techniques.
  • Different climates and biomes make no difference in how you farm or what you farm.
• Problem 2: Farming is currently a front-loaded challenge
  
  • We have a "learning cliff" currently - you have to know everything to get anywhere in the game.
  • Once you know everything, the game is too easy, all problems have been solved.
  • The goal should be to make something simple to start with, so any random new player survives their first year, but progressively more difficult as time goes on, challenging those who have mastered the basics
  • It should be possible for players to get by without knowing terribly many tricks, but with rewards for the player who truly masters the system, in much the same way as dwarfputing.
• Problem 3: Deep without micromanagement or pointless complexity
  
  • All automatable things should be automated.
  • Anything that is a simple "what's the quickest way to make the most food" math problem is a calculation, not a choice, and that is automatable.
  • Once all automatable functions are out of the way, high-level management choices should be made as a matter of a strategy, based upon comparing difficult-to-compare-mathematically things, such as short-term payoffs vs. long time setting up but better long-term payoffs.
  • The game needs to do the basic math calculations itself, but leave the grand strategy to players.
  • To keep players engaged, occasional challenges should arise to bring player attention back to the farms, but not in a micromanagement way.
• Problem 4: Creating a Medieval Fantasy Ecosystem Simulation
  
  • While abstracted, the game should leave players with the sense that this is an actual ecosystem, where the changes they make impact their world.
  • This can mean clear-cutting a forest leading to soil erosion that means no more "free trees", for example.
  • This can also empower players to purposefully alter the ecosystem to better suit their needs if they have a mastery of the concept.
  • If the game models real-world systems, even if abstracted, it creates learning opportunities through play.

So, rather than an overview of what each button does, let's instead go over what you would want to do when you are first setting up a fort.

"I just embarked, I want to farm plump helmets, since those are traditionally fast and easy to farm."
OK, so dig yourself some nice open space in your new fortress for your farm to actually occupy.  Once that's done, you want to open up your land management menu, and get to designating the actual farming zone.

You'd open the menu with "l" and need to start designating the farm with "w" to bring up the second tab, the farm designation window.  From here, zone out your farming plot.  There's no real need for "building" a farming plot in this system, it's just designated as the land that your dwarves will work when they are trying to grow something. 

To actually get them working, open up the scheduler window by hitting "e" to activate that tab.  We will assume you actually brought plump helmet spawn, and haven't eaten them all or something silly at this point.  So, what you need to do is create a new farming block, for a good three or four months, and then scroll over, and select the plump helmet seeds.  In all odds, if you just dug out the place for this farm plot, you will need to add to the biomass and water levels of this plot to make it suitable to the plump helmets' growth.  Plump helmets can survive with little existing nutrients and even soil of any kind, but grow on rotting material like logs, so you can simply add several logs to the farm plot, and keep them wet to both grow plump helmets, and also build up the soil nutrients, paving the way for future underground crops to be planted in the same area.  To do this, you need to add some permissables - use "z" to start adding permissables of providing water by bucket and logs to grow those plump helmets on.  Use the + key to keep adding the number of logs you will permit the dwarves to apply to the ground until the ♠ symbol turns green.  Make sure the ≈ symbol is green as well by adding to the number of permissable buckets of water to apply to the soil, and use the * key to add ten bucketfuls at a time, or - until you hit a negative number, which should make an ∞ symbol appear, designating infinite uses of that (presumably plentiful) resource.

This means you have set up a schedule for that farm to be used for plump helmet farming for several months, and have given permission for the dwarves to use the resources they need to run that farm.  Dwarves will determine whether they need all the materials they are permitted to use or not at the time, but permissables give you a chance to throttle how much they are allowed to use if you have some other priorities for those resources.  The logs you are using, for example, are potentially useful in other industry, and you wouldn't want to throw all your logs at just farming if there are other uses you have for those logs.

Speaking of which, you probably don't have enough logs to sustain that farm forever.  You need to designate some area for clear-cutting to generate the wood you need for your farms and future industry.  The land management menu has replaced the old designation menu for the purposes of designating trees for cutting down.  Now, you designate a zone where trees grow (similar to the plot designation), and then set the area to be continually chopped down for lumber.  You do this with the "a" Forestry menu.  You can also set it for herbalism at the same time to gather some easy food to tide you over until your farms are working at full capacity, and get you the seeds for aboveground farming.  Herbalism, in fact, is a skill with a simultaniously more defined and less defined role now that farms and wild soil are made to be fairly similar.  Herbalism easily supports small numbers of dwarves that can manage to keep their harvesting areas safe from skulking goblin ambushers, but at the same time, plundering too many plants from the soil without returning nutrients can gradually lead to a more barren landscape with less plants overall.

Also, while you don't need to flood your fields anymore, if you have bucket brigades watering your fields, then the closer you can get a replinishable basin of water to your farms, the less distance the dwarves have to ferry water by bucket from the basin to the fields.  Maybe later, you can improve this system by building irrigation pipes connected to a pump or sluice that will take the "bucket brigade" out of the picture entirely.

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Now let's assume you're ready for aboveground farming with some of those seeds you gathered.  You've walled in an area, and also managed to cut down or gather all the trees and shrubs in the area, and need to have a denser, more sustainable food source.  Now you can start really building your first self-sustainable farm system, one which doesn't require outside resources aside from the manure the living creatures in your fortress generates.  To do this, zone another farm in this aboveground area. 

Why do you need to go through this trouble?  Well, plump helmets require a constant supply of more trees cut down to serve as their biomass.  This means that eventually, you're going to be needing hundreds of trees cut down each year to just keep your plump helmet farms going, not even counting the rest of your industry.  That can get unsustainable fast.

Now, you're going to need to schedule a more complex system than you did before with the plump helmets.  First, make a block to harvest some staple crop in.  Let's assume it's wheat, and wheat takes 8 months to grow, starting from Slate/mid-Spring to Timber/late-Fall.  Wheat is a "heavy feeder", and needs plenty of nutrients in the soil to grow.  We'll assume that the soil just happened to already be mostly ready for the purposes of this demonstration, but you might need to spread several layers of fertilizer or grow some preliminary crop to prepare the soil for this wheat.  Growing wheat will probably require permissables set throughout the year for several applications of manure, water, and it will probably be a good idea to give permission to till the soil before the wheat is planted (this kills weeds). 

Now, because wheat is a heavy feeder, it will severely deplete the soil's nutrients, so we need to have a system in place that lets soil fertility climb back up, and it needs to do this without claiming hundreds of trees per year that you may not have on hand to keep feeding your agriculture industry. This means crop rotations.  You can first try to at least maximize your growing season, however, and try to grow something small and light-feeding, like onions, as a winter crop.  Light feeders won't consume as many of your resources, and may grow faster than some heavy feeders, but generally aren't as economically valuable or capable of feeding as many people as your heavy feeding staple crops usually will.  They will, however, give you a chance to really use up the last of your soil fertility while you go ahead and start adding more manure and biomass to allow soil bacteria a chance to break down your fertlizers into usable nutrients while still giving you something of economic value from your fields.

Now you need to schedule what you do for the next year, and we want this year to be based around the idea of replinishing the soil for the wheat the next year.  This means it's going to be a two-year crop rotation cycle, which has the advantage of simplicity, although it isn't as efficient as more advanced crop rotation cycles. 

Instead of merely "fallowing" the field, which lets weeds take over, we will instead schedule clover, which can serve as fodder for livestock.  Clover is a soil replinisher, it actually adds some nutrients to the soil as it grows, although it still does take up some other nutrients.  During the time that clover grows, however, we can send in our cattle and livestock as grazing herds, and let those cattle leave their own fresh fertilizer behind on the fields as they eat - giving us back some of those nutrients that the clover wasn't replinishing all by itself.  We can grow clover for part of this "set-aside year", and another replinishing crop that produces an edible crop, like peanuts or soy beans, during their particular season.  During all of this time, we keep on adding manure, helping push up the soil fertility in anticipation of the heavy feeders to come next year.

At the end of this second year, we can then set the scheduler to be on "repeat".  The wheat will then be slated to be grown the next year, and before it is planted, the soil will be tilled, fertilizers will be added, the wheat will be sewn, and wheat will be grown, watered, and harvested, all on auto-pilot by the dwarves.  A single scheduling session at the start of the first year is all you need to do to set this up.

Of course, that doesn't mean that farms will run perfectly forever - pests may strike, fertilizers may run low, droughts may occur, but generally, you can leave the day-to-day operation of the farm in the hands of the farmer dwarves, while you can focus on other aspects of the game.

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This doesn't mean we have to stop at the simple, however, as the point of this is to allow players to enjoy far more variety in their fields, and to have more complex and nuanced systems, without actually forcing the player to perform any more complex a calculation than they actually want to perform.

We could also try to have a farm plot that grows alternative or more complex sets of crops - for example, orchards of fruit trees that require bees for their pollenation, and which must be spaced apart from one another to have space to grow, with grasses or even legumes growing in the spaces between to both maximize land use and restore nutrients to the soil.  So long as cattle only eat the grasses, and not the trees or their fruits, their fertilizers will be applied directly to the soil the orchards are using without any significant use of labor.  If the orchards are attacked by pests that destroy your fruit harvests, you can even try growing flowers that repell the enemy vermin, or attract insects that prey upon the vermin that are eating your fruits.

This requires use of the scheduler to create a polyculture - growing more than one plant on a single farm plot.  Using the scheduler, you need to add more than one type of seed to be planted, and the ratio of the seeds that you will plant. 

This would take care, however, as the soil conditions would need to be suitable for all those plants at the same time - so plants must be selected that require very similar growing conditions, but which won't actually compete with one another over space.

Meanwhile, another player may try their hand at a fishery and a chinampa, creating a floating "raft" to place their farms upon over the fisheries, so that fish and crops can be grown in a single location. 

I get a little carried away naming things, sometimes.

The green thingy is a glass irrigation pump.  The brass irrigation plumbing has been built, and the dwarf is smiling about it, I guess.