Where I become unsure and don't know enough about soil science and ecology is whether or not it is possible if you recycle 100% of nutrients (including humanure) on site in the optimal way. If a forest were cleared in slash and burn style and the ground was planted in annual vegetables, wouldn't leaching from rain and irrigation severely increase, and even with 100% nutrient recycling, the land would eventually be warn out? Now, what if that same land is cleared in the same way, but instead planted in a forest garden. Leaching should be less than the annual veggie garden, but wouldn't it still be far greater than the native forest, and therefore the land would eventually be worn out from leaching without inputs? I can't imagine how a human made forest garden could ever trap as many nutrients from leaching as a native forest. Even the most complex forest garden is nothing like a real forest.
From the little I have read of ancient sustainable agriculture systems, it seems they are generally located in an area that receives inputs from nature or inputs that can be easily gathered. Flooding, hand gathering river silt and other sources, etc. If it's true than an on site close loop system is impossible, then we need to imagine systems with sustainable inputs. The nutrients leaching out of the soil are just a part of a larger loop, so you need to figure out how to reconnect the other end of that back to your land. One example I can think of is a site within proximity to the sea. Nutrients leach to the sea and can be recovered in the form of seaweed, fish, shellfish, etc.
I guess I have a lot more reading to do about soil science and ecology Any recommendations? I've just had this burning question for a while, and had to get it off my mind. Thanks
we have brought things into our forest systems to try to add fertility to areas that are planted, with the same type of thinking that you are mentioning here.
i have brought branches and prunings onto the land and when we dug (and are still digging deeper) our pond area, we used the silty soil at the bottom of the pond to cover branches and woody materials on the forest floor and paths..covering things that will rot with fertile silty soil from the pond bottoms..and clay mix.
it all breaks down and mixes in with the autumn leavevs and branches and twigs and manures that get deposited in our forest..so yeah..i think if you have a way to add decent materials to your forest areas you should go ahead and do it.
esp if you are also removing some things from your food forest gardens..etc.
we also remove some of the junk trees and brush, burying a lot of it..and we replace with more productive trees and shrubs and plants..that provide food to the wildlife as well as us.
this also brings in more wildlife which then deposits more "gifts" into the forest.
a forest that is short on wildlife will never be as fertile as one that is overrun with a myriad of wildlife..
now that we have opened up our forest some more, burying some of the brush and saplings under some pond scum and levelling it off..we have had an amazing increase of wildlife going through this area..including a lot of bear, whitetail deer, coyote, fox, and smaller mammels of course, opossum, skunks, mink, shrews, rabbits, etc...and they all leave their gifts
"Permaculture: a designer's manual" by Bill Mollison has a good chapter on soils.
minerals are going to leave, it's really not reasonable to try to stop that completely. slowing that process down makes a lot of sense. bringing minerals back isn't evil, though, so long as it's done in a responsible and thoughtful way. and there are thoughtful and responsible ways to do that. remember that more minerals are also becoming available through weathering or breaking down through other means of both rock and the mineral component of soils. somebody would really have to work hard to exhaust that sink.
You can actually gain fertility over time if you aren't exporting products, because plants produce fertility from water and sunlight, and deep-rooted plants bring minerals from deep beneath the soil. There's no leaching of nutrients if the soil has optimum humus and is well-mulched, in other words emulating a natural forest.
I think there's always going to be some leaching (or oxidation in some settings) of nutrients no matter how excellent the dirt is, even (or especially) in a natural forest. reducing that leaching is useful. reducing it to zero is next to impossible. but your point is important, Ludi: the organic matter in dirt is largely made of water and CO[sub]2[/sub], which means it can be replaced as long as other elements are available to facilitate the process.
we should also remember that soils in some "natural forests" leach nutrients at extremely high rates. tropical rain-forests are one well-known example, though they are almost synonymous with the word "biodiversity" for a lot of folks. if I'm not mistaken, the quick leaching quality of those soils is what led to the development of terra preta by folks trying to garden there. maybe an improvement over the natural forest soil; maybe not.
in the much longer term, "sustainable" doesn't really mean "sustainable forever". tectonics and volcanism are pretty substantial game changers that eventually effect pretty much all the land on Earth. unless the sun goes nova first. probably not something many folks should be factoring into designs or land management, but maybe worth considering in thought experiments like this one.
As is often mentioned, seafood can be a good way to import minerals. Oftentimes, as in the case of oysters grown near the mouth of a river, it is a direct way to capture and return nutrients that have leached away from farm fields.
In terms of leaching, I don't think you can talk in generalities but need to focus on specific nutrients (Ca, K, Mg, micros) and how much of them you have in your mineral soils. THen you can talk about pools and availability.
Then consider the historic input of ocean nutrients from anadromous fish migration in the PNW.
Consider just picking up a good soil textbook.. 'Nature and Property of Soils' is a good tome with lots of food for thought, and a much more quantitative basis for the kind of question you are asking... Also, 'Agroforesty for soil management' is good for tropics.
You can actually gain fertility over time if you aren't exporting products, because plants produce fertility from water and sunlight..., and deep-rooted plants bring minerals from deep beneath the soil.
Photosynthesis only produces sugar and oxygen from water and carbon dioxide. See http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookps.html
... and deep-rooted plants bring minerals from deep beneath the soil.
While this is true, if the native soil on the site is lacking and not very suitable for agriculture, than the subsoil and rocks are not going to have the correct amount of minerals for productive agriculture, since they are the basis of the native soil.
Photosynthesis only produces sugar and oxygen from water and carbon dioxide.
From your link:
"Plants may be viewed as carbon sinks, removing carbon dioxide from the atmosphere and oceans by fixing it into organic chemicals. Plants also produce some carbon dioxide by their respiration, but this is quickly used by photosynthesis. Plants also convert energy from light into chemical energy of C-C covalent bonds. Animals are carbon dioxide producers that derive their energy from carbohydrates and other chemicals produced by plants by the process of photosynthesis."
Which decompose to become humus.