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Maybe a better way to put this question is:

Given an isolated garden bed, that had no external inputs except light, water and air. If you were to grow plants in it, compost those plants and add them back to the garden. Over time (with no external inputs) would you end up with more nutrients over time or less?

To add context, I have a yard with soil, a few trees, lots of grass and a couple garden beds. I rake the leaves, cut the grass and compost them. Do the trees and grass only take nutrients from the soil, or do they create new nutrients that I can compost and add to garden. Meaning over time my yard as an enclosed system, does it lose or gain nutrients; assuming I also recycle all fruit and vegetable back through the compost.

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Check out the Wikipedia plant nutrition article. Plants take carbon, oxygen and hydrogen from the atmosphere, the remainder from the soil. (There are exceptions - some plants can make use of atmospheric nitrogen; some plants derive some of their nutrients from trapping and consuming animals.) Plants cannot manufacture chemical elements, so all these will need to be taken in from their surroundings. If you recycle everything back into your garden as compost, over time your garden should be, more or less, a stable system. Of course, if an animal dies in your garden, the decaying body will increase the overall nutrient level.

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  • That's a great article link. So I guess the answer is a big "it depends" on what nutrients you're talking about and what plants as well. e.g. Overtime carbon will increase because it is grabbed form the atmosphere. Nitrogen may not, because it's pulled from the soil, except some plants can get it from the air. So it's a very complex answer. thanks
    – Dan
    Apr 24 '20 at 20:22
  • some nutrients do come in with the rain and the wind, but not enough to offset losses from harvesting. so it really depends upon how much you harvest...
    – flowerbug
    Apr 25 '20 at 14:18
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You are missing the fact that air is just as vital as an "external input" as light and water. Plants process carbon dioxide from the air directly, and specialist bacteria also convert nitrogen to forms useable by plants. (There is also a small amount of nitrogen converted by lightning strikes in the atmosphere, but that's probably not very important).

Almost all the increase in the mass of vegetation as it grows comes from the air, not from the soil.

Probably the most important elements (by quantity) that are "recycled" are phosphorous and calcium. Calcium is typically removed from the system by animals whose bones etc end up elsewhere when they die. \This is critical when feeding animals on grass - consider the weight of the bones in a herd of cows that are slaughtered for meat, for example.

These elements may be present in the soil in sufficient quantities to last a long time (decades or centuries) but there are many other essential trace elements which could be depleted.

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  • "These elements may be present in the soil in sufficient quantities to last a long time ..." Very true. I'm a self-employed gardener. One of my contracts includes cutting a 1000m2 lawn in north London, which I've done for the past twenty years. I collect the clippings, dispose of them off site, and have never applied any sort of feed. Year after year, that grass is the most vigorous I've ever encountered. To such an extent that if I get the timing wrong in the spring, it's a real struggle to mow. The answer must lie in the soil :-)
    – Peter4075
    Apr 24 '20 at 12:03
  • I did forget to mention in air as an external input, but I was thinking about it when considering this question.
    – Dan
    Apr 24 '20 at 14:09
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Over time a well balanced ecosystem will gain nutrients. Fertile top-soil is a natural occurrence. If it was a zero sum endeavor the entire planet would be a desert. Soil can be thought of as a living entity. It's filled with bacteria, fungi, insects and other living creatures. Also, legumes through their symbiotic relationship with soil-dwelling bacteria actually add nitrogen to the soil. Of course there are certain nutrients, especially minerals that must be already present in the soil.

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  • 1
    "If it was a zero sum endeavor the entire planet would be a desert" - that's not true. I'm pretty sure the earth as whole is a closed system. But within that system some areas are more fertile than others and that can change over time as nutrients and such are moved by natural an human forces
    – Dan
    Apr 24 '20 at 14:11
  • That's my point exactly. The earth is a closed system. The cosmos is not introducing additional fertilizer to the system and the system sustains itself. It seems like most deserts exist due to a lack of water. If water was added they eventually would sustain life and eventually have nutrient rich top-soil. Apr 24 '20 at 14:38
  • @TroyTurley I think you’re mis-using the term zero-sum. A closed system is zero-sum: the total sum of increases in one place must, by necessity, be cancelled out by the sum of decreases in another place, such that the total sum is 0. A non-zero-sum system is one in which the total quantity of resources changes over time (resources are permanently and irreplaceably consumed or lost outside the system, and/or new resources are generated or added from outside the system, and these two do not equal one another and cancel out). Of course, a garden probably isn’t really closed...
    – KRyan
    Apr 24 '20 at 19:17
  • The critical part here is that zero-sum implies given an initial state. It does not imply "the initial state must be empty, i.e. a vacuum or void with no mass or energy. Zero-sum instead allows (and basically usually has) an initial state that is given. Zero-sum refers to what then happens along the time dimension. Apr 25 '20 at 14:20
  • @TroyTurley Actually the Earth isn't exactly closed sum - 40,000 metric tons of space dust fall to the Earth every year. Not a lot on human timespans, but still a non-zero amount.
    – Michael
    Apr 26 '20 at 4:01
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An important thing that is relevant to your question but maybe not a nutrient per se is carbon. Growing plants and composting them into the soil will increase the soil's organic matter, which will make the soil more conducive to growing plants even though plants don't take up much carbon from the soil (most of their carbon comes from the air).

If you have nitrogen fixers among your plants, then growing them and composting them will increase soil nitrogen.

For all other nutrients (and for nitrogen, if you don't have nitrogen-fixing plants), no, growing and composting plants only recycles what's already there and doesn't add anything new.

Edit: over very long times, minerals in the soil and bedrock will break down to release additional nutrients. This is relevant to geologists but not gardeners.

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Not zero-sum. The garden would on average lose nutrients over time. An example? Australia. Nitrates can be somewhat replenished by rain, but phosphates decline.

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    in soil science there is some basic idea of the carrying capacity of an area, this is related to the mineral composition of the bedrock but also relates to the surrounding lay of the terrain too... :)
    – flowerbug
    Apr 25 '20 at 14:20

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