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According to the principle of mass conservation I think the amount of the soil should be reduced when plants grow. But this reduction is not typically observed. For example, some trees (big trees) grow so fast that if the soil was reducing due to the growing trees, it seems like no soil should be left around them (i.e. the soil surface had to go noticeably down). But it didn't occur. So, which matters are reduced by growing plants?

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    $\begingroup$ You could even say that the sun has to lose 1.5 million tons of rest mass per second to allow life on Earth, in the first place... and it's been doing that (with some variation) for over 4 billion years... $\endgroup$ – CuriousOne Jul 13 '16 at 7:30
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    $\begingroup$ Have you ever used a fertilizer in your garden? That's more or less what is lost from the ground. Of course, in nature, even that is replenished over time - e.g. through decomposition of dead plants (and animals), erosion (calcium carbonate/oxides washing into the soil), nitrogen-fixing bacteria... And don't forget that naturally, these are distributed or even dissolved in the soil (and water), so even when they're gone, the soil level doesn't necessarily drop noticeably. If you ever tried compacting soil, you know how sparse it can be - which in turn helps absorb rain and prevent erosion. $\endgroup$ – Luaan Jul 13 '16 at 10:14
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    $\begingroup$ Since soil includes the organic matter component composed of decaying plant/animal material (for the purposes of simplified discussion we'll only consider the non-living organic matter as soil), and since plant roots die back at certain times of the year or under certain conditions (adding organic matter to the soil), it could be argued that the amount of soil actually increases as plants grow. $\endgroup$ – That Idiot Jul 13 '16 at 11:19
  • $\begingroup$ @ThatIdiot Really? So wonderful! I never expected increasing the soil. Thank you very much for teaching! $\endgroup$ – lucas Jul 13 '16 at 11:22
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    $\begingroup$ @lucas My comment oversimplifies things. While the plants are shedding roots and adding organic matter (OM), the soil food web of fungi, bacteria, nematodes, etc. continually "consume" this OM and respire the carbon dioxide back into the atmosphere. If these processes are in balance, OM content tends to remain stable in the soil. However if it becomes unbalanced (from changes on soil moisture or nitrogen inputs among other things), the OM content change as well. BUT the question was limited to the effect of plant growth, and plants do add OM to the soil. $\endgroup$ – That Idiot Jul 13 '16 at 11:32
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The vast majority of the mass of a plant is carbon-based which is obtained directly from the air via photosynthesis. So trees are, in a loose sense, solidified air!

And most of the mass that comes from the ground is water which, of course, is constantly being replaced when it rains (or by Charlie with her watering can).

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  • $\begingroup$ Thank you very much because of your attention! Your answer makes sense! $\endgroup$ – lucas Jul 13 '16 at 7:07
  • $\begingroup$ I suspect that oxygen actually constitutes the majority of plant mass (as it does for the mass in animals. Many carbohydrates like glucose actually have about numerical parity of C and O, and plants contain significant amounts of water). So, water and air mix to build up trees. $\endgroup$ – Chieron Jul 13 '16 at 8:50
  • $\begingroup$ @Chieron I suspect you're right. What I meant by 'carbon' was 'carbon-based' - updated. $\endgroup$ – lemon Jul 13 '16 at 8:57
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    $\begingroup$ Man, the world would end if it wasn't for Charlie... $\endgroup$ – MKII Jul 13 '16 at 10:25
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    $\begingroup$ @lucas if you think this answer is satisfactory, please mark it as accepted. $\endgroup$ – DanielSank Jul 13 '16 at 10:58
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The soil is the source of a small quantity of vital elements like magnesium (component of the chlorophyll), phosphorus (DNA), sulphur (some proteins) and more. As the plant grows, these are removed from the soil, so its mass shrinks.

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  • $\begingroup$ DNA (all amino acids) also need fixed nitrogen, which comes through soil and groundwater. $\endgroup$ – Whit3rd Jul 13 '16 at 8:45
  • $\begingroup$ @Whit3rd: Amino acids make up proteins, not DNA. DNA is built from a skeleton of sugars and phosphate, with (nitrogen-containing) nucleobases attached. $\endgroup$ – Henning Makholm Jul 13 '16 at 9:50
  • $\begingroup$ Thank you because of your attention! @Martín-Blas Pérez Pinilla $\endgroup$ – lucas Jul 13 '16 at 10:01
  • $\begingroup$ I've extended / formed your answer a little bit to improve its chance to survive the review process of the site. I hope there is nothing added which you hadn't. $\endgroup$ – peterh Jul 13 '16 at 10:51
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As said in other answers, CO2 is pulled from the air, and H2O is constantly resupplied. Then there is Nitrogen, which is either taken from the ground, or from the air, depending on the plant.

Leaving meat eating plants aside, all the other stuff has to come from the soil, but how much is it? This is actually pretty easy to estimate yourself:

Just burn the tree/plant, and all the water will evaporate and all the carbon will be burned, and all that is left are the minerals that that plant pulled from the soil.

So if you ever had a nice campfire burning for quite a while, you get a pretty good idea that while being a fraction of the trees mass/volume, it is still quite some material.

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    $\begingroup$ "Just burn the tree/plant, and all the water will evaporate and all the carbon will be burned, and all that is left are the minerals that that plant pulled from the soil." Which plant is this that provides a complete combustion? Might be useful for alternate fuels here soon. $\endgroup$ – jwillis0720 Jul 13 '16 at 9:29
  • $\begingroup$ @jwillis0720: not sure what you are after, but I constantly burn my garden waste to ashes, and I am pretty sure only traces of carbon and water are left in the ashes, which gives a pretty good approximation. $\endgroup$ – PlasmaHH Jul 13 '16 at 9:36
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    $\begingroup$ Actually, unless you're really thorough, lots of the ash from burning e.g. wood is still carbon. You can tell roughly how much by the colour - the blacker the ash, the more carbon is left. If your ash is white, it is indeed mostly the minerals - mostly calcium carbonate/oxide, potash, tiny amounts of phosphates and trace amounts of stuff like iron. Depending on the kind of wood, the ("pure") ash is something around 1% of the dry mass of the wood. Needless to say, wood ash is a great fertilizer :) $\endgroup$ – Luaan Jul 13 '16 at 10:09
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    $\begingroup$ "all that is left are the minerals that that plant pulled from the soil" is also probably not quite right, since nitrogen is also pulled from the soil and at least a fraction of it will go up as nitrogen oxides. $\endgroup$ – Emilio Pisanty Jul 13 '16 at 10:19
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    $\begingroup$ @EmilioPisanty: As mentioned, there are different sources for nitrogen, so it may or not be from the soil, depending on the plant. After all, its just a way to estimate. Also afaik the majority of nitrogen will be gone after burning, but I really never did any research or experimentation on that. There will also be things like sulfur that is taken from the soil and burned away, but again, its a suggestion for an estimate for the minerals taken from the soil, not an accurate measurement. $\endgroup$ – PlasmaHH Jul 13 '16 at 10:26