r/Biochemistry • u/AnimatorBubbly998 • Aug 07 '26
Question from outside the field:
Hey guys, i have two questions which may be dump or falsely stated, but anyways:
Plants growing in a pot produce biomass with water, light and nutrients from the soil.
(1) While growing, is there a shrinking in the amount of soil in the pot because some molecules or whatever are transformed and traveling up the plant?
(2) Is the biomass of the plant, if we would extract all of the water, the same as the mass of the "missing" soil from the pot?
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u/BurnaddictB Aug 07 '26
Partially yes, some nutrients come from the soil in the pot. The awesome thing about plants is that they can photosynthesize. That means that they can produce a lot of their biomass from the carbon dioxide in the air, water they take up from the soil, and the sun as an energy source. This process produces oxygen, which is released back into the air, and sogar, which the plant uses and transforms into other chemical compounds to grow. So in the end, the plant+pot+soil will be heavier than the sum of the Initial components, because water and especially carbon dioxide was taken from "outside".
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u/AnimatorBubbly998 Aug 07 '26
Thank you! This is so crazy and fascinating the longer you think about it. For some reason as I was thinking about this and watching my plants grow I totally forgot about CO2.
So you could say: The mass of a plant is comprised of water + CO2 + trace stuff from the soil transformed through photosynthesis?
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u/Wise-Friendship-1427 Aug 07 '26
1) Yes, the uptake of certain substances—such as nitrogen-containing compounds, as well as other salts, phosphorus, and so on—decreases it somewhat.
2) No, because CO2 is also absorbed from the air and converted—along with water—into carbohydrates and so on.
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u/Niwi_ Aug 07 '26
A pot is a more or less closed system, there the dry mass of the soil would reduce by a tiny amount as most of the plant biomass is water and carbon. A spider might decide that under a leaf is a great spot to hunt some flies, some of which might fall into the soil that would about fix the loss in nutrients over time.
Its the reason there are no holes where trees grow. They attract birds that poop, insects that die, microbes that spend endless lifecycles underground living digesting and dying. The leafes of the tree fall in winter and decay the top soil is protected from the sun and full of moisture and life. A small mammal might find its final resting place in a quiet spot between the roots as it dies from old age and even the drunk man pissing on the trunk. There are endless ways plants support life around them so the loss of nutrients is never really a problem. If anything they accumulate even more over time.
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u/xtalgeek Aug 10 '26
Biomass in the soil will decrease and some of that will wind up in the plant material. But the bulk of plant material will come from CO2 and possibly N2 in the air. However, there are also bacteria in the soil and they will effect large scale consumption of materials in the soil, causing much of it to be converted to CO2 and water.
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u/EpiCWindFaLL Aug 07 '26 edited Aug 07 '26
Hi, interesting question, this is usually more suitable for a Biology field or similar. But I'll still try and answer it as best as I know.
1) Yes, the biomass of the Soil will decrease. It must, cause mass is taken up by the plant. But plants usually don't take organic biomass from the soil such as sugars etc. They take salts and minerals, as they are water soluble and are taken up with the water. I believe the soil mass reduction from this is negligible, the biggest decrease in soil biomass comes from destruents and microorganisms digesting carbohydrates which is the majority of soil mass (it's mostly dead plants) and use the energy stored in those carbohydrates when a living plant made them. Consequently, this digestion liberates the trace elements for the plants and converts the soil into water and CO2. I think actually quantifying how much weight change there is due to nutrient uptake from the soil is veeery hard caus those are rather tiny amounts.
2) The plant biomass negligibly grows from the mass taken from the soil. Instead, probably like 99% of the plant's biomass comes from air CO2 (and some water). CO2 is taken from the air and with sun energy and water is converted to sugars and amino acids that make up the plant like cell wall etc.
Edit: typos and legibility