I’ve read about how producing food on an industrial scale is taking nutrients out of the soil faster than they can be replenished, and causes certain food (tomatoes are a common example) to taste more bland than they did years ago and you need to eat more to receive the necessary amount of nutrients.
If there are a finite amount of nutrient resources in the soil, and plants use them to grow and then we (in)directly eat plants and receive the nutrients which we expend as energy throughout our daily lives, doesn’t the work of moving and living deplete the energy of some nutrient forever? A movement of a muscle cannot be reclaimed and while the muscle can decompose and put nutrients back into the food chain, the action of the muscle itself cannot. Therefore, given an infinite amount of time, wouldn’t the nutrients in the soil on the entire planet be finite and could eventually all be absorbed, consumed, expended, and depleted?
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This will be an incomplete answer, but enough to get thing started. Probably the most important thing to realize is that soil is not a static thing, i.e., it forms and is modified through a mixture of physical, chemical, and biologic processes. When talking about the deleterious effects of intensive agriculture on soil, we have to clarify which part(s) of the soil formation and/or modification process are being disrupted, i.e., are we just talking about nutrient depletion (e.g., Tan et al., 2003) where (in the simplest possible terms) we might imagine that the physical structure (e.g., thickness, porosity, etc.) of soil is not being modified but specific key nutrients (e.g., phosphorous, etc.) are being depleted or broader “land degradation”, which itself includes a variety of processes (e.g., Hossain et al., 2020), but can include significant changes in soil volume, biotic content, microorganism populations, porosity, etc., all which may have negative effects on the ability for that soil to be used for agriculture.
For kind of all of the above, the key issue often boils down to rates. I.e., soil forms at a given rate, which varies based on the conditions in a particular area. In natural settings, the rate at which soil forms and soil is removed via erosion will approach some quasi-equilibrium. In terms of the nutrient content of the soil, this is also usually in some quasi-equilibrium, i.e., the “key nutrients” in the sense of limiting factors for plant growth will in part be derived by the soil formation process itself (i.e., weathering of bedrock) and in part derived from biologic activity itself (e.g., nitrogen fixing bacteria in the soil, etc.), but in general, in a natural state the system will tend toward a state where the plant populations growing in an area (in terms of type and volume) will be what the soil can support given the background conditions. Also of importance is that a lot of the biomatter (and nutrients taken up by the plants) don’t move significantly, i.e., a plant grows, dies, and ends up being decomposed back into the soil returning many of the nutrients it took up. Even if we account for animals consuming the plants, a vast majority of them will also die in effectively a similar area, so there isn’t necessarily a massive flux out of the system of key elements.
When we throw agriculture into the mix though, effectively we are disrupting all different parts of that balance. A lot of agricultural processes cause soil erosion at rates that vastly outpace soil production. Similarly, many agricultural processed disrupt other parts of the soil (e.g., chemicals for pest control may also impact soil microbiota that are important for modulating nutrients in the soil). We are also often growing a greater density of (crop) plants in an area than would be the case for a more natural setting and we are also removing large portions of the (nutrient rich) biomatter completely from the ecosystem in question.
So ultimately, the short answer is that soil (and the nutrient content of it) is not really a renewable resource on a human agricultural timescale and process (because it’s proceeding at a rate and volume that often outpaces the natural rates and it exports a lot of the nutrients that are taken up by the plant from the soil), but it is closer to a renewable resource on a long-term timescale and in a more natural ecosystem. In that sense, nutrient depletion and/or land degradation are not inevitable if we’re considering a mostly “natural” system. That’s not to say that in a natural system you would never have areas that went from being more to less productive (or vice versa), it happens all the time as the conditions that form and modify soil change, but it’s not as though globally, we would expect all systems to be moving toward lower and lower nutrient content of soil if we were considering a more undisturbed state.
The answer by u/CrustalTrudger is great, but I’d just add one more thing related to your specific point about the movement of muscles.
You are correct that using your muscles (or indeed using energy from your food in any way) releases energy, by breaking down chemical bonds. While that energy is lost to the environment, the basic elements are not and just take different chemical forms.
In a completely closed system that would not be sustainable. However the Earth isn’t a closed system. The sun pumps in energy, and then plants use that energy through photosynthesis to store energy in chemical bonds that can move up the food chain.