My geology textbook tells me that soil is not renewable, and I agree with this, but there was some question in my class as to whether this is true.
Some soils take more than a human lifetime to regenerate. However, in crop production, it seems as if soil can be regenerated with additives.
In the scientific community of soil scientists, is soil considered a renewable resource by most of those scientists? Is there strong evidence to support this?
Soil is an interesting case because although it is non-renewable (at any useful rate) as a 'bulk material' once removed from the ground, the nutrient content of soil can be renewed with fertilizers.
What a soil-scientist would understand as 'soil' is ultimately produced from the physical and chemical breakdown of solid bedrock at the base of the soil horizon. The rate at which this happens for natural soil production can vary substantially depending on the climatic conditions and other factors, but typically could range from 0.1 to 2.0 mm/yr.
In many intensively farmed regions, (top)soil is being removed by erosion much faster than it is being replaced by natural process. Removal of vegetation cover is enough to expose bare soil to rainsplash erosion at rates much greater than it is renewed. Once soil is bare, it becomes much more susceptible to erosion.
I think the additives you are referring to replenish the nutrient content of the soil, and not the the bulk material that would be produced by bedrock decomposition. With careful management, the fertility of existing soil can be maintained. But if the soil is allowed to be washed off or erode, for all practical purposes, the rate of replenishment is not fast enough for it to be classed as renewable in that sense.
This site has links to more aspects surrounding this issue.
There are several interesting components in soil:
The carbon content, mostly in the form of humic acids - a proxy for how well the soil an store nutrients and water and for wether there's a living soil microbiome (earthwormes etc.). Soil carbon is also removed by biological processes outside of special circumstances like peat foramtion. Carbon is added by decaying plant matter and other organic matter, like faeces of animal (wether natural or manure applied as fertilizer)
The macronutrient content, what we think of as fertilizer - usually N, P, K, (and to a lesser extent) S are considered macronutrients that plants need. There is also the question on how available those nutrients are. Macronutrients are added as fertilizer. Macronutrients that come as part of organic matter (e.g. N in proteines) must first be digested by something. This is why earthworms and the like are important. Macronutrients can be washed out and aer used by plants.
Micronutrients - trace elements, salts etc. that plants need. Again, it is important to understand that not so much the total amount in a given volume of earth is interesting but the amount actually available to a plant (Mg tied up in the middle of some rock won't help). I think these cycle like macronutrients, but the slow breakdown of sand and rock is also an important source. But this is a slow process. On intensivly used fields, these will be added in addition to fertilizer. This gives you a hint that the processes supplying micronutrients happen slowly.
So if you think of soil as the sum of these three categories you can look at how fast or slow each is replenished and used. FArmers will watch for the nutrient balances as well as for the carbon balance of their fields.
However all of these influence each other, soil is really complex. While you can argue that soil is renewable since in some cases all three broad categories of stuff will be replenished eventually, IMO this misses the point how slowly this happens. It also misses the more important point that soil is not simply a mixture of stuff but ideally a living system where things happen.
