soil building: what am I missing?

So... hundreds of years to build an inch of topsoil

I read that statement frequently and it has me scratching my head. Am I to assume that this inch is averaged over a huge surface area (like the surface area of all land on earth) to reach that number? Or are we adding all the mineral content from leaf litter that cannot be turned into a gas by any fungal or bacterial process? Only including the material that cannot be leeched out by rainfall over time? I ask because it takes a floodplain only a single large weather event to add feet of topsoil. It takes only a short time for dust from the sahara to build up a continent away. only a few years for a paved road to return to a meadow when the cars stop rolling... not 500 years. more like 5.

So... what exactly is being measured when you hear a scientist say hundreds of years per inch of "soil"? What exactly is "soil" in this context?

I think it's the creation of topsoil. Your examples are not about creating but translocating. A river does not create topsoil, it moves it from one place to another. Thus one area is suffereing errosion while another is getting silted up.

That inch per century, as Abbey says, is for the creation of topsoil out of what was there already. It is generally illustrated as in a forest, which much of the world would naturally be without human disturbance, and I presume a gently sloped upland so that erosion and deposition are not factors. The process of roots breaking up mineral soils and moving the nutrients, and plants transforming them into richer forms before dropping them on the surface, is slow but steady. Roots living, growing and dying in the ground also puts organic matter deep under the surface and adds to the topsoil building process.

I'm confident that the amount of soil being created cannot be accurately measured, and it can't even be guessed at with any confidence. The Earth is way too big, and way too variable from place to place, to even attempt those sorts of calculations.

Paul Lutz wrote:So... hundreds of years to build an inch of topsoil

I read that statement frequently and it has me scratching my head. Am I to assume that this inch is averaged over a huge surface area (like the surface area of all land on earth) to reach that number? Or are we adding all the mineral content from leaf litter that cannot be turned into a gas by any fungal or bacterial process? Only including the material that cannot be leeched out by rainfall over time? I ask because it takes a floodplain only a single large weather event to add feet of topsoil. It takes only a short time for dust from the sahara to build up a continent away. only a few years for a paved road to return to a meadow when the cars stop rolling... not 500 years. more like 5.

So... what exactly is being measured when you hear a scientist say hundreds of years per inch of "soil"? What exactly is "soil" in this context?

They are talking about the supposition that all soil is built from natural erosion (water and wind). This is only a mathematical extrapolation not fact.

Flood plains are where soil lands after being eroded and carried down mountains and hills, this will accumulate at a very high rate of speed (think of the Nile River flood events recorded through out history).

It takes a blacktop road 25 years to go away in moist places, much longer in a desert. It takes over 50 years for a concrete road to really start breaking down.

Soil can be rapidly built if you are helping the earth mother along (in one year you can build 3" or more of carbon rich new soil simply by planting the right things, letting them grow to fruit then crimping or cutting these crops and letting them rot in place).

The length of time to build "soil" is relevant only if you are wanting to build some in a particular place, for a particular reason.  It can be measured but to do so you have to do it in a controlled environment.

Redhawk