Why is forest soil generally not that deep?

I've always read that forests have the best soil, but in every forest that I've dug under the leaf litter to see the soil, the topsoil only lasts like 1-2 feet and then it turns into orange clay. There's a small old growth forest-preserve near my house and its even the case there.

I don't know, I figured since forests are supposedly the most efficient natural system, that the topsoil (the portion which is black) would be deeper (I've heard this one farm has 22 feet deep topsoil or something ridiculous like that).

Also, I always struggle to find earthworms in forest soils where my sheet mulched beds have tons.

Can anyone explain this.

Forests ARE most efficient in cycling nutrients. That's why there is almost no topsoil at all. The more efficient, the less topsoil. Look at the rainforests. No topsoil but trees, fruits, nuts in abundance.

Look at Chernozam: Rich dark soil. 30 inches deep! But only shrubs and grass. The best soil to grow things in IS Chernozem which is rich dark humus + loess. This type of soil naturally forms when you have 1. Loess + 2. lush vegetation in spring, a dry summer killing all the vegetation and then a hard cold winter where micro- and macroorganisms tuck in on the dead vegetation. It's accumulated abundance of humus. But it is not a forest. It's a steppe, a plain.

This is the process you emulate when you mulch your veggie garden. Except that you spread the dead vegetation to let the micro- and macroorganisms enrich your soil. You skip nature's part with the dry summer and the extreme cold winter. Your veggie garden is crap in efficiency compared to a forest. That's why you actually can build topsoil in your veggie beds.

Hope that added to the confusion! 

Two feet is actually a pretty good amount. How far down did you check in the forest preserve? The vast majority of forests in the world are not very old; most have been damaged by humans at one time or another. Also, conifer forest won't have as much soil as deciduous forest. The ecosystem with the most soil is prairie.

Efficient means faster cycling of nutrients, which means it's not in the soil long enough to create deep, nutrient rich soils- it goes back through the microbes, fungi, plants and animals instead. Thus, in a rain forest, the soil is "poor" but that's because the nutrients are being cycled back into living things very quickly.

Also, two feet isn't that shabby, depending on where you are. On the palouse, yah, two feet isn't much, but out in Montesano? People are quite pleased if they even have 8 inches.

There's also the issue of fungi at play here. Forest soils are said to have micro-organism mixes with higher fungi to bacteria ratios than prairies or meadows. The synergy of plant roots with a whole host of mycorhiza and fungal partners creates incredible possibility with very little.

Dunkelheit wrote:
Forests ARE most efficient in cycling nutrients. That's why there is almost no topsoil at all.

Kirk Hutchison wrote:
Also, conifer forest won't have as much soil as deciduous forest.

It really all depends. Cool, moist evergreen forests (like coastal Pacific NW) are even better than prairies at accumulating organic matter, and so are peat bogs. If the heat, moisture, and oxygen levels are all high, then organic matter will not survive long in the soil.

Charlie Michaels wrote:I'

Also, I always struggle to find earthworms in forest soils where my sheet mulched beds have tons.

Can anyone explain this.

That much I can.  earth worms were wiped out in much of north america in the last ice age and earthworms don't move very fast on their own so most forest still don't have earth worms.  Earth worms and especially compost type earth worms are actually harmful to forests because the forests have evolved to live without them.  They eat and destroy too much of the forest floor litter leaving no mulch to hold moisture and that can then end up killing forest thru earthworm induced drought because they ate the mulch holding the moisture in.