The modern method of building is to wipe all the top soil away from a lot, build a house, and bring back 3 or 4 inches or soil or just sod and call it good.
This, of course, is not good for gardening, and many people want to increase the depth of the top soil on their property. But how? I see advice posted how you just top dress your lawn, add an inch a year, or whatever. So you slowly bury your driveway and sidewalk? Other people bemoan the thought of digging up everything, digging out some of the fill soil, and then adding new top soil. Common knowledge is that there is no other way, that top soil is like fossil fuels, a nonrenewable resource we just use up. Common knowledge is wrong, you can in fact, make it.
How is top soil formed to begin with, in the great American plains, where it is many feet deep, how did it form? Well, they say 1% of decaying organic matter ends up as stable humus in the soil. So over thousands of years as grass lived, and died, and animals grazed, and pooped, it formed. Dead grass and animal droppings? Is that it? Not quite.
We often do not think about what happens beneath the soil, but things happen, many important things. We see plants above ground dropping leaves, and twigs, and shedding organic matter all over, but it also happens beneath the soil. Roots die off, they’re abandoned by their plant and they rot. When grasses grow up big and tall they need long deep roots, when the bison wander by and munch that grass down, they don’t need those roots anymore and some are abandoned. As those roots, which in some grasses can reach down over 10 feet deep (imagine, grass roots over 10 feet deep). The roots decay and add organic matter deep within the soil, once they decay they create channels for water, air, and nutrient infiltration.
So actually, under ideal circumstances, you can add a cm or more (I’ve heard of much more) of top soil under your lawn without using a shovel by simply planting the right things and fertilizing appropriately so they grow vigorously, on an annual basis. The first thing you’ll want to do is make sure you’re planting an appropriate grass. Kentucky bluegrass is probably the most famous turf grass type, but it is horrible for practical purposes, my son happens to be allergic to it, but if that wasn’t enough it is very shallowly rooted, with roots only about 4 inches deep, this also means, in addition to not breaking and improving your subsoil, it needs frequently watering.
In contrast, tall fescue (hardy in zone 4 and warmer) can grow roots 3 feet deep, and bermuda grass (zones 8-10) can grow roots 10 feet deep. Native buffalograss can reach 8 feet of depth and worths as a turf grass.
I am a big fan of planting clovers in lawns, all clovers can develop feet deep root systems, of course perennial white dutch clover, while shallower rooted compared to other clovers, is perennial and I’m a fan of seeding it in all lawns. In additions of course to the roots, they fix nitrogen helping green your lawn, and helping that grass grow better, so it can send down even deeper roots.
I know root depth is not the top of your list normally when picking plants, but in this case, research it and use it to guide your decisions.
Depth of root is also often to height of top growth, with turf grasses anyways, so if you want deep roots, let your lawn get longer. In the heat of summer this can help stave off drought, but for the specific purpose we’re talking about you also want them long, the longer the better. And then, every once in awhile, you want to let the bison graze on it, to create some destructive rebirth. Don’t have bison? Try a lawn mower, though honestly animal grazing is best as they’ll add some animal fertilizer and maybe tear up and aerate some.
So, if you want to improve your subsoil under your lawn plant deeply rooted grass varieties and include clover as well, then let it grow long between mowings (or grazings) and over time the soil will improve.
Now, suppose you want to do the same for a perennial bed or a vegetable garden, you have even better tools are your disposal. Tall perennial grasses are even more deeply rooted, many perennial flowers and other shrubs are also deeply rooted, especially native plants, some of which can grow roots up to 20 feet deep if such a depth is possible in your soil. If you have a particularly hard subsoil you want to break up you can plant daikon radish, the roots of which grow with such intense power it can break through many compacted soils. It is best planted in the fall if you’re doing so for this purpose, and you could of course harvest it but for soil health it is better to let it just die and decompose.
To prepare a new perennial bed you could plant some of these things for a year, or in the case of daikon radish you could tuck them in and among the existing plants. Make sure you fertilize so the daikon don’t starve the existing plants of nutrients. If your perennial bed is an ornamental grass one the problem will take care of itself more or less (though check, some types of ornamental grasses are not deeply rooted). But otherwise we’re talking about sacrificial plantings, things you do not intend to keep for perpetuity, and you want to grow for a year or two or three just so they can work on your soil.
You should definitely never allow dirt to just sit, always make sure something is planting and working in there, so that means plant a cover crop over winter in your veggie garden. Alfalfa is a great plant for this purpose, it has deep strong roots, and as a legume will add nitrogen to the soil. If you want to improve your dirt you can plant it, let it grow, then till it in the next season as a green manure.
I read that there is research that the root systems of plants contribute twice as much organic material to the soil during the growing season as what remains in the root system at the end of the growing season, some roots excrete various compounds and binders that help form good soils, in addition to what they contribute when they decompose. All this organic matter feeds life in the soil.
I know, this can seem like a lot, plant something, wait years, get better soil, but waiting years does beat waiting millenia, and we’re not talking about a huge monetary investment. I mean we’re talking about seeds here. Compared to yards and yards of topsoil, or renting heavy machinery, seeds are relatively inexpensive, and no one (well, no one reading this blog I’m sure), can afford to purchase enough topsoil to cover an entire homestead or farm or large market garden. So the only option is good stewardship over a long time period, but you will be rewarded in the end. Every year you’re actively planting in your soil it will get a little better, but if you plant the right things it will get a lot better.