Building soil over rock

Ok, so here's the situation.  I live on top of a hill, where most of the soil... and even dirt... has eroded away, leaving mainly rock.

The good news is a local tree service will drop off as much bark chips as we want, and our local grocery store will give us all the cardboard we could want - so, oodles of potential organic matter for free.  But the thing is, after I build up some soil I don't want it all to run down the hill again.

So if I do go ahead and build it up like that, what kinds of things should I plant into it to hang onto it long-term?

A few questions first;
- what size are we talking about
- what is the rainfall annually
- does it all rain at once
- Is the site protected from winds etc
- how steep is the land that rises to the point under consideration

Trees with great big, spreading roots would probably help to keep the soil in place.

You may also consider building terraces to keep the soil more level, so erosion won't happen as easily.  Maybe you could move the rocks to make swales horizontally across the hillside as a simple terrace system?  That would also have the benefit of giving you definite pathways.

I'm thinking some kind of terracing would help in the short-term, and long-term, you'll want trees.

I agree. Small stone walls on contour, or following k-lines. That's a lot of work, though.

If you don't have the resources, maybe you could plant adapted bushes in contour lines as a first step. They will make a living wall both over and under the ground.
Then, I'd suggest to plant also slow growing trees after the initial bushes.
Slow growing trees may be able to anchor themselves in the stone and even break it apart if you develop a healthy soil above. Plant many one year forestal saplings, following the contour lines, then cut the weakest when they become crowded.
If your hill is too sloped for this, maybe you can level a small line with a pickaxe where your saplings can hold.

Or if you want even less work, try just accumulating debris in the contour lines: dead branches, rocks, garden gnomes...
(I prefer k-lines over contour lines because I don't have to deal with the overflow.)

Planting a grain like rye would help hold soil in place for the short term. I've used it on the sides of my hugels with good results. Any plants that have a large, fast growing root system would work, though. Even after the plants die back in the summer drought or with the fall frosts, the roots persist for a while. If you're not needing to plant anything specific, one of the aggressive, running grasses would probably be great.

Always keeping 3inch+ of mulch will soften the impact of the raindrop so there is less erosion. It will also absorb and slowly release the water vs just having runoff.  You can also make mulch berms every 20ft on contour, it will stop and catch the eroded soil. The  closer you make the berms, the less momentum it will have and so the easier it will be to catch it. You can also make said "berms on contour" our of rocks or logs and sticks or out of living shrubs, or a combinations of all the above.

Instead of bark mulch you could also try some "invasive living plant" such as mint, it will hold onto the soil tightly. Or really just any plant.  

Vetiver grass would be good for what you want to do:


source


source

As someone else mentioned ryegrass would work too:


source

A handy infographic:


source

https://www.harvestingrainwater.com/2016/05/evolutions-on-mr-phiris-water-harvesting-plantation-1995-2016/

In the link above you can find the experience of someone who went down that path nearly 30 years ago.

Abraham Palma wrote:(I prefer k-lines over contour lines because I don't have to deal with the overflow.)

Can you explain what a k-line is?  I don't think I've heard the term before, and it sounds like something interesting to learn about.

You guys are totally right.  Quick-spreading groundcovers would be a great way to keep the soil in place right away, and bushes would be a great first step before adding trees.

In other words, a polyculture with plants that fill every niche may be helpful?  What a surprise!  (Laugh.)

Emily Sorensen wrote:Can you explain what a k-line is?  I don't think I've heard the term before, and it sounds like something interesting to learn about.

I think his use of k-line is the same as keyline, which is explained in one of the paragraphs in here.

Yes. K-line = keyline.

You can follow the method, or you can make it simpler: the idea is that you lead water from a valley towards the mound of the hill. Since most of the water goes to the valleys, causing erosion, by sending it in the opposite direction you are hydrating the driest side of the hill where it can be absorbed and removing it from the valley where it might not be absorbed fully.