How can I prevent stones from rising to the soil surface each year?

Question

My allotment has a lot of flint stone (UK, Bucks area). I am preparing the beds for the first time, and have been reducing the amount of stone as much as possible by raking and removing stones as I can. I have been told that there is a virtually limitless amount of stone further down that rises up over winter (I guess when the soil is wet enough, the larger stone rises to the surface?).

How much truth is there in the rumour that they keep resurfacing?

Is there any where to reduce or stop them rising up? If I were to dig down a good way and put a weed membrane would it stop the stones, or is it likely to cause issues with the soil structure/roots etc?

Or do I need to just keep pulling stones out, and expect to repeat the exercise every year?

Answer 1

Well, in my area, unless you add about 6" of soil every year, the stones will rise to the surface each spring. Around here, we call it the 'crop'. I've heard it is frost action working on the stones in the subsoil, but I'm not sure that's the cause. In any case, it's a chore you can expect to have each spring, and eventually you'll get a system down to do it efficiently.

I am leery of putting a substantial amount of anything man-made into the soil, I wouldn't recommend doing the weed membrane thing. While it may help with the rocks, it may also do something you don't expect, and it will be a big pain to remove, even more so than to install. Around here, we have a ton of night crawlers that like going down a lot further than that, so it would seal a good bit of them under, and keep the rest shallow.

When you remove rocks, it leaves a void, which the soil refills. eventually, the soil level will be lower than it was before, by a good margin. To compensate, I always count how many barrow/trailer loads of rocks I'd raked out, and replace them with at least their weight in decomposed organic matter (compost). This is less dense, but will settle and pack down considerably overtime. This will keep the ground at least level, and improve the soil at the same time. It is better for a garden bed to be humped up, rather than inverted.

Answer 2

It's called "granular convection" and there's really nothing you can do about it except continue to remove the stones as they come up. The winter cold and the water in the soil help this along - I'm forever removing stones - some rather large - from our garden beds and pastures. I pulled a stone out that was easily 7 inches long and 5 inches across a few weeks ago. It hadn't been at the surface last year. Such is life.

I agree with the others - pull the stones, add organic matter. Rinse and repeat. Save the stones for a rock garden or border.

I'd absolutely avoid adding that membrane to try to deal with the rocks. It would work for weed suppression (probably) but it'll be a pain otherwise.

The stones are just a reality of life. I don't worry much about them. I pick them out, pile them up and move on. That would be my suggestion to you. As long as the soil otherwise is rich in organic matter (not all clay like my pastures) it won't be hard to remove them.

Answer 3

Agree with J Musser's answer - where I lived, the soil was very stoney, and yes, they do surface every year, and you notice them particularly after winter. I used to wonder if it was the rotation of the planet that brought them up (you know how if you shake a container of nuts, the largest ones come to the top) but however it happens, it does happen. Initially there were an awful lot of stones and I riddled frequently - I used the stones as hardcore beneath my patio when I laid that, and the more decorative ones got inserted in the mortar between stones. After the first 2-3 years they were a bit less, and I didn't worry about it too much, raking off the ones on the top when necessary, then digging over, and maybe riddling certain areas if I wanted to plant seeds.

As yours is an allotment, you will probably be emending the soil every year with humus rich materials anyway - smaller pieces of stone in the soil aren't an issue unless you're growing root crops, which may fork much more in stoney ground. And I, too, wouldn't recommend the membrane idea.

Also bear in mind that a degree of stoniness isn't necessarily a bad thing - it's very good for drainage, and over a very long time, stones contribute minerals to the soil as they gradually erode.

Answer 4

I live in New England and harvest a fresh crop of stones every spring. I've heard that this is why settlers headed out west when the Ohio River Valley was "discovered" by the Europeans. There may be other factors at work, but for us, it is largely a consequence of freezing (we call it frost heave). When the ground freezes it expands, however, the rocks do not, they just get pushed in the direction of least resistance which is obviously up unless they are at the frost boundary. Theoretically, you'd eventually get all of the rocks above that boundary if you replaced the volume of the rocks with soil every year and got your rock-free layer as deep as the frost boundary. This phenomenon is why we have to build footings for our houses that extend below the frost boundary, which explains why almost everyone has a basement.

Answer 5

In my experience, it is the larger stones/gravel that sink into the soil while the fines come up! The only reason to ever use landscape fabric is to save your graveled areas that are normally walked upon. If you make 'beds' for your plants raising them by double digging and you don't walk on them, you shouldn't have rocks rising to the top.

The main thing to worry about is drainage and that you don't have a solid layer of stone that prevents good drainage. Double digging to make planting beds is the best way to discover what is happening beneath the top few inches of soil. I even use weeds, old sod turned upside down and fill my beds with that covering with a few more inches of soil.

Soil IS basically just tiny rocks. Healthy top soil includes decomposed organic matter that attracts micro and macro organisms. After I double dig a plant bed it is at least a few feet higher, if not up to 4' high. Very soon this bed is only 12" or less but I never dig in it again except to plant plants. I just keep topping it with decomposed organic mulch that keeps weeds down and feeds all the organisms that continue to do the aerating/mixing of soil and organic matter FOR YOU. I pull out any rock that is bigger than my fist and relax about the rest.

It is important to find out if you have a layer of clay or rock that prevents drainage. I dig down at least 2 to 3' periodically while I make plant beds with double digging. If there is clay or rock I bust through it. This combined with the raised, fluffy beds and a trench all around the plant bed (vegetable row) gives all the drainage necessary. The trench also prevents erosion from torrential downpours...

Otherwise, all these answers are excellent. Except don't use rocks for borders. That is like providing condominiums for slugs, pillbugs, earwigs or whatever. Looks terrible in my opinion...grin! They are great to use for 'dry wells' if you need to direct surface water to somewhere it can get back into the water table without erosion...or around the foundation of your home if you don't have gutters to collect water. Stops the splashing of soil on your siding.

Answer 6

Here's what makes these stones mysteriously appear. Stones are better conductors of heat than soil, so the stone conducts heat away from the warmer soil beneath it. That colder soil under the rock then freezes before other dirt at the same depth.

Remember that when water freezes it expands. So, when the water in the soil under the rock freezes, it expands and pushes the rock up a little.

When the ground thaws a space is left under the stone which fills with dirt, so the stone rests a little higher. Over a period of time this repeated freezing, expanding, upward push, and filling underneath eventually shoves the rock to the surface.