Improving clay soil

[Your Letters 2]

I have dry, clay soil.  I have a brick patio and then a grassy area with some evergreens behind the grass.  I wanted to make the evergreen bed bigger to include more of the area between the evergreens and the pavers.  The problem is the soil.  I am afraid to add compost to the planting holes for fear of creating a "bath tub" effect.  But the soil is so bad that I think I need to amend it more than just mulching it every year. Or should I just pick out plants for dry clay?  Which seems a little boring...  I realize I should plant small plants, mulch it well, water well etc.  I do need to extend the sprinklers to hit that area better, but that would just make it wet, clay soil.  . Any advice would be appreciated.

- K.R. -

[Janet Macunovich 32]

It's true that planting into a hard packed clay soil can create a bunch of sinks that become soggy as water falls in that couldn't infiltrate the soil between holes. But if you improve a whole area -- wide ways and also deep enough that water can drain down and away --  and then water carefully (more on that in a bit) a clay area can be a great garden. Especially if it's been dry for a long time, the aeration and wetting you do will release a lot of nutrients that have been just sitting.

 

And you can improve it gradually or quickly -- more labor or more time.

 

Making soil better doesn't work by rote, however. It works if you first figure out why it's bad to begin with. A common situation is where the surface is a fill layer that has been packed hard so that water doesn't readily work its way down into the soil and that layer sits above a subsoil layer of a different texture or density so the two layers' pores don't work together to pass moisture UP, either. In that case, breaking up the top layer and roughing up the surface of the subsoil to create a transition layer with intermediate pore size can work like magic. It takes digging to find out what's going on under the surface -- how far down the hard packed clay extends, and what's below it. Sometimes we dig 8 inches and can see that the soil below has a different color and feel -- grittier meaning it has more sand in it, for instance, and more yellow or brown meaning air moves through it well. Sometimes we dig deeper, and sometimes we find the compressed top layer is only a few inches thick.

 

You don't have to break it into tiny pieces, just break it into blocks, leave it laying rough, and top dress with compost that will cause the sides of each block to be colonized by fungi and bacteria and soil animals that mix the soil particles and create crumbly loam.

 

Then, careful watering comes into play. Turn a sprinkler on and watch. How long can the soil absorb water before it begins to run off and puddle? How much water has been applied in that time? (We measure that with a straight-sided collection can, such as an empty tuna can.) We experiment to see if we can coax the soil to take in more water by applying it more slowly, or in interrupted spurts. Whatever the upshot, we don't water past the soil's ability to accept it, and we don't water again until it begins to feel dry a couple of inches below the surface. That might mean we can only apply 1/3 inch of water at a time and may have to water two or three times a week. Over time, the soil becomes more absorbent and hosts more roots that can soak up water, and the watering changes.

 

Of course, there are clay soils that just go down and down. If a drainage test there shows that water will just sit for a day or more, anywhere in the top 18 inches, then we do have to restrict the planting to species that can take that. (For instance, blue arctic willow can live there -- that's Salix purpurea -- but yews will die -- Taxus varieties.) Or we have to build up, make a raised bed deep enough to create 18 inches of well drained stuff above the soggy layer.

 

To see what I mean by breaking the soil into blocks, check our Improving Clay article. For digging to see what's beneath hard clay, look at the photo below but also check back here -- we will be posting our Challenging Garden: Difficult Sites webinar soon (its printed notes are already available with 8 pages of plants lists, from our Presentations to download page), and I'll update this post to let you know it's there to provide more illustrations. We don't find a lot of other soil improvement images on the Internet -- it's not a very sexy subject and the photos can be, well, dirty. The Royal Horticultural Society has an outline for working with clay that's mostly words but very good.

 

By the way, very late winter and early fall are the times to work with clay. Don't wait until April if you're in a region like we are, where rains begin then. Working wet clay is not only difficult but can mangle it further and thus delay the organic reconfiguration I mentioned above -- the action of fungi/bacteria/soil animals.

 

Here's clay we were investigating for poor growth in the bed. We found the hard pan layer B just about five inches down under the original soil plus some imported stuff A. Important thing -- below that compressed layer that's blocking drainage and air flow, there's layer C of dark brown, workable clay loam. Dark means good organic matter and the fact that it's brown rather than gray or blue means air does move through it so it's worth digging through the hard pan to help roots be able to use it.

[drupnorth 0]

Looks like my soil up north. I know I'm SUPPOSED to have sand like my neighbors but I have clay courtesy of Mother Nature. I have learned to deal with it over the last few years-

Trial and error, trial and error.

[Margaret Thele 16]

In my first 'real' garden I invested a tidy sum on fruit trees, dug big holes, heavily amended soil into clay - only to have them all rot and not take because of the "flower pot" - effect. The holes collected the extra rainwater and provided a soggy mess.