I’m sure anybody who lives in Washington County knows what I’m talking about when I say “wretched clay?” That stuff where we lose our boots in the winter and can’t break it with a pick axe in the summer. We may not all call our dirt by that name, but I’m sure we all have some nasty names for it. The big question is “How do we improve it?” The problem is the structure of clay soil. Picture a billion thin layers of sticky muck that won’t let air or water pass by each layer. In the winter our poor plants drown and in the summer the water runs right off the soil so our flowers die of thirst.
The solution to every gardener’s nightmare is to change the structure of the soil. We can turn our clay soil into a delight with a technique perfected by Maurice Horn, owner of Joy Creek Nursery in Scappoose, Oregon. By adding 4 inches of organic amendments and 2 inches of quarter-ten gravel, (sharp-edged gravel that is between the size of ¼” and 1/10th”) and then rototilling it into the heavy clay, the organic matter will mix with the clay, and then wrap around all the minute pieces of gravel creating a structure more like a brownie mix with nuts and chocolate chips. This improved tilth allows the roots to penetrate deeper into the soil enabling plants to go longer without watering.
I have been aware of this technique for a long time and have talked about it to both clients and contractors. The concept is so radical that even people in the green industry don’t want to believe it. I’ve had a couple of contractors question my wisdom over it. However, I’ve seen what happens to heavy clay after a few years if only organic material is added. It converts from friable soil back to heavy clay. The quarter-ten gravel is a permanent solution that will not fade away with time. We will still need to add compost but we won’t be trying to grow plants in concrete. It’s important that you use quarter-ten gravel and not quarter-minus gravel . Quarter –minus gravel is ¼” all the way down to fine sand. If you add fine sand to clay you will yield concrete. No one wants that.
In my own garden, I added the quarter-ten gravel to the beds six years after the garden was installed. I didn’t do it at first because I didn’t think I’d need it in my raised beds. Boy was I wrong. The garden was lush for five years, then the soil started to harden and the plants suffered. So I worked a half inch of the gravel into the garden beds. Then I laid an inch of the quarter-ten gravel over my sad lawns and added an inch of compost on top of the gravel. I reseeded the lawns with great results. I still add quarter-ten gravel into the hole when I’m planting new plants. It never hurts to add a little more.
Ann has lived and practiced landscape design in Tualatin Valley since 1993. Please feel free to contact Ann via www.ann.nickerson.net or by phone at 503-846-1352 with your comments or questions.