Interfacing is the phenomenon in which distinct soil layers do not transfer water from one soil layer to the next until the upper or top soil layers first become saturated. For example: if you were to put down a two inch layer of top soil on your garden plot and then water it, the top two inch layer of soil would absorb the water and actually hold it, keeping it from going to the next layer of soil beneath it. Once the top layer of soil becomes saturated (meaning it can no longer hold more water) only then will it allow water to descend past it to the next succeeding layer of soil. As you can imagine, this is a problem we want to avoid; we want water to percolate evenly down the soil profile and not soil layer by soil layer. You’ll know if you’re experiencing interfacing in your garden because the top layer of soil in your garden will be swampy but as you dig further down in your garden you’ll notice that lower soil layers are completely dry.
The best way to avoid interfacing is to not add any additional soil to your garden area but simply use the already existing soil in your garden. Of course, this is not often practical nor is it very much fun because, as gardeners, we want to improve our soil, and enjoy doing it. In order to add top soil or soil amendments without creating interfacing there are a couple of things you can do . . .
1) Be sure to till in or evenly mix soil amendments or top soil with the already existing soil you’re adding them to. This prevents distinct layers of soil and thus prevents interfacing.
2) If you’re planting trees or shrubs, always fill in the hole you dug with the original soil. If you want to add soil amendments for your tree/shrub then mix it in evenly with the original soil before filling the hole back in.
3) If you have really crummy soil and you don’t think a simply addition of soil amendments tilled in will do the trick then add a new layer of soil but make it really thick: many thin layers of soil will create a lot of interfacing in just a small amount of space; larger layers of soil will have a lot more volume or space so that the effects of interfacing are spread out over a greater area and are thus considerably less problematic.
If you already have a garden area that experiences interfacing and it would be too costly to rework the entire soil area, there is still an option for you: deep tine aeration. Aerators are typically used for lawn areas to break-up excessive thatch or relieve soil compaction which is a common problem for sports fields or playgrounds. Since aerators do this by inserting a hollow metal tines into the soil and pulling out a plug of soil the length of the inserted tine, this has a tendency to break through soil layers and thus break up interfacing. The deeper the tine, the better. You can aerate lawns pretty much any time during the year. It’s best to aerate your garden during the off-season when your soil is bare, but try to aerate before the ground freezes so the aerator’s tines can still sink into the soil and pull out soil plugs.
