I am just about to start building some terraces on a site that varies from 12-15% slope, more or less. We are going to build the terraces with a slight tilt from one extremity to the other (about 1%). We are going to tilt the terraces back into the slope at about 1% as well, and we are planning to make humus storage ditches à la Sepp Holtzer.
We read that he does not make them by removing any material, but rather by compacting the back of the terrace into a sort of ditch, using the bucket of the excavator, thereby reducing the soaking in and encouraging slow flow and evaporation of the water in the ditch.
My question is the following. When building a terrace, the first thing you do is to remove the top soil. Then, once you have shaped the terrace, the topsoil gets put back. So does the humus storage ditch get built after the topsoil is put back or before? Is the ditch made from compacted topsoil or compacted subsoil that does not get covered with topsoil afterward?
Our subsoil is silty sand with lots of rocks. I can't see that compacting it will work. Will compacting the topsoil into a ditch work?
You bring up a couple of good points about Holzer style terracing.
Sepp stressed that it is very important that the terraces pitch away from the slope, not into it. This should be at least a 3 degree slope. If you build terraces that tilt into the slope there is potential for a landslide. This is not a very resilient structure and nature will promptly return it to a more stable state.
Visiting his new place and the Krameterhof I noticed that he builds everything with extreme caution. He knows it will never fail, this is what enables the earthworks to last for generations.
This summer I did two terracing projects with humus ditches. In both cases we made terraces with the aforementioned slope from the clay layer and built the humus ditch when adding the humus layer.
As for compaction; it depends upon the soil and aggregate type.
Interesting. It's good to know what Sepp does on his land. But I imagine these things vary depending on soil composition.
In Geoff Lawton's course, he recommends to tilt the terraces back into the slope with a 2% grade, and for the terraces themselves to be graded from end to end so there is never just water sitting and soaking in. This is exactly what we have done here. I also read that it depends on your soil. We have an extremely solid composition of silty sand and rock. Sepp doesn't recommend humus storage ditches in clay soil because if landslide risk, but I haven't actually found anywhere in his book talking about how he grades the terrace.
With our humus storage ditches being graded to guide water down to the end of our terraces or into our ponds, I can't really imagine that our terraces will become unstabilised. Also, our humus storage ditches will become more compacted with time since they are doubling as walking paths.
I think your spot on, approaches vary substantially depending on soil composition. That's interesting to know that Geoff grades them the other way. Sounds like the slope of the terraces from end to end is probably more important than the grade of the terrace itself. Sepp definitely does the same thing with each terrace sloping into another so water is never pooling other than in the ponds. If you've got this part I'd imagine that you are good. Sepp is also working with crazy steep slopes. The woodlot at the krameterhof is absolutely unreal how steep it is. It was a terraced slope that was definitely steeper than 45 degrees. 12-15% slope is not exactly crazy steep. I'm excited to hear how it turns out!
For sure I have pictures!!! Tomorrow is our last day of earthworks, so give me a few days to finish planting and mulching and to recuperate (it is exhausting to be with an excavator for five days), and I would love to share what we've done here.
Also, after I posted my last reply, I reread Sepp's section on working with water, and in the very last paragraph he mentions the type of soil we have (page 28). I feel reassured. The only thing I can say is that we maybe should have done a better job of connecting the ends of our terraces, but we are working with a lot of constraints on this property, so we have done what we could. We're going to have to build a few staircases here and there.
Anyway, pictures will follow (before and afters)! Thanks for the help!
Ok, here it is! Sorry for the delay. We have been madly getting ready for winter - we were still pooping in a bucket outside until two weeks ago. Now we also have heating in the house, so there is time and energy for sharing our hard work with you guys!
Also, in case you are interested, you can check out my final design project for Geoff Lawton's online PDC:
Tilting the terrace in or out of the slope is dependent on the soil type first followed by the amount and intensity of rainfall. Take into account the subsoil because it's more likely to slip in heavy rain events than the topsoil when fully saturated.
The ultimate goal of farming is not the growing of crops, but the cultivation and perfection of human beings. - Masanobu Fukuoka