Soil samples for cob.

We just recently purchased a property and we had plans from the beginning to build a straw bale/cob hybrid house. We were excited to inspect the soil to see if it was suitable. I have gardened in different zones all my life and I have never seen soil quite like it. It is mostly woodland and has a slow grade to the top of a hill. The best way I could describe the soil is a this sticky clay completely gritty ,mixed with sand. It is sticky and gritty. and will hardly break apart under pressure. In the woods it is dark in color., I am assuming from the many years of leaf litter. on the top of the hill I was even more astonished because it is a sticky yellow clay full of fine sand and what looks to be fin pea gravel. I am not sure how to amend it either for growing or building. and I also show in photo's a sample of dried soil (I guess you could call it a rock)from the upper terrain, and it shows all of this clay,sand and pea gravel dried into a small brick. On the lower levels we collected another rock of another kind. Large rock appear to be a slate type. I am really quite bewildered. Id love to have your advise on how to prepare cob out of this. And also any ideas on planting in it. (The soil sample got kind of mixed in all the hiking, but before there was a definite yellow clay and dark brown clay)

Here are pics

More pick, rocks

In the lose soil at the top of the hill the pea gravel could be filtered through a screen for walks or anything. The soil reminded me of a natural quikkret, with out the cement. Also in the pictures you can see the sparkle from the sand.

Hi Kim,

First the rocks; it looks like you've got a metasedimentary conglomerate(a sedimentary rock that has been partially metamorphosed) and chert(kind of like flint)

For the clay;

First perform a sediment test with a mason jar and water, measuring the height of each layer. Fill the quart or larger jar with soil and water, then add 1 tsp salt attach the lid and shake vigorously. Allow to stand for 2 days in order for the clay to separate from the silt. Measure and this will give you rough percentages.

Second is the clay test;
Make a 1/2" roll of soil, like a sausage, it should not be too wet and sticky. Roll it out in your hand to 1/8"-1/4" and measure how long the thread will build before it breaks.
10-12" high clay
2-4" with difficulty is low clay

Now, since it looks like you've got pretty good building soil, mix in the straw etc. and try different hydration rates on a ball of mix, dropping them from shoulder height onto the ground. The splats should mostly hold together, but break into a few heavy pieces. One piece-too wet, lots of pieces-too dry. Now test the mix on a sample wall. Test every batch of mix this same way and you're ready to go.

OK , so we did the tests. The clay is very sticky. It does not break apart easy at all. And the quart jar shows the soil to be 2/3 sand, 1/3 clay. And little else. So what is the advise you cob builders would suggest for amending to build.

Kim,
I think you are probably ready to make a test wall. The ratios sound close to optimal; add straw and give 'er a go.

I have seen a lot of cob in pictures. It always looks brown or reddish. I have not seen any cob with black clay. It might turn out to be pretty ugly. But the option to put a lime plaster on it seems likely. In the jar test it was really evident how dark the clay and sand both are. In my mind I am wondering what geographical event would have caused this varied soil types.

Kim Bowen wrote:I have seen a lot of cob in pictures. It always looks brown or reddish. I have not seen any cob with black clay. It might turn out to be pretty ugly. But the option to put a lime plaster on it seems likely. In the jar test it was really evident how dark the clay and sand both are. In my mind I am wondering what geographical event would have caused this varied soil types.

HI Kim,

I don't know where you live, so it's difficult to say. The photos of rocks indicate that you have a fair bit of volcanic activity and that the clay is probably from hydrothermal/chemical alteration of, here I am guessing, granite(indicated by the mica in the clay).
Let me k ow your geologic area and I might have more guesses for you.

We are in Logan County, KY bordering Tenn. We don't have nearby Volcanos, but who knows what blew in from someplace else.