Here is the recipe
2 parts horse manure (or chopped straw)
2.5 Earth mix (Screened so its fine with no stones)
1 Part water premixed with at least 25% Nopal cactus juice.
STEP 1. Harvest Nopal cactus (Chumberia in Spanish) paddles, a cactus that is actually edible. When you have harvested the paddles, the next step is to cut them small and then smash them with a lump or sledge hammer. I use a large metal or flexible plastic bucket to do this. Then mix it 50 50 with water and let it sit for two days (up to six days is fine).
You can work out you proportions more or less by doing some simple soil samples and mixing them with water. Stones will settle first then sand, clay, silt and organic matter. If you have too much clay it will crack. If you have too little there won’t be enough to coat the sand and let the mix "go off" and stick to the wall.
STEP 7. Then you want to use a flexible plastic trowel or Japanese trowel to work the surface and give it a nice uniform finish (The budget version is to use old shampoo or yogurt pots cut into strips). The idea is to work the clay to the surface and push the sand down, you can spray a fine mist of water if needed. The fished result is a nice uniform plaster.
You can use different colour clays depending on the tone you are going for. This mix could be used on any wall, it does not have to be necessarily an earth wall.
That's it! Let me know on facebook if you try this plaster or if you have any questions.
Matt Prosser 31.05.2018