I'd say you need to add sand. I've done lots of earth plaster in Iowa, USA. I throw lumps of clay into a barrel about half full of water, let it sit for a few days to soften. I stir it up with a paint mixer in a drill. You don't have to mix the entire barrel at once. I just do the top layer, then add more clay or water as needed. I mix until it looks like thick chocolate milk. I dip a dry finger in. When it covers my fingerprint completely, it's at a good point. It can be just slightly thicker, no harm. I strain it through a screen to get small rocks and twigs out. Then I mix it with sand. Again, I use a drill. If it's too dry, you won't be able to make it turn, add more clay slip. Too wet, and the the surface stays mostly level. When it's right, the mix forms a sort of tornado shaped hole in a 5 gallon bucket of plaster. Fiber of any sort helps to stop cracking. I consider manure just a source of fiber. I haven't found it helps with waterproofing. Experiment with different amounts of sand. You'll find some samples crack. (needs more sand) Some are too brittle. (too much sand) and some will have minimal cracking and be tough to break apart. That's what you want. I'd start with one to one slip to sand, it's different with each type of clay you have. I've always used the plaster immediately, I don't see any reason to wait. If you can give the wall protection from rain with a bigger roof overhangs, that will help too. I made a simple finish coat out of one part lime, three parts sand. That sheds the rain just fine for me. Good luck on your project.