Earth Plasters and Aliz - Carol Crews

Plastering inside and/or outside walls of buildings made with alternative materials has become both science and art. Carol Crews offers some of her tips from personal experience:
Basics of plastering the wall with mud.

Clay is essential for stickiness. Do a shake test by placing your sifted earth in a jar of water with salt added and shaking it thoroughly. When it settles, the clay will be the top layer, the silt is in the middle, and the heavier particles of sand will have sunk to the bottom. Plaster dirt should be at least 20% clay. Even at this percentage, you may wish to add manure or flour paste to make it stickier.

When plastering straw bales, I find it easiest to first spread a thin layer of mud with a high clay content and no additional sand or straw on the bales, to lock into the straw and provide a surface upon which the next layer can adhere. If you use this technique on tight bales, you can avoid using stucco netting. Even when chicken wire or lath are used, the smooth mud will penetrate the metal and leave no air spaces to cause future cracks. You don't have to wait for this to dry before applying the thicker layer of plaster with straw added.

On rough cob or adobe and for the next layer on straw bales, I like to use a plaster with high clay content and lots of straw mixed to a slippery, easily spreadable consistency. This can be applied with the hands to a dampened wall and is very good for filling in depressions. (It gets rid of your own depression too because it feels so good to sling that mud around.) The condition of your walls and how much shaping they need will determine whether to use long straw, chopped straw or a combination. The thicker the layer needs to be, the more long straw it should have. Don't trowel this layer down smooth, but get it as flat as you can with your hands and let it dry out thoroughly. It will make lots of little cracks and provide a perfect surface for the next layer to adhere to.

Always try out an earth plaster you are not familiar with by making a test patch of several square feet. Clays differ in their shrinkage rates and if it cracks too much, you need to add sand and more straw. I've seen some plasters dry into the sort of cracks you find on the bottom of a dry lake bed, and fall off the wall. This happens more often to a trowelled surface because there is less surface area to release moisture than if it's left rough.
For more information: Network Earth