I agree with Rob that tar seal is bad news. I think mud seal is good news though. Soil has beneficial micro organisms that keep the nasties at bay. Also it's good to block sunlight off the trunk wound. Ofcourse best is to not nick the tree in the first place. Really bad news is to repeatedly nick the tree. I think that really weakens the plants and invites disease problems.
I've never heard of using mud before. I will have to look into that, it sounds interesting.
I got the mud as sealant idea from an article in Organic Gardening magazine. They reviewed the tar seal prodcuts and found them not only ineffective but actually injurious to the plants. They suggested instead using local moistened soil on the large cuts. This article must have been 25-30 years ago. So i'm surprised anybody continues to buy those spray on tar seal sprays for pruning cuts?
You are using the clay in the mud. What happens when the mud dries? Won't break off or wash off? Do you use plastic bag to hold it onto the tree? thanks
Clay can be a healing agent for people
Remember that not all soils are clay soils. One of my orchards has clay soil and the other not. Both work equally well. I don't use a plastic bag. Once the mud cakes up it holds long enough for tree to heal. I suppose a torrential hurricane style rain could wash it off right after applying. Agreed clay is good stuff for both plants and animals, but what i'm mostly after are beneficial soil bacterias and fungi that are in soil, to help keep the bad guys out. Also all soils will help to block harsh sun rays. If your soil is pure sand then this won't work so well. You will have to mix your sandy soil with some compost.