cool nice work I am taking a bit of a different approach to doing this currently And I hope to make mine a little more effective. Yours has some issues due to every 6 Bog Earth ( I assume you are making ) You only gain 2 dirt Now how you went about filtering that two dirt I dont know its been a royal pain in my butt that's for sure.
So that's only 2 dirt per 6 peat made. for your tree farm.
Your Tree farm needs 9 dirt per X humus So the Peat farm is not going to keep up. the sand input to the Peat Farm is Not enough and That is why You seem to be lacking Bog Earth and your Peat farm is mostly dirt. If I was you and this is some thing I am doing I am using a cobble generator a block breaker and a pulverizer to make sand from cobble and feeding my system that way ( I don't have a tree farm for my setup i have 2 sugarcane farms making plantballs that way.)
when i looked at a way to create plantballs I seen that the tree farm required a lot to maintain and that was just some large pain for me because there is no effective way to make dirt. That I know of
I think from reading above that Steve's carts may have a nice way to chop trees must look into it.
Also I am having major issues with making too much sand and Bog Earth due to the dirt expanding and not removing the extra 2 it makes from sand. :/ I am now in the process of coding a ComputerCraft Program to count the dirt and monitor the whole thing so no items build up and spill out over the place.