Soil Mix in Grow-Boxes

Q. I have made several beds. I am using the original garden soil to which I have added 10% vermiculite and 10% peat moss by volume. My top soil was so shallow (about 5 inches) that I really need to add another
couple of inches of something to get the mix to the top of the frames.

Would it be all right to fill the bed the rest of the way with fresh pine sawdust I have a ready? It is the size of dry oatmeal. Could there be an acid or heat problem with too much pine dust?

A. Several problems are associated with using the natural soil in your Grow-Boxes.
1. The weight of the dirt is much greater than the organic materials we recommend for 65-75% of the mix. This will tend to push your walls out.

2. The soil will tend to get very hard and compact.

3. The soil may have disease, almost certainly has weed seeds, and likely has some bugs in it. So by using it you continue problems that we try to avoid in the Mittleider Method.

There is no problem in using pine sawdust, but the size of what you describe makes it sound more like shavings or chips, rather than sawdust. You are inviting problems of having the materials sour if they are too large, and they don't absorb water, the way sawdust does, so I can't recommend anything larger than regular sawdust.