Q. I was looking for a fertilizer substitute in my country (tropical), and I mentioned I was trying to grow in sand+sawdust mix. The person I talked to mentioned that the sawdust will decompose with time, leaving me with only sand. I recall reading on your website that the planting mediums do not require to be replaced, but what he said made sense. What are your experiences with this?
A. Organic materials will, indeed, decompose over time, and become less useful. They do not disapear altogether, but you will need to supplement them occasionally. Sawdust is slower to decompose, and thus useful for a longer time than peatmoss. And perlite – if you can get it – lasts a very long time. Coconut husks last well, but rice hulls decompose rather fast.
Dr. Mittleider has had the same Grow-Boxes in his backyard garden for over 25 years, and has never replaced the materials, to my knowledge. He has supplemented whenever necessary. When we say the materials don’t need to be replaced, we mean that so long as there is no disease present, you can continue to use them – supplementing as necessary to keep the box full of soil mix.
Also, in a tropical country, organic materials will decompose faster than they do in colder climates, because not much decomposition happens when materials are frozen.