Q. When I told someone I was growing in a sand+sawdust mix, he said 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. What is your experience with this?
A. Organic materials will, indeed, decompose over time. They do not disapear altogether, but you will need to supplement them occasionally. Sawdust is slower to decompose, and thus is useful for a longer time than peatmoss. And perlite – if you can get it inexpensively – lasts a very long time. Coconut husks also hold up well, but rice hulls decompose rather fast.
Dr. Mittleider has had the same Grow-Boxes in his backyard garden for over 25 years, and never totally replaced the materials, to my knowledge. He has supplemented regularly, however.
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 during the 4-6 months of winter.
Organic Fertilizing & Nitrogen Deficiency
Q. Sometimes I have seen gardens with compost and manure as the fertilizer of choice become very yellow. What causes this, and how do I avoid that happening to my garden?
A. What you have seen is “Induced Nitrogen Deficiency.” Soil amendments, including straw, tree bark, shavings or sawdust, peat moss, and manure (almost always containing a large percentage of bedding straw or sawdust) can induce a nitrogen deficiency on plants. The reason is that these materials are very high in carbon content, and therefore adding them into the soil raises the carbon to nitrogen ratio.
The carbon to nitrogen ratio is the amount of carbon in relation to the amount of nitrogen in the soil. This ratio should be 10:1 or lower. When the soil has ten parts of carbon, it should have at least one part of nitrogen or the plants will not be able to obtain the nitrogen they need. When carbonatious soil amendments are added, the amount of carbon is raised in relation to nitrogen. Micro-organisms in the soil attempt to break down the carbonatious material and in this process they use some of the nitrogen from the soil, making the ratio even worse. The micro-organisms have the ability to take the nitrogen before the plant can, so oftentimes adding soil amendments induces a nitrogen deficiency for the plant population. Therefore, whenever soil amendments are used, it is important to add some nitrogen, to bring the carbon to nitrogen ratio back to a ten to one, so that both the plant and the micro-organisms requirements are satisfied.