Richard R. MacMahon, Ph.D.
One of the most basic and fundamental aspects of organic gardening is the use of compost. Adding compost to the soil increases the organic content of the soil, improves the structure of the soil, permits better water retention in the soil and facilitates mineral uptake by plants. It may also add a small amount of nitrogen to the soil, but it is not a fertilizer in the common sense. Excessive amounts of compost may actually result in nitrogen deficiency. If you are going to have an organic garden you must understand what compost is and how it is made.
Compost is partially decomposed organic material, mostly derived from plants. Compost is usually not purchased, but rather made at the garden site. If you do purchase compost, it is often sold as “humus” or “loam”. But it is so easy to prepare that there is little advantage to buying it, other than time. Compost is prepared by simply piling scrap vegetable materials together. These can be grass clippings, leaves, waste foods (but vegetables only!), weeds and any other plant materials you can think of. A small amount of soil is mixed in to furnish the correct microbes, some water is added to dampen the entire pile and then you just let the pile sit until it has partly decomposed or “composted”. Sounds simple, right? But there are a few other considerations that we had best mention.
If you just leave the pile sitting there, it may blow away or be disturbed in some other way. You need to make a compost bin to contain the materials. We have a store near our school that is only too glad to give away pallets. Pallets are ideal for a compost bin because they have large slits that let air into the compost. We have obtained materials for several bins and it has not cost us anything. All we did was to lash four pallets together with nylon cord to form a bin. You can also buy a number of different kinds of compost bins, but it is good for the class to work together recycling old pallets.
The more finely divided the plant material at the beginning, the quicker the compost will form. We have had good luck with running a rotary lawn mower over a pile of leaves and weeds, and catching the resulting chopped-up materials in the bag. This material is then added to the compost bin.
The one problem you may have with your material is getting it too compacted and/or too wet. If this happens, you do not get compost, you get rotted plants. And this material will not help your garden. So you must aerate the compost. This is why a bin that has slits in the side is ideal. In order to aerate the compost you normally turn the entire mass in the bin over with a spading fork every few days. We arranged our bins so that the front of the bin swung open like a door. This made turning the compost very easy.
As the compost forms in the bin, the vegetation is decomposing. Soil micro-organisms will feed off of the vegetable matter and this will raise the temperature of the inside of the compost pile to from 120âF to 160âF. This decomposition needs to be aerobic, thus the need for turning the compost. You must supply water but if you over-water you will eliminate the needed air and decomposition will become anaerobic.
Finally, when the compost is finished, it will be black and crumbly. Not quite like soil, but definitely no longer identifiable plant material. Usually at this stage we run the compost through a sieve made of hardware cloth and some 2 X 4’s. (Available at any building supply store. They are easily constructed. Do not make the frame too large or the students will have a difficult time shaking it.). The sieved material we add to the garden, the residue we put back as starter material for the next batch of compost.