Brown composts are older dead plant material. Examples of this would include straw, old leaves, sawdust, branch or twig pieces, old grass clippings, old stalks or stems, wood chips or shavings, etc. The main point is that brown composts are older composts. Aged, or older composts, have unique properties that need to be addressed so we use them wisely. They are a great source of organic matter but they are lacking in one critical nutrient: nitrogen.
What happens to brown composts once they are in a soil is they begin to break down. This breaking down process is done by soil microbes such as bacteria or fungi in the soil who feed on the organic matter — in this case, the brown compost. The breakdown of organic matter in soils is desirable and so we want the soil microbes to proceed with the decomposition process. The problem with brown composts, since they are nitrogen poor, is that they soon run out of nitrogen which the soil microbes need in order to keep feeding on the brown compost. As the microbes run out of nitrogen they start to starve and die. The microbial population never completely dies out because as some begin to die, the nitrogen from the dead microbes is released and made available again to the surviving microbes such that the population begins to climb again.
So microbial populations on brown composts rise and fall and rise and fall. The results of a fluctuating microbial population due to nitrogen deficiency are two fold: (1) the decomposition process is greatly slowed, and (2) the microbes will scavenge for other nitrogen sources in order to stay alive (this is the more detrimental result, as far as your garden is concerned). That means that microbes will actually take nitrogen out of your garden soil in order keep themselves alive while they continue to feed on and breakdown brown compost. The end result is the creation of a competitive interference between soil microbes and your plants’ roots for the nutrient nitrogen. This of course will lead to a nitrogen deficiency in your plants which will greatly stunt their growth and their ability to produce for you. We do not want this problem.
Now don’t shy away from brown composts given these facts because this problem is easily remedied by simply supplementing brown composts by mixing brown composts with other more nitrogen-rich organic matter sources such as manures or green composts for example (remember green=nitrogen), or by adding a little nitrogen in the form of a fertilizer. If you add nitrogen to your brown composts the microbes will be able to continuously feed without scavenging for nitrogen in your garden soil. I’ve pulled together a graph to illustrate the principles we’ve been discussing with respect to brown composts. The red line represents a soil microbial population on a brown compost that is not supplemented with any nitrogen. Notice how the microbial population fluctuates and never reaches its full decomposing potential. This brown compost would take a long time to decompose.
As an example, have you ever noticed that forests often have leaf litter on the forest floor even though it’s not autumn or have you ever noticed how dead pine needles are always perpetually underneath pine trees en masse. Well, that’s because leaf litter, including pine needles, are a brown compost, and without some kind of additional nitrogen source it takes soil microbes forever to break them down. It takes months, and sometimes years, for the most nitrogen-poor brown composts to decompose. The blue line in the graph, by contrast, represents a microbial population on a brown compost that is supplemented with nitrogen. The microbial population is able to continue feeding unimpeded, and best of all, these microbes would not need to scavenge for nitrogen in your garden soil.
Another way to help in the decomposition process is to increase the surface area of the compost. This is done by breaking down the compost mechanically into smaller pieces such as shredding leaves with a string trimmer or lawn mower, or by breaking up twigs into finer bits and pieces. This simply exposes more area on the compost which gives the soil microbes more access to the compost as a food source. Some composts are already very finely chopped-up like sawdust or grass clippings and therefore require no further mechanical handling of the material.
Before I move onto other organic matter sources I want to mention that one of the big benefits to brown composts is that you can often get them totally for free. Autumn leaves are an awesome example of this because they are usually around in great abundance. Don’t throw away your autumn leaves, put them in your garden. If your property has few trees on it, or if you have a really big garden that could use a ton of autumn leaves, be a good neighbor and ask your neighbors if you could serve them by collecting their leaves for them. And then, of course, instead of throwing the leaves away, put the leaves in your garden. Trees, and the autumn leaves they shed, are fantastic because they produce a reliable, free source of organic matter every year, and once more, usually you don’t have to drive or go anywhere to get it. Trees aside, you can often get sawdust or straw for free from people who work with wood or a local farmer who has straw but doesn’t have a need for it. There are a lot of possibilities for getting brown composts for free or nearly free.