Q. When composting, we are told to mix “browns” and “greens” together for a good balance of carbon from the “browns” and nitrogen from the “greens”. I am puzzled. All living things have both until they are composted. To me, the browns add fluffy aeration to the soil while the greens clump together in the compost pile. Is it possible that the mix of browns and greens is as much for texture as it is for carbons and nitrogen?
A. This is a huge question that requires a lot more space than this column permits. Compost is used as an amendment for soils for two reasons; because it can positively change the chemistry and structure of soil. I will try to answer it more completely in my blog.
Browns and GreensThe terms “browns” and ‘greens” are a simplification for the average person to make it easier to choose the correct plant ingredients when making compost. Dry wood, or sawdust made from wood is about 50% carbon by weight. The amount of nitrogen in sawdust is about 400 - 500 times less than the carbon. So, the carbon to nitrogen ratio of sawdust is about 400 – 500 carbons for every single nitrogen (brown).
A good compost should have only 40 carbons or less for every nitrogen or less. Extra nitrogen must be added to this sawdust from “green”. It just so happens that food waste (greens) has about 20 carbons for each nitrogen. This is about the same as coffee grounds, which happens to be “brown”. The “brown and green” rule doesn’t always work!
Using Fertilizers As a Greens SubstituteAnother option is to add nitrogen to the carbon or "browns" with nitrogen fertilizer such as 21-0-0 or 46-0-0. A much smaller volume of fertilizer is needed than "greens". This may not sit very well with some people such as the "organic crowd". The plant doesn't care where the nitrogen comes from but there are potential contaminants in mineral fertilizer products. Ornamental trees and shrubs that constitute "green waste" may be sprayed with pesticides. Sometimes it is impossible to know.
Animal ManureMany composts use farm animal manure, rich in nitrogen, in combination with wood or paper products rich in carbon. Farm animal manures can be as low as 12 carbons for each nitrogen. Human manure can be as low as 6 carbons for each nitrogen. On top of that, animal manures are easier to collect and transport for composting.
When “browns” and “greens” are mixed together in the right proportion, voilà. The compost has the magical carbon to nitrogen ratio less than 40:1. I usually aim for a carbon to nitrogen ratio close to 20:1.
Best to Carbon to Nitrogen Ratio
Finished compost with a ratio of carbons to nitrogen of 40 (C:N = 40) has a small amount of nitrogen to give to plants. Compost with C:N = 20 has much more nitrogen to provide and can act similar to a fertilizer when applied to the soil near plants. But carbon and nitrogen aren’t the only “fertilizers” supplied by compost. The composting process releases all the nutrients contained in the ingredients. What goes in, must come out.
So much for the chemistry. Compost also changes the structure of a soil. It acts very similar to peat moss and coir, making it more “fluffy”, while providing many more nutrients to the plants.
I recommend the following:
• Learn how to estimate the C:N ratios and use it as a guideline about how much nitrogen to add to a compost
• Use a variety of feedstocks for composting since plants have a range of nutrients locked inside them
• Garbage in, garbage out. You cant make quality compost by adding feedstocks loaded with contaminants.• I don’t agree about not composting animal carcasses or animal by-products. It can be done. On farm composting of dead animals is a common practice. But it must be done correctly.