What Decomposes Faster: Grass Clippings or Oak Leaves?

Dry grass clippings in a layer before spreading on garden beds to prevent odor.

Composting is an excellent way to recycle yard waste and food scraps into usable soil amendment and nutrients for your plants. Composting is easy to do, and you don't need a special bin to accomplish it. You do need a good mixture of organic materials for a balanced compost. The major components of compost are nitrogen and carbon. Nitrogen can be though of as the green, moist parts of compost. Carbon is the dryer, woody aspects of the compost heap. When combined, the two fuel a fast decomposition, but alone, they also decompose, however the rate is slowed.

Carbon vs. Nitrogen

The oak leaves in this case are the carbon and the grass is the nitrogen. The optimum ration of carbon to nitrogen for composting is between 25:1 and 30:1. At a higher carbon range, the heat production, which is the energy that fuels decomposition, drops and composting slows. Higher rates of nitrogen may cause a compost pile to get moldy or rot. This is because nitrogen materials tend to be wet and fresher than carbon materials.

Leaves vs. Grass

Nitrogen decomposes at a higher temperature than carbon, which means it will break down more quickly into compost. The grass will decompose faster than the oak leaves if it doesn't mold first. The addition of dryer items to the pile of grass will prevent mold and enhance the composting process by providing a balance of carbon and nitrogen. Aerobic bacteria are the most important decomposers and release nitrogen but use carbon to fuel their activities. This means that more leaves or carbon must be added to a compost pile to keep it active and healthy. Excess nitrogen in the form of grass or other green items can cause a pile to become smelly and even rot because the organisms are also releasing nitrogen.

Ratio for Composting

Grass is mostly nitrogen and requires a considerable amount of carbon added to make a perfectly balanced compost. The ratio for grass is 20:1. This may seem like a high amount of nitrogen, but the ration for carbon illustrates the difference. The leaves will compost with a ratio of 60:1. The leaves can handle a high amount of nitrogen, but the grass cannot be paired with carbon at such high levels. A good balance will enhance decomposition and keep the aerobic bacteria from releasing toxins into the compost.

Particle Size

Decomposition and the formation of compost is affected by air circulation, temperature and moisture. It is also dependent upon the size of the items being composted. The smaller the particles, the faster the items break down. The more exposed surface area, the better able microbes and bacteria can access the item to assist in decomposition. Grass clippings are already small enough, but it's a good idea to shred the leaves a little bit. This can be done by blowing or raking them into the lawn and then mowing them with the grass. The result is a blend of carbon and nitrogen that can start an excellent compost pile.

References

• Composting 101: A Balancing Act (Carbon-to-Nitrogen Ratios)
• Pennsylvania Department of Environmental Protection: How to Make Compost
• University of Illinois Extension: The Science of Composting