Soil organic matter is comprised of plant debris such as dead roots, leaves, and stems, along with bacteria, fungi and insects, which help break down this debris. Many people also use animal manures, make their own compost or grow cover crops, all of which can be good additions to the soil. Once organic matter is added to the soil it usually doesn’t just sit there but continues to break down, so it needs to be added regularly.
The benefits of soil organic matter include improving aggregation and structure of soils, increased infiltration of moisture, better water-holding capacity of sandy soils and better drainage of high-clay soils. Organic matter also reduces compaction while providing a slow release of nutrients to plants and animals. However, it is important to understand that most of the organic matter we are accustomed to adding to our soils is not high in nutrients and while the material provides some nutrients, if you desire good plant growth additional fertilization is necessary.
There are various soil organisms that aid in the decomposition of organic materials and through this process release nutrients. A major decomposer of animal and plant debris is bacteria. Some bacteria are often found close to the soil surface and require higher moisture levels. Then there are different bacteria, which oxidize minerals, releasing nutrients and making those available to plants such as the oxidation of ammonium to nitrate nitrogen.
Other organisms, including fungi, protozoans, nematodes, mites and other insects, also help. Of course, don’t forget the earthworms, which mix organic matter residues, improve soil aeration, and increase water movement. And if you don’t see earthworms in your soil it likely means you don’t have enough organic matter, since they need that to exist. If you provide organic matter they will come.
People often ask me how much organic matter their garden soils should have and can they add too much. The answer to the latter question is no, you can’t really add too much. And the reality is that even if you tried, other forces aren’t going to allow it. This is partially due to the difficulty in acquiring large quantities of organic matter and to the difficulty in maintaining it in your soil.
Obviously, the type of residue you use affects how fast organic matter breaks down in your soil. For instance, grass clippings and chopped up leaves decompose fairly rapidly no matter what you do. Other forces affecting decomposition include temperature. With an increase in temperature there is an increase in decomposition. Also, the more your planting area is aerated, tilled or otherwise worked increases decomposition.
Plant nutrients other than nitrogen have little affect, but nitrogen, or the lack of it, can have a big effect because the decomposition of organic matter requires nitrogen. If you add your plant debris directly to the garden you need to have nitrogen available in the soil to decompose the plant material. If adding that organic matter while plants are growing you also could need additional nitrogen for the plant growth.
So how much organic matter is in our soils? For native Utah soils this varies from 0.25 percent to 2 percent, while ideal soils have levels of 5-10 percent. So if you want organic matter in your soil its necessary to add it regularly to maintain the organic content and improve soil conditions.
So how much should you add each year? When using truly composted material, which most people don’t, you should consider a yearly application of at least one inch per year or two to three inches of material if you are in dire need of organic matter. Many people think buying a few bags of material is enough, but one inch of composted materials represents three cubic yards when spread over a 1,000-square-foot area.
Since most people don’t add fully composted material to their soil you have to add considerably more uncomposted material because plant debris shrinks considerably during decomposition.
If you would like to know more about these topics check out this publication on organic matter at extension.usu.edu/files/publications/publication/AG_Soils_2003-05.pdf.
Thought for the day: “The reason why worry kills more people than work is that more people worry than work.” —Robert Frost.
For more information about these topics call the Utah State University Extension Grand County office at 259-7558 or email Mike Johnson at mike.johnson@usu.edu.