By Stephen Janak
County Extension Agent – Ag/NR
I recently made a farm visit after receiving a call from a couple who needed help understanding their soil test results and were having problems with their garden being less than productive. The cause of their limited production eluded the three of us for longer than I care to admit. I commended them on their efforts to raise a healthy and productive garden, and indeed they were very agriculturally well-read and had a good understanding of ecology. The garden site had very sandy soil, susceptible to the loss of nutrients through leaching, or the loss of fertility as nutrients are carried downward through the soil profile by water. In an effort to improve their soil, the landowners were adding leaves and woodchips as well as a small amount of chicken manure. I was excited to see people who understood that organic matter (leaves, woodchips) was important to soil health, the infiltration and storage of water, and the retention of nutrients. I was very confused then, when they said that the productivity of the garden had declined in response to their efforts.
This situation reminded me how important it is to know that the soil is in fact a living organism, and not just a substance to hold plants upright. You see, soils are full of millions of tiny organisms known generally as microorganisms, or microbes. They include bacteria, fungi, algae, and protozoa. These organisms are responsible for the breakdown and decomposition of organic matter such as leaves, grass clippings, and wood particles. Through this process, nutrients locked away in organic matter become available to plants once again. This is known as the mineralization of nutrients and is part of the nutrient cycle. The most important elements for microbe activity are carbon and nitrogen. They must have both elements in the proper amounts to flourish and reproduce quickly. When the amount of carbon available in the organic matter is significantly greater than the amount of nitrogen, microbes must then use additional nitrogen from the soil that would otherwise be available for plant growth. The optimal carbon to nitrogen ratio is somewhere between 15:1 and 30:1. Woodchips, however have a ratio of 200:1 or more, and most leaves are somewhere around 60:1.
To summarize, the garden in question had extremely low levels of plant-available nitrogen because the microbes were using it all to break down the woodchips and leaves. To compensate, additional nitrogen must be added either through synthetic fertilizers or manures which are high in nitrogen. This will not only supply available nitrogen for plants to use, but will also feed the microbes and speed the decomposition process.
It is important to remember that every management decision we make on the land will have effects on other parts of the ecosystem. It is imperative to manage land using a systems approach, instead of managing each aspect of the ecosystem separately.
The best way to add organic matter to soil is to start a compost pile. A mixture of woodchips, leaves, grass clippings and soil, if kept moist but not soggy and turned or flipped every couple of weeks, can be ready to use in about eight months. Start composting now with your fall garden in mind. For more information, go to aggie-horticulture.tamu.edu and search “composting,” or call the Extension office at 979-732-2082.