I grew up in Northern Illinois. As a kid, I never really considered it, but I was surrounded by some of the most beautiful black soil on the planet. At 13, my family moved to Southern California. The climate of course is great, but the soil… not so great. This is especially true in any of the modern housing tracts where the topsoil (such as it is, or was) is graded, compacted, and otherwise abused during construction.
The reason that the soil near my childhood home has that wonderful black color is because of the organic matter it contains. Over the eons, plants deposited organic matter on the soil surface as a natural mulch. This organic matter slowly decomposed through the action of a whole host of microscopic organisms such as protozoa, fungi, and bacteria. Insects, worms and other animals helped to incorporate this organic matter into the soil. Over time, the organic matter was transformed to a relatively stable material called “humus”.
The relationship between the various organisms that live in the soil is sometimes called the soil foodweb and has been recognized as an important factor in overall soil health, and therefore plant health.
Soil organic matter has many benefits for plants: Organic matter is a natural fertilizer, it improves the nutrient holding capacity of soil (the so-called cation exchange capacity), it makes plant nutrients more available for uptake by roots, it improves water absorption and drainage, and supports the beneficial organisms in the soil ecosystem. Organic matter improves the water holding capacity of sandy soil and loosens heavy clay soils. Soil organic matter also benefits the environment because soils with adequate organic matter absorb and hold more water, and therefore less water runs off into lakes and streams.
In a previous posting I discussed the benefits of mulches and the types of organic materials used for mulches. Mulching is one way to improve the organic matter content of your garden soil. However, mulch is only applied to the surface of the soil and relies on the same processes for incorporation as occurs in nature. You can increase the organic content of your soil more quickly by incorporating organic materials into your soil.
Similar organic materials can be used for incorporation as are used for mulching. However, there is one major thing to consider, and that is carbon-nitrogen ratio of the product. When organic matter is incorporated into the soil, microorganisms use the carbon in the material as a food source. The microorganisms also need nitrogen to grow and multiply. If there is not enough nitrogen in the organic matter, then the microorganisms take-up whatever other nitrogen is present in the soil. Plants also need nitrogen to grow, but the microorganisms are much more effective at using the available nitrogen. So if there’s too much carbon and not enough nitrogen in the soil, the microbes will use all the nitrogen and the plants won’t get enough and will turn yellow. Eventually, the microorganisms complete their job of decomposition and there will be a net release of nitrogen which your plants can take advantage of, but this may take several weeks or months.
This may all sound very complicated, but the bottom line is that you generally want to avoid incorporating “raw” organic matter into your soil, such as sawdust or uncomposted leaves. Some of the best material to use is compost, because it has already undergone the decomposition process in the compost pile. Most high quality composts will act as a natural fertilizer in your garden and will support the community of microorganisms in your soil.
When should organic matter be added to your soil? As often as you like, but generally the most convenient time to add it is when preparing areas for planting. After the garden is already planted, it can be difficult to incorporate organic matter without disturbing plant roots. At that point mulching is probably an easier option.
How much organic matter should you add to your soil? The amount added is not that critical. Start with a layer about 2 to 4 inches deep and work it into the top 6 inches of soil. Large amounts of organic matter can generally be added to the soil with no problems. One exception is materials containing large amounts of manure. These are relatively high in fertilizer value and can burn plants if too much is used.
If you have trouble deciding what product to use, talk to your local garden center. They will be familiar with the different materials that are available and work best in your area.
So add some organic matter to your soil and improve your little corner of the world. You will be joining a long list of people that have recognized the importance of soil.
“To be a successful farmer one must first know the nature of the soil.” Xenophon, Oeconomicus, 400 B.C.
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