Wednesday, September 28, 2011

Today I Learned (TIL)

Today I learned that compost isn't a fertilizer, it's a soil amendment. In other words, compost doesn't feed your plants, it feeds your soil. Yes, soil needs food, nom nom nom. Soil is hungry. Soil needs to eat because soil is ALIVE! Good soil, anyway. Somewhere in the back of my head, I knew this. I've heard about soil microbes. But somehow I had it in my head that the compost I work into my garden beds every season was feeding my plants. Although compost does have some nutrient value, its primary purpose is not to provide nutrients to plants -- at least not directly. Compost feeds the soil microbes; in turn, the microbes convert the compost into plant food. Or as Skip Richter, former Texas AgriLife Extension Director for Travis County, says, "Good soil fertilizes itself."

Teaspoon of soil
This is how it works: in each teaspoon of soil are millions and millions of microscopic good-guy organisms: bacteria, fungi, algae, protozoa, nematodes (most of which are not disease-causing, but beneficial), and something called Actinomycetes, which are bacteria that have some fungal-like properties. Add it all up, and you can have more than 75 pounds of these critters in 1000 square feet of healthy soil, not counting the critters you can see, like earthworms and pillbugs and ground beetles. Every day, these invisible life forms are hard at work in your soil, doing important jobs like decomposing plant material, fighting disease-causing organisms, breaking down chemicals, and making nutrients available to plants. Soil microbes fix nitrogen, chelate minerals, and produce nitrogen waste, which plants can then take up through their roots.

So what makes soil microbes happy? Carbon-rich organic matter, particularly plant material, green or brown. For example, kitchen scraps. Or composted leaves. Or aged manure, which is what's left after an animal eats, ruminates and eliminates plant matter. Or cover crops, which are planted and turned into the soil before the next season's planting. Organic matter improves the structure of the soil, making soil more permeable to water and air, and improving its ability to maintain pH (a balance between acidity and alkalinity). This creates an environment where soil microbes can thrive and flourish.

Good soil teeming with microbes makes for happy plants. Some soil microbes have a symbiotic relationship with plant roots: the roots secrete stuff that the microbes like, and the microbes create a protective shield around the roots, or convert a particular nutrient to a form the plant can absorb and use. Other microbes break down stuff like leaves and manure and kitchen scraps and grass clippings and insect shells, releasing the nutrients into the soil. Some microbes eat other microbes, keeping checks and balances on the system.

As the foundation of a garden, nothing can replace or make up for lack of good soil. But creating good soil is a process and there are few short cuts. In forests, soil microbes turn layers of decomposing leaves into nutrient-rich humus; rain percolating through the leaf cover acts as a natural "compost tea." A natural, gradual sort of rototilling occurs as the roots of understory plants continually die and regenerate, releasing nutrients into the soil and creating small pockets for air and water to flow through. This process goes on for years, often decades. We can mimic this process by adding compost to our garden beds, mulching with leaves, planting cover crops, keeping tilling to a minimum, and wisely using water, fertilizers and pesticides. But even with good cultural practices, it can take 3 - 5 years to turn poor soil into good soil. So be patient and keep at it.

For more information on soil biology and soil organisms, visit the United States Department of Agriculture's Natural Resources Conservation Service's Soil Biology website.

(Today I Learned features a nugget of information I learned during training to become a Travis County Master Gardener.)

Words and photos © 2009-2011 Caroline Homer for "The Shovel-Ready Garden". Unauthorized reproduction is prohibited.


  1. Great photo! I thought it was stock photography for a minute and a helpful explanation of the difference in soil amendments and fertilizer. Just curious, what time are your Master Gardener classes? I'd like to take them in Bexar county, but they are always offered int he middle of the day (for retired people, I guess).

  2. This is a very informative and well written post. Thank you!

  3. Clearly, you are psyched about this fabulous program! Wonderful post. Aren't you having a ball?!

  4. Abbey, my classes are from 9 to 5 every Tuesday for 11 weeks. Although I work full-time, my employer is allowing me to flex my time to complete the course.

    Diana, I am truly having a blast. So much good information!

  5. I missed this class...this was good info. I need to go back and listen to the session. I've always wondered if the compost should be worked into the soil but it sounds like just tossing it on top of the beds is sufficient. Along with Abbey, your photo is cool.

  6. Seems like there's two schools of thought, Cat. One school recommends working the compost into the top 6" or so of soil, to avoid the "bathtub" effect (clay acting as a barrier under amended soil) and create a transition soil layer. Others champion the no-till method, where compost is layered over the top of the existing soil season after season with no tilling and breaks down over time. There are advocates for both methods. I think the "work it in" method might be best for areas with clay soil and little rain (cough) but I'm just too old and lazy to till.