Soil is teeming with life. For all the plants and insects we can see, there are millions of organisms in every square inch of soil invisible to the human eye. Every garden has a unique soil composition. For several years, I have been gardening in a Seattle community garden on a concrete rooftop several stories off the ground. The original lightweight soil mix, trucked in and lifted onto the roof, has been transformed. Planting and composting, worms, insects, micro-organisms and of course the gardeners themselves have transformed the plain soil that was spread onto barren concrete into a patchwork of productive garden plots. How can such an unusual garden location develop a healthy soil ecosystem?
A productive rooftop garden
I have been pleasantly surprised at the general success that my rooftop garden has had by simply adding compost and organic fertilizer over the growing season. After the last harvest, I turn over the garden debris into the soil, including stalks, leaves and unused produce, by burying it in shallow trenches. The soil health improves, and every year there is a bountiful harvest and more worms and good insects that bring the soil to life. The soil is black and crumbly, a good indication that it is what soil experts call “friable.”
Soil health varies from plot to plot. Last year I moved to a new garden plot, and not knowing what condition the soil was in, I got a soil test. The King Conservation District does free soil tests, you just gather a few soil samples, mix them together, and mail it in. Results come back in a few weeks, showing the quality of nutrients in soil. The numbers on the chart are deceptively simple, giving us just a glimpse of the activities of a complex soil ecosystem.
Most gardeners have heard of the holy grail of nutrients that is on the side of fertilizer bags, NPK, which are nitrogen, phosphorus and potassium. Additionally, plants require calcium, magnesium and sulfur, and several other micro nutrients for growth. Like many gardeners, I try to bring those nutrients to the garden with organic fertilizer, like cottonseed meal, bone meal, fish emulsifier, or others. It gives the soil a good kick start in the spring, and at intervals during the growing season. But fertilizer does not only have to come from outside sources. When the soil is well fed by compost, turned cover crops, and its own ecosystem, it can produce nutrients. How do we get the soil to do the work of providing the necessary nutrients?
Plants and soil have been working together for eons, teaming up to benefit each other, as can be observed in a forest or grassland ecosystem. We can encourage a healthy ecosystem in our gardens by creating an environment that supports the bacteria and fungi that supply nutrients to plants. To get those microbes to work for you and feed your plants, you need to feed the microbes. The microbes in the soil are a part of the chain of nutrients. They eat compost and recycle discarded plant material, and in their life cycle, they store nutrients in their bodies, which then become available to plants when they die. The more microbes in the soil, the more nutrients are stored for future use, rather than leaching out of the soil. Diversity in the microbe population also helps to control disease. Competition among microbes keeps rogue populations of bacteria in check, preventing them from wreaking havoc on plants. A healthy diverse soil community will pay back in healthy garden plants.
A couple of fascinating and very readable books on the subject that I would highly recommend: Soil Building, A Down-to-Earth Approach and Teaming with Microbes, A Gardener’s Guide to the Soil Food Web.
According to the Rodale Institute, the web of living creatures in the soil include fungi and bacteria that recycle nutrients, protozoa, which feed on bacteria, nematodes that also recycle nutrients, and microarthropod predators, which are preyed upon by earthworms and larger insects. Pests and disease are sometimes unfortunately in the soil as well. I have seen my share of slugs, molds, and other diseases in the garden. Having a diversity of organisms that keep the soil in balance will help to keep the unwanted ones from ravaging the plants.
All of these creatures are part of a complex community that build the soil and provide nutrients to plants. In turn, the plants provide carbohydrates and proteins through their roots for the microbes. Bacteria and fungi are fed by the products from the plant roots, which are in turn consumed by nematodes, arthropods and protozoa, which are consumed in the food chain. Through this process, nutrients are consumed and retained within the ecosystem. The specific microbes that are hanging around the plant roots are not there by accident–the plant is orchestrating the system by attracting the organisms that it wants with enticing root secretions. The plant attracts organisms that will feed it the nutrients it needs.
A robust microbial community has a cyclical system of benefits for its members that recycles and retains nutrients. This has a great advantage over chemical fertilizers, the majority of which is not absorbed and drain out of the soil. The best way to provide nutrients to your soil is to keep the microbes happy and fed so that they do the work of releasing nutrients continuously.
Results of the soil test
The tricky thing about a soil test is that it only measures the soluble nutrients that are immediately available to plants. All of the potential nutrients that are tied up in the decaying matter and organisms of living soil is completely missed by the test. To understand how much nutrient value your soil really has, it is important to know the history of your soil and the amount and quality of the organic matter. Living soil builds up over years of plant growth and decay, application of compost and growth of microorganisms. It makes the soil appear black and crumbly. While the majority of soil is made of minerals and pore space filled with air and water, the most important part, the organic matter, is typically only 3-6% in gardens and farms. This seemingly small amount of material is working hard to build a nutrient dense soil.
Adding even more mystery to the soil test is the elusiveness of nitrogen. It doesn’t stick to the soil for long and easily leaches away. What is measured in the test will not capture the true amount of nitrogen stored in organic matter. Instead of relying on the soil test, the amount of nitrogen needed for the season is based on what the plants need and what is estimated to be in the organic matter that will be continuously released. On the other hand, the amount of potassium, phosphate and other available nutrients are more readily determined by the soil report, since they tend to stick in the soil.
Since I just moved to this new plot last year, I don’t know the prior history of building organic matter. Having added lots of compost last season, I had hoped for a good soil report. It turns out the soil had another story to tell.
The soil laboratory rated the soil as if it was in a container, the container being the garden rooftop. Here are the highlights:
- pH 5.8. My soil report said that this was in the optimal range. Yeah!
- Calcium – 120 ppm. This is also in the optimal range!
- Nitrogen – 13 ppm, which is really low compared to the optimal 70-200 ppm range.
- Phosphate – 9 ppm, low for the optimal 15-25 ppm range.
- Potassium – 28 ppm, very low for optimal 100-150 ppm range.
Since the growing season has already started, the soil will need an immediate recharge of nutrients. The compost amendments that I will continue to add to build the living soil fertility will take time to release nutrients. The garden needs organic fertilizers that will have a faster effect. Here are a few recommendations from the soil report that can be added a few weeks before planting:
- Nitrogen: Blood, Fish, cottonseed or feather meal all provide nitrogen.
- Phosphorus: Soft Rock Phosphate or bone meal will provide this nutrient through slow release microbial action. While there are liquid quick release phosphate fertilizers, they are not organic.
- Potassium: Add sulfate of potash for slow release potassium. Liquid kelp will provide a quick source of potassium.
Fortunately, all of the above are available at Walt’s Organic Fertilizer a local Seattle company that manufacturers organic products. A combination of products can be mixed and matched for the needed nutrients.
Investment in soil
Gardening is a very intensive use of the soil. Constant watering and harvesting of produce take nutrients that constantly need to be replaced. Keeping value in the soil means giving as much in composting as is taken in harvesting. Seeing the results of this soil test was a wake up call. My garden not only needs fertilizer, but it also needs to increase organic matter for a more nutrient dense soil. That means some serious composting and cover crops. Building a living soil is an investment in the future. When the soil is nurtured, it thrives and grows to support a robust ecosystem and vibrant garden.
My plot is small in comparison with all of the agricultural land around the world. The principles for healthy garden soil, when applied at a greater agricultural scale, can help to heal our depleted soils. Healthy soil is an ecosystem that sequesters carbon, contributes to biodiversity and fights against disease. It reduces fertilizer applications and related runoff of excess nutrients. On our rooftop urban garden oasis, we community gardeners can do our part to contribute to an abundant ecological diversity of living soil.