Soil Testing Part II—Digging Deeper
Knowing the amount of nitrogen, phosphorus and potassium (N-P-K) in your garden soil is the first step in evaluating the health of your soil. These are primary elements affecting plant growth and the very basic measures of what your garden soil has to offer the crops you grow. Unfortunately, solely looking at N-P-K as the indicators of your soil fertility is greatly oversimplifying the matter. To better understand this complex topic you’ve got to dig a little deeper and look at what else is going on underground—specifically at the network of life.
In a typical organic garden, natural substances such as manures, plant debris, kitchen scraps, bone and blood meals and other organic matter are the sources of the nutrition that plants require: N-P-K along with an extensive list of trace minerals. The nutrients in these organic materials are mostly locked up in complex molecules, but in this form the plants cannot absorb them. To make the minerals available for the plants, the large organic compounds must first be broken down into simpler, inorganic forms. That task falls on tiny, non-plant lifeforms in the soil—a diverse range of microscopic bacterial and fungal life that consumes and digests the organic matter. It’s their excretions that contain the nutrients in a form that plants can assimilate. This population of microbes is just one layer of the vast network of life in the soil, and their act of transforming the soil’s nutrients into bio-available forms is one of the innumerable functions they provide. The microbes are also responsible for producing enzymes, proteins, hormones and other compounds that stimulate plant growth or contribute to the plants’ immune responses.
Plants require a rich and diverse assortment of microscopic communities in their root zones, forming symbiotic and parasitic relationships. In fact, healthy soil can contain more microbes in one gram of soil than the entire human population on Earth! These hard-working organisms spend their lives scavenging, digesting, and breaking down the organic materials in the soil and then excreting simpler compounds, which the plants use. In return, some of the organisms also feed off the carbohydrates and other compounds that the plants’ roots exude. This very simplified explanation of the process is referred to as nutrient cycling, where the energy and nutrients in organic matter is converted into usable forms for plants. These microbes are the bottom of the subterranean food chain, which moves up to predatory protozoa and arthropods, and on to an enormous variety of progressively more complex creepy-crawlies, finally through the soil-dwelling animals like moles and gophers. With this intricate network of life in the soil comes a healthy, balanced, sustainable ecosystem.
Why don’t we just give the plants the nutrients they need in a form that they can absorb?
Good question. That’s what chemical fertilizers accomplish! Inorganic or chemical fertilizers provide nutrients in a readily usable form for plants—a sort of shot-in-the-arm of nutrition. It doesn’t sound all that bad until you acknowledge that this type of practice essentially bypasses the soil-dwelling community. This can lead to severe imbalances in the system such as overgrowths of detrimental fungi and bacteria, which would naturally be kept in check when the rest of the community is fed and healthy. When these overgrowths occur, you’ll frequently see their adverse effects in your crops: powdery mildew, black spot, blight, etc. Conventional farming techniques sometimes use toxic pesticides to treat the symptoms, where organic practices see the garden ecosystem as a whole that is self-sustaining and self-balancing.
How does this relate to soil testing?
Armed with the knowledge that it’s the soil’s residents who do the work of nutrient cycling and feeding the plants, the logical conclusion is that if we were able to measure the beneficial organisms in the soil, we could get a better indicator of our garden’s health. Interestingly enough, there is a fairly simple method to do just that. We observe the activity of the organisms by measuring their respiration. That’s right, these beings breathe, and by observing this exchange of gasses, we can gauge their health. With the help of a simple-to-use test kit, a sample of soil is sealed in a test chamber with a test probe that reacts to exposure of carbon dioxide (CO₂) by changing color. Since CO₂ is released when our subterranean microbes breathe, measuring its presence will give us a good indication of the biological activity in the soil sample. Soil that’s full of life has the innate potential to release nutrients from organic matter for crops to utilize.
So, what if my soil is depleted of life?
Poor soil management results ultimately in poor plant growth, reduced yields, weak flavor, lower overall quality and less nutrition in the harvest. We can build beneficial biological communities with cultures from healthy compost, mycorrhizae and compost teas. Balanced compost contains the life that the soil needs to support plants, so amending the garden with compost is a great start. If you’re unsure of your compost’s vitality, the same sort of test to measure soil life is available to measure compost life! Brewing compost tea from good quality compost is a very effective way to create a population explosion of the biology from the compost to then inoculate your soil. Mycorrhizae is available commercially in various forms. These microbes form symbiotic relationships with the plants’ roots and add to the effective soil life. Adding any of these to the soil along with ample organic matter will help build a strong biological community to support your crops. Additionally, practicing crop rotation and planting cover crops are two more methods of fostering healthy soil.
Now we know, soil fertility is so much more than N-P-K. Although this explanation is still a greatly simplified one, it’s one step closer to understanding the big picture. It’s the soil microorganisms that are the key to turning minerals in the soil into usable forms for plants. So, a truly healthy garden has not only robust plant growth, but also a diverse assortment of invisible life underground.
Author: Kat B.