By Bill Chapman, PhD
Research Soil Scientist —
In this life there are two kinds of creatures—those which digest their food inside their gut (most of the big animals) and those which digest their food on the outside (most of the rest of life on earth). Outside of the oceans, most of the external digestion of food takes place in the soil. In fact some people would argue that guts of bigger creatures are simply sacks for carrying soil around to aid in digestion. Whether in the soil or in a gut, we know that microbes are responsible for much of the final breakdown of food. Some would say that soil is like the stomach of the earth and just like our stomachs, it needs to be fed.
Most gardeners are familiar with nutrients like nitrogen, phosphorus, and potassium. Many are under the mistaken notion that putting those nutrients into the soil is all there is to feeding the soil. Just as with people, the major nutrient that the earth needs is energy. Energy fuels all the processes of life and soil gets all its energy from the organic things that fall onto and get incorporated into the soil. Energy (think calories) has bad rap with people as so many of us have excess reserves a-plenty. However, due to modern practices, it is rare indeed to find a fat soil. All the energy-rich leaves, stems, trash, and manure—unused parts that used to go back into the soil are often burned, taken away for recycling, or, heaven forbid, composted before returning the remnants to the soil. While the remnants may be high in some nutrients, they are not high in the most important nutrient, which is energy.
Composting needs a special mention. Composting was designed to heat up organic matter to rid it of pathogens and weed seeds before returning it to the soil. If there are no weeds or diseases to contend with then as far as the soil is concerned, it would be much happier to be converting your vegetable matter into rich humus rather than having it done in your bioreactor. The reason that soil needs to break down your organic matter itself is that it derives energy from the process. Composting should only be done to the point of killing the undesired organisms, but that is a story for another day.
So, if you feed your soil with energy, what do you get in return? Research from Colorado State University reveals the typical amounts of various organisms living in the soil in Table 1.
As Table 1 shows (and it is by no means complete) soil is very much alive. Soil needs energy and in return for that energy, soil structure is improved, diseases and some pests are suppressed, nutrients are released as plants need them, and the tilth of the soil is improved which means that generally everything about the garden’s soil gets better. Each one of these topics could be discussed in detail but just to explain briefly:
Soil structure refers to the way that soil is bound into aggregates. Aggregates are small lumps and they are good because they allow air, water, and roots to move freely in the soil. One of the first signs of unhealthy soil is that it becomes dense and heavy. Not feeding your soil properly in combination with excessive cultivation breaks down the structure and speeds up its decline. Fungal hyphae, glues exuded by bacteria, and digesting of the soil by mites, worms, and others all serve to bind the soil into stronger aggregates, which resist breakdown from tillage. Progressive gardeners have been known to feed molasses to their soil to improve structure, but for most of us returning fresh organic (not overly composted) matter, manure, or green manure (plowed in crops) is sufficient to maintain structure.
Suppressive soil refers to the ability of soils to keep certain diseases or pests in check. This phenomenon has been known of for hundreds of years and research has shown that suppressiveness stems from populations of certain organisms that live within the soil. Suppressiveness can be transferred from one fat soil to another but not so easily to a skinny soil. One of the best ways to boost your soil’s suppressiveness is to plant a green manure crop like mustard which is plowed into the soil before it goes to seed. There are many advantages to using locally acquired energy in your garden rather than bringing it in from afar, and perhaps those will be discussed another time.
We all know about adding nutrients to the soil but there are some benefits to adding nutrients as undigested organic matter. As organic matter breaks down (as, for example, in composting) the nutrients are mineralized which means they become available for every creature to use. Some of these readily available nutrients may be washed away, others may be denitrified and lost to the atmosphere, and weeds may grab some of the nutrients intended for your vegetables. By keeping your nutrients in organic form you increase the odds that they will end up where you want them, which is in your crop.
Tilth is a mysterious quantity. It refers to all the qualities that allow a soil to foster plant growth. It refers to structure, aeration, the ability of roots to penetrate the soil, ease of water to infiltrate and stay in the soil, and other properties besides. Farmers from the dawn of farming have understood that the key to managing soil tilth lies in managing soil organic matter. No less a personage than Thomas Jefferson wrote on the extreme importance of regular additions of organic matter to the soil.
Feed energy to your soil and it will become fat and happy—and you may, too.
Bill Chapman has a PhD from UBC. He is a soil scientist with the BC Ministry of Forests who studies all aspects of plant interactions with an emphasis on how soil organisms influence plant growth and disease. Bill, Louisa and their family are in their third decade of Cariboo living. They can often be found picking mushrooms in the area and if approached will bore you with endless stories on how fungi are essential to the growth and well-being of plants and therefore all terrestrial life.
