This is the first in a series of articles I am developing on the gardening aspects of real food. These articles are the abbreviated version of what may become an ebook. If you might be interested in such an ebook, please email me here and I'll let you know when it's available.
Food plants grow in soil. Understanding what soil is and how it contributes to food is important for anyone who eats, and especially important for anyone who endeavors to eat real food.
What is soil?
For the purposes of this article, we’re really talking about topsoil. Topsoil is a mix of solids, liquids, and gases in the form of minerals, organic matter, water, and air. The composition of topsoil changes constantly. Water runs in and out and evaporates. Organic matter breaks down and is used up. Healthy topsoil is dynamic.
In addition to these basic ingredients, topsoil is home to vast numbers of living organisms. Some such as moles, earthworms, and insect larvae, are easy to see with the naked eye. But most of the organisms in soil are microorganisms. They include organisms such as fungi, bacteria, and nematodes.
We’ll learn more about the organic aspects of soil when we get to the Compost article. But in the mean time, it is important to understand that all the parts of the soil serve special functions. If you consider the question “Which makes music, the musician or the instrument?” you’ll see that it’s not one or the other- they must work together to make music. I hope to show you how healthy soil is similar. Healthy plants are the product of healthy soils where all the parts are working together.
Soil solids include both mineral and organic matter. They provide the bulk of soil and the medium that holds water and air where plant roots can reach them.
Mineral solids include rocks, sand, silt, and clay and makes up roughly 45% of optimal topsoil. A soil’s texture describes the proportion of specific mineral solids in the soil.
Soil that is predominantly clay is often referred to as “heavy” soil. These soils retain a lot of water and are generally high in nutrients. However, the particles are packed close together leaving little room for air. Clay soils feel slippery or sticky when wet and can bake solid, and even crack, when dry. Amending clay soils will improve water drainage, allow for better root penetration, and improve the soil’s ability to support microorganisms.
The “solution” to clay-heavy soil is to add composted organic matter. Recommended amounts range from 3-8” inches deep across the surface area you intend to plant. This composted material should be dug or tilled into the top several inches of soil; however, going deeper than a shovel’s depth is not necessary.
Soil that is predominantly sand feels gritty and has low water- and nutrient-holding capacities. Sandy soil has the opposite problem from clay soil in that the spaces between the sand grains are too large, allowing water to drain away too quickly and offering little stability or nourishment for plant roots. Amending sandy soils will improve water retention and increase nutrient content.
The solution to sand-heavy soil is the same as for clay-heavy soil: composted organic matter. However, because sandy soil has little nutritive value of its own and poor particle adherence, a greater quantity of compost must be added over a longer period of time. Planning to incorporate a 2-3” depth of compost across the surface area you intend to garden, applied 3-4 times a couple weeks apart might be a good guide. It’s a lot of work up front; but after the initial garden building, maintenance should be pretty simple and pain-free.
Silty soil feels silky like fine ground powder or flour when dry and slippery when wet. Similar to clay soils, silty soil has small pore size and poor drainage. On the soil particle size spectrum, silt tends to fall between the super small size of clay particles and the relatively large size of sand particles. Silty soils are improved by the addition of compost; however, smaller quantities are generally needed than for either clay or sandy soils.
Organic matter soil solids include humus, biomass, and residues and by-products. Humus is organic matter that has completed the composting process and become stable. It is a complex and not yet fully understood substance, yet is vitally important to both soil structure and health. One primary function of humus is water retention. Biomass refers to the living aspect of soil and includes creatures ranging in size from microscopic bacteria to soil-dwelling vertebrates such as moles. The organisms that comprise biomass are responsible for breaking down residues and improving soil structure by creating tunnels that allow water movement and oxygen transport underground. Residues include un-decomposed dead plant material and creature corpses. By-products are substances that some plants and soil creatures release into the soil. Residues and by-products provide nutrients and energy to soil organisms and help hold soil particles together in clumps. Compost produced in the garden is often a combination of these components.
Gardeners rarely have soil composed of only clay, sand, or silt. However, the relative proportions of these materials suggest whether or not a particular garden site needs to be amended to provide better soil structure and if so how. Soil with a good mix of clay, sand, silt, and organic material is called loam. Loam is the gardener’s goal.
The liquid component of soil is called the soil solution and is composed of water and dissolved materials. The soil solution is where plants obtain nutrients. An important aspect of soil solution is pH.
pH is a scale from 0 to 14 that indicates whether a soil solution is neutral, acidic, or basic (alkaline). A neutral pH reading is 7. Acidic readings are less than 7, while basic readings are greater than 7. Most garden vegetables prefer a pH between 5.5 and 7.5, but it’s a good idea to consult a pH chart regarding the specific plants you intend to grow because, to a plant, that two point spread is sometimes pretty significant. Home pH test kits are available, but pH is also tested as a part of a regular soil test which would also provide information about nutrient content, as well as amendment recommendations. Some garden centers, and all extension agencies, can either provide you with a soil test or help you get one.
The air in soil is pretty similar to atmospheric air, with the exception that soil air contains a bit higher concentration of carbon dioxide and a very slightly lower concentration of oxygen. One of the main reasons for this difference is that the soil is teeming with living creatures that take in oxygen and release carbon dioxide. Because oxygen moves into the soil and carbon dioxide moves out of the soil through tiny pores, the exchange happens somewhat slower than the conversion taking place in the soil. Plants adapted to growing on well-drained soil also require oxygen for their roots to survive.
Why do I need to know this?
Gardeners need at least a basic understanding of the components and workings of the soil because soil is plants’ life support system and every change humans make to the soil affects the health and productivity of the soil and the organisms dependent upon it. Humans are one of those dependent organisms. Most foods humans eat can be traced back to the soil, with the partial exception of things like seafood (although it’s all connected, thanks to nifty things like the water cycle and erosion).
Next up… More on soil tests and composting…
This post is my entry for today's Fight Back Friday hosted by Food Renegade.
This post is my entry for today's Fight Back Friday hosted by Food Renegade.