Building Great Garden Soil


By Skip Richter

Contributing Editor

I love soil! My affections for soil go way back to the smell of freshly turned earth in our family garden, and mashing red South Texas clay loam soil in my hands to form the walls and ceilings of a clay fort for plastic army soldiers to use as a refuge. Mashed marbles of soil when dried made awesome exploding slingshot ammo too! I learned then that mashing soil destroys its structure so that when it dries it will be almost as hard as concrete — very unlike the crumbly, friable structure of our family’s garden soil.

Many people refer to soil as dirt, a term more properly reserved for soil out of place, like the footprints made by bare feet traipsing across my mom’s kitchen floor after a day of fort-building and slingshot ammo production. Soil, on the other hand, is an amazing universe of minerals and living things, much of which we have hardly begun to understand. It is physics, chemistry and biology interacting to create the stuff life is made of. Man was formed out of the dust of the ground and all living things return to the soil eventually, a point made by Henry Wadsworth Longfellow in “A Psalm of Life,” when he states, “Dust thou art, to dust returnest.”

One of my favorite quotes is, “Despite his artistic pretensions, his sophistication and his many accomplishments, man owes his existence to a six-inch layer of topsoil and the fact that it rains.” There is a wealth of truth in this simple statement. Our food, fiber, building materials and much more come from the soil. Where the soil is favorable, plants and people thrive. Where it is limited, life struggles to exist. Let’s consider some of the aspects of soil and how we can build the soil in our gardens for maximum productivity, whether we’re growing flowers, vegetables, fruits or a landscape.

Soil Chemistry

The more we learn about the soil, the more we realize how much we don’t know. A soil class will teach you that soil is made up of minerals, water, air and organic matter. Scientists can elaborate on the chemical properties of various elements in the soil and how they interact to affect the soil’s physical characteristics such as structure.

Soil contains the nutrients that plants use to support growth, flowering and productivity. The macro nutrients (nitrogen, phosphorus and potassium) are needed in the largest quantities, while the secondary nutrients (calcium, magnesium and sulfur) are needed in smaller amounts. Micronutrients include iron, manganese, zinc, boron, copper, molybdenum and chlorine. These are as important to plant growth as the big three, but are only needed in very small amounts.

A deficiency of any one nutrient limits plant performance, sort of like the old adage that a chain is only as strong as its weakest link. But it isn’t just the nutrients considered essential for plant growth that matter. Cobalt, for example, may not be in fertilizer, but it is needed to form vitamin B-12. This is one reason why organic matter is an important way to enrich the soil; but I’ll get to that in a moment.

The ratio between nutrients is also important. An excess of one nutrient can cause problems with another. Phosphorus, for example, can tie up iron, making it unavailable to growing plants.

Soil pH is another important aspect of the soil. Soil pH is a measure of the hydrogen-ion content of the soil, but it is important because it affects the availability of various nutrients. High (alkaline) and low (acidic) pH levels can both result in nutrient deficiency symptoms in your garden even though those nutrients are present in the soil. This is why on high pH soils pecans often suffer from zinc deficiency, and new growth on azaleas and blueberries lacks a healthy, deep green color, even if these two nutrients are present in the soil.

If you haven’t had a soil test in the past few years, it would be helpful to have your soil tested now to determine its nutrient content and pH. This will enable you to make any needed adjustments so that the nutrients your plants need will be available to them for optimum growth and production. You can contact your County Extension office for help with having your soil tested or go online to where the forms and instructions are available.

While fertilizer additions are important, I’ll focus on general soil-building in this article and leave fertilizer and fertilizing practices for another time.

Soil Texture, Structure

Texture describes the size of particles that make up a particular type of soil. Sand, silt and clay are three basic texture types. Sand particles are like large boulders next to the tiny silt particles and the infinitely smaller clay particles. Sand drains well but doesn’t hold moisture or nutrients well. Clay on the other hand drains slowly but holds water and nutrients very well.

Structure is the way the soil particles group together. Some clays are very “massive,” forming solid, impervious layers or blocks that are as hard as concrete when dry. Such impervious clays may be great for lining a farm pond, but are not what we need in the garden.

Clay with good structure, on the other hand, has particles that group into clusters, giving it a loose structure that we refer to as “friable.” Grab a clod of clay with good structure and it will easily crumble in your hand. Clays with good structure are better aerated and drain better than those with poor structure. Roots thrive in soil with good structure.

Organic Matter

Organic matter is a small part of soil by percentage, but important nevertheless. It helps sand hold water and nutrients better and helps clay form better structure, thus improving the quality of both for better plant root growth. Our Texas soils are notoriously low (less than 1 percent) in organic-matter content. In our warm climate, if the soil is moist, organic matter decomposes into humus as fast as the cycle of nature can produce it. In order to increase our garden soil’s organic matter content, we need to add more on a regular basis. Organic matter also provides food for the microbes and larger creatures that live in the soil, which is perhaps its most important function.

The Life of the Soil

Microbes are the part of the soil that amazes me most. They are the life of the soil and do dramatic things to change the soil and help plants to thrive. Microbes and other forms of soil life include bacteria, actinomycetes, archaea, protozoa, nematodes (good ones) and fungi. Add to this arthropods, such as mites that shred organic matter, and earthworms that plow the soil, open up air channels and decompose organic matter and you have a lot of life in the soil. Let’s take a closer look at the wide cast of characters that make up the life of the soil and what they do to benefit our plants.

Bacteria are so tiny that 250,000 to 500,000 can fit inside the period at the end of this sentence. But they do a huge job as they feed on sugars and simple carbohydrates from plant material in the soil as well as plant-root exudates. Some bacteria take nitrogen from the air in the soil and turn it into a form plants can use. Others create antibiotic compounds that fight diseases. The triple antibiotic ointment we use on cuts and scrapes are produced from antibiotics made by several types of soil bacteria!

Actinomycetes are a type of bacteria that gives soil its wonderful “earthy” smell. They can break down cellulose, which comprises one half of a plant’s mass, and chitin, which is found in insect exoskeletons and the cell walls of fungi.

Archaea are very similar to bacteria in many ways. We are just beginning to learn about these microbes, but they also are part of nature’s recycling program that turns once-living plant and animal materials back into soil. By the way, remember the comment about cobalt and vitamin B-12? Neither plants nor animals can produce B-12, but only bacteria and archaea have the enzymes needed to synthesize this critical vitamin.

Protozoa are organisms that feed on bacteria, fungi and other protozoa. In fact, one protozoa can eat as many as 10,000 bacteria a day! Their waste materials release nutrients back into the soil solution, where plant roots can take it up. Scientists estimate that up to 80 percent of a plant’s nitrogen comes from protozoan waste.

When a gardener hears the term “nematodes,” they generally think of the bad kind that forms swollen areas on the roots of susceptible plants like okra and tomatoes. However, there are “good nematodes” that feed on bacteria, fungi and other organisms. A single teaspoon of garden soil may have 20 bacterial-feeding and 20 fungal-feeding nematodes. Like any life form, as they eat they also excrete and in the process they mineralize nutrients, making them available to plants.

Fungi are the primary decay agents in soil. They do the tough work that others can’t — breaking down complex organic compounds such as lignin, chitin, hair, fingernails and even bones! A teaspoon of rich garden soil can have several yards of fungal strands. Mycorrhizal fungi grow in association with plant roots, greatly expanding the roots’ ability to take in nutrients and moisture, while helping to defend the root against attack by disease organisms.

In addition to releasing the nutrients in dead plant matter, microbes also secrete substances that help glue soil particles together, forming better structure. Their activity produces organic acids that help dissolve soil minerals and release more nutrients into a nutrient-rich “soup” on the surface of soil particles. This creates a perfect environment for plant roots to grow and thrive. Regular additions of compost help moderate extremes in soil pH as well as lessen the effects of nutrient deficiencies and excesses.

A rich garden soil can have 75 pounds of living organism per 1,000 square feet. A handful of rich garden soil has more living organisms than there are people on the planet! That is a lot of creatures building the soil and enhancing root growth. Scientists are just scratching the surface of learning all the ways that various microbes enhance the soil for plants to thrive and become more productive. So now let’s consider how we can make the soil a place where good microbes can thrive.

Compost Builds Content

No single ingredient or fertilizer can make the positive difference in your garden soil that compost can. Nature was designed to make its own compost both in forests and grasslands. In a forest, the tree leaves and branches fall to the soil surface and slowly decompose back into the soil. This is a smorgasbord for microbes which work in concert with each other, breaking this dead plant matter down and building the soil in the process. Undisturbed forest soil is some of the richest soil around.

Approximately 75 percent of the nutrients that a tree took up during the growing season are in the fallen leaves. Thus these composted materials are perfect for replenishing nutrients back into the soil. So never let a leaf leave your property, and ask your neighbors for theirs, too! I am an avid leaf collector — by the bag full. It seems that I can never get enough leaves for mulching and composting over the course of a year.

Grasslands also build the soil. Individual grass roots live about a year. Then they die and decompose, leaving a channel through the soil and decomposing organic matter to fuel microbial growth. Consider the annual process of root growth, death and decomposition, and you can see why the soil in undisturbed grasslands becomes richer and richer over time.

In these natural settings microbial populations explode and organic matter is recycled rapidly, sustaining plant growth and health. We can learn how to build our garden soil from these natural systems so that it becomes a rich foundation to stimulate plant growth and production.

Building Soil

When we add organic matter to the soil we create the environment where microbes can do their thing. There are several ways to do this.

The fast track to rich soil is to simply add well-decomposed compost. Spread a couple of inches over the soil and work it in with a rototiller or spade. I have found that it is best to make such amendments a few weeks prior to planting to allow the soil time to mellow and settle in so plant growth will be better.

Keep in mind that it is best to work the soil when it is moist. If it is too dry you can rattle your teeth out banging a rototiller across a would-be garden spot in an effort to break up the soil. If, on the other hand, the soil is wet, rototilling will destroy the structure and be a very messy endeavor. Take advantage of dry periods in the fall or spring to prepare your soil for the next season. That way a rainy spell around planting time won’t delay your plans. If your soil is very dry, give it a good soaking and then wait a couple of days for the water to soak down and the surface to dry a bit. That way your soil will be in prime condition for spading, rototilling and incorporating organic matter and nutrients.

While it is best to mix well-decomposed organic matter into the soil there is another technique known as “sheet mulching” in which undecomposed materials are mixed into the soil and allowed to decompose prior to planting. You’ll need several weeks to a couple of months between sheet mulching and planting, depending on the materials you use, but it can be an easy way to build the soil. Lay down a layer of leaves about 2 or 3 inches thick. This works better if the leaves are shredded but you can also use whole leaves. Rototill them into the soil. Then lay down another layer of leaves and repeat the process.

Wet the entire area and finish by covering the area with a layer of mulch to hold in moisture and deter weeds. In a few weeks to a month or so the organic materials will be largely decomposed and the soil ready for planting without further rototilling. I like to use this in the late fall for a winter fallow period or in early summer for a summer fallow period. Other organic materials such as grass clippings, manure and rotted hay (best if shredded first) can also be used for sheet composting.

A third approach, the “constant mulch” system, mimics the forest floor. A layer of mulch is maintained around shrubs and trees or in perennial beds by adding more mulch to the surface as it decomposes. The surface will always look like dry leaves, but where the organic matter contacts the soil and conditions are moist the microbes will be actively decomposing the mulch into its nutrient components. This is a slow process but the easiest way to improve soil around growing plants.

Green manure crops are a fourth way to build soil. They are species grown to turn under the soil to increase the soil’s organic matter content. These include cereal rye, ryegrass, clover, vetch, sudangrass, mustard and southern peas. If you have soil areas that won’t be planted for a season, consider planting a green manure crop to build the soil during this period.

However you go about it, take time this year to build your soil by adding organic matter. Avoid working the soil when it is wet or stepping on planting beds so you don’t destroy the loose structure. Maintain moderate moisture to keep microbes thriving. With a little attention and care you can build a very rich, productive garden or landscape soil over time.

So if you are struggling with an unproductive garden, banging a rototiller over the concrete clay, or watching nutrients and water disappear into a lifeless sand, now is the time to turn it all around. Remember to start with a few inches of compost, nature’s living fertilizer and add another inch or so every time you transition from one planting season to another. You’ll find your gardens more healthy and productive, and gardening a whole lot more rewarding too. Everyone will think you just have a green thumb, but you (and your microbes) will know the real secret!

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