Soil seems simple at first—it's that dark-colored stuff we stick seeds and plants in. But it's actually the most important factor in gardening success (assuming the sun doesn't go out). Soil is where savvy gardeners focus a lot of effort. Healthy soil supports healthy plants.

As we think more about soil, we encounter concepts like drainage, soil organic matter, and microbial life, which are critical to our success as gardeners. A few primary functions we know but don't always think about are also part of any discussion about soil.

Characteristics of Garden Soil

We can describe a dog by size, breed, color, or temperament. We can describe our garden soil the same way, in this case by drainage (well-drained vs. poorly drained), levels of organic material in the soil, and even how active and healthy our soil microbiome is. Fortunately, if you have adequate drainage and soil organic material, an active soil microbial environment will soon follow. 

Drainage 

We often discuss drainage in gardening. Whether planting vegetables, flowers, or even trees, most garden plants need soil with good drainage. But what does that mean? You've likely seen water ponding in the low spot of a field after a rain. If the water doesn't quickly seep into the ground and seemingly disappear, that site has poor drainage. It's one of several reasons gardeners try to avoid low spots when selecting a site for a new bed.

Drainage, at its root, is the ability of water to move through the soil profile instead of staying in the top layer of soil, causing problems. While some water is held by soil particles and will later be used by plants, excess water needs a place to go. If it can't drain, water sits there, creating muddy, wet conditions unsuitable for most plants. Drainage is affected by the soil composition, the amount of organic matter present, the elevation and position in the landscape, and often by human activity.

Too much water in the soil from poor drainage means there's little or no oxygen getting down to the roots of the plants. Like us, plant roots need oxygen to respirate, burning the carbohydrates formed during photosynthesis to produce energy to carry out their functions. If there's too much water in the soil, they can't get oxygen and they drown. If you've heard a plant doesn't like wet feet, that's a saying for not tolerating soggy conditions. Some wetland plants have adapted to grow in saturated soils, but most vegetables and flowers we grow in our gardens have not. 

Factors affecting Drainage

The soil's proportion of sand, silt, and clay particles greatly influences how fast it drains. Clay soils are well known for their poor drainage. Clay particles are very small, meaning that the surface area of a given soil volume is very high (lots of sites for water droplets to attach), and channels between soil aggregates are tiny. Clay is so good at restricting the free movement of water that it's used as a liner in ponds and dams.

Sand particles are large, so the channels and paths between sand particles are wider, and the surface area of the soil particles in the volume is much lower. You've watched sand on the beach seem to drain in a matter of seconds after every wave recedes. Loamy soil is in the middle drainage wise. Few soils are all one particle type or another; most are a mix. The dominant soil particle type will have a large effect on the rate of drainage in your garden. Sandy soils will drain quickly, all else held equally, and soil with a high percentage of clay will drain more slowly. 

Topography and the position of a garden in the landscape can greatly affect drainage. A garden bed located partway up a slope will likely experience better drainage than a site at the bottom. For many of us, our yards are fairly level and free from this influence. For yards with topography, it's one of the important aspects of planning a garden layout. 

Organic matter, which we'll discuss next, is important for drainage and nutrient storage. Higher organic matter levels facilitate increased water storage available to plants and increase the flow of excess unstored water through the soil profile. Excess drainage in sandy soils can be slowed (improved in this case) by adding organic material to the soil. Poorly drained clay soils can be improved (better drainage) by working in copious amounts of organic matter. It's part of the reason gardeners love adding compost—it's virtually a cure-all for garden problems. 

Soil Organic Matter 

Hang around gardening folks long enough, and you'll hear the term soil organic matter. In this case, organic doesn't mean without synthetic chemicals. When we talk about organic material in soil, it's the remains of things that once were living (but now aren't). That organic matter can be partially or mostly decomposed, or fresh and easily identified. 

For example, pine needles that fell from the trees to the forest floor five years ago might be broken down to humus and duff, but those that fell two years ago are still recognizable. Both count as soil organic matter if they've been worked down into the soil profile by weather, animals, insects, or even people. 

Soil organic matter is important for retaining and storing water and nutrients in the soil, improving soil structure which aids water movement and air exchange, and providing more nutrients as it breaks down. Adding compost is the gardener's favorite method of increasing soil organic matter, and a garden soil high in organic matter will be loose, fertile, and productive. Adding a little organic matter annually to your garden will pay dividends for years.

Soil Microbial Life

Mars and the moon are full of weathered rock material, which resembles soil, but it wouldn't do a gardener much good. While the minerals would be present, there is no microbial life in that soil, and that's where the magic happens. 

The soil under our feet is absolutely crawling and alive with billions (and trillions, and whatever comes after that) of microorganisms: bacteria, fungi, insects, protozoa, actinomycetes, and more. New gardeners are often shocked to learn that we want bacteria in our garden soil. We often revolt, thinking of the "germs" that inspired our parents to chide us to wash our hands. But in fact, uncountable numbers of helpful bacteria are present in healthy soil, performing their functions and enhancing the conditions for plants to live. Some even partner with plants, swapping nutrients in a system beneficial to both. 

As gardeners, we want to encourage this plethora of life beneath the surface by eliminating the use of harsh chemicals, opting to reduce tillage when possible, sheltering the soil surface with mulch, and adding organic matter (food for the little folks) to the soil. 

Primary Functions of Soil in the Garden 

While it seems obvious that soil has functions in the garden, defining them can help us better understand what we are working toward and how our plants interact with the soil we plopped them in.

Water and Gas Exchange

We know plants take up water from the soil with their roots and that water is held in the soil in tiny channels and attached to soil particles. A bucket full of rocks (with a hole in the bottom) won't hold much water, but a bucket full of good garden soil does. Those channels and pathways between soil particles, including some beautiful ones left by friendly garden worms, also allow air exchange. 

While we commonly think of plants as breathing in carbon dioxide and breathing out oxygen, that's not the only gas game going. Plant roots need oxygen just like the rest of the plant. Soil with good structure supports the movement of air to plant roots. If the soil is too compacted or waterlogged, they can't get it. 

Physical Support

Most plants grow in the soil (some are aerophytes, and of course, there is hydroponics). As schoolchildren, we learn to draw the roots below the soil surface and the leaves, stems, and flowers up above in the sunlight. Those roots anchor the plant, holding it up and providing a sturdy base while the plant grows upwards against gravity, sends out branches, leans over to grab a bit more sunlight, or sways in the wind. 

If you've seen a tree that tipped over in the wind, you may have noticed a giant rootball ripped up from the ground—there wasn't enough support to handle the load. The same can happen in the garden; tall plants like sunflowers can run out of support and flop over. The artificially maintained loose and weed-free soil in our gardens doesn't provide as much support for a tall plant, which is why we sometimes need to stake them. 

Providing Nutrients and Minerals

We often call fertilizer plant food, even though plants are autotrophs and make their own food. Fertilizers are minerals and nutrients that plants use to build new cell walls, form enzymes and hormones, and perform hundreds of other functions. Plants growing in soil high in organic matter and fertility don't need fertilizing, but many times, we are withdrawing more from the soil than gets replaced naturally and need to add nutrients back. 

Nutrients and minerals are stored in the soil as readily plant-available cations and anions and as larger chunks of material waiting to be broken down by weathering or soil microbes. Soil low in these minerals and nutrients is said to be poor, while a sufficient amount to support vigorous plant growth is said to be rich. 

The soil in our garden is a complex system. When we nudge it in the right direction, the rewards are great. Paying attention to drainage and organic material and encouraging our soil microbial friends will help our soil better provide those basic functions of water and gas exchange, support, and nourishment necessary for a flourishing garden.