What Plants Need: Soil

Soil

Gardeners spend lots of time digging, weeding, fertilizing, transplanting and working at a host of other chores. Sometimes it’s helpful to take a moment to think about what we’re doing. After all, what are we trying to accomplish by gardening in the first place? We all have our personal reasons for getting a little dirt under our nails now and then, but if any of us are successful, it’s because we are doing a pretty decent job of replicating the conditions that plants require for survival. These conditions include three basic ingredients that gardeners can control: soil, water and light. Of these, perhaps the most naturally variable is soil.

What is Soil?

Soil is made of mineral fragments, organic matter, air space and water. The mineral fragments, from the tiniest clay particles to the largest stones, are pieces of local geologic formations that have been weathered away through the forces of wind, water, ice and even rugged life forms such as lichens and bacteria that live on the surface of rocks.

Organic matter is slowly added to the weathered minerals. Colonizer plant species such as lichens, mosses, grasses and ferns are the first to arrive on barren sand, clay and even in rock crevasses. As generations of these colonizers live and die, their decayed remains, as well as the remains of microbes, insects, fungi and other decomposers are added to the soil horizon by successive generations, and new introductions are added by wind, water or deposited by passersby. The roots of new plants reach ever deeper into the soil to extract nutrients, and by doing so create channels for moisture, air and other organisms to utilize.

As the mineral and organic components interact physically, metabolically and chemically, over time increasingly “needy” types of plants can make their homes there. Early colonizers are adapted to thrive in lean environments with low fertility and frequent drought. When the soil components mellow to a suitable texture, depth and richness of fertility, shrubs and trees will ultimately gain introduction. It is important to note, however, that the soil building process is ongoing; that is, both the weathering of rock and the addition of organic matter happen without cease.

In The Garden

If you have kept a potted plant or container garden for any amount of time, you may have noticed that the texture of the potting mix changes drastically within a year or so, from coarse and crumbly to fine and dusty. Most potting soil is considered “soilless,” meaning that it is comprised of nearly 100% sterilized organic matter which rapidly decomposes when plants are growing in it. That is what happens to the organic matter in garden soil as well. The lesson here is that the organic content of the soil is volatile and must be replenished regularly, especially in warm, moist regions. There are several ways to accomplish this.

• Use bagged mulch and soil amendments such as peat moss, manure and the like, purchased from the local garden center.
• Make your own compost with weeds, garden waste and kitchen scraps.
• Use ground up leaves and grass clippings as mulch.
• Plant “green manure” cover crops in otherwise inactive garden beds.

The organic matter in garden soil is the medium that supports beneficial microbes that keep plants strong and healthy as they create more soil. While it’s not the only factor, a high percentage of organic content sets the stage for good things to happen in the soil.

Mineral content is far slower to deteriorate, although it eventually does. Lime is a good example. Gardeners with acidic soil, like the red clay where I live, must apply lime periodically to make the soil less acidic for many types of plants to thrive. The pH adjusting effect of ground up limestone goes away after a couple of years, and it must be reapplied. The same is true for other elements such as iron, sulphur, magnesium and others. Soil testing determines the levels of these nutrients present in the garden. Local quarry operations are good sources for rock dust, a byproduct of stone grinding operations, which may be used to replenish depleted mineral levels.

Take Away This

The gardener’s role in soil building is to help it happen more quickly and efficiently, because we tend to desire a greater density of plants and more consistency than nature generally offers. The quick answer: monitor the organic and mineral content of your soil and supplement both regularly. Also, learn to make great compost. It will save you lots of money while growing beautiful, healthy plants.

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