May we never refer to it as dirt again…

There are a couple ways to look at soil: one is as a static, inert growing medium and the other is as a dynamic, living environment of its own that affects the environment above ground. It is the latter context that scientists use mostly today. Understanding soil basics first facilitates better understanding of this much grander context.

World Soil Day is December 5, 2018

What is soil?

Unweathered geological material, mineral, or organic matter is the original source of soil, referred to as its “parent” material. Over time, and with the effects of climate and biological activity, the parent material breaks down to make up about half of the total mass of soil. The other half is made up of varying proportions of air and water. The specific qualities of a given soil—along with climate and surface features (slope or rock, for example)—determine what thrives in it.

It can take up to 200 years to produce
1 cm of soil.

According to the National Resources Conservation Service, there are over 1300 different soil types in Texas, and they vary widely throughout the state.

The General Soil Map of Texas  shown above, is the go-to for an overview of soil types. The Woodlands, for the most part, sits in area 51, which is a small portion of the Western Coastal Plain and Flatwoods region. This area covers about 16.1 million acres, dissected by many streams. It has many kinds of upland soils, which tend to be deep, light-colored, acid sands and loams over loamy and clayey subsoils that support pine-hardwood vegetation characterized by the ubiquitous loblolly pine.

Breaking it down

By simple observation, many clues can be gleaned about a particular soil’s properties.

Color. A soil’s color helps determine how much organic material it has, the various minerals present, and its ability to drain. The darker the soil, the more organic material it has. Darkness can also indicate slower drainage. Conversely, lighter soil is an indication that it’s lower in organic content and more highly leached.

Color can also give clues to the soil’s mineral content. Iron minerals are by far the greatest contributor to soil color variation, creating yellow, red, grey, black or brown hues.

Texture. Soil texture is important because it determines how well (or not) water drains through it and if it creates pockets of air. Texture is defined by the relative mix of three components:

• sand, being coarse;
• silt, being fine; and
• clay, being finest.

The degree of coarseness or fineness provides clues to soil’s productivity. The coarser the soil:

• the faster it drains;
• the less water and more air it holds;
• the faster it warms; and
• the more easily it can be worked or penetrated by roots.

Depth. More can be learned from the soil’s “horizons.” Horizons are simply layers of differing composition. If there are key things to note about horizons, it’s that the A horizon, or surface, is the most fertile and has the best structure of all the other horizons., making it vital for plant life. Its depth can vary between just 2 inches to more than 12 inches thick. The B horizon, or subsoil, dictates how well water drains. Its depth too, varies between 2 and 12 inches. A subsoil of mostly clay will impair drainage and root growth. And these issues only increase the more shallow the surface soil is above it.

If there’s good news for the gardener who has less-than-ideal conditions in surface soil and subsoil horizons, it’s that improvements can be made by adding organic material, especially compost.

The value of loam

The relative proportion of sand, silt and clay gives soil what is referred to as its class. If there is a “perfect” soil class, it would be what’s called loam. Generally, loam is made up of equal parts sand, silt, and clay.

The best soil texture for growing plants is
what is called “loam.”

Loam = (<52% sand) + (28-50% silt) + ( 7-27% clay)

Loam soils are best for plant growth because the different-sized particles leave spaces in the soil for air and water to flow and roots to penetrate. The roots then can feed on the minerals in the suspended water. It retains enough water to keep the soil moist, but its texture is porous, allowing water to flow through slowly enough for the plants to access it, but fast enough to keep soil from getting waterlogged.

Tiny air pockets in soil are critical to support the animals that live in the soil, like worms and many types of bacteria. And one of the ways that plants get air is by absorbing it through their roots. Without air at a plant’s roots it would suffocate.

Deep sands do not hold moisture well and are often infertile. Clay holds moisture better than sands and may be more fertile, but they tend to swell when they get wet, which may limit the movement of water and roots. Clay cracks when they dry and the clods become very hard and difficult to penetrate.

Soil is alive

The key to understating soil’s properties is that they can determine the life it holds. Soil is a dynamic, interconnected, living thing—there’s a whole universe of life underneath our feet. It’s a web of energy conversions that fuels and makes possible life above ground. How big and diverse that universe is, is an easy measurement of how healthy the soil is.

As a living thing, soil quality is referred to as its “health.” The healthier the soil, the more it can:

• Sustain plant and animal productivity and biodiversity;
• Maintain or improve air and water quality; and
• Support human health and habitation.

Healthy soil teams with life and supports its own food web as shown in the chart and illustration below, both from Soil Biology Primer, by Soil and Water Conservation Society.

Billions of bacteria, fungi, and other microbes that live within soil are the foundation of an elegant symbiotic ecosystem.  Healthy soil makes possible clean air and water, bountiful crops and forests, productive grazing land, diverse wildlife and beautiful landscapes.

Keep soil alive with organic material

Organic material in soil not only greatly improves its structural qualities it also helps create the right conditions for the life it supports. By helping maintain favorable temperature and moisture in the soil, earthworms, insects, bacteria, fungi and other organisms thrive. These in turn, break down the organic material into nutrients that make plant life flourish.

Up to half our household waste could be composted to nurture the soil.

Tips for maintaining healthy soil:

• Disturb it less
• Minimize compaction
• Diversify soil biota with plant diversity
• Keep roots growing in it year-round
• Keep it covered
• Compost, compost, compost

To learn more about soil, check out these great resources:

Healthy Soils Are… PDF series of fact sheets

Soil Biology Primer, by Soil and Water Conservation Society