The Origins of Soil
Ancient soils were formed almost exactly like today’s soils are: Parent materials, like debris and rocks, were broken down by microorganisms, worms, insects, burrowing animals and even other plants.
Rainfall, temperature and topography also played a part in the outcome, so the broken-down results differed depending on the climate and location—just like today. As life grew and developed, organic matter mixed with the resulting dark-colored surface to form what we now call top soil. That process continues today right in your backyard.
The Role and Function of Soil
Soil makes up the earth’s surface and is where plants develop and grow. It’s composed of both organic and inorganic materials. The main purpose of soil is to act as a reservoir for plants and to provide water and nutrients whenever needed. Plants anchor themselves into the soil via their root systems, which in turn helps the soil remain in place instead of eroding away.
Unfortunately, in most urban landscapes, this natural version of top soil is no longer present. It’s either been taken from the land by construction companies and weather, or it’s been disturbed, inverted and compacted during construction. Inversion moves the subsoils with less nutrients and value to the surface. Turf grass that takes root in this type of subpar soil requires dedicated, proper lawn maintenance to reach its full potential.
The Properties of Soil
There are four principal components of soil, each with its own important role to play:
- Organic matter
Minerals in Soil
The largest percentage of soil (approximately 50%) is actually made up of rock that’s been broken down over a long period of time by water, wind and chemical processes—this is how soil gets its minerals. Depending on the components of the parent material (rock), the components of the soil will have large or small portions and sizes of sand, silt and clay.
The exact percentages and composites of these three components tell us whether soil is heavy, light, poor, good, clay, sandy or loamy (balanced). Each component has its pros and cons:
- Sand. Physically the largest particle in soil—it’s visible to the naked eye—sand allows water and air to travel freely through the soil. Despite this water-friendly aspect, sandy soils have the lowest capacity for holding water and nutrients. They can be found along coastal regions and localized areas such as the Great Lakes.
- Clay. Clay is physically the smallest of the three particles. These soils are smooth when dry and extremely sticky when wet. Soils with a high percentage of clay are considered heavy soils. They suffer from compaction, but they do hold an enormous amount of water and nutrients, rendering them capable of long-term turf grass sustenance. However, once dried out, clay soils are difficult to remoisturize.
- Silt. Coming in right between clay and sand in size, silt soil feels powdery and smooth. Even when wet, silty soils retain this smooth texture without getting sticky, giving them a higher capacity to hold water than sand. But due to the smallish particles of silt, air and water movement are reduced within the soils.
Organic Matter in Soil
The organic matter found in soil is made up of both decaying and living animal and plant matter. If anything in your soil was ever alive—or still is—it’s organic matter. Despite making up a tiny fraction of soil, organic matter is one of its most critical components.
Organic matter is crucial not only because it stores and releases water and nutrients to turf grass, but it also boosts soil structure and improves gas exchange and water infiltration, making it a vital helping hand for lawn maintenance.
As the fibrous roots of turf grass mature, decay and regrow, more organic matter is formed in the soil. Thus, the simple act of maintaining a healthy lawn builds healthier soil over time by creating its own organic matter.
Also adding another layer of organic matter to soil, the organisms that live and interact with your lawn soil on a daily basis yield great benefits for your lawn. These include bacteria, fungi, springtails and earthworms. Earthworms are especially beneficial, improving aeration, drainage and soil structure with their activity.
These soil organisms also help release stored nutrients from the soil so your turf grass can access them easily. Unfortunately, if your lawn is rooted in the disturbed subsoil so common to urban areas, it’s also probably lacking the proper populations of soil organisms to produce these lawn benefits.
For comprehensive lawn maintenance plans, you can figure out how much organic matter is in your turf soil. First, determine how much of it was present when the grass was established. Then add in the yearly decay rate of plant and animal materials.
Water and Air in the Soil
The way that water and air move through—and are held by—soil depends on the soil’s texture. There are two major concepts that should help you in both soil selection and ongoing lawn maintenance: soil porosity and soil permeability.
- Soil porosity. Soil porosity is a measure of the amount of pore space in your soil. Pore space is where water and air are found in soil. Essentially, all the space inside of soil that isn’t mineral or organic matter is pore space. Turf grass reaches its full potential when growing in soil with a balanced mixture of small and large pores (made from the ideal blend of sand, silt and clay), leaving enough space for water and air to move freely throughout the soil without waterlogging or drying out.
- Soil permeability. This measures the ability of water to flow through soil pores. Sandy soils contain many large pores, meaning water rapidly flows through. These soils never get waterlogged but can dry out quickly because they have a low capacity to hold water. On the other hand, clay soils contain tons of small pores, so they hold more water. Unfortunately, this also leaves them prone to waterlogging, thus reducing microbial activity and airflow.