Whether you’re a commercial farmer or hobbyist gardener, having a basic understanding of the different types of soil is crucial for ensuring your plants thrive.
Soil can be categorized into many different types — each with its own characteristics, limitations, and benefits — but how many main categories there are depends on who you ask.
What Is Soil?
According to the Soil Science Society of America, soil is “the unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.”
Soil is also formally defined as the surface matter that’s been subjected to environmental factors of climate, macro- and microorganisms, and relief conditioning.
Top soil, by definition, is the upper, outermost layer of soil. As such, topsoil is usually the top 5 to 10 inches of surface dirt and is composed of organic matter, water, air, and mineral particles.
Potting soil is a type of soil mixture of peat, loam, sand, and nutrients that help vegetables, herbs, fruit, and plants grow to their maximum potential.
Commercial potting soil is specifically blended to reduce the risk of plant-borne diseases, but certain mixtures have caused human illnesses due to the chemicals and pesticides used in the blends. Organic potting soils have become a popular alternative as more people have moved away from chemical-laden gardening solutions.
Soil performs seven critical functions in an ecosystem:
- Soil is a growth medium for plants.
- Soil modifies the atmosphere by absorbing and emitting gases and dust.
- Soil provides a habitat for animals and organisms.
- Soil holds, releases, absorbs, purifies, and alters the earth’s water.
- Soil processes recycled nutrients such as carbon.
- Soil acts as an engineering media for constructing roadbeds, foundations, buildings, dams, and other manmade structures.
- Soil serves as a living filter to clean water before it moves into an aquifer.
Types of Soil
Soil can be categorized into three basic types:
These three types of soil are considered soil separates. They are divided by particle size with clay particles being the smallest and sand being the largest. Sand particles are visible to the naked eye. Viewing individual clay particles, however, requires an electron microscope.
To make the topic even more interesting, scientists classify soil into 12 types of textures. These textural classes are:
- Loamy sand.
- Sandy loam.
- Sandy clay loam.
- Sandy clay.
- Clay loam.
- Silty clay.
- Silty clay loam.
- Silt loam.
As you can see, these 12 textures are simply a combination of the three overall soil types: Sand, silt, and clay. Each soil profile contains varying amounts of each.
But what about loam? Loam, in fact, is a soil composed of mostly silt, sand, and a smaller amount of clay, making it just another combination of the main three soil types. The different soil textures are often represented in a texture triangle to make visualizing the subtle differences easier.
Of course, there are many types of soil that go beyond these categories. The tundra soil type, for example, is a unique soil created from the ground’s freezing and thawing.
During the warmer months, water accumulates underground and later freezes, driving the soil upward. Permafrost is the thick subsurface soil layer that remains frozen year-round.
Even though permafrost and tundra soil act differently than soils in other parts of the world, they still consist of a mixture of soil combined with rock particles and water (or ice, in this case).
Why Is Weathering Important to Soil?
Weathering refers to the set of processes operating at or near the earth’s surface that break down preexisting rock and minerals. There are two possible outcomes of weathering.
Either the source material is broken into smaller pieces or the material is both broken into smaller pieces and changed into new material.
In other words, the weathering process literally creates soil. Although soil forms continuously through weathering, it also occurs slowly as rocks are gradually broken down through:
- Physical weathering.
- Chemical weathering.
- Biological weathering.
In physical weathering, rocks are broken down via temperature changes, frost, collisions, and other mechanical actions. Chemical weathering occurs when rocks experience a change in their chemical makeup, which can happen when they react with air, water, or chemicals.
Biological weathering refers to the breakdown of rocks by living things, including animals and plants. Burrowing animals, for instance, frequently help air and water get into the rock. Plant roots can also grow into rock cracks and make them split.
Soil properties vary depending on how long the soil has been weathered.
Maximizing Garden Soil
Whether you’re using potting soil or top soil for your garden, soil testing can help you achieve the ideal soil profile for growing. The main thing to keep in mind is the importance of phosphorus in soil.
Phosphorus is an important macro-element required for plant nutrition. It plays a key role in metabolic processes like energy transfer, photosynthesis, and carbohydrate breakdown. Unfortunately, phosphorus levels are quite low in natural soil, which is why many gardeners and farmers opt to fertilize their plants.
A soil test kit can improve your growing efforts in a number of ways. Aside from optimizing crop production, soil testing can help protect the environment from excess fertilizers and runoff, improve the soil’s nutritional balance, diagnose plant issues, and conserve energy by helping you use less fertilizer than you think you might need.
Even the most basic soil test kit can help you determine the soil’s pH level and fertility, allowing you to plan a solid nutrient management program.
Fresh tilled soil is also more conducive to making soil amendments. Tilling is a type of cultivation that prepares the soil about 8 to 10 inches deep. After tilling the soil, a gardener can mix the appropriate amount of potting soil or other organic amendments required to improve the soil and help it reach its desired nutritional profile.
In some cases, you may not want to disturb the earthworms and other microorganisms more than necessary, so you may choose a shallower tiller for the job.
But why till if you already have fertile soil? Over time, the soil becomes compacted due to rainfall, foot traffic, and other factors, making it harder for air and water to penetrate the topsoil. Tilling your garden on a regular basis also loosens the soil to give your root vegetables more room to spread.
Understanding the different types of soil can help you better manage your garden and choose plants that are ideal for your specific blend. Consider this knowledge as an important aspect of your gardening practice.