Exploring Different Soil Types
Soil Types in Our Yards
The type of soil in your yard makes a big difference in what kind of plants and trees are able to grow. Soil varies in how well it drains, which nutrients are present and available, and how acidic or alkaline it is. The properties of soil vary greatly depending on their location and the types of greenery that are there, meaning it can even vary from yard to yard. When it comes to the soil in our yards, there are lots of things we can do to encourage good-quality soil that nurtures trees to grow big and strong.
The Different Types of Soil
There are a few main components of soil, and the particular composition influences how the soil will interact with plants. These materials include organic matter (like decaying leaves and insects), biotic life (like bacteria, worms, and other decomposers), and granular stone components that provide the soil with its long-term structure (like clay and sand, which are used to classify the different types of soil). The main soil types include clay, sand, silt, and loam, but they may also include gravel and chalk, as well as peat (partially decayed organic matter).
Properties of Each Soil Type
The texture of soil refers to the combination of grain sizes of the rock material. This material affects the ability of soil to drain and/or retain moisture, which has an effect on how plants grow. When plants are able to grow in a given soil, that material eventually decomposes and builds the soil’s bulk, containing nutrients and affecting the pH level of the ground (which, in turn, affects the availability of the nutrients that may be present at any given time). A healthy soil also contains fungi, bacteria, worms, and other decomposers that help build the topsoil, make nutrients available, and provide solid bulk for the soil to retain moisture and remain in place over the long term.
The soil types have properties specific to their texture:
Clay – Clay is made of the smallest stone particles, which stick together tightly. Clay-heavy soil holds lots of moisture, but it can also dry out and harden. In either case, since clay is so dense, moisture isn’t absorbed quickly, making it the most impermeable soil type.
Silt – Silt grains are larger and looser than clay, but not quite as large as sand. Silt absorbs water well, but due to its small grain size, wet silt doesn’t hold its form for very long. However, silt is nutrient-rich and drains better than clay soil.
Sand – Sandy soil has larger grains than silt, but they are smaller than pebbles. Sand grains are easily felt when you run this soil through your fingers, and the coarse material creates a fast-draining soil that doesn’t hold moisture very well.
Loam – Loamy soil has the best mix of clay, silt, and sand. The clay will hold on to moisture, but the silt and sand help break up the clay so that excess water can drain. On the other hand, the clay holds the soil together, and the ground can maintain its form much better than sand- or silt-dominant soil.
Gravel – Soil with lots of gravel, pebbles, or other large stones will drain very quickly, and only plants with the deepest roots will thrive. If your yard has lots of gravel in it, building the organic material in the topsoil will help the ground retain more moisture and nutrients over time.
Chalk – Chalky soil occurs over limestone bedrock, which is present across Nashville. The bedrock that underlies a location contributes the stone components of the topsoil, since the topsoil is formed over and rests on the bedrock itself. Limestone and chalk are alkaline due to high calcium levels, which can make it difficult for plants to get the iron and other nutrients they need.
Peat – In swampy areas that remain wet for most of the year, the highly acidic conditions may preserve the plant material due to a lack of decomposers that are able to survive in the acidic ground. This nutrient-rich material is sometimes applied as a soil amendment to add drainage and moisture-retention capacity, acidity, nitrogen, carbon, and other nutrients to the soil.
Soil may also be classified according to its stability or firmness. These soil types are important for classifying productivity as well as suitability for excavation or gradient leveling in construction.
Type A Soil – Type A soil classification is the most productive and stable soil type. It has high organic matter content and a moderate amount of clay, allowing it to hold moisture and keep its form on sloping land.
Type B Soil – Type B soil isn’t as productive as type A because it has less organic matter and more clay, allowing it to compact much more easily. Alternatively, it may have more silt, which doesn’t hold its form very well. This type of soil can’t hold a steep slope.
Type C Soil – Type C soil is the least productive soil, with the lowest level of organic material and the coarsest material, like sand or gravel, which doesn’t hold on to moisture well at all. Neither does it maintain a firm structure, requiring a wide slope if dug into.
Benefits of Different Soil Types for Trees
The amount of drainage, nutrients, and moisture retention of each soil type impacts the types of plants that grow in that type of ground. Sandy soil, for example, which is usually found near bodies of water, drains quickly so that the topsoil doesn’t become waterlogged. Grasses, bushes, and trees that grow well in sandy soil have deeper roots to reach deeper into the ground for water since the topsoil dries out quickly. Trees that grow well in sandy soil include the red oak, red cedar, persimmon, and ginkgo.
Clay-heavy soils, on the other hand, retain lots of water and don’t drain very fast. This results in moisture-rich ground that may be on the more acidic side. Trees that grow well in moist, slightly acidic soil include white oak, maple, elm, poplar, and bald cypress.
Loamy soil is the best of all worlds, and most of the trees mentioned above can grow in this type of soil since it has the ideal mixture of moisture retention and healthy drainage. These conditions also make loam the most fertile type of soil because it can host both vegetation and the decomposers in the soil that process organic material, make nutrients available for the plants with a balanced pH, and provide the right balance of moisture and oxygen that the roots of plants need to survive. Of course, the topsoil’s actual composition and conditions will vary from property to property based on the present materials and the managerial strategies of the land parcel’s caretakers.
Tips and Tricks for Working With Different Soil Types
Whether you have clay-heavy or too-sandy soil, there are actions you can take to assess and improve the texture and fertility of your soil to encourage the healthiest growing environment for the plants in your yard.
Examine the Soil – Looking at and feeling the soil to know whether it’s too compacted, too dry, too wet, or too pale is the first step in assessing what your yard might need. This basic assessment can easily tell you whether your soil is sandy or clay-heavy by its color and texture. Take a look at your trees, grass, and other vegetation; if plants don’t seem to be growing well in a particular location, it may mean the soil could use some attention.
Test the Soil – If you’re noticing your trees, grass, or other plants are yellowing, browning, wilting, or generally looking weak, the pH or nutrient levels may be out of balance. Using a moisture probe, conducting a home pH and nutrient test, or sending in a soil sample to your local University of Tennessee extension office can give you a reading on your soil that you can’t see with the naked eye or feel with your hands.
Amend the Soil – Once you have an idea of the material composition, pH, and nutrient levels, you can take steps to add organic materials to the topsoil that will aerate, provide bulk, replenish nutrients, and balance the acidity or alkalinity of the ground in favor of your plants. Since plants absorb nutrients from the surrounding ground, they must be replenished naturally or with amendments. Compost is one of the best soil amendments you can add to build the quality and quantity of soil.
Aerate the Soil – Clay-heavy soils may need to be aerated by adding some sand and/or fresh compost to the topsoil to break up the clay material and introduce porous space. Even for loamy types of soil, if the ground becomes compacted from the mower or heavy foot traffic, roots have a difficult time getting the moisture, oxygen, and nutrients they need. Water may have a difficult time penetrating the ground and lead to pooling or runoff. Adding compost yearly can help build well-aerated soil that’s heavy in insect life (who tunnel and move soil around regularly) without using an aeration tool that can damage roots. Yard caretakers should also avoid using heavy mowers over tree roots to keep the soil from compacting (and to protect the roots and bark).
Apply Mulch – Both sandy soil and clay-heavy soils need organic material to fill in the spaces between the grains. Mulches like woodchips and compost build the soil over time, breaking up clay soil and filling in sandy soil with solid organic materials, humic acid, and woody materials that provide structure and nutrients to the ground. An application of mulch in the fall around your trees will provide insulation in the cool months, breaking down slowly over the seasons to release nutrients to the tree’s roots throughout the following year. Over several years, regular mulch applications will build the topsoil and support your tree’s healthy growth.
Soil Quality and Trees
In nature, only trees and other plants that are suited to certain soil conditions will be able to grow there, but they’ll build up the soil by dropping leaves, adding to the soil bulk and cycling the nutrients they need to survive. On our home properties, however, we can make decisions to amend the soil to make it more suitable for a wider range of plants to grow. The best way to improve your soil is by doing it slowly and working with what you have as a base.
If you have sandy ground, choosing a tree that likes sandy soil will work best even if you improve it to a more loamy type. Similarly, a tree that likes clay soil will do best in a clay-heavy yard, even when you amend the soil to drain a bit better. Working with nature’s process, rather than against it, results in the best and most sustainable ecological outcomes. With good maintenance habits, however, improvements can always be made, and since nature is constantly changing, expanding, and contracting, there’s always something you can do to intervene and support your yard’s greenery!
Once you know your yard’s soil type and whether any amendments are required to support fertility, you can choose one or more species of appropriate trees to plant and hold that soil in place. Browse the wide selection of trees that we have available through the Nashville Tree Conservation Corps’ tree sale, and consider volunteering with us to support the city’s growing canopy. Subscribe to our newsletter to stay informed on the different aspects of tree maintenance in residential and city settings to keep Nashville’s urban forest robust and resilient!