What Is Soil?
Soil is made up of organic matter as well as rock particles, clay and minerals. It is generally understood as the top layer of the earth’s ground covering. When we speak of soil for gardening and planting, there are several specific types. Planting soil can be upgraded or amended to better accommodate certain kinds of plants. Some varieties of soil hold more water while other types are better for promoting drainage or have different mineral compositions. Pairing plants with their ideal soil type will help ensure gardening success.
Layers of Soil
The ground that we walk on is made up of five main layers of soil. These include the following (starting from the top):
- Organic Surface or Humus Layer: The soil surface that your feet touch. This layer is composed largely of decaying organic matter.
- Surface Soil or Topsoil: This layer goes down approximately 5 to 10 inches and contains the most minerals and microorganisms needed for plant root growth and health, as well as the most organic matter (other than the humus layer). This is the layer that most concerns gardeners.
- Subsoil: This layer of soil is composed of some organic matter, as well as iron and clay.
- Substratum: This layer is located below the subsoil, and it’s the most highly compacted.
- Bedrock: The bottom-most layer composed mainly of hard rock.
Types of Soil
There are three primary types of soil, determined by the amount of clay, silt or sand particles present.
- Clay soil is used in pottery and ceramics and contains a high percentage of clay and silt. The particles are small and cling together, holding water and nutrients well. However, clay soil is susceptible to compaction, which can make it difficult for the moisture and nutrients to reach plant roots and for roots to penetrate the soil. You can identify clay soil by its sticky, slippery feel and its tendency to cling to garden tools.
- Sandy soil is composed of larger, coarser particles. It drains quickly, but it isn't effective at holding moisture and nutrients. This type of soil feels rough and doesn't hold together well. However, cacti and succulents (particularly those grown in pots) grow best in a sandier soil mix.
- Loam has a good balance of clay, silt, sand and organic material. It's the best type of soil for gardening, drainage and retention of moisture and nutrients. Loam holds its shape when you squeeze it lightly and is easier to dig than other types.
The soil in your landscape will likely not be ideal initially, but soil amendments can help you improve it, allowing your plants to thrive.
Soil Amendments
To amend soil means to improve it with additional materials. These soil amendments or soil conditioners improve the physical nature of soil. They reduce compaction, aerating the soil to allow water and nutrients to more easily move through it and reach plant roots. Some soil amendments also add nutrients to the soil, help retain moisture and help maintain the correct pH balance.
Common soil amendments include:
- Garden soil is a soil amendment for in-ground use rather than use in containers or raised beds. Work it into your existing soil to improve aeration and moisture retention. Garden soil may also include fertilizer to feed your plants.
- Sphagnum peat moss absorbs water, slowly releasing it for use by plant roots. It lightens clay soil, providing aeration, and adds mass to sandy soil, helping prevent the leaching of nutrients. Don't confuse sphagnum peat moss with decorative sphagnum moss, which is primarily a floral design product.
- Composted manure is an odorless farm byproduct. In addition to improving aeration and moisture retention, it enriches the soil. Dehydrated manure is a similar product that contains less moisture.
- Mushroom compost is a mixture of straw, peat moss and other organic components, formulated for use in commercial mushroom production. The mixture is used for one round of growing and then packaged as an amendment for the home garden.
- Topsoil is intended for repairs to your garden or your lawn – filling holes, leveling ground, correcting eroded areas, etc. Don’t use topsoil for planting or potting unless you blend it with other amendments such as composted manure, vermiculite, perlite, peat moss and aged wood bark.
Additional amendments include:
- Lime - raises soil pH, reducing acidity
- Sulfur - lowers soil pH, increasing acidity
- Gypsum - improves aeration of compacted soil, helping it drain more efficiently
- Perlite - improves aeration and drainage
- Vermiculite - improves moisture retention and aeration
Many gardeners choose to make their own soil amendment by composting. Read Making Compost for tips on creating this nutrient-rich amendment.
Potting Mix
Potting mix, sometimes called potting soil, is the best choice for potting or repotting and for use with container plants. Potting mixes contain organic matter as well as elements for aeration and moisture retention, such as perlite and vermiculite. Potting mix also supports good root growth. Some potting mixes contain fertilizer. You can find mixes designed for use in raised beds as well as mixes for specific plants, including African violets, orchids and cacti. For tips, see Choosing the Best Potting Soil for Container Gardens and Raised Beds.
Soil Calculator
Use our Mulch and Soil Calculator to estimate the amount of soil you need for your planting or lawn project. You can buy soil in bags and, depending on your location, some types of soil and amendments may also be available in bulk. The area you can cover with the soil varies based on the desired depth and the type of soil.
Testing Your Soil
You can determine some qualities of your soil by observing its consistency and how well it drains, but a soil test tells you less obvious information. Test your soil with a home kit or send soil samples to your local Cooperative Extension office. Results tell you what nutrients your soil needs as well as the soil pH level, allowing you to adjust for your plants. See Test and Improve Your Soil for more information on soil testing.