Soil is nature’s pipeline for moving water. Compacted soil causes stormwater runoff, almost as much as a driveway or rooftop, due to the fact that very little water can be absorbed. Much of the soil in the St. Louis region is a compacted silt topsoil or exposed clay subsoil that may need amending and aeration. If your location was constructed less than five years ago, expect the soil to have little ability to absorb rainwater.
Options to Address Compacted Soils:
- Top-dress your planting beds with 1–3″ of well-aged compost to improve lightly to moderately compacted soils. Earthworms and other soil organisms will gradually move it down into the soil which will loosen the soil and make it more absorbent. Don't add too much compost if using a native plant palette since many native plants do not naturally grow in highly organic soils.
- Another option is to till the soil lightly and incorporate the compost into the soil.
- More extreme physical aeration may be necessary to repair highly compacted soils. Consider hiring a professional landscaper to do vertical mulching—drilling deep holes in your planting beds or around your trees and backfilling them with compost.
- You can purchase commercially available topsoil and apply it to the surface of your soil.
- Inorganic amendment options that will help restore the capacity of your soil to infiltrate water include adding calcined clay, and/or expanded gypsum. Incorporating sand can actually reduce the permeability of your soil and is not recommended.
- Replace turf grass with native plants and trees that have robust root structures. The deep and/or fibrous roots of these plants physically break up the soil, allowing oxygen, earthworms, and other soil organisms to follow, thus decompacting the soil.
- Add an organic shredded leaf or shredded hardwood mulch when establishing new plants and each year to further decompact your soil. Mulch should be added in late fall, after the first hard frost, in a 2–3″ layer around herbaceous plants and a 3–4″ layer around shrubs and trees. Some gardeners first add a 1–2″ layer of compost topped with a 1–2″ layer of mulch (50/50 ratio). Additional mulch may be added throughout the growing season in the event it is depleted or when planting new plants.
Over time, bacteria and fungi, which are fed by plant root secretions, break down organic matter in the soil. This process increases pore space in the soil by aggregating soil particles into small clumps, thus decompacting soil, preventing compaction from reoccurring, and improving soil porosity and permeability.
Inorganic Soil Amendments
Both calcined clay and expanded shale (hadite) are the result of high firing (at 1200 degrees). They form ceramic-like granules that work to physically break up clay soils. Because they are somewhat absorbent, the soil’s moisture holding capacity is also increased. Calcine clay is electrically active, aids in conserving nutrients in the soil and increases the porosity and permeability of soil. Unlike natural clay, it does not expand and contract as it becomes wetter or drier.
Organic Amendments
Organic amendments are natural materials of either plant or animal origin such as well-aged manures, compost, leaves, mulch etc.
Compost
Compost, often referred to as organic matter (OM), is the best way to improve the soil. Beneficial microorganisms in the compost help bring nutrients to plant roots and also are fed by plant root secretions. At the same time, these microorganisms work to combine soil particles which in turn improves soil infiltration rates.
The addition of compost improves the soil structure to bring it closer to a category of a loam soil. In most soils, the ideal amount of organic matter is between 5 and 10 percent. It is important to keep in mind what types of plants you will be planting. Many, including a large number of native plants, grow best in a less organic soil (approximately 5 percent organic matter). Missouri glade and upland prairie species, for example, grow in a rocky-sandy soil with a very low percentage of organic matter while vegetables grow best in a more organic soil (10–15 percent organic matter).
Many garden centers and plant nurseries carry commercial ready-to-use compost. If homemade compost is used, be sure the compost has finished composting before use since unfinished compost may rob the soil of nitrogen. A finished compost has organic matter that is well broken-down and is a dark brown color throughout.
Topsoil
In general, the top layer of soil is considered to be topsoil. The topsoil layer is, on average, about 6 to 10 inches thick to a couple of feet thick, depending on its geographical location. It has a higher organic matter content than lower soil layers and is where the soil’s biological activity takes place. Topsoil consists of minerals, microorganisms and decomposing organic matter. A good topsoil should have a loose, open structure so that it is well-draining, a medium-dark brown color and have a good level of moisture retention. Gardeners can purchase locally harvested topsoil and have it blended with compost in a ratio recommended by soil test results for their property. In many cases, topsoil may already have a high organic matter content and can be used as is without further amending. Much of the commercially available bagged “topsoil” products are blended with minerals and organic matter and bagged when the soil particle size is relatively small. These products can be very high in organic matter (rich, dark brown, almost black, in color) and are meant to be blended with the existing topsoil.
Mulch
Mulch is any organic material that is spread over the surface of the soil, the most common being shredded bark or shredded leaves. It is considered a soil amendment, even though it is not tilled or turned into the soil, because it continues to decompose and improves the soil over time. Shredded hardwood bark and shredded leaf mulch are the most commonly used mulches.
The benefits of adding mulch are:
- helps prevent and reverse soil compaction
- provides insulation from temperature extremes of the seasons
- reduces evaporation at soil surface and conserves moisture in the soil
- helps control weeds and weed seed germination
- adds organic matter into the soil through decomposition of mulch
- reduces soil erosion
- improves the overall health of the plants and soil
- improves aesthetics of the planting bed and provides a manicured look
Organic mulches:
Shredded hardwood bark mulch – Comprised of hardwood (predominantly oak species) bark only, this comes in a few forms: shredded, twice or triple ground. Since Eastern Missouri is in a primarily hardwood forest (oak-hickory), this is the most commonly available mulch. It is less expensive than other tree species from farther away or than treated/dyed mulches.
Shredded leaf mulch – This is readily available and free mulch every autumn, unless you have no trees on your property. Leaves may be collected and run through a shredder (or mowed and collected in the mower bag) and spread over the surface as a mulch. Consider using shredded leaves as mulch rather than removing them from the property. Shredded leaf mulch may also be purchased from a composting facility.
Hardwood mulch – Chipped hardwood mulch, also referred to as wood chips, is coarsely chipped wood. It is not separated from the bark and is less expensive. In general, wood chip mulches have a high carbon to nitrogen ratio which means that in the process of decomposing they may temporarily reduce the supply of soil nitrogen for plant uptake.
Pine or cypress bark – The shredded bark of pine and cypress are harvested from farther away, typically bagged and are shipped over longer distances. Thus, they are less sustainable and frequently cost more than locally produced hardwood bark mulch.
Note: In rain gardens and bioswales with high volume, high velocity water and/or steep slopes, organic mulches such as shredded bark are not recommended since they easily wash away and expose the soil to erosion. Instead, use stone and gravel in high energy water areas.