WORKING WITH YOUR SOIL
Your soil, what is it like?
Gardeners must be concerned with two types of soil properties: physical properties and chemical properties.
Physical: Much of the soil in Chatham County is heavy with high clay content. Low permeability makes clay slow to absorb water. Once it gets wet, it drains poorly and is slow to dry out. If the soil has been subjected to compaction from construction equipment or even regular foot traffic, it may be insufficiently aerated and plants may not,thrive. The ability of the soil to absorb and hold water and air is included among its physical properties. Heavy clay soils may be very difficult to dig and work.
Chemical: If the soil has never been farmed or gardened (or even if it has), it may lack nutrients and may not readily yield nutrients to plants even when fertilizers are applied. This nutrient status is referred to as its chemical property. Native soils may be too acid for plants to thrive.
How can you improve the physical properties?
We improve the physical properties of heavy soils by increasing the size of pore space in the soil to improve permeability and drainage. Pores are the spaces between grains of soil where water and air are held. Soils with small soil particles such as clay also have small pore spaces making them difficult to work, difficult to wet, and lacking in aeration.
To increase pore size we need to increase the size and mix of soil particles.
Since we already have abundant small clay particles, we add material with larger particle size. Sand and clay may be mixed to make brick; but it is not practical to add enough sand to a clay soil to improve its physical properties. Peat moss is not used on clay soils, because it is also slow to absorb water and slow to dry out. Materials most effectively used include pine bark humus, composted leaves, and pea gravel (3/8” or smaller).
These materials must be dug or tilled in to make a good mix. Tillage may require heavy equipment. Remember that trees you hope to save may be permanently injured from having their roots tilled. It may be easier to remove live trees before gardens are in place than to remove dead ones after.
It is generally not desirable for any “topsoil” or soil amendment to be placed on top of the existing soil without tilling it in. A uniform rooting medium is preferable to trying to grow plants in a thin layer of “topsoil.” It may be easier to till and work with the soil you have than to garden in a thin topdressing. A top dressing deeper than four inches may also be sufficient to kill trees by limiting air available to their roots.
Creating gardens under established trees may require creation of small raised beds that allow plants adequate root room to get established while minimizing injury to tree roots.
How can you improve soil fertility? Learn what you need with a soil test!
The question we ask is not what kind of fertilizer a plant needs. We ask what the soil is able to supply and what we need to supplement. How do we know the nutrient status of our soil?
The North Carolina Department of Agriculture provides soil analysis at no charge. But the analysis is no better than the quality of the soil sample you submit.
For complete directions on how to take a sample, see: https://chatham.ces.ncsu.edu/soil-testing-for-lawns-and-gardens/
Once you get a soil report it will tell you if you need lime, how much lime, and what kind of fertilizer. Lime may not be essential. Lime buffers soil acidity by raising pH, a measure of acidity or alkalinity. Application of lime can affect whether plant nutrients are available for plant use. To be most effective lime must be tilled into the soil. Otherwise it may take years for it to be effective in the plant root zone.
Your soil report will also suggest what kind of fertilizer will best supplement your soil’s natural fertility. When to apply that fertilizer will depend on what you plan to grow.
- Annual bedding plants are fertilized frequently throughout their growing season.
- Perennials may be fertilized the first year, perhaps no more.
- Vegetables are fertilized when they are planted and usually again several weeks later.
- Trees and shrubs are fertilized in the fall or winter during early years; later they may be fertilized only every two to four years.
- Lawns are fertilized on a schedule based on the specific type of grass.
For more information on fertilizer scheduling, taking a soil sample, or interpreting your soil analysis, contact the Master Gardener VolunteersSM at 919-545-2715 or firstname.lastname@example.org.