A soil amendment, sometimes called a soil conditioner, is any material added to a soil to improve its physical and chemical properties, such as water retention, permeability, drainage, aeration, structure, and nutrient holding capacity. The goal is to provide a better environment for roots to grow.
To work properly, an amendment must be thoroughly mixed into the existing native soil. If it is merely backfilled into a planting hole, then its effectiveness is reduced and it may interfere with water and air movement as well as root growth. Thorough mixing is especially important in North Georgia’s heavy clay soils. Most soil amendments can be incorporated seasonally before planting annual flowers or vegetable gardens. Never try to add soil amendments after plants are established. This could bury established roots too deeply and smother your plants. Many types of trees prefer having shallow roots near the surface if the ground is compacted or drains too slowly, which is an adaptation to our clay soils.
Amending the soil is not the same thing as mulching, although some mulch materials are composted to make soil amendments. Mulches are left on the soil surface. The purpose of mulch is to reduce evaporation and runoff, inhibit weed growth, and create an attractive appearance. Mulches also moderate soil temperature, helping to warm soils in the spring and cool them in the summer. Mulches should not be incorporated into the soil as amendments unless they have fully decomposed or properly composted to the point that they no longer include coarse woody material.
There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was once alive. Organic amendments include sphagnum peat moss, composted pine bark, food waste, grass clippings, straw, manure, biosolids, sawdust and wood ash. Inorganic amendments, on the other hand, are either mined or man-made. Inorganic amendments include vermiculite, perlite, pea gravel, and sand.
Not all these soil amendments are recommended by UGA Extension. These are merely examples. Wood ash, an organic amendment, is high in both pH and soluble salts. Be sure to check your soil pH by doing a soil test before and after using wood ash as a soil amendment. Also, don’t add sand or gravel to clay soil, as this can form a layered soil profile that won’t drain properly.
Organic amendments increase soil organic matter content and offer many benefits. Organic matter improves soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments even contain plant nutrients and act as time-release fertilizers. Organic matter also is an important energy source for beneficial bacteria, fungi and earthworms that live in the soil.
For heavy clay soil, it is generally recommended that you add soil amendments at a rate of 25% by volume to realize its full potential benefits. This is the equivalent of adding a 3” layer of soil amendment and then tilling to incorporate it with native soils 12” deep. One cubic yard (nine 3-cubic feet bags) of soil amendment will cover an area of about 100 square feet. This is generally more practical for small, raised bed vegetable gardens. At this application rate it may not be economically feasible to apply organic matter to all landscape plantings. Preference should be given to annual flower beds and herbaceous perennials prior to planting. Lawns can also benefit from soil amendments prior to planting.
Fresh manure can harm plants due to elevated ammonia levels. To avoid this problem, use only aged manure at least six months old. Food-borne pathogens are another potential problem with fresh manure, especially in vegetable gardens. Most home composting systems do not sustain high enough temperatures to kill potential pathogens or weed seeds. Home-composted products containing manure are best used in flower gardens, shrub borders and other nonfood gardens. Commercial composts are generally safer and are made primarily from leaf or wood products alone or in combination with manures or biosolids. Another major concern of using manures is where they were collected. Manures that were taken from pastures treated with certain weed killing herbicides might contain residues of these chemicals that could carry over into your garden. These chemical residues are often strong enough to still kill weeds as well as your vegetables in some situations. Be sure to find out about your source of manure before you use it. If you are unsure, test a small area with the manure using a sensitive plant such as tomatoes. Within a couple of weeks, if the manure has any chemical residues present, your plants will likely show symptoms as their leaves become distorted, twisted, and stunted. It’s always better to lose one plant than an entire garden.
