Adapted from the Durham County Master Gardener Urban Horticulture Notes No. 2 and No. 3 and Managing Soil Tilth: Texture, Structure, and Pore Space
Clay soil particles pack too tightly to form the large pore channels where oxygen and roots are found. When organic matter is added, the particles aggregate into larger clumps that pack loosely, improving drainage and air and root penetration. Organic matter also fosters beneficial soil life (microorganisms and earthworms) that further improve structure. Note that structural improvement cannot be achieved in a single session or season because there are limits to how much organic matter can be added (50% by volume) without creating a drainage barrier.
Well-rotted compost is the best single organic amendment for clay, but mixtures of organic amendments are also a good choice. Although compost also enriches the soil, it releases nutrients slowly and often does not contain enough nutrients to supply all the needs of growing plants. The structural benefits of compost are numerous:
- creates a better environment for root growth
- increases the ability of the soil to hold and release essential nutrients
- promotes the activity of earthworms and soil microorganisms beneficial to plant growth
- improves seed emergence and water infiltration by reducing soil crusting
- Pine bark
Finely-ground pine bark, sold as “pine bark soil conditioner”, is another excellent amendment and is a native and renewable resource. A pea-size grind (¼–½″) is best. Pine bark soil conditioner is available in bags or by the truckload. Straight pine bark is a better value than pine bark mixed with other ingredients. Other soil conditioner products are often too fine or coarse to provide the needed pore space.
Composted manure has no odor. It is excellent for improving soil structure and supporting beneficial organisms like earthworms and also supplies some minor nutrients. Manure is only a modest fertilizer, with an NPK of about 1-1-1. Make sure that the manure has been tested to be low in salt.
Manure N P K Cow, dried 1.3 .9 .8 Hen, fresh 1.1 .9 .5 Horse, fresh .6 .3 .3
Recently, herbicide carryover in compost and composted manure has become a problem. An Extension factsheet discusses herbicides of concern, sensitive plants, and sources to avoid. The US Composting Council has a certification program that tests compost for many features including pH, soluble salts, nutrient value, fecal pathogens, and heavy metals.
Inorganic amendments like gravel complement the functions of organic amendments. They are permanent additions used to improve drainage, to make soil more resistant to compaction and, to varying extents, to moderate temperature and help conserve moisture. Inorganic amendments should be incorporated into the soil just like organic amendments, ideally raising the level of the bed. Putting gravel in the bottom of a planting hole does not improve drainage — the hole still forms a “bathtub without a drain”.
The best size is a pea gravel called “78” that is about ⅜″ diameter. As an added bonus, sharp gravel seems to deter tunneling moles and voles.
- Expanded shale
Sold under brand names such as “Perma-Till”, it functions like gravel but is lighter in weight and more expensive.