Adapted from Durham Urban Horticulture Notes No. 2 and No. 3
Both amendments and mulches can be either organic, such as ground bark, or inorganic, such as gravel. Although amendments and mulches are often similar materials, amendments have a finer texture and are worked into the soil to improve tilth in the plant root zone, while mulches are placed on top of the soil to conserve moisture, moderate soil temperature, reduce weed seed germination, protect soil structure and reduce erosion. After organic mulch decomposes, it can be worked into the soil as an amendment.
Adapted from CMG GardenNotes #241 Soil Amendments and CO CES Fact Sheet Choosing a Soil Amendment.
To improve aeration, infiltration, and structure in clay soils, consider both permeability and water retention. Ideally, use a mix of materials
Expanded clay shale
Pea gravel ‘78’
|* not recommended|
Will the product be used repeatedly to improve the soil over time, as in a vegetable garden, or as a single application before planting lawns, perennials or shrubs? If the latter, consider adding an inorganic amendment as well.
Products that decompose rapidly, like grass clippings and manure, give quick results, while products that decompose slowly, like fine pine bark, give longer-lasting results. Use a combination of materials to get both rapid and persistent benefits.
Products containing manure and/or biosolids are often high in salts, while plant-based products are low in salts. Application rates and frequency will depend on the salt content of the material and the depth to which it will be tilled.
Soil amendments can tie up nitrogen until they decompose. Supplemental nitrogen should be added to reflect the decomposition rate.
~ 6 months
up to 10 years
Dried grass clippings
Aged wood chips
Adapted from Durham 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:
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.
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.
Sold under brand names such as “Perma-Till”, it functions like gravel but is lighter in weight and more expensive.
Peat moss is not a good amendment for clay soils. It retains too much water initially, then decays rapidly and leaves the soil no better than before it was added.
Sand is not a good amendment for clay soils. Any mixture less than 70% sand/30% clay actually packs more densely than straight clay. This makes a readily compactable soil that is worse than clay or subsoil.
Gypsum is useful in the alkaline clay soils of the Western US but is not effective in our acidic clay.
Fresh manure is too salty to use near plants. The salt draws water from the plant, drying out roots and causing burned edges on leaves. However, fresh manure is a good nitrogen source in the compost pile to offset high-carbon materials like dried leaves and plant stems. Don’t use human or pet waste as these can transmit diseases to humans. Cow, horse, rabbit, and chicken manure are fine.
These have a high carbon content and therefore use a lot of nitrogen while decomposing. If you incorporate either without adding enough compensating nitrogen, none will be available for your plants. In addition, wood chips can take years to decompose, and some types of wood can raise soil pH undesirably. Barn bedding sometimes is made of wood chips and even though the manure provides additional nitrogen for decomposition, bedding should not be tilled into your garden until it is well rotted. Of course, undecomposed materials can be used on top of the soil as mulch. Hardwood chips break down faster than pine chips.
There are no laws regulating the content of topsoil, so buyer beware. Purchased topsoil is frequently no better than your existing soil. Sometimes it contains too much sand, and unsterilized topsoil can contain weed seeds. Purchased topsoil is useful for creating raised beds (the caveats above still apply), but for most landscaping needs it is better to amend the existing soil with organic materials. Whether you use purchased topsoil or make your own amended soil, be sure to mix the new materials thoroughly with the native soil. If new soil is just spread over the existing soil, plants will not root into the clay underneath and will dry out in hot weather.
Undecomposed organic matter (such as wood chips or shredded leaves) is fine as mulch, but should not be worked into the soil during the growing season.
Anaerobic decomposition is often a problem with materials high in cellulose such as leaves or hardwood chips. Anaerobic decomposition is rare for pine bark because lignin is the main component rather than cellulose. Pine bark is unique because it can be used without composting as either mulch or compost. Only the particle size differs.