Kinds of Soil Amendments

Here are some widely-available soil amendments:

Animal manures of all types have long been used as soil conditioners. All the commonly used manures have modest amounts of the major nutrients (nitrogen, phosphorus, and potassium). Chicken manure is the most potent, so use smaller amounts of it.

All manures work better if they are composted first.

Manure often carries weed seeds, depending on where the animals were grazing.

The amount of salt the animal has eaten influences the quality of the maure. Bagged steer manure is usually from feedlot steers, which are fed diets high in salt, so the manure is high in salt. If your soil has a salt problem, you should probably avoid it. If you don’t have a salt problem, the salt in the manure will wash out with ordinary watering without causing harm.

Wood by-products , such as ground bark and sawdust, are widely sold. Only finely-ground materials are suitable for amendments. Coarse wood products, such as bark chunks or shredded bark, takes too long to decompose, and are better used as a mulch.

Wood by-products are frequently fortified with nitrogen. The process of decomposition requires nitrogen; if organic materials are not already decomposed, the bacteria that decompose them will use nitrogen from the soil to aid in the process, a phenomenon called nitrogen draft. If you use sawdust as a soil conditioner, add a nitrogen supplement as well. For each 100 pounds of sawdust, add 1 pound of actual nitrogen. If you’re putting 2 inches of sawdust onto a 100-square-foot area, add 1/2 pound of nitrogen. For more information, see Nitrogen Draft.

Lawn clippings are both plentiful and free if you have a lawn. Moderate amounts of lawn clippings can be worked directly into the soil without composting. But if your grass has gone to seed or if it is a variety that grows from its stems, such as bermudagrass, don’t use it in your garden as an amendment without composting it first. Don’t use clippings that have been sprayed with an herbicide.

Compost , either made in your own yard or purchased, is an excellent soil amendment. When properly managed, the high heat of decomposition (140 to 160 degrees) will kill most weed seeds, insects, and disease organisms.

Leaf mold , decomposed leaf material, is often sold commercially. This material may be acidic, depending on the leaves used, making it a favored amendment for acid-loving plants such as azaleas and blueberries. Make sure no poison oak or poison ivy has been included in the mix.

Peat moss (from ancient swamps) is commonly available at nurseries. It is peat derived from moss and is an excellent soil amendment.

(Sphagnum moss is sometimes confused with peat moss; it is dried, but not decomposed, moss. Sphagnum moss is used as a packing material, for making hanging baskets, and to cover the surface of pots; but it is not used as a soil amendment.)

Peat moss is expensive, but it retains moisture well, drains superbly, and is lightweight. It has an acidic effect (pH of 3.5 to 4.5) that may be desirable for certain plants, or you can neutralize this acidity by adding 5 pounds of lime per 100 square feet when peat moss is used 4 inches deep.

Peat moss is the longest lasting of the organic mulches, minimizing shrinkage of soil volume that would be caused by further decomposition.

In some areas, other types of peat are available, such as sedge peat; but these other types are inferior to peat moss as an amendment in the garden.

Vermiculite and perlite are mineral pieces whose size has been expanded by heating.

Vermiculite has a porous structure. It retains water and nutrients very well, but it is fragile and keeps its structure for only a few months in the soil. It sometimes contains small amounts of asbestos, a carcinogen.

Perlite particles are impervious to water and don’t retain water or nutrients. It lasts much longer in the soil than vermiculite. Both perlite and vermiculite allow for good soil aeration and are lightweight, making them ideal for container soils, especially when a sterile soil rather than ordinary garden soil is desired. A sterile soil will reduce plant pathogen problems when seeds are germinating.

Sand is often added to heavy clay soil in an effort to improve drainage, but it may have the opposite effect. Unless you make the resulting blend more than 80% sand, the mix will have worse drainage than the clay alone. The clay fills in the spaces between the sand particles, acting like a cement. Sand, especially coarse sand, improves drainage in a container mix that does not include any clay soil.

Lime and sulfur are added to soils to adjust the pH, or acidity. Lime makes soils less acid, and sulfur makes it more acid. Lime is often used in the East, where summer rain causes the soil to be acid. Sulfur is often used in the arid West, where alkaline soils are common.

Agricultural by-products include a wide variety of materials. These amendments are available in some regions:

Peanut hulls are an excellent amendment.

Tobacco stems work well but should be kept away from plants in the same family (tomatoes, potatoes, peppers, nicotiana) because of virus problems.

Cottonseed meal has fairly high levels of plant nutrients.

Cocoa bean hulls hold water well, have a pleasing appearance (and aroma) as an amendment or mulch, and are acidic.

Rice hulls are sometimes available in the South and West. They make a good amendment, but are too lightweight to use as a mulch.

Bagasse is a sugar-cane by-product used as an amendment and mulch in the South.

Apple and grape pomace —the leftover skins, seeds, and stems of these fruits after processing—has been used with good results.

Ground corncobs are available in the Midwest.

Straw , a traditional soil amendment and mulch, must be supplemented with nitrogen when worked directly into the soil; it often contains weed or grain seeds.