A soil amendment is any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. The goal is to provide a better environment for roots.
To do its work, an amendment must be thoroughly mixed into the soil. If it is merely buried, its effectiveness is reduced, and it will interfere with water and air movement and root growth.
Amending a soil is not the same thing as mulching, although many mulches also are used as amendments. A mulch is left on the soil surface. Its purpose is to reduce evaporation and runoff, inhibit weed growth, and create an attractive appearance. Mulches also moderate soil temperature. Organic mulches may be incorporated into the soil as amendments after they have decomposed to the point that they no longer serve their purpose.
Organic vs. Inorganic Amendments
There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was alive. Inorganic amendments, on the other hand, are either mined or man-made. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, manure, biosolids, sawdust and wood ash. Inorganic amendments include vermiculite, perlite, tire chunks, pea gravel and sand.
Not all of the above are recommended. These are merely examples. Wood ash, an organic amendment, is high in both pH and salt. It can magnify common soil problems and should not be used as a soil amendment. Don’t add sand to clay soil — this creates a soil structure similar to concrete.
Organic amendments increase soil organic matter content and offer many benefits. Over time, organic matter improves soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments contain plant nutrients and act as organic fertilizers. Organic matter also is an important energy source for bacteria, fungi and earthworms that live in the soil.
Ideally, the landscape and garden soils are improved to 4-5% organic matter. At this level, themineralization (release) of nitrogen from the organic matter will be adequate for most plants without additional fertilizers. Many cities now require that the landscape soils be brought up to this level in new developments as a water conservation technique. With the improved aeration and deeper rooting, plants are more efficient in capturing rain events.
Where the soil amendments may be high in salts, the rate is limited due to the salt problem. Salt burn of roots and death of landscape and garden plants is common from over application of salty soil amendments.
Wood products can tie up nitrogen in the soil and cause nitrogen deficiency in plants. Microorganisms in the soil use nitrogen to break down the wood. Over several months to years, as microorganisms complete the rapid decomposition process, the nitrogen is released and again becomes available to plants. This hazard is greatest with sawdust, because it has a greater surface area than wood chips.
Compost wood products, before using them as soil amendments. For these products to decompose rapidly, add a nitrogen source to the compost pile. This could be plant residues high in nitrogen (such as grass clippings or manure), or a nitrogen fertilizer. Do not use uncomposted wood products or sawdust as a soil amendment. It is slow to break down, ties up nitrogen, interferes with seedbed preparation, and interferes with soil and water movement through the soil profile.
Fresh manure can harm plants due to elevated ammonia levels. To avoid this problem, use only aged or composted manure.
Human pathogens, including E coli, are another potential problem with fresh manure, especially on vegetable gardens. For vegetables with direct contact with the soil, fresh manure must be applied at least four months prior to harvest. For other fruits and vegetables, fresh manure must be applied at least three month prior to harvest. In simple words, fresh manure would be only fall applied for the spring garden.
Aged manure refers to manure that has been piled for at least six months. Excessive ammonia will have escaped. Salt levels may be higher as the salts concentrate in the decomposing material, or may be leach out with high rainfall. Weed seeds will be viable.
Composted manure technically refers to manure that has been through multiple active heating cycles and turned in between. If heated above 145 degrees F, it will kill pathogens and weed seeds. In composted manure, the organic matter is stabilized (through the rapid decomposition process) making it an ideal soil amendment. Salt level may be concentrated or may be leach out with high rainfall.
As a point of clarification, composts and manures are not regulated. Many commercially available products are labeled as “composted.” However, this does not mean that it has been through the active decomposition process.
Compost refers to decomposed organic matter. It is not regulated, so there is no standard about the state of decomposition. In commercially available products the term “compost” is often used generically, and does not infer that the product has been through the actively heating, decomposition process.
A wide variety of compost products are available in bagged and bulk products. These may be a combination of plant-based compost, manure-based composts, biosolids, and other agriculture by-products (such as chicken feathers).
Manure-based composts are often high in salts. Use with caution.
Compost made solely from plant-based products (such as wood chips and yard wastes) are low in salts. These are preferred over manure based composts which are often higher in salts. However, they are generally more expensive.