Soil Amendments

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, helping to warm soils in the spring and cool them in the summer. Mulches may be incorporated into the soil as amendments after they have decomposed to the point that they no longer serve their purpose.

Quick facts of soil amendment are :

1. Soil amendments improve the physical properties of soils.
2. Amendments are mixed into the soil. Mulches are placed on the soil surface.
3. The best soil amendments increase water- and nutrient-holding capacity and improve aeration and water infiltration.
4. Wood products can tie up nitrogen in the soil.
5. Sphagnum peat is superior to Colorado mountain peat.
6. When using biosolids, choose Grade 1 biosolids.

There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that is or 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.

Bassalt Rock is a kind of porous rock which can be used as Soil Amendment

Bassalt Rock after be grinded to be gravel size then be applicated on the farm

Not all of the above are recommended as soil amendments. 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. 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.

Application rates of soil amendments is according to the condition of the soil. If the soil has less than 3 percent organic matter, then apply 3 cubic yards of the chosen organic amendment per 1,000 square feet. To avoid salt buildup, do not apply more than this. Retest the soil before deciding whether to add more soil amendment.

There are at least four factors to consider in selecting a soil amendment :

1. how long the amendment will last in the soil,
2. soil texture,
3. soil salinity and plant sensitivities to salts, and
4. salt content and pH of the amendment.

Laboratory tests can determine the salt content, pH and organic matter of organic amendments. The quality of bulk organic amendments for large-scale landscape uses can then be determined. 

The amendment which be choosen depends on the goals, such as :

1. To improve soil physical properties quickly, choose an amendment that decomposes rapidly.
2. For a long-lasting improvement to the soil, choose an amendment that decomposes slowly.
3. For a quick improvement that lasts a long time, choose a combination of amendments.

Table 1: Decomposition rate of various amendments.
Amendment	Decomposition rate
Grass clippings, manures	Rapid decomposition (days to weeks)
Composts	Moderate decomposition (about six months)
Wood chips (redwood, cedar), hardwood bark, peat	Slow decomposition (possibly years)

Soil texture, or the way a soil feels, reflects the size of the soil particles. Sandy soils have large soil particles and feel gritty. Clay soils have small soil particles and feel sticky. Both sandy soils and clay soils are a challenge for Gardeners. Loam soils have the ideal mixture of different size soil particles.

When amending sandy soils, the goal is to increase the soil's ability to hold moisture and store nutrients. To achieve this, use organic amendments that are well decomposed, like composts or aged manures.

With clay soils, the goal is to improve soil aggregation, increase porosity and permeability, and improve aeration and drainage. Fibrous amendments like peat, wood chips, tree bark or straw are most effective in this situation.

Use Tables 2 and 3 for more specific recommendations. Because sandy soils have low water retention, choose an amendment with high water retention, like peat, compost or vermiculite. Clay soils have low permeability, so choose an amendment with high permeability, like wood chips, hardwood bark or perlite. Vermiculite is not a good choice for clay soils because of its high water retention.

Table 2: Permeability and water retention of various soil types.
Soil Texture	Permeability	Water Retention
Sand	high	low
Loam	medium	medium
Silt	low	high
Clay	low	high

Table 3: Permeability and water retention of various soil amendments.
Amendment	Permeability	Water Retention
Fibrous    Peat    Wood chips    Hardwood bark	low-medium high high	very high low-medium low-medium
Humus    Compost    Aged manure	low-medium low-medium	medium-high medium
Inorganic    Vermiculite    Perlite	high high	high low

Some forms of compost and manures can be high in salts. Avoid these amendments in soils that are already high in salts (above 3 mmhos/cm) or when growing plants that are sensitive to salts. Raspberry, strawberry, bean, carrot, onion, Kentucky bluegrass, maple, pine, viburnum and many other landscape plants are salt sensitive. In such cases, choose sphagnum peat or ground leaves instead of compost or manures.Always beware of salts in soil amendments. High salt content and high pH are common problems in the arid land soil. Therefore, avoid amendments that are high in salts or that have a high pH. Amendments high in salts and/or pH include wood ash, peat and composted manures. An amendment with up to 10 mmhos/cm total salts is acceptable if well mixed into low-salt soils (less than 1 mmhos/cm). Amendments with a salt content greater than 10 mmhos/cm are questionable. Choose a low-salt amendment for soils testing high in salts.

Sphagnum peat and compost made from purely plant sources are low in salts and are good choices for amending sandy soils. Ask for an analysis of the organic amendments that you are considering, and choose your amendments wisely. If no analysis is available, test a small amount of the amendment before purchasing a large quantity.