Choosing an Amendment

Factors to Consider When Choosing an Amendment

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

  • how long the amendment will last in the soil,
  • soil texture,
  • soil salinity and plant sensitivities to salts, and
  • 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.

Longevity of the Amendment

The amendment you choose depends on your goals.

  • Are you trying to improve soil physical properties quickly? Choose an amendment that decomposes rapidly.
  • Do you want a long-lasting improvement to your soil? Choose an amendment that decomposes slowly.
  • Do you want a quick improvement that lasts a long time? Choose a combination of amendments.
Table 1: Routine application rate for soil amendments.
Site Depth of soil amendment prior to incorporationA
(based on an incorporation depth of 6-8 inches)B
Plant-based composts and other soil amendments low in saltsC Manure, manure-based compost, biosolids, biosolid-based compost and other soil amendments that may be high in saltsD
One-time application to new landscapes prior to planting trees, shrubs, perennials, and laws. 2-3 inches 1 inchE
Annual application to vegetable garden and annual flowerbeds First three years 2-3 inches 1 inchE
Fourth year and beyond 1-2 inches 1 inchE
A Three cubic yards (87 bushel) covers 1,000 square feet approximately 1 inch deep.B Cultivate the soil amendment into the top 6-8 inches of soil. On compacted/clayey soils, anything less may result in a shallow rooting depth predisposing plants to reduced growth, low vigor, and low stress tolerance. Rate should be adjusted if incorporation depth is different.C Plant-based compost are derived solely from plant materials (leaves, grass clippings, wood chips, and other yard wastes). Use this application rate for other soil amendments know to be low in salt.D Use this application rate for any soil amendment with maure or biosolids, unless the salt content is actually known, by soil test, to be low. Excessive salts are common in many commercially available bagged and bulk products. Use with caution.

E For soil amendments with high salts, this routine application rate may be too high. Use with caution.

Soil Texture

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

Soil Salinity and Plant Sensitivity to Salts

Many forms of compost made with manure, and biosolids are 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 plant-based composts or sphagnum peat.

Salt Content and pH of the Amendment

Always beware of salts in soil amendments. Amendments frequently high in salts and/or pH include wood ash, mountain peat and manures, and manure-based compost, biosolids, and biosolid-based compost.

An amendment with up to 10 mmhos/cm total salts is acceptable if mixed well 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.

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.

Use caution as the salt content in compost may vary from batch to batch.

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