Soil Amendments

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Terms

The term soil amendment refers to any material mixed into a soil.  Mulch refers to a material placed on the soil surface.  By legal definition, soil amendments make no legal claims about nutrient content or other helpful (or harmful) effects that it will have on the soil and plant growth.  In Colorado, the term compost is also unregulated, and could refer to any soil amendment regardless of microorganism activity. 

By legal definition, the term fertilizer refers to soil amendments that guarantee the minimum percentages of nutrients (at least the minimum percentage of nitrogen, phosphate, and potash). 

An organic fertilizer refers to soil amendments derived from natural sources that guarantee the minimum percentages of nitrogen, phosphate, and potash.  These should not be confused with products approved for use by the USDA National Organic Program.  The federal Certified Organic Label, USDA Organic, allows only certain regulated products as listed by the Organic Materials Review Institute (OMRI).  For additional information on certified organic soil amendments and fertilizers, refer to the web site at www.omri.org.

Many gardeners apply organic soil amendments, such as compost or manure, which most often do not meet the legal requirements as a “fertilizer” and generally add only small quantities of plant nutrients.

Managing Soil Texture and Structure

Routine applications of organic matter should be considered an essential component of gardening and soil management.  Organic matter improves the water and nutrient holding capacity of coarse-textured sandy soil.  On a fine-textured clayey soil, the organic matter over time glues the tiny clay particles into larger chunks or aggregates creating large pore space.  This improves water infiltration and drainage, air infiltration (often the most limiting aspect of plant growth), and allows for deeper rooting depths (allowing the plant to tap a larger supply of water and nutrients).  For additional discussion, refer to the CMG GardenNotes #213, Managing Soil Tilth.

Using organic soil amendments is a great way to turn otherwise useless products, like fall leaves and livestock manure, into compost for improving soil tilth.

When using organic soil amendments, it is important to understand that only a portion of the nutrients in the product are available to plants in any one growing season.  Soil microorganisms must process the organic compounds into chemical ions (NO3-, NH4+, HPO4-2, H2PO4-, K+) before plants can use them.

Cultivate or hand-turn the organic matter thoroughly into the soil.  Never leave it in chunks as this will interfere with root growth and water movement.

Selecting Soil Amendments

For details on selecting a soil amendment, see the CSU Extension Fact Sheet #7.235, Choosing a Soil Amendment.

  • Desired results – In selecting soil amendments, first consider the desired results.  To improve the water and nutrient holding capacity on sandy, gravelly, and decomposed granite soils, select well decomposed materials like finished compost, aged manure, and peat.  To improve aeration and infiltration (improve structure on clayey soils) select fibrous materials like composted wood chips, peat and straw.

  • Potential for routine applications – Another important consideration is the potential for routine applications to improve the soil over time, as in a vegetable garden or annual flowerbed.  In many landscape settings, the amendment is a one-time application added before planting lawns, perennials, trees and shrubs.

  • Longevity of the product merits consideration.  Produces that decompose rapidly (like grass clippings and manure) give quick results, while products that decompose slowly (like wood chips, bark chips and peat) provide longer lasting results.  For quick improvement that last, use a combination of materials.

  • Salts are a primary consideration.  Products made with manure and/or biosolids are often very high in salts.  Salt level may actually increase in the composting process, although water moving through the compost pile leaches out the salts.  Use with caution!   Plant-based products are naturally low in salts.

    Routine application rates depend on the salt potential of the material and the depth to which it will be cultivated into the soil.  Table 1 gives standard rates.

Table 1.
Routine Application Rate for Compost
Site
Incorporation Depth 2
Depth of compost before incorporation 1
Plant-Based Compost
and other compost know to be low in salts 3
Compost Made with Manure or Biosolids for which the salt content in unknown 4
One-time application - such as lawn area to be planted
6-8 inches
2-3 inches
1 inch
Annual applications to vegetable and flower gardens - first three years
6-8 inches
2-3 inches
1 inch
Annual applications to vegetable and flower gardens - fourth year and beyond
6-8 inches
1-2 inches
1 inch

1. Three cubic yards (67 bushels) covers 1,000 square feet approximately 1 inch deep.

2. Cultivate compost into the top 6-8 inches of the 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.  If the incorporation depth is different, adjust the rate accordingly.

3. Plant based composts are derived solely from plant materials (leaves, grass clippings, wood chips and other wards wastes).  Use this application rate also for other compost known, by soil test, to be low in salts.

4. Use this application rate for any compost made with manure or biosolids unless the salt content is known, by soil test, to be low.  Excessive salts are common in many commercially available products sold in Colorado.



In purchasing products, gardeners need to understand that there are no regulations about the quality of the product, salt content or other beneficial or harmful qualities.  Voluntary standards for bulk products may help in product evaluation.  Use with caution!  Many of the soil amendments sold in Colorado are high in salts!

  • Need for nitrogen fertilizer – Soil microorganisms release nitrogen tied-up in organic matter over a period of time.  Release rates from compost are very slow, over a period of years.  The need for nitrogen fertilizer is based on the soil organic content.  As the soil organic content increases, the need for fertilizer decreases.  [Table 2]

Table 2.
Need for Nitrogen Fertilizer Based on Soil Organic Content
Soil Organic Content
Routine Application Rate for Gardens
1%
2 pounds actual nitrogen per 1,000 square feet
2-3%
1 pound actual nitrogen per 1,000 square feet
4-5%
0


Over Amending

Over-amending is a common problem.  Some gardeners try to fix their soil limitations by adding large quantities of amendment in a single season.  This can result in following problems:

  • High salts
  • High nitrogen
  • Low nitrogen (from the tie-up of nitrogen due to a carbon to nitrogen ratio imbalance)
  • Holding too much water
  • High ammonia (burns roots and leaves)

Problems may also arise, over time, from the continual application of high rates.  This can result in the following problems:

  • High salts
  • Excessive nitrogen, phosphorus and potassium
  • Ground water contamination
  • Micronutrient imbalance

Evaluating the Quality of Organic Amendments

The quality of organic amendments can be determined by both visual evaluation and laboratory testing.

Visual Evaluation

  • Color – Dark brown to black
  • Odor – Earthy, no ammonia smell
  • Texture – Less than ½ inch particle size; lawn top dressing less than ¼ inch
  • Foreign materials – Less than 1% and smaller than ½ inch size
  • Uniformity within the batch
  • Consistency between different batches
  • Raw materials – Concern of heavy metals (biosolids), human pathogens (manure), and salts (manure and biosolids)
  • Weed seeds – Test by germinating some material

Laboratory Testing

  • C:N ratio – Less than 20 to 1 acceptable; 10-12 to 1 is better
  • Ash content – (This measurement of the mineral portion after the organic matter is burned off will determine if soil was a primary part of the mix.) 
  • 20-30% common
  • Keep below 50%
  • If greater than 50-60% it probably contains a lot of soil
  • Bulk density – Less than 1.0 gm/cc
  • pH – 6.0 to 7.8 
  • May be higher in manure
  • Near neutral (6.8 to 7.2) is best
  • Salts – Acceptable levels depend on use
  • Potting grade: < 2.5 mmhos/cm
  • Potting media amendment: < 6 mmhos/cm
  • Top dressing: < 5 mmhos/cm
  • Soil amendment in a low salt soil: <10 mmhos/cm
  • Sodium – Sodium adsorption ratio less than 13%
  • Ammonium – Less than 1/3 of total nitrogen.  If higher, it may not be finished composting.
  • Heavy metals – A concern with biosolids but regulated by application permits.
  • Pesticide residues – Rarely a problem since they breakdown in composting. 
  • PathogensEscherichia coli and other human pathogens are a potential in manure.
  • Nutrient content varies greatly from product to product.
  • Germination test – Seeds are started to check potential of toxic chemicals.
  • Stability (respiration rate) vs. maturity – Relative measurement of the completeness of microbial activity.  If microorganisms are highly active, they may consume oxygen in the root zone causing root problems.
  • Bacterial and fungal diversity – Some compost has been found to suppress plant diseases.  This is a high-tech field with commercial applications.

Examples of soil amendments

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.

Peat

Sphagnum peat is a good soil amendment, especially for sandy soils, which will retain more water after sphagnum peat application.  Sphagnum peat is generally acidic (i.e., low pH) and may help gardeners grow plants that require a more acidic soil.  Sphagnum peat is harvested from bogs in Canada and the northern United States.  The bogs can be revegetated after harvest and grow back relatively quickly in this moist environment.  In recent years however, harvest rates have become so high that it is raising questions on renewability.

Colorado mountain peat is not an acceptable soil amendment.  It often is too fine in texture and generally has a higher pH.  Mountain peat is mined from high-altitude wetlands that will take hundreds of years to rejuvenate, if ever.  This mining is extremely disruptive to hydrologic cycles and mountain ecosystems.

Biosolids

Biosolids (sewage sludge) are a way to add slow release nutrients and organic matter to soil.  They are available from some communities or sewer treatment districts in bulk and from garden stores in bags.

Some biosolids are extremely high in salts.  For example, tests on MetroGro report a salt content of 38.3 dS/m (38.3 mmhos/cm), which is considerably above acceptable tolerances for soil amendments.  (A soil amendment above 10 dS/m is considered questionable.)  For details on salty soil amendments, refer to CMG GardenNotes #224, Saline Soils.

Biosolids typically have a 5-6% nitrogen content.  Annual applications should be made only when the biosolids and garden soil are routinely tested for salt content.

Compost, Manure, Cover Crops, and Green Manure Crops

Compost -- Homemade or commercial compost is a great source of organic matter for the garden. Compost provides a food source for beneficial soil organisms, enhancing the soil food web and releasing nutrients over the long term. For additional information, refer to CMG GardenNotes #243, Using Compost in the Home Garden and #246, Making Compost.

Manure -- For some gardeners, manure is readily available as a source of organic matter to build soils and add small amounts of nutrients. However, follow precautions with manure applications or they could become more detrimental than beneficial. For additional information, refer to CMG GardenNotes #242, Using Manure in the Home Garden.

Green Manure and Cover Crops -- A cover crop is simply a high numbers of plants (usually specific annual, biennial, or perennial grasses and/or legumes) growing and covering the soil surface. When the cover crop is tilled into the soil it is referred to as a green manure crop. For additional information, refer to CMG GardenNotes #244, Cover Crops and Green Manure Crops.

Worm castings

Versatile worm castings can be used in potted plants, soil mixes, and in garden beds.  Worm castings pose no threat of burning potted plants.  Worms should have digested the batch of vermicompost for 4 months to ensure that microbial oxygen consumption has diminished sufficiently.

Red worm castings are the feces from compost worms.  It has a slow release performance due to a mucus covering that is slowly degraded with microorganism activity.  It contains highly available forms of plant nutrients that are water-soluble, has a neutral pH, and contains trace elements, enzymes, and beneficial microorganisms.  The release time for nutrients is around 4 months.  For continual release of nutrients, repeat application at 4-month intervals.

Some batches made from livestock manure may have high salts depending if the animals producing the manure had access to a salt lick, and if the vermicompost maker leached them or not.

Castings can be applied as a top dressing, 1/4 inch deep, on potted plants, as 25% of a soil mix (1 to 4 mix) or tilled into a garden at 1 gallon per 13 square feet or 7.5 gallons (1 cubic foot) per 100 square feet.  Due to the high cost in Colorado, they are generally used in small gardens or potting mixes. 

Perlite and Vermiculite

Perlite and vermiculite are common inorganic amendments used in potting soils and planter mixes. 

Vermiculite is made from heat expanded silica (mica).  It is used to increase pore space and has a high water holding capacity.  Perlite is made from heat expanded volcanic rock.  It is used to increase pore space and has a low water holding capacity.

Summary: Considerations in Selecting Soil Amendments

Gardeners often inquire about the best amendments to use for their situation.  There really is not a best.  What is practical and available varies from gardener to gardener and within different communities.  The important aspects are that 1) soils are routine amended improving soil tilth and 2) the gardener follows the limitations for the specific product used.  The following summarizes considerations in selecting soil amendments:

  • Cost

  • Local availability
  • Cost of product
  • Size of area to be treated (quantity needed)
  • Depth of incorporation (application rate / quantity needed)
  • Transportation costs

  • Need for fertilizer after amending

  • Soil organic content

  • Precautions with specific products

  • Salts (manure and biosolids)
  • Weed seeds (manure and compost)
  • Plant pathogens (compost)
  • Human pathogens (manure)

  • Alternatives to amending

  • Potential to incorporate amendments
  • Accepting a reduction in plant growth and vigor
  • Accepting increased maintenance requirements
  • Selecting plants more tolerant of poor soils
  • Avoid crowding plants competing for limited soil resources
  • Mulching with organic mulch to slowly improve soil over time
  • Container and raised-bed gardening
  • Preventing compaction forces

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Authors: David Whiting (CSU Extension, retired) with Carl Wilson (CSU Extension, retired), and Jean Reeder, Ph.D. (USDA-ARS, retired). Artwork by David Whiting; used by permission.

  • CMG GardenNotes are available online at www.cmg.colostate.edu
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  • Copyright. 2013-2014. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised October 2014

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