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 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 a soil amendment that guarantees the minimum percentages of nutrients (at least the minimum percentage of nitrogen, phosphate, and potash). 

An organic fertilizer refers to a soil amendment derived from natural sources that guarantees 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 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 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.

Application rate depends on the product.  Excessive salt levels are a common issue on manure, biosolids, and compost made with manure or biosolids.  On these products, do not exceed 1 inch per year (cultivated into the upper six to eight inches of soil) unless the product is know to be low in salts.  Many commercially available products in Colorado have high salts!

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 a soil amendment

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

Evaluating the quality of organic amendments

The quality of organic amendments can be determined by both visual evaluation and lab 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 residue – Rarely a problem since they breakdown in composting.  Long-term residues of chlordane have shown up in some samples.
Pathogens – E-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 active, it will 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.

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.

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 acid (i.e., low pH) and can 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 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.

Under state and federal laws, use of biosolids is carefully monitored to prevent toxic levels of heavy metals.  Special permits are required by the city (or sewer treatment district) or the farmer applying biosolids.  To protect from metals and pathogens, use only the top grade biosolids, Class A (federal standard) or Class 1 (state standard). 

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

Compost, cover crops, green manure crops, manure, and organic fertilizers

For details on these soil amendments, refer to the following CMG GardenNotes:

• #241,   Soil Amendments
• #242,   Using Manure
• #243,   Using Compost
• #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 which 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, ¼ 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. 

Additional Information – CMG GardenNotes on Soils, Fertilizers and Soil Amendments:

#211 The Living Soil
#212 Earthworms
#213 Managing Soil Tilth
#214 Estimating Soil Texture
#215 Soil Compaction
#216 Mulching with Wood/Bark Chips, Grass Clippings and Rock
#217 Soil Drainage
#221 Soil Tests
#222 Soil pH
#223 Iron Chlorosis
#224 Saline Soils
#231 Plant Nutrition
#232 Understanding Fertilizers
#233 Calculating Fertilizer Rates
#234 Organic Fertilizers
#241 Soil Amendments
#242 Using Manure
#243 Using Compost
#244 Cover Crops and Green Manure Crops

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