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Choose the performance need(s) you are interested in to see a list of
characteristics you should look for in terms of feedstock type, salinity,
composting method, stability/maturity, nutrients and pH.
- Feedstock Type (Compost Ingredients)
The nutrients in the feedstock will determine the available nutrients in the final
product. Compost made from manures and biosolids is frequently higher in nitrogen (N) than
that made primarily from yard trimmings or wood.
- Salinity
Not a significant factor
- Effect of Composting Method
The anaerobic compost process (exposed to little or no oxygen) generates significant
amounts of ammonia (NH3) that are released into the atmosphere, leaving less
nitrogen in the compost product. Compost that is produced through an aerobic process
(exposed to adequate amounts of oxygen) generates less volatile ammonia.
- Stability/Maturity
Stable and mature compost contains a variety of available macronutrients, such as carbon
(C), nitrogen (N), phosphorus (P) and potassium (K). Compost also contains
micronutrients, such as copper (Cu), iron (Fe) and zinc (Zn). Unstable compost can
immobilize nitrogen and make it unavailable for plant use. See section on assessing stability/maturity.
- Nutrients
Most compost suppliers will give an analysis of major nutrient content in compost.
However, not all of the nutrients are available for plant use.
- pH
The pH of the growing medium plays a large role in the availability of plant nutrients.
In general, the pH of the compost should be greater than 5. However, pH of
the soil should be taken into account.
- Feedstock Type (Compost Ingredients)
Some research suggests that carbon-rich feedstocks produce a compost with a higher fungal
content and that nitrogen-rich feedstocks produce a compost with a higher bacterial
content. More diverse feedstock may result in a more diverse population of
microorganisms. Carbon-rich feedstocks include leaves and yard trimmings.
Nitrogen-rich feedstocks include manures.
- Salinity
Not a significant factor
- Effect of Composting Method
Aerobically composted material is more likely to contain beneficial microorganisms than
anaerobically digested or processed materials. Large compost piles may
be more difficult to keep aerobic. However, large piles may be kept
aerobic through frequent turning. Refer to the "Compost
Quality Standards" publication listed on the previous page for information
on desirable microbiological test results or the Compost Quality
Guidelines provided by Organic Ag Advisors and BBC Laboratories, Inc.
- Stability/Maturity
In general, beneficial microorganisms are found in mature compost. See assessing stability/maturity for information on
assessing maturity.
- Nutrients
Not a significant factor.
- pH
Alkaline (pH greater than 7) soils tend to be dominated by bacteria. Acidic (pH
less than 7) soils tend to be dominated by fungi.
- Feedstock Type (Compost Ingredients)
Compost made from animal manure may not be appropriate for seed germination and
transplants since it is frequently higher in salinity than compost made from yard
trimmings and woody debris. Salinity should be assessed through lab analysis.
- Salinity
.75-3 dS/m (deciSiemens/meter or millimhos/centimeter) is best for seedlings and young,
tender plants. This should be the salinity of the final growing medium after compost
has been applied and blended.
- Effect of Composting Method
Aerobically composted material is most appropriate for seed germination and/or
transplants. Compost that is intentionally, or unintentionally, anaerobically
composted may contain ammonia and volatile organic acids that are considered toxic to
plants. A smell of ammonia can indicate an immature and/or anaerobically composted
material.
- Stability/Maturity
Stable and mature compost in combination with fertile soil or potting mix can be safely
used for seed germination and/or transplants. Immature composts usually contain more
growth-inhibiting substances that can be toxic to plants. A germination test is
recommended prior to compost use. See assessing
compost stability/maturity.
- Nutrients
Too much N can sometimes kill seedlings. In most cases, compost should be blended
with soil prior to use for new growth. A standard blend for nursery use is 15% to
25% compost mixed with a standard wood residual.
- pH
pH less than 5 is likely to be immature and contain growth-inhibiting substances that can
be toxic to plants. Best results are likely with compost that is neutral to alkaline
(pH greater than or equal to 7).
- Feedstock Type (Compost Ingredients)
The type of feedstock composted will determine the likelihood and the type of
noxious weed seeds and pathogens that may be present prior to composting. E. coli
and salmonella are most prevalent in manure and biosolid feedstock. Seeds would be
most likely in yard trimming feedstock.
- Salinity
Not a significant factor.
- Effect of Composting Method
Most weed seeds and pathogens are killed by high temperatures during the thermophilic
stage of the composting process. However, some weed seeds are resistant to high
temperature. Pursuant to State regulations (Title 14, California Code of
Regulations, Section 17868.3), compost must be exposed to high temperatures for specified
periods of time to kill weed seeds and pathogens:
Windrow method (elongated piles of compostable material that is turned on a
periodic basis): 131 degrees F (55 degrees C) for 15 days with a minimum of 5 turnings.
In-vessel method (compostable material that is enclosed in a container): 131
degrees F (55 degrees C) for 3 days.
Aerated static pile method (compostable material exposed to an air distribution
system that either blows or draws air through the material): 131 degrees F (55 degrees C)
for 3 days.
- Stability/Maturity
Stable compost is unlikely to contain viable weed seed or plant pathogens.
- Nutrients
Not a significant factor.
- pH
Not a significant factor.
- Feedstock Type (Compost Ingredients)
Contaminants in the compost product are dependent upon the type and cleanliness of the
feedstock used and the level of grinding and screening. Visual inspection may
identify some contaminants, such as plastic bags.
Trace Elements: Biosolids may be significantly higher in trace elements,
including heavy metals, than yard trimmings. However, heavy metals in compost
produced by a permitted facility cannot exceed U.S. EPA Part 503 maximum levels.
Glass, Plastic, and Metal Objects: Yard trimmings vary in the degree of inert
contamination. Plastic sheeting from trash bags is common. However, most of
these contaminants can be removed by the processor through adequate screening.
- Salinity
Not a significant factor.
- Effect of Composting Method
Not a significant factor.
- Stability/Maturity
Not a significant factor.
- Nutrients
Not a significant factor.
- pH
Not a significant factor.
- Feedstock Type (Compost Ingredients)
Most compost is high in organic matter. Higher organic matter content in the compost
will increase water-holding capacity. It also improves soil structure in both clay
and sandy soils. Because it can break up clay soils, organic matter can help improve
drainage. Organic matter also increases cation exchange capacity (CEC). Soils
with a high CEC hold onto nutrients and reduce leaching to groundwater. Soil added
to the compost may decrease total organic matter.
- Salinity
Not a significant factor.
- Effect of Composting Method
Compost that is ground and/or screened to a smaller particle size (less than ½ inch)
may improve cation exchange capacity (CEC).
- Stability/Maturity
Not a significant factor.
- Nutrients
Not a significant factor.
- pH
An acidic compost (pH less than 7) can have a higher cation exchange capacity (CEC) than
an alkaline compost. At high rates of application, the CEC from compost may be a
significant proportion of the total CEC in the soil, especially on sandy substrates.
- Feedstock Type (Compost Ingredients)
The composition of the feedstock will determine the salinity of the final compost.
The compost process actually concentrates salts so the finished product is higher in
salinity than the starting material. Compost made from animal manure may not be
appropriate in specific situations since it is frequently higher in salinity than compost
made primarily from yard trimmings or woody debris.
- Salinity
Certain crops may not tolerate high-salinity compost. For most established crops,
electrical conductivity (used to measure salinity) of the growing medium (after compost
has been applied and blended) should not exceed 5 dS/m.
- Effect of Composting Method
Not a significant factor.
- Stability/Maturity
Not a significant factor.
- Nutrients
Not a significant factor.
- pH
Not a significant factor.
Compost Performance Requirements
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