[USCC] technical definitions of finished compost

[David Schellinger]

Hi Cathy.  

Definitions of composting processes and compost will differ significantly
between educators and industry personnel.  This has a lot to do with the
fact that no two composting processes are the same.  Feedstocks compost
differently, and numerous environmental, chemical and physical factors
combine to influence the composting process.  Slight changes in composting
process management also influence the rate of composting and the properties
of the end product.  However, two fundamental principals are discussed in
most composting educational materials:  compost stability and compost
maturity.    

A simple way to express the science of compost stability when feedstock
nutrient balance is optimized in the initial feedstock mix (Carbon:Nitrogen
= 30:1 to 40:1):  when the readily available organic carbon sources for
microbiological energy production have been depleted from the feedstocks to
the point where microbiological populations and metabolic activity are not
sufficient to maintain thermophilic conditions in the materials.  In short,
as materials become stable, the average temperature in the materials
decreases over time, and pore space oxygen usually increases due to reduced
microbiological use.  If insufficient nitrogen is added to the initial
feedstock mix, elevated degradable organic carbon may remain in the finished
compost.  When added to soils or potting mixes, the organic carbon may
degrade in the soil and utilize nutrients that are meant for plant use.  

Though many experts may differ with this assessment, maturity of composted
materials can only occur after compost has become relatively stable.  Some
experts determine immature compost as containing significant concentrations
of organic acids produced during composting.  Because these acidic compounds
are readily available organic carbon sources for microbiological use, the
presence of the organic acids denotes instability.  However, significant
concentrations of ammonia or sulfur compounds, elevated or low pH levels, or
excessive electrical conductivity (soluble salts) may be expected in stabile
compost, and these properties will influence the ability of plant seeds to
germinate and plants to survive in the compost.  Feedstock mixes with low
C:N (<30:1) will typically contain more elevated plant toxins in the stable
compost.  The curing process relies on low levels of microbiological
activity, activity from slow growing microorganisms and increased aeration
properties in the compost to reduce the harmful levels of the phytotoxic
compounds.  The cured compost is also called mature compost.  

Typically, large scale composting operations will observe temperatures in
composting materials in excess of 145 degrees F for periods of time and over
several turning cycles.  This temperature and turning activity is important
for reducing the viable weed seed in the finished compost.  Temperatures of
131 degrees various periods and turning cycles is used to destroy pathogenic
microorganisms in feedstocks containing animal mortalities, food waste,
human or animal excrement, or other materials.  In small scale processes
such as backyard composting systems it is often difficult to achieve even
130 degrees F.  Therefore, backyard composting of animal wastes or meat
products is not recommended.  

For further assistance I would recommend obtaining the valuable literature
available on the US Composting Council web site.  I hope I've been of some
assistance.

Dave Schellinger



-----Original Message-----
From: Cathy Gill [mailto:cathy at athenaes.com] 
Sent: Wednesday, November 28, 2007 2:58 PM
To: Compost Discussion List
Subject: [USCC] technical definitions of finished compost

Hi all! 

You've been so helpful in the past so I was wondering if you wouldn't mind
answering a couple of more questions for me.

What is the technical definition of when compost is "finished"? does it have
to reach a certain temperature and then cool? Or is it based on levels of
microorganisms that are active? I am very interested in knowing the science
behind this.

Also, I have read somewhere that, at least in a home setting, compost that
is finished may still need to be cured. How does one know when it is cured?
What happens during the curing process? 

Thanks in advance!

Cathy Gill

AthenaES
1450 S. Rolling Road
Baltimore, MD 21227
410-455-6319
www.athenaes.com
cathy at athenaes.com