Agriculture Reference
In-Depth Information
As temperature gradually declines to around 40 °C the mesophilic microorgan-
isms recolonize the pile and the composted materials enter the curing phase. The
rate of oxygen consumption declines to the point where compost can be stockpiled
without turning. During this phase, organic materials continue to decompose and
are converted to biologically-stable humic-like substances. A long curing phase is
needed if the compost is unfinished or immature; this can happen if the pile has
received too little oxygen or too little/much moisture. There is no clearly defined
time for the curing phase but common practices in commercial composting opera-
tions range from one to four months (Cooperband
2002
). Maturation phase could be
controlled by windrow turning and moisture check for obtaining stabilized organic
matter in good conditions especially for the compost screened into smaller sizes.
Compost is considered finished when the raw feedstock is no longer actively
decomposing and are biologically and chemically stable. When the temperature at
the center of the pile returns to near-ambient levels and oxygen concentration in the
middle of the pile remain greater than 10-15 % for several days, compost is con-
sidered stable or finished. These measurements should be taken when the compost
pile has at least 50 % moisture content by weight (Cooperband
2002
). In general,
the composting process could be completed in a 3- to 4-month period. The litera-
ture provides different scientific works that discuss methods to determine compost
stability or parameters to be considered to assess when composting process is fin-
ished. Compost stability assessment through CO
2
production or molecular oxygen
demand indices are much more important in the industrial processes monitoring
than in the on-farm composting as they indicate the minimum period to finish a
process and to start a new one. During on-farm composting the temperature trend is
the most practical way to decide when a composting process is finished, however,
a germination assay should be carried out especially if compost will be used as a
nursery substrate component.
1.3.1
Starting Mix C/N Ratio: The Compost Secret
Decomposing microorganisms: bacteria (including actinomycetes) and fungi are
the main actors in the composting process, they require beside C and N other mac-
ro- and micronutrients for their growth. Carbon compounds provide energy for the
metabolism and in most of the cases it is in excess while the N is the critical limiting
element for microbial growth: if it is supplied in an insufficient amount the decom-
position process will slow down, if it supplied in excess N will be volatilized as
ammonia or leached as nitrate. It can be assumed that a mixture of plant materials
should contain the essential level of P, K, S, and other trace elements, however, type
and source of input feedstock affect the microbial community during the compost-
ing process according to their starting C/N ratio.
The aerobic metabolism of a generic decomposer microorganism growing on
glucose substrate might have a net yield of 0.1-0.2 g of new microbial mass
per each gram of substrate (Haug
1993
). Based on this assumption, it has been
