The Composting Process - USU

1 October 1995 Michael G. Pace, Graduate Research Assistant; Bruce E. Miller, Assistant Professor,Kathryn L. Farrell-Poe, Extension Environmental Engineer; Ag. Systems Tech. & Ed. is the aerobic, or oxygen-requiring, decompositionof organic materials by microorganisms under controlledconditions. During Composting , the microorganisms consumeoxygen (O2) while feeding on organic matter (Figure 1). Activecomposting generates considerable heat, and large quantities ofcarbon dioxide (CO2) and water vapor are released into the CO2 and water losses can amount to half the weight of theinitial materials, thereby reducing the volume and mass ofthe final 1. The Composting ProcessWhat Happens During CompostingComposting may begin as soon as the raw materials aremixed together. During the initial stages of the Process , oxygenand the easily degradable components of the raw materials arerapidly consumed by the temperature of the windrow or pile is directly related tothe microorganism activity of the windrow and is a goodindicator of what is going on inside.

2 The temperature of thecomposting materials generally follows a pattern of rapidincrease to 120-140oF where it is maintained for several weeksdepending on the materials (Figure 2). As active compostingslows, temperatures will gradually drop until the compostreaches ambient air curing period usually follows the active compostingperiod. During the curing period, the materials will continue toslowly decompose. Materials continue to break down until thelast easily decomposed raw materials are consumed by theremaining microorganisms. At this point, the compost becomesrelatively stable and easy to 2. Compost TemperatureFactors Affecting the Composting Process1. Oxygen and AerationAerobic Composting consumes large amounts of oxygen,particularly during the initial stages. If the supply of oxygen islimited, the Composting Process may turn anaerobic, which is amuch slower and odorous Process . A minimum oxygen concen-tration of 5% within the pore spaces of the compost is necessaryfor aerobic Composting .

3 Oxygen levels within the windrows orpiles may be replenished by turning the materials over with afront-end loader, or by means of mechanical agitation with aspecial compost C:N RatioCarbon (C), nitrogen (N), phosphorous (P), and potassium(K) are the primary nutrients required by the microorganismsinvol-ved in Composting . Microorganisms use carbon for bothenergy and growth, while nitrogen is essential for protein pro-duction and reproduction. The ratio of carbon to nitrogen isreferred to as the C:N ratio. An appropriate C:N ratio usuallyensures that the other required nutrients are present in materials blended to provide a C:N ratio of 25:1 to 30:1are ideal for active Composting , although initial C:N ratios from20:1 up to 40:1 consistently give good Composting results. ForC:N ratios below 20:1, the available carbon is fully utilizedwithout stabilizing all of the nitrogen which can lead to theproduction of excess ammonia and unpleasant odors. For C:Nratios above 40:1, not enough N is available for the growth ofmicroorganisms and the Composting Process slows 013.

4 MoistureMoisture is necessary to support the metabolic processes ofthe microbes. Composting materials should be maintainedwithin a range of 40% to 65% moisture. Experience has shownthat the Composting Process becomes inhibited when themoisture content is below 40%. Water displaces much of the airin the pore spaces of the Composting materials when themoisture content is above 65%. This limits air movement andleads to anaerobic conditions. Moisture content generallydecreases as Composting proceeds; therefore, you may need toadd additional water to the compost. As a rule of thumb, thematerials are too wet if water can be squeezed out of a handfuland too dry if the handful does not feel moist to the Particle SizeThe rate of aerobic decomposition increases with smallerparticle size. Smaller particles, however, may reduce theeffectiveness of oxygen movement within the pile or Composting conditions are usually obtained withparticle sizes ranging from 1/8 to 2 inches average TemperatureComposting will essentially take place within two tempera-ture ranges known as mesophilic (50-105oF) and thermophilic(over 105oF).

5 Although mesophilic temperatures allow effectivecomposting, experts suggest maintaining temperatures between110o and 150o. The thermophilic temperatures are desirablebecause they destroy more pathogens, weed seeds and fly larvaein the Composting the temperature of your compost pile is in the mesophilicrange, try mixing the pile. If the temperature still does notreach the thermophilic range, review the factors describedabove to determine whether one or more of the factors is limi-ting the Composting Process . If you are still unable to increasethe compost s temperature, the active stage of Composting maybe TimeThe length of time required to transform raw materials intocompost depends upon the factors listed above. In general, theentire decomposition and stabilization of materials may beaccomplished within a few weeks under favorable conditions;but, research at Utah State University has shown that 10-14weeks of active Composting for dairy cattle waste is more com-mon. Active Composting will change depending upon theamount of natural moisture or water added to the compost,turning frequency, materials being composted, and 1.

6 Guidelines for Composting Range*Preferred RangeCarbon to nitrogen (C:N)ratio20:1-40:125:1-30:1 Moisture content40-65%**50-60%Oxygen concentrationsGreater than 5%Much greater than 5%Particle size (diameter in inches)1/8-1/2 Varies** (oF)110-150130-140 Source: On-Farm Composting Handbook, Northeast Regional Agricultural Engineering Service (NRAES-54)*The recommendations are for rapid Composting . Conditions outside of these ranges can also yield successful results.**Depends on the specific materials, pile size, and/or weather State University is an Equal Opportunity/AffirmativeAction InstitutionIssued in furtherance of Cooperative Extension work, Acts of May 8 andJune 30, 1914, in cooperation with the Department of Agriculture,Robert L. Gilliland, Vice President and Director, Cooperative Extension Service,Utah State University. (EP/10-95/HBH)CuringWhen windrows or piles no longer reheat after turning, thecuring stage begins. The curing stage of compost usually lasts 3to 4 weeks.

7 Curing is a very important and often neglected partof the Composting Process . Curing occurs at mesophilictemperatures. The importance of curing increases if the activecomposting stage is either shortened or poorly compost can contain high levels of organic acid, ahigh C:N ratio, and other characteristics which can be damag-ing to crops and as a soil amendment vs. fertilizerMost plant nutrients in compost are in an organic compost is not high in nitrogen, phosphorous, orpotassium, (it contains approximately 2% of each) thesenutrients are released slowly over a long period of become available to plant roots at a slower rate withcompost compared to inorganic fertilizers, therefore thenutrients are less likely to leach out of the soil. Only a fractionof the nitrogen, phosphorus, and potassium applied as compostis usable by the crop the first year with more becoming avail-able in the years that real benefit of adding compost to the soil lies in itsability to increase soil organic matter levels.

8 Research studiesat the Connecticut Agricultural Experiment Stationa haveshown that a 1 inch thick layer of leaf compost annually appliedand incorporated into the soil over a 12 year period increasedthe organic matter content from to The samestudies have shown that the water holding capacity of the soilwas increased from inches to inches of water per foot ofsoil after seven years of compost Benefits of Compost include: Improved manure handling Possible saleable product Improved land application Weed seed destruction Pathogen destruction Lower risk of pollution problems Excellent soil conditioner Possible revenue from tipping feesa Maynard, A. A. & Hill, D. E. (1994). Impact of compost on vegetable , 35(3), 66-67.