III. UNDERLYING SCIENTIFIC PRINCIPLES
A. Temperature
At any given time, the temperature of the pile reflects the balance betweenmicrobial heat generation and the loss of heat to the surroundings. The rateof heat generation is a function of factors such as temperature, oxygen,water, nutrients, and the remaining concentration of easily biodegradableorganic materials. The rate of heat loss is a function of factors such asambient temperature, wind velocity, and pile size and shape.
Temperature is a powerful determinant of the rate of decomposition. Temperaturesof less than 20øC (68øF) slow decomposition. Temperatures above60øC (140øF), which is hotter than the setting of most homehot water heaters, are also unfavorable because they kill most of the desirablemicroorganisms. The range of favorable temperatures is approximately 20 to60øC (or about 70 to 140øF). Precise control over temperatureusually is not essential for leaf composting, but gross departure from thedesired range should be avoided. Maintenance of the proper temperature, alongwith oxygenation, is the basic consideration underlying the recommendationsfor windrow size and turning operations (see Sections III.C and V). If precisemeasurements of pile temperature are required, the County Extension Officeshould be consulted (see Appendix D).
Grass clippings are a more "energetic" material (capable of generating moreheat) than leaves, and are produced and composted during the warmer partof the year. Overheating is thus more likely and under heating less likelythan for leaves. This, along with the need for increased oxygen supply, iswhy a smaller pile size is recommended when grass clippings are includedin a windrow.
B. Oxygen
Composting is basically an aerobic process (requires oxygen), although anaerobic(without oxygen) activity also may occur to a significant extent. Most ofthe heat produced in composting results from the biodegradation of organicmaterials with consumption of oxygen and production of carbon dioxide andwater. Thus, the pile must be sufficiently porous to allow oxygen (from theair) in and carbon dioxide out. For this reason, materials should be placedloosely in the piles and compaction should be avoided.
In the absence of oxygen, anaerobic conditions occur. This can lead to odorproduction and slowed rates of decomposition.
C. Windrow Size and Turning
For leaves, control over process temperature and oxygen content can be exercisedto a useful extent (though they are not optimized) through windrow size andturning. A basic problem is to reconcile the needs for oxygenation and heatconservation, which are somewhat in conflict. The need for oxygenation favorssmall windrows to minimize the distance that air must penetrate within thepile. In contrast, the need for heat conservation, especially in the winter,argues for large windrows for greater insulation. Excessively large windrows,however, might result in excessively high temperatures and anaerobic conditions.These requirements can be reconciled in part by management of windrow sizeand turning. Specific recommendations are given in Section V. For almostall composting, windrows should be no more than 6 feet high and 12-14 feetwide.
D. Water
Water is essential for biological functions in general, and composting isno exception. Adding water (when needed) at the start of composting is veryimportant to insure adequate moisture throughout the pile at the time ofits formation and thereafter. Rainfall, even if heavy, penetrates the pileonly slowly and cannot be relied upon to remedy initial dryness. Similarly,once a pile is formed, the interior material is not easily wetted by applyingwater to the surface. Unless a pile is turned during or shortly after wetting,much of the water will simply evaporate to the air. Initial dryness is acommon and serious cause of slow leaf composting rates, and as such shouldbe prevented. An initial moisture content of at least 50% (wet weight basis)is recommended.
Leaves also can be excessively wet, slowing oxygen penetration (see SectionIII.B). This condition is self-correcting, as excess water drains from thepile. Depending on weather conditions prior to collection, the leaves mightbe sufficiently moist upon receipt, but this cannot be relied upon in routineoperation. In general, it is better to start with a pile that is too wetthan to risk dryness.
Specific recommendations for providing a water supply and for adding waterprior to windrow formation are given in Sections IV.I and V.B.3 and AppendixA.
E. pH
Fresh leaves are close to being chemically neutral (neither acidic nor basic,pH near 7), which is desirable for rapid microbial activity. However, withthe onset of decomposition even prior to composting, the production of organicacids causes the pH to decline to suboptimal levels, possibly to as low as4.2 if extensive anaerobic conditions develop. The pH subsequently recoversto a neutral or slightly alkaline level (perhaps pH 7.5) as the acids decomposein the presence of oxygen. A persistently acidic pH is indicative of prolongedanaerobic conditions. Adjustment of the pH with limestone or other additivesis not ordinarily necessary.
Composting of high nitrogen materials such as grass clippings may lead topHs as high as 8.5-9.5 as ammonia is released. Mixing with leaves will helpcontrol this excessive pH rise, as well as reduce ammonia loss.
F. Inorganic Nutrients
Microbial activity also requires a variety of other elements, such as nitrogenand phosphorus. Leaves have a high carbon-to-nitrogen ratio (C/N), whichcan slow microbial action early in the composting period. This nutritionalimbalance rights itself as carbon is lost in the form of carbon dioxide,while nitrogen is conserved within the system. Supplementation with nitrogenat the outset would accelerate decomposition, but this measure is not generallynecessary. It might be justified, in conjunction with other measures, ifthe resultant saving in process time were essential for the success of theoverall operation (see Section V.D). The increased rate of decompositionfrom nitrogen addition could lead to other problems, such as an increasedneed for oxygen supply, which would also then have to be addressed. Otherwise,slow decomposition and odors might result. Appreciable deficiency of othernutrients such as phosphorus is not likely.
Supplementing the end-product compost with nitrogen, phosphorus and potassiumwould increase its quality in terms of plant nutrition. This benefit hasto be weighed against the costs of such additions.
Grass clippings, on the other hand, contain excess nitrogen and thus havean undesirably low C/N. Unless sufficient available carbon (such as fromleaves) is added, ammonia will be lost from the material, producing potentialodor problems. The nitrogen also may contaminate ground or surface waters.
G. Microorganisms
Microorganisms found on leaves and yard trimmings are fully capable of startingthe composting process and carrying it forward. A variety of commercial"inocula", "starters", and "bioaugmentation" products are offered for sale,and based on testimonials, these are often claimed to be beneficial. However,there is no support for these claims in scientific journals. Properly controlledexperimentation indicates that inoculation has no useful effect on the process.Therefore, such products should not be purchased for leaf or yard trimmingscomposting operations.
H. Leaf Type
Maple leaves decompose more rapidly than oak leaves, and other leaf typesdoubtless differ in this respect. Mixtures would ordinarily be received ata leaf composting facility, and no specific recommendation is made basedsolely on leaf type.
I. Pregrinding
Pregrinding or shredding of leaves make them more susceptible to microbialattack, potentially speeding up the composting process. This is not desirablein most cases, unless provision has been made for very frequent turning orblowers to supply the extra oxygen that will be needed, and remove the extraheat that will be generated. It is normally not recommended, and the guidelinesgiven later assume no pregrinding. If any pregrinding is done, smaller pilesare recommended. The equipment typically used for the final shredding offinished compost (see Section V. B.8) usually is not suitable for shreddingof leaves prior to composting.
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