A. Temperature

At any given time, the temperature of the pile reflects the balance between microbial heat generation and the loss of heat to the surroundings. The rate of heat generation is a function of factors such as temperature, oxygen, water, nutrients, and the remaining concentration of easily biodegradable organic materials. The rate of heat loss is a function of factors such as ambient temperature, wind velocity, and pile size and shape.

Temperature is a powerful determinant of the rate of decomposition. Temperatures of less than 20øC (68øF) slow decomposition. Temperatures above 60øC (140øF), which is hotter than the setting of most home hot water heaters, are also unfavorable because they kill most of the desirable microorganisms. The range of favorable temperatures is approximately 20 to 60øC (or about 70 to 140øF). Precise control over temperature usually is not essential for leaf composting, but gross departure from the desired range should be avoided. Maintenance of the proper temperature, along with oxygenation, is the basic consideration underlying the recommendations for windrow size and turning operations (see Sections III.C and V). If precise measurements of pile temperature are required, the County Extension Office should be consulted (see Appendix D).

Grass clippings are a more "energetic" material (capable of generating more heat) than leaves, and are produced and composted during the warmer part of the year. Overheating is thus more likely and under heating less likely than for leaves. This, along with the need for increased oxygen supply, is why a smaller pile size is recommended when grass clippings are included in 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 of the heat produced in composting results from the biodegradation of organic materials with consumption of oxygen and production of carbon dioxide and water. Thus, the pile must be sufficiently porous to allow oxygen (from the air) in and carbon dioxide out. For this reason, materials should be placed loosely in the piles and compaction should be avoided.

In the absence of oxygen, anaerobic conditions occur. This can lead to odor production and slowed rates of decomposition. 

C. Windrow Size and Turning

For leaves, control over process temperature and oxygen content can be exercised to a useful extent (though they are not optimized) through windrow size and turning. A basic problem is to reconcile the needs for oxygenation and heat conservation, which are somewhat in conflict. The need for oxygenation favors small windrows to minimize the distance that air must penetrate within the pile. 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 size and turning. Specific recommendations are given in Section V. For almost all composting, windrows should be no more than 6 feet high and 12-14 feet wide.

 D. Water

Water is essential for biological functions in general, and composting is no exception. Adding water (when needed) at the start of composting is very important to insure adequate moisture throughout the pile at the time of its formation and thereafter. Rainfall, even if heavy, penetrates the pile only slowly and cannot be relied upon to remedy initial dryness. Similarly, once a pile is formed, the interior material is not easily wetted by applying water 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 a common and serious cause of slow leaf composting rates, and as such should be prevented. An initial moisture content of at least 50% (wet weight basis) is recommended.

Leaves also can be excessively wet, slowing oxygen penetration (see Section III.B). This condition is self-correcting, as excess water drains from the pile. Depending on weather conditions prior to collection, the leaves might be sufficiently moist upon receipt, but this cannot be relied upon in routine operation. In general, it is better to start with a pile that is too wet than to risk dryness.

Specific recommendations for providing a water supply and for adding water prior to windrow formation are given in Sections IV.I and V.B.3 and Appendix A.

 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, with the onset of decomposition even prior to composting, the production of organic acids causes the pH to decline to suboptimal levels, possibly to as low as 4.2 if extensive anaerobic conditions develop. The pH subsequently recovers to a neutral or slightly alkaline level (perhaps pH 7.5) as the acids decompose in the presence of oxygen. A persistently acidic pH is indicative of prolonged anaerobic conditions. Adjustment of the pH with limestone or other additives is not ordinarily necessary.

Composting of high nitrogen materials such as grass clippings may lead to pHs as high as 8.5-9.5 as ammonia is released. Mixing with leaves will help control this excessive pH rise, as well as reduce ammonia loss.

 F. Inorganic Nutrients

Microbial activity also requires a variety of other elements, such as nitrogen and phosphorus. Leaves have a high carbon-to-nitrogen ratio (C/N), which can slow microbial action early in the composting period. This nutritional imbalance rights itself as carbon is lost in the form of carbon dioxide, while nitrogen is conserved within the system. Supplementation with nitrogen at the outset would accelerate decomposition, but this measure is not generally necessary. It might be justified, in conjunction with other measures, if the resultant saving in process time were essential for the success of the overall operation (see Section V.D). The increased rate of decomposition from nitrogen addition could lead to other problems, such as an increased need for oxygen supply, which would also then have to be addressed. Otherwise, slow decomposition and odors might result. Appreciable deficiency of other nutrients such as phosphorus is not likely.

Supplementing the end-product compost with nitrogen, phosphorus and potassium would increase its quality in terms of plant nutrition. This benefit has to be weighed against the costs of such additions.

Grass clippings, on the other hand, contain excess nitrogen and thus have an undesirably low C/N. Unless sufficient available carbon (such as from leaves) is added, ammonia will be lost from the material, producing potential odor problems. The nitrogen also may contaminate ground or surface waters.

 G. Microorganisms

Microorganisms found on leaves and yard trimmings are fully capable of starting the 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 controlled experimentation indicates that inoculation has no useful effect on the process. Therefore, such products should not be purchased for leaf or yard trimmings composting operations.

 H. Leaf Type

Maple leaves decompose more rapidly than oak leaves, and other leaf types doubtless differ in this respect. Mixtures would ordinarily be received at a leaf composting facility, and no specific recommendation is made based solely on leaf type.

 I. Pregrinding

Pregrinding or shredding of leaves make them more susceptible to microbial attack, potentially speeding up the composting process. This is not desirable in most cases, unless provision has been made for very frequent turning or blowers to supply the extra oxygen that will be needed, and remove the extra heat that will be generated. It is normally not recommended, and the guidelines given later assume no pregrinding. If any pregrinding is done, smaller piles are recommended. The equipment typically used for the final shredding of finished compost (see Section V. B.8) usually is not suitable for shredding of leaves prior to composting.

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