Is Composting an Earth Friendly Method of Recycling Plant and Food Scrap Waste?by Terry Green on 05/28/13
How often have you heard that aerobic composting is Nature’s way of dealing with organic waste, that it is a sustainable and earth-friendly method for recycling organic plant-based waste such as food scrap, plant yard and agricultural debris back into soil? This blog looks at some of the limitations of composting in recycling organic waste back into soil relative to available alternative technologies available and used by Nature in doing the same job.
In Nature, detritus (dead plant and animal matter) gets recycled passively and with little energy input principally by a combination of scavenging, detritivore, herbivore and microbial metabolism of the waste. Nutrients in the waste recycle without human intervention into animal, insect, invertebrate and microbial biomass, and additionally end up back into soil. Aerobic oxidation akin to composting occurs only at the surface of decaying detritus lying on the ground. The availability of oxygen beneath the upper layers of the detritus is simply too low to support significant aerobic microbial activity.
Beneath the upper layer of the decaying detritus undergoing aerobic oxidation, facultative anaerobic microbes (microbes able to grow and survive in oxygen rich- and poor environments), and anaerobic microbes (microbes intolerant and unable to survive in oxygen-rich environments), actively grow off the waste, breaking up cellular structures and facilitating depolymerization of large macromolecules while also metabolizing and mineralizing smaller organic products released during the decomposition process. Carbon, nitrogen, phosphorous, trace minerals, and other essential elements necessary for the growth of healthy plants, pass with water released from disrupted cells directly into the soil as the cellular structures in the decaying detritus break down. Further processing of the nutrients taken up in the soil ensues.
Synergism among soil microfauna, including many opportunistic insects and invertebrates, all feeding off the decaying waste passed to the soil, increases as soil microfauna grow and thrive on nutrients delivered to the soil in this manner. Soil beneath and proximate to the decaying detritus also receives the benefits of plant available nutrients re-entering the soil. Additionally, nutrients entering the soil support and promote the growth of soil microfauna (including microbes involved in fixation of nitrogen, soil mites, invertebrates, etc.) beneath the decaying waste, which collectively by growing in population and diversity, amplify the turnover of detritus as it passes onto and into the soil.
Figure 1 is an example of healthy soil. It is not just finely ground dirt. On the contrary, healthy soil is rich in organic matter in varying states of decomposition. The organic matter in the soil is beneficial to the fertility of the soil not simply because of its nutrient value, but also because it provides water-retentive structure and channels water and gas exchange percolating through the soil, and it also is a rich feedstock in promoting the growth of beneficial microfauna noted above. Soil mites (see Fig. 1b), barely visible to the naked eye, are just one example of the many essential microfauna having a role in improving mineralization of detritus falling on the ground.
Fig. 1. An example of healthy, fertile soil. 1(a), note the presence of a rich-variety of organic matter in the form of detritus (leaves, wood and fibrous plant debris in varying stages of decay) which enhance water retaining capacity, which create channels for water percolation and gas exchange into the soil, and which also serve as feedstock in supporting healthy and abundant growth of a variety of soil microfauna; 1(b), soil mites (see arrows) can be seen feeding on detritus in the soil. Copyright (c) 2013 Terry Green, All rights reserved.
Composting, on the other hand, is a distortion of the natural process by which organic matter decays in Nature. Here aeration is stressed to hurry along the decomposition process as quickly as possible in ramping up the microbial oxidation aspect of the decomposition process that occurs in Nature. Yet it is obvious that composting, defined and practiced using microbial oxidative activity as the means of breaking down organic matter, is not the principal means Nature resorts to in recycling organic matter. Nowhere will you observe leaves, grass or woody debris piling up on the forest floor get fluffed up and aerated in the wild. Unlike the passive and less energy intensive recycling processes occurring in the wild, composting is energy intensive, requiring substantial work in forcibly turning over organic waste in order to ensure adequate oxygenation of the waste. Composting also requires constant intervention and management. Without human intervention and continuous energy input, microbial oxidation of organic waste would quickly wind down except at the very outer periphery of the compost pile. Composting in its simplest form is simply a process of controlled combustion – an incineration of organic waste, albeit at a slower rate and temperature than would otherwise be the case using an open flame.
Temperatures at the core of a compost pile typically exceed 160 F (71 C) as organic matter undergoes oxidation, temperatures not typically reached in the natural decomposition processes. Large amounts of carbon in the form of carbon dioxide, nitrogen in the form of nitrous oxide, nitric oxide, and ammonia, and to a lesser extent sulfur in the form of sulfur dioxide, discharge into the atmosphere. This loss in carbon, nitrogen and sulfur, subtracts away nutrient value that otherwise would have found its way back to the soil in replenishing that drawn from the soil. Copious amounts of water, itself a valuable resource, are also consumed in operating compost piles to keep them from overheating, and from drying out.On a quantitative basis, the venting of greenhouse gases and washout of nutrient solutes during composting operations are as much as 50+% of the waste’s starting mass (see Mass balances and life cycle inventory of home composting of organic waste).
Although pollution accompanying composting is not at first glance visually apparent, it occurs as would be expected of an incineration process, polluting principally the atmosphere with greenhouse gases and corrosive chemical byproducts. Given its off venting of gases into the atmosphere, the large amount of energy and labor required in maintaining a composting operation relative to Nature’s ways of passively recycling organic matter, composting, in short, cannot objectively be viewed as a fully sustainable recycling technology akin to that carried out in the wild.
In earlier blogs (see Black Soldier Fly Processing of Biodegradable Wastes, Processing Waste | Let the Black Soldier Fly Have At It!, and Recycling Food Scrap? | What's With the Stink?) we described how beneficial Black Soldier fly larvae efficiently decompose food scrap and agricultural debris, and how synergism between Black Soldier fly larvae and anaerobic fermentation processes work together to process these types of waste. These recycling technologies, insect-mediation combined with anaerobic processing of waste, appear in closer alignment with the way organic wastes are recycled in Nature. We furthermore noted how leachate can be used to treat and improve soil fertility (see Black Soldier Fly Processed Food Scap | Foilant and Soil Applications, Amending Soil With Black Soldier Fly Processed Food Scrap Leachate, and Soldier Fly Food Scrap Leachate | A Treasure Trove Amended in Soil).
One very interesting finding in returning food scrap and agricultural wastes back to soil using these alternate recycling technologies is the prospect of biologically fertilizing soils wherein production of essential nutrients such as plant available nitrogen and phosphorous is induced to occur within the soil in a more sustained manner, reducing the need for buying and adding NPK fertilizers back into the soil. Certainly, in the case of plant based organic wastes such as food scrap, yard and agricultural debris, when it comes to improving soil quality, we could learn a lot by paying closer attention to Nature’s example on how to recycle organic wastes into soil.
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