Composting and the utilization of compost stand as powerful tools within nutrient management strategies, holding the potential to not only enhance crop growth but also mitigate the risk of environmental pollution when executed effectively.
Compost, at its core, is a blend of organic residues encompassing elements such as manure, animal carcasses, and straw, among others. This mixture is methodically piled, blended, and hydrated to initiate thermophilic decomposition, characterized by elevated temperatures ranging from 113 to 160 degrees Fahrenheit (F) (SSSA, 1997).
While this discussion centers on composting animal manures, it's worth noting that information regarding composting animal carcasses can be found in NDSU Extension publications titled "Animal Carcass Disposal Options" (NM1422) and "4 Easy Steps for Composting Dead Livestock" (AS1781).
The essence of composting lies in the consistent infusion of oxygen, which stimulates aerobic microorganisms. These microorganisms feed on organic components, transforming the amassed organic matter into a remarkably stable, nutrient-rich soil amendment (Larney and Blackshaw, 2003). This enriched compost can serve as a fertilizer on agricultural fields, enhance soil structure, substitute peat in horticulture, and act as a catalyst for enzyme activities (Steger et al., 2007).
Advantages of Compost
Introducing compost to the soil has multifaceted benefits. It augments fertility, augments water retention capacity, optimizes bulk density, and bolsters biological properties (Flavel and Murphy, 2006). Furthermore, the olfactory impact is mitigated, and the potential for fly egg survival is curtailed due to the high temperatures achieved during microbial degradation (Larney et al., 2006). Importantly, composting serves as an effective means to eliminate internal parasites present in manure, significantly reducing the threat of health issues in animals. Parasite eggs are swiftly eradicated when temperatures exceed 104 F, leading to population reduction and preventing further spread (Nielsen et al., 2007).
Controlling weed proliferation is another advantage linked to composting. Certain weed seeds can pass through animals and flourish in manure-applied croplands. However, in properly composted manure, weed seed viability is significantly diminished, potentially reducing the necessity for extensive herbicide use or tillage for weed control. For instance, research conducted by Larney and Blackshaw (2003) demonstrated that weed seeds, including downy brome, false cleavers, foxtail barley, scentless chamomile, wild mustard, and wild oat, failed to germinate after 21 days of composting.
Size Reduction and Pathogen Elimination
Composting entails a considerable reduction in manure volume and density, often reaching a range of 50-65% (Figures 1 and 2). This reduction translates to diminished hauling costs. Notably, a case study by Wiederholt et al. (2009) compared the energy demands of a feedlot operation using raw manure versus composted manure for agricultural field applications. The study concluded that composting and applying livestock compost is more energy-efficient compared to transporting raw manure. The volume decrease, facilitated by composting, offset the energy consumed during composting. The energy ratio between raw manure and composted manure stood at 1.56-to-1 energy units.
Pathogen Control and Site Selection
The pathogen-killing potential of composting is noteworthy. Grewal (2006) conducted a study comparing pathogen survival rates in simulated composted dairy manure, dairy manure pack, and liquid dairy lagoon. After just three days of composting at 131 F, pathogens like Escherichia coli, Salmonella, and Listeria monocytogenes were undetectable. However, Salmonella persisted for 28 days in manure pack and lagoon simulations. Clearly, composting holds promise as an effective pathogen mitigation strategy.
Selecting an appropriate composting site is essential. The chosen area should possess efficient drainage without the risk of runoff or leachate reaching water bodies. Ideally, the site should drain into a containment pond. The location should be well-drained, have slopes of 2-4%, consist of packed soil, gravel, or concrete, and be free from flood plains or soil textures coarser than a sandy loam. Windrows, the piles of composting materials, should align parallel to the slope to prevent runoff blockage and facilitate access.
Composting Process and Aeration
The heart of composting lies in the role of microorganisms indigenous to manure. Through proper management, these microorganisms are nurtured to perform the decomposing process. Piles of manure are formed, with the carbon-to-nitrogen (C/N) ratio ideally maintained at 30-to-1. Adequate hydration is crucial, with around 50% of the pore space requiring water. The pile should be kept aerobic, ensuring the presence of oxygen (Rynk et al., 1992).
The formation of windrows serves as a common approach to composting. These windrows' dimensions are influenced by pad length and turning equipment size. The internal temperature of the pile should surpass 120 degrees F after a day or two (Figures 3 and 6).
Maintaining an appropriate C/N ratio, ideally ranging from 20-to-1 to 40-to-1, is pivotal. This ratio is necessary due to approximately 50% of metabolized carbon being released as carbon dioxide. It prevents nitrogen loss and ammonia volatilization while fostering microbial activity (Miller, 1996). Monitoring moisture levels is crucial, with the "wet rag test" being a handy technique. If the compost feels akin to a wrung-out wet rag, moisture levels are adequate (Rynk et al., 1992).
Adequate aeration is achieved through regular turning of the windrow. Oxygen is incorporated, promoting microorganism activity and improving the pile's homogeneity. Several methods can be employed to turn the pile, including windrow turners or bucket tractors. Turners mix the compost using an auger, rotary drum, or elevating conveyor. Turning intervals depend on the composting stage, with greenhouse gas emissions minimized by strategic turning (Michel, 2009).
Assessing Compost Maturity and Nutrient Management
Determining compost maturity is vital before application. Various testing methods exist, including laboratory analysis and colormetric kits measuring carbon dioxide and ammonia emissions. Nutrient management is key, with compost serving as both a soil enhancer and a source of fertilizer for crop production. Given the variance between compost nutrient availability and that of commercial fertilizers, careful assessment and supplementation are essential.
Applying compost calls for calibration, ensuring accurate nutrient distribution and minimizing pollution risks. The incorporation of compost into nutrient management strategies necessitates a thorough understanding of compost's properties, nutrient content, and compatibility with crop requirements.