Written by Wesley Steeves

Introduction

Compost is described as “the transformation of biodegradable material… into humic substances” and “a way of obtaining a stable product from biological oxidative transformation, similar to that which naturally occurs in the soil” (de Bertoldi et al. 1983). Composting has changed little from when the ancient Romans began intentionally using the process to increase soil fertility (Cooperband 2000). In composting, organic residues, such as animal manure and straw, are blended to promote decomposition by thermophilic microbes (NDSU 2016). Compost can improve soil structure, increase biological activity in the soil, and fertilize plants (NDSU 2016).

The fertilizer property of compost is essential in organic agriculture since compost contains a host of essential nutrients for healthy plant growth, packed into a stable and nontoxic form (NDSU 2016). Many of these compost nutrients cannot be substituted in organic agriculture as their synthetic counterparts are not permitted in Canadian organic agriculture. Furthermore, the additional qualities of compost can be even harder to find a substitute for. The physical characteristics of soil can be greatly enhanced with the addition of compost in such ways as water-holding capacity and bulk density (NDSU 2016). Due to the high temperatures achieved during the composting process, weed seeds and insect eggs are drastically reduced in the organic matter used, allowing for the recycling of organic matter as well as reducing the need for herbicide or tillage for weed control (NDSU 2016).

For compost to be effective, it must be of high quality. This means it must be mature, stable, and pathogen-free, with reasonably low quantities of trace elements and contaminants (Wichuk and McCartney 2010). The quality of compost is influenced by the organic materials used, as well as the level of management and duration of the composting process (Huang et al. 2017). In this chapter, a technique for creating high-quality compost will be outlined, some of the biological aspects will be detailed, and its agricultural value will be summarized.

Making Compost

Under optimal conditions, composting requires nearly one month to be considered complete (de Bertoldi et al. 1983). This time is not easily reduced due to the biological limitations of the microbial process. Compost can be made in a closed system to provide controlled conditions, or outdoors in a windrow (long pile) in the same amount of time (de Bertoldi et al. 1983). Optimal composting conditions are created by providing a favourable environment for aerobic microbes through the right balance of food, water, and air (Cooperband 2000). Organic matter must be combined at a carbon-to-nitrogen ratio (C:N) between 25:1 and 35:1, moisture content must be 45-60% of the mass, oxygen concentration above 5%, particle size less than 2 cm, bulk density below 600 kg/m³, and a pH of 5.5-8.5 (Cooperband 2000). The amount of material being composted in a single pile is also relevant.

This section will describe the factors that govern these requirements and how they can be adjusted. The carbon-to-nitrogen ratio affects how nitrogen will be used. Too much nitrogen means mineralization will occur, meaning the nitrogen will be converted to minerals such as ammonium and nitrate (Cooperband 2000). These mineral forms of nitrogen are less stable and more likely to be lost to the atmosphere (de Bertoldi et al. 1983). This is why enough carbon must be present to keep nitrogen bound in microbial biomass. However, too much carbon slows the rate of decomposition (Cooperband 2000; de Bertoldi et al. 1983). Different organic materials have different C:N ratios. For example, animal manures contain lower ratio quotients compared to plant material, and green plant materials have more nitrogen than brown (Cooperband 2000). These materials can be mixed to adjust the C:N ratio of the compost.

Moisture is important for microbe survival. Low moisture will lead to decreased microbial populations, and at the same time increases the possibility of spontaneous combustion due to the buildup of heat generated from aerobic decomposition (Cooperband 2000). On the other hand, too much moisture reduces the pore space available for oxygen, altering the environment to favour anaerobic microbes. Moisture content can be adjusted by adding organic materials with different water-holding capacities (Cooperband 2000) or pore sizes. As an example, adding woody material will decrease moisture content by increasing pore space, allowing water to drain more freely and leaving more space for oxygen.

Particle size influences pore size and surface area. Increased surface area increases the rate at which microbes can oxidize the organic matter (de Bertoldi et al. 1983). The material must be sufficiently small to facilitate quick decomposition, but not so small as to negate the required level of airflow and moisture content (de Bertoldi et al. 1982). Even with adequate pore space and bulk density, oxygen is not able to penetrate the pile of organic matter at the rate at which it is consumed during the decomposition process (Cooperband 2000). Oxygen must be introduced to the pile. The easiest way to accomplish this is by simply mixing the material regularly but air can also be forced through the pile (de Bertoldi et al. 1982).

Composting can occur at a pH between 3 and 11, but 5.5-8 is optimal; bacteria grow well close to a neutral pH while fungi excel in acidic conditions (de Bertoldi et al. 1982). PH is also important as ammonia will be volatilized above a pH of 7.5, resulting in nitrogen loss (Cooperband 2000). Unfortunately, adjusting the pH of a compost pile is not easily accomplished (de Bertoldi et al. 1983) but could be adjusted before the composting process begins.

The size of the pile being composted is another important factor as it affects the temperature the pile can maintain and the volume of oxygen present; temperature is important since most microbes function optimally between 49-60°C (Cooperband 2000). The minimum size for a compost pile to maintain an adequate temperature is about 0.76m³.

Biology and Properties of Compost

Composting transitions between multiple stages. These stages are differentiated by the organisms that are the most active and can be simplified into three main periods based on the temperature of the pile: a fast initial rise in temperature, a sustained high temperature, then a slow return to the surrounding temperature (de Bertoldi et al. 1982; Sanchez et al. 2017; Wichuk and McCartney 2010). Carbon compounds such as sugars and organic acids are broken down quickly through exothermic reactions by heterotrophic microflora, creating a buildup of heat in the compost pile (de Bertoldi et al. 1982). Higher temperatures favour aerobic thermophiles capable of breaking down more complex compounds such as pectin and starch (de Bertoldi et al. 1982). The temperature decreases as these compounds are used up and higher fungi become dominant, decomposing tough compounds such as lignin; the decomposition of cellulose occurs throughout the process but peaks in the third period (de Bertoldi et al. 1982).

At the end of the third period, the compost is then stable and mature. Stability refers to resistance to further decomposition and maturity is the compost’s viability as a finished product (Wichuk and McCartney 2010). Viability refers to the effectiveness and lack of negative effects of the compost. Unstable compost can lead to fires during storage or ruptured packaging from heat and gas buildup. Immature compost can produce phytotoxic effects that negatively impact plant germination and growth (Wichuk and McCartney 2010). Stable compost properties depend on the materials used to create it. The majority of compost production uses animal manure as a main ingredient since it is a waste product high in minerals required for plant growth, but not all manure sources are equal (Huang et al. 2017). Besides containing varying levels of plant nutrients, different manures require different lengths of time to reach maturity. For example, cattle manure requires 36 days, chicken manure 56, and swine manure takes 70 days to be considered mature (Huang et al. 2017). C:N ratios also differ significantly, as chicken manure needs relatively more additional material to reach an ideal C:N ratio compared to cattle manure.

Significance of compost in agriculture

Given the restricted use of chemically processed fertilizers in organic production (OMAFRA), compost is an essential source of nutrients (Raviv et al. 2004). Compost is also an excellent way to increase soil organic matter, which improves soil quality and plant growth making it prevalent in conventional agriculture where high carbon loss rates are common due to repeated tillage and synthetic fertilizer (Scotti et al. 2016). Beyond the agricultural benefits, increasing soil organic matter sequesters the carbon that would otherwise act as a greenhouse gas (Erhart et al. 2016). More than a tonne of carbon can be sequestered per hectare of agricultural land every year, whereas using mineral fertilizer alone results in a net loss of soil carbon (Erhart et al. 2016).

A final point of significance for compost in organic agriculture is the source from which it is derived. Compost made mainly from manure is recycled material. Making manure compost is a way of maximizing the value of a byproduct created by animal agriculture.

References

Cooperband LR, 2000. Composting: Art and science of organic waste conversion to a valuable soil resource. Lab. Med. 31(5), 282-290.

de Bertoldi M, Vallini G, Pera A, 1983. The biology of composting: A review. Waste Manag. Res. 1(2), 157-176.

de Bertoldi M, Vallini G, Pera A, Zucconi F. 1982. Comparison of three windrow system. Biocycle. 23(2), 45-50.

Erhart E, Schmid H, Hartl W, Hulsbergen KJ, 2016. Humus, nitrogen and energy balances, and greenhouse gas emissions in a long-term field experiment with compost compared with mineral fertilization. Soil Res. 54, 254-263

Huang J, Yu Z, Gao H, Yan X, Chang J, Wang C, Hu J, Zhang L. 2017. Chemical structures and characteristics of animal manures and composts during composting and assessment of maturity indices. PLoS ONE. 12(6), e0178110. https://doi. org/10.1371/journal.pone.0178110

North Dakota State University Extension Services 2016:  Composting Animal Manures:  A guide to the process and management of animal manure compost.  (NM1478, Revised Feb. 2022).  https://www.ndsu.edu/agriculture/extension/publications/composting-animal-manures-guide-process-and-management-animal-manure-compost

Raviv M, Medina S, Krasnovsky A, Ziadna H. 2004. Organic matter and nitrogen conservation in manure compost for organic agriculture. Compost Sci. Util. 12(1), 6-10

Sanchez OJ, Ospina DA, Montoya S. 2017. Compost supplementation with nutrients and microorganisms in composting process. J. Waste Manag. 69, 136-153

Scotti R, Piccolo A, Palese AM, Pane C, Celano G, Zaccardelli M, Spaccini R. 2016. On-farm compost: a useful tool to improve soil quality under intensive farming systems. Appl. Soil Ecol. 107, 13-23

Wichuk KM, McCartney D. 2010. Compost stability and maturity evaluation – a literature review. Can. J. Civ. Eng. 37, 1505-1523