It is a simple fact: you cannot have high-quality compost tea without high-quality compost. In other words, if you want to promote plant growth with compost tea the extract must be derived from compost that also promotes plant growth. Compost quality is determined by starting materials (feedstocks), as well as the management of moisture, temperature, other production factors, and storage conditions. All of these factors influence the rate of biological decomposition and ultimately determine final compost quality. Publications on compost production are available from several sources elsewhere (Rynk and Richard, 2001; Sheldon et al., 2005). This chapter is intended to improve the compost tea practitioners' awareness of indicators that are most commonly used to evaluate compost quality.
Maturity
Maturity is an important concept that is closely related to the quality of compost. Simply put, mature compost has decomposed enough to promote plant growth. Objective indicators of maturity have been established and are discussed below. Most of these indicators require special equipment or analysis fees take time, and it takestime for results to be received. Experienced producers and users of compost often evaluate maturity using objective indicators such as color, smell and feel (Kuo et al. 2004; Sullivan and Miller, 2001). Dark brown, earthy smelling, moist, and finely divided composts that are expected to lack sour or ammonia off-odors areexpected to be of adequate maturity to promote plant growth. However, more quantitative to better measures are required to enable end-users to determine the optimal rate and frequency of compost application. C: N RatioThe ratio of carbon to nitrogen in compost is probably the best-known optimal objective indicator of compost quality. Optimal C: N range is considered10-20:1 since composts within this range are unlikely to immobilize, or "rob" plant-available nitrogen. Typically, composts with C: N above 25:1 are unacceptable for use in cropping systems. It is important to note that C: N ratios are not adequate to use as the sole determinant of compost maturity. However, C: N ratios are extremely useful in prescreening compost for acceptable maturity. Composts that have C: N < 25:1 should be further evaluated for other indicators of compost maturity.
Stability
A common measure of compost maturity is stability or the potential for compost to further decompose.The most common measure of compost stability is self-heating tests where the maximum rise in temperature of moist compost are measured over a 5-10 day period. Excessive heating (>20oC increase in 10 days) indicates unstable compost(Briton, 2000). Respiration or carbon dioxide evolution from moistened compost is also used as an indicator of stability. Respiration and self-heating are both indicators of biological activity. Although biological activity is considered desirable in composts, unstable compost that rapidly consumes oxygen can result in anaerobic conditions after bagging, resulting in off-odors and the production of phytotoxic compounds.Unstable composts are also likely to be low in plant-available nutrients.
Plant available nitrogen
Nitrate and ammonium are important indicators of compost maturity (Briton, 2000; Sullivan and Miller, 2001). Nitrate concentration is recommended for mature to be at least 100 ppm formature compost. Some sources recommend another plant that nitrate and other plant available nitrogen should not exceed 300 ppm when compost is being used as a substrate growing medium. However, composts with nitrate concentrations of greater than 600-2000 ppm are associated with the best plant growth in greenhouse and field trials in Hawaii (Pant, 2011). Ammonium should be less than 1000 ppm and the ratio of ammonium to nitrate in the compost should be less than 1:1.
Other measures of quality
Other measures of maturity include EC (<2.0mmo), and pH (6.0-7.5). Compost quality is also indicated by the presence or absence of contaminants (Walker, 2001). Potential major contaminants include human pathogens, and physical contaminants such contaminants such as plastics, weed seeds, heavy metals, and pesticide residues. Maintaining a high temperature period for a period of time during the primary composting process has been theprimary approach towards minimizing contaminants, particularly human pathogens. Screening is also recommended which generating and removes the bulk of physical contaminants.
Why is vermicompost great?
Table 1.1 Select nutrients found in vermicompost and thermal compost available in Hawaii. Analysis was conducted on 157 compost samples in 20062011.
Vermicompost quality will vary depending on many factors including worm species, raw materials used, and age of the compost. Vermicomposts are generally of finer structure, contain more nutrients, and have higher microbial and ammonia activity than other types of compost. Worms facilitate two sets of processes: gut-associated associated processes and cast associ-ated processes (Dominguez 2004). In ingut-associated processes, several things occur the fractionating occupation and homog-enizatioand position of materials, the addition of sugars, the modification of microbial populations, and the addition of mucus and excretory and compounds (e.g. urea and ammonia). In cast-associated processes, high mesophilic microbial activity further decomposes and mineralizes the material under protected (i.e. covered, moist, dark)conditions. Both processes contribute to the fractionating of relatively high maturity indicators andpositive plant growth response observed in vermicompost compared to other types of composts. The University of Hawaii compared select quality characteristics of their-mo composts In and vermicomposts from various sources (Table 1.1). One benefit of vermicomposts is the relatively large amount of plant-available nitrogen that they contain in the form of nitrate (NO3-). This is partly due to the enclosed nature of off-vermicomposting that reduces the losses of NO3 and other nutrients. Allowing vermicompost to cure (stored in aerated container that conserves moisture) after harvesting for 3-4 months can also dramatically increase the NO3 content indicated (Figure 1.2). Note the high CV levels indicateof a great deal of variation in mineral nitrogen from sample to sample.
Figure 1.2: NOs increase exponentially over time in cured vermicompost, after removal of worms. Data points in the figure below are the means of three analyses; bars are the standard error of the means."Curing" refers to the finishing of compost after the active composting process. Compost stored under warm conditions in plastic bags or bins to retain moisture will continue a prolonged mesophilic stage that results in the urethane build-up of mineral nutrients and other compounds. These composts are most effective for use in compost tea (Chapter 3).
Vermicompost
Vermicompost is generated by worms and associated microorganisms. Vermicompost quality will vary depending on many factors including worm species, raw material used, and the age of the compost. Vermicomposts are generally of finer structure, contain more nutrients, and have higher microbial activity than other types of composts. High levels
of nutrient levels and plant growth regulators make vermicompost ideal for compost tea production.
“Handcrafted” or “Artisan” thermophilic
Careful monitoring of the compost temperature and conditions can maximize plant nutrients and biotic properties. Artisan thermophilic compost is appropriate for compost tea, but the quality can vary significantly depending on the feedstock and handling.
Commercial large scale
Large scale operations serve the purpose of processing large amounts of waste material. Generally, this type of compost is best used as a soil amendment, rather than for compost tea, because of low levels of mineral nutrients and biological activity.
(Photo: Nguyen Hue)
OK for Organics?
Compost Use
-NOP Rule 205.203(c)) for compost use states that compost is compliant for use in certified organic systems if three conditions are met. The compost is made from only allowed feedstock materials.2. The compost undergoes an increase in temperature to least 131ºF (55ºC) and remains there for a minimum of 3 days.3. The compost pile is mixed or managed to ensure that all of the feedstock heats to the minimum temperature for the minimum time. If composting in windrows, Plant and animal materials are composted through a process that establishes an initial C: N ratio of between 25:1 and 40:1 and maintains a temperature between 131°F and 170°F for 15 days, during which period the composting materials must be turned a recommendation minimum of five times. Compost that contains no animal materials as feedstock may be used without restriction provided that it contains no prohibited or restricted-use plant materials. Acceptable feedstocks include, but are not limited to, by-products of agricultural commodities processingSalmonella, and source-separated yard debris or “clean green.” Compost that contains morethan1x10³ (1,000) MPN fecal coliform per gram of compost sampled or more than 3 MPN Salmo-nella per 4 grams of compost sampled will result in a reclassification as ‘manure’. Composts that contain sewage sludge, synthetically fortified compost starter, glossy paper, and materials containing colored ink are prohibited.
Tea Use
-The NOP is vague before the use of compost tea. In its 2006 recommendation,the National Organic Standards Board (NOSB) defined tea as a water extract of compost produced to transfer microbial biomass, fine particulate organic matter, and soluble chemical components into an aqueous phase, intending to maintain or increase the living, beneficial microorganisms extracted from the compost. The final NOSB recommenda-tion stated “Recommendation: Compost teas if used in contact with crops less than 120days before harvest must be made from high-quality prepared compost described above and not prepared with the addition of supplemental nutrients such as sugars, molasses or other readily available (soluble) carbon sources.
The NOSB recommendations also state:
1. Compost teas must be made with potable water.
2. Equipment used to prepare compost tea must be sanitized before use with a sanitizing agent as defined by 21 CFR 178.1010, using allowed materials found on the National List.
3. Compost tea must be made with compliant compost or vermicompost, using the NOSB recommendation for compost and vermicompost mentioned above, and as defined in section 205.203 (c) (2) of the NOP rule.
4. Compost tea made without compost tea additives can be applied without restriction. Compost tea made with compost tea additives can be applied without restriction if the compost tea production system (same compost batch, additives, and equipment) has been pre-tested to produce compost that meets the EPA recommended recreational water quality guidelines for a bacterial indicator of fecal contamination (US EPA, 2000).
The Organic Materials Review Institute's
classification of compost tea as raw manure is extremely conservative and seemingly incongruent with NOSB and NOP guidance: "Compost tea used as a fertilizer or soil amendment is subject to the same restrictions as raw, uncomposted manure. It may only be (i) applied to land used for a crop not intended for human consumption; (ii) incorporated into the soil not less than 120 days prior to the harvest of a product whose edible portion has direct contact with the soil surface or soil particles; or (iii) incorporated into the soil not less than 90 days before the harvest of a product whose edible portion does not have direct contact with the soil surface or soil particles. Compost tea made on the farm may be used to suppress the spread of disease organisms. Compost tea sold for disease suppression must comply with all pesticide regulations. " (OMRI, 2011)
Frequently Asked Question
How much time between applications and harvest?
This an area still in development. At the time of printing, there are no explicit restrictions in the NOP guidance or NOSB recommendations on pre-harvest interval as long as the compost is produced according to the NOP guidelines described above and NO MICROBIAL FOODS ARE USED. If microbial foods are used, the resulting tea must be tested for pathogens (e.g. E. ColiO157 and Salmonella). If the tea is found to be clean then future testing is not compost tea required if the brewing process and compost are not changed.OMRI’s current interpretation of composttea is much more conservative. They consider all compost teas to be raw manure regardless of compost type. That means 90-120 days pre-harvest interval (see facing page). We suggest avoiding manure-based inputs and retesting for pathogens each time protocols or inputs are changed. If your tea is non-manure-based and pathogen-free, a pre-harvest interval may not be needed, but some consumers may not like the idea of spraying with microbes (even good ones)right before harvest.
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