Biology and Ecology of Earthworm Species for Vermicomposting PDF

Chapter 3 Biology and Ecology of Earthworm Species Used for Vermicomposting Jorge Dominguez and Clive A. Edwards CONTENTS I Introduction.........

Chapter 3 Biology and Ecology of Earthworm Species Used for Vermicomposting Jorge Dominguez and Clive A. Edwards CONTENTS I Introduction......................................................................................................28 II Earthworm Species Suitable for Vermicomposting......................................... 29 A Temperate Species..................................................................................... 29 1 Eisenia fetida (Savigny 1826) and Eisenia andrei (Bouché 1972).......29 2 Dendrodrilus rubidus (Savigny 1826)................................................. 32 3 Dendrobaena veneta (Rosa 1886)....................................................... 32 4 Lumbricus rubellus (Hoffmeister 1843).............................................. 33 5 Drawida nepalensis (Michaelsen 1907).............................................. 33 B Tropical Species.........................................................................................34 1 Eudrilus eugeniae (Kinberg 1867)......................................................34 2 Perionyx excavatus (Perrier 1872).......................................................34 3 Polypheretima elongata (Perrier 1872)............................................... 35 III Influence of Environmental Factors on Survival and Growth of Earthworms.................................................................................................. 35 A Temperature............................................................................................... 36 B Moisture Content....................................................................................... 36 C pH.............................................................................................................. 36 D Aeration..................................................................................................... 37 E Ammonia and Salts.................................................................................... 37 IV Predators, Parasites, and Pathogens of Earthworms........................................ 38 Acknowledgment...................................................................................................... 38 References................................................................................................................. 38 © 2011 by Taylor & Francis Group, LLC 27 28 Jorge Dominguez and Clive A. Edwards I INTRODUCTION Earthworms are macroscopic clitellate oligochaete annelids that live in soil. They are segmented worms, bilaterally symmetrical, with an external gland (cli- tellum) for producing the egg case (cocoon), a sensory lobe in front of the mouth (prostomium), and an anus at the end of the animal body, with a small number of bristles (setae) on each segment. They are hermaphrodite animals, and reproduc- tion normally occurs through copulation and cross-fertilization, following which each of the mated individuals produces cocoons containing 1–20 fertilized ova. The resistant cocoons, which are tiny and roughly lemon-shaped, with shape dif- fering between species, are usually deposited near the soil surface, except in dry weather when they are laid at deeper layers. Cocoons hatch after an incubation period that varies according to the earthworm species and environmental condi- tions. Hatchling earthworms, unpigmented and only a few millimeters in length on emerging from the cocoons, gain their adult pigmentation within a few days. Assuming favorable conditions, they reach sexual maturity within several weeks after emergence. Mature individuals of most vermicomposting species can be distinguished easily by the presence of the clitellum, the pale- or dark-colored swollen band located behind the genital pores. The clitellum secretes the fibrous cocoon, and the clitellar gland cells produce a nutritive albuminous fluid that fills the cocoon. Earthworms display indeterminate growth and can continue to grow in size after completing their sexual development although they do not add segments. According to Reynolds and Wetzel (2004), there are more than 8300 species in the Oligochaeta, of which about half are terrestrial earthworms. The most common earthworms in Europe, North America, western Asia, and many other parts of the world belong to the family Lumbricidae, whereas in West Africa, many of the common earthworms belong to the family Eudrilidae. In South Africa there is the Microchaetidae, in Australia and other parts of eastern Asia, the Megascolecidae, and the family Glossoscolecidae predominate in Central and South America. Different species of earthworms have different life histories, occupy different ecological niches, and have been classified, on the basis of their feeding and bur- rowing strategies, into three ecological categories: epigeic, anecic, and endogeic (Bouché 1977). Endogeic (soil feeders) and anecic species (burrowers) live in the soil and consume a mixture of soil and organic matter, and thus excrete organ- omineral feces. Epigeic species of earthworms are litter dwellers and litter trans- formers; they live in organic soil horizons, in or near the surface litter, and feed primarily on coarse particulate organic matter. They ingest large amounts of unde- composed litter and excrete holorganic fecal pellets. These species are small in body size and uniformly pigmented with high metabolic and reproductive rates, which represent adaptations to the highly variable environmental conditions at the soil surface. In tropical regions, epigeic earthworms can also be found in the axils of Bromeliaceae plants. © 2011 by Taylor & Francis Group, LLC BIOLOGY AND ECOLOGY OF EARTHWORM SPECIES USED FOR VERMICOMPOSTING 29 II EARTHWORM SPECIES SUITABLE FOR VERMICOMPOSTING Epigeic species of earthworms, with their natural ability to colonize organic wastes; high rates of consumption, digestion, and assimilation of organic matter; tol- erance to a wide range of environmental factors; short life cycles; high-reproductive rates; and endurance and tolerance of handling, show good potential for vermicom- posting. Few earthworm species display all these characteristics, and in fact only five have been used extensively in vermicomposting Eisenia andrei (Savigny), Eisenia fetida (Bouché), Dendrobaena veneta (Savigny), and, to a lesser extent, Perionyx excavatus (Perrier), and Eudrilus eugeniae (Kinberg). Characteristics and life his- tory aspects of eight common species of earthworms are summarized in Table 3.1. A Temperate Species 1 Eisenia fetida (Savigny 1826) and Eisenia andrei (Bouché 1972) These lumbricid earthworm species are those most commonly used in vermi- composting and vermiculture mainly because they are ubiquitous with a worldwide distribution and colonize organic substrates naturally, their life cycles are short, they have a wide temperature- and moisture-tolerance range, and they are resilient earth- worms that can be readily handled. E. fetida corresponds to the striped or banded morph, with the area around the intersegmental groove having no pigmentation and appearing pale or yellow; hence, its common names of ‘‘brandling’’ or ‘‘tiger’’ earthworm; whereas E. andrei, the common ‘‘red’’ worm, corresponds to the uniformly reddish morph. Aside from the differences in pigmentation, the two species are morphologically similar and their overall requirements the same. Their reproductive performances and life cycles do not differ significantly, although growth rates and cocoon production are slightly higher in E. andrei. The problem of their taxonomic status remained unresolved for a long time, and moreover in much of the current literature both species are termed indiscriminately as E. fetida or E. foetida, the latter an illegal or inaccu- rate emendation of the former name, and it is often not clear which of the two spe- cies is being referred to. We have confirmed that they are two different biological species, reproductively isolated, and that they are also two different phylogenetic species. The reproductive isolation was confirmed after studying the offspring via- bility from inter- and intraspecific crosses of both species (Domínguez et al. 2005). Additionally, fully resolved and well-supported phylogenetic trees based on mito- chondrial (COI) and nuclear DNA sequences (28S) confirmed that they are different phylogenetic species (Pérez-Losada et al. 2005). This evidence implies important considerations; thus in vermiculture or vermicomposting E. andrei is more often recommended since its growth and reproduction rates are higher. Moreover, the two species are syntopic, commonly living in mixed colonies in dung and vermicompost heaps, and therefore hybridization is possible. The existence of postcopula but not precopula isolation in sympatric populations clearly affects the population dynamics © 2011 by Taylor & Francis Group, LLC  30 Table 3.1a Comparison of Some Aspects of the Biology of the Vermicomposting Species Eisenia fetida Eisenia andrei Dendrobaena rubida Dendrobaena veneta Color Brown and buff bands Red Reddish purple Reddish and purple bands Size of adult earthworms 4–8 mm × 50–100 mm 4–8 mm × 50–100 mm 3–4 mm × 35–60 mm 5–7 mm × 50–80 mm (0.016–0.03 in × 1.9 × 3.0 in) (0.016–0.03 in × 1.9 × 3.0 in) (0.11–0.16 in × (0.2–0.27 in × 1.3–2.4 in) 2–3.15 in) Mean weight of adults 0.55 g (0.01 oz) 0.55 g (0.01 oz) 0.25 g (0.008 oz) 0.92 g (0.032 oz) Time to maturity (days) 28–30 21–28 54 65 © 2011 by Taylor & Francis Group, LLC Number of cocoons day–1 0.35–0.5 0.35–0.5 0.20 0.28 Mean size of cocoons 4.85 mm × 2.82 mm 4.8 mm × 2.82 mm 3.19 mm × 1.97 mm 3.14 mm × 1.93 mm (0.03 × 0.11 in) (0.03 × 0.11 in) (0.12 × 0.07 in) (0.12 × 0.075 in) Incubation time (days) 18–26 18–26 15–40 42.1 Hatching viability (%) 73–80 72 85 20 Number of worms 2.5–3.8 2.5–3.8 1.67 1.10 cocoon–1 Self-fertilization + + + — Life cycle (days) 45–51 45–51 75 100–150 Limits and optimal Tª 25°C (0°C–35°C) 25°C (0°C–35°C) — 25°C (15°C–25°C) (77°F)(32°F–95°F) (77°F)(32°F–95°F) (77°F)(59°F–77°F) Limits and optimal 80%–85% (70%–90%) 80%–85% (70%–90%) — 75% (65%–85%) moisture Jorge Dominguez and Clive A. Edwards  Table 3.1b Comparison of Some Aspects of the Biology of the Vermicomposting Species Drawida nepalensis Eudrilus eugeniae Perionyx excavatus Lumbricus rubellus Color — Reddish brown Reddish brown Reddish brown Size of adult earthworms — 5–7 mm × 80–190 mm 4–5 mm × 45–70 mm 4 mm × 70–150 mm Mean weight of adults 0.82 g (0.02 oz) 2.7–3.5 g (0.09–0.12 oz) 0.5–0.6 g (0.01–0.02 oz) 0.80 g (0.02 oz) Drawida nepalensis Eudrilus eugeniae Perionyx excavatus Lumbricus rubellus Time to maturity (days) 34–42 40–49 28–42 74–91 Number of cocoons day–1 0.15 0.42–0.51 1.1–1.4 0.07–0.25 mm © 2011 by Taylor & Francis Group, LLC (0.003–0.01 in) Mean size of cocoons — — — 3.50 mm × 2.46 mm (0.13 × 0.10 in) Incubation time (days) 24 12–16 18 35–40 Hatching viability (%) 75–88 75–84 90 60–70 Number of worms 1.93 2–2.7 1–1.1 1 cocoon–1 Self-fertilization + — — — Life cycle (days) 100–120 50–70 40–50 120–170 Limits and optimal — 25°C (16°C–30°C) 25°C–37°C (77°F–99°F) — temperature 77°F(61°F–86°F) Limits and optimal — 80% (70%–85%) — — moisture BIOLOGY AND ECOLOGY OF EARTHWORM SPECIES USED FOR VERMICOMPOSTING 31 32 Jorge Dominguez and Clive A. Edwards by reducing the fitness of the individuals. For this reason, in applied aspects it is important to keep the two species separated (Domínguez et al. 2005) although they often occur in cultures together. The life cycle and population biology of E. fetida and E. andrei in different organic wastes have been investigated by several authors (Watanabe and Tsukamoto 1976; Hartenstein et al. 1979; Edwards 1988; Reinecke and Viljoen 1990; Domínguez et al. 1997; Domínguez and Edwards 1997; Domínguez et al. 2000; Monroy et al. 2006). The optimum temperature for growth of both species is 25°C(68°F), and although they can tolerate a wide range of moisture conditions, the optimum moisture content for these species is 85%. In optimum conditions the length of their life cycles (from newly-laid cocoon through clitellate adult earthworm) ranges from 45 to 51 days. The time for hatchlings to reach sexual maturity varies from 21 to 30 days. Copulation in these species, which takes place beneath the soil or waste surface, has been men- tioned by various authors since 1845 and has been observed more often than in any other megadrile species. Cocoon laying starts 48 hours after copulation, and the rate of cocoon production is 0.35–0.5 day–1. The hatching viability is 72%–82%, and the incubation period ranges from 18 to 26 days. The number of young earthworms hatching from viable cocoons varies from 2.5 to 3.8 depending on the temperature. In controlled conditions, the average life span is 594 days at 18°C(64.4°F) and 589 days at 28°C(82.4°F) with a maximum life expectancy between 4.5 and 5 years, although under natural conditions it may be considerably shorter. 2 Dendrodrilus rubidus (Savigny 1826) Dendrodrilus rubidus is a holarctic earthworm species belonging to the family Lumbricidae with a relatively cosmopolitan distribution. It is an epigeic earthworm with a clear preference for highly organic soils, and it has also been found in organic substrates such as rotting wood and straw, pine litter, compost, peat, and nearby sew- age tanks and manure. Although some specific aspects of their biology have been investigated (Gates 1972; Sims and Gerard 1985; Bengtsson et al. 1986; Cluzeau and Fayolle 1989; Elvira et al. 1996), it is not an earthworm usually used in vermicom- posting or vermiculture. The mean time for hatchlings of D. rubidus to reach sexual maturity is 51 days, and the mean rate of cocoon production is 0.2–0.4 cocoons earthworm–1 day–1 (Bengtsson et al. 1986; Cluzeau and Fayolle 1989; Elvira et al. 1996). Hatching success for cocoons is 85%, with a mean incubation time of 22 days and an average of 1.7 hatchlings (between 1 and 3) emerging from each cocoon (Elvira et al. 1996). Dendrodrilus rubidus can complete its life cycle within 75 days, and according to Cluzeau and Fayolle (1989), one of the factors that contribute to the high reproduction rate of this species is that its reproduction may be biparental, amphimitic, or uniparental, either by parthenogenesis or by self-fertilization. 3 Dendrobaena veneta (Rosa 1886) This species is a large earthworm that can also survive in soil with potential for use in vermiculture; although it is not very prolific, it grows very rapidly (Edwards © 2011 by Taylor & Francis Group, LLC BIOLOGY AND ECOLOGY OF EARTHWORM SPECIES USED FOR VERMICOMPOSTING 33 1988; Viljoen et al. 1991). A number of commercial vermicomposting companies use this species, and it may have particular potential for protein-production systems and for breeding for use in field soil improvement. Dendrobaena veneta (also sometimes called Eisenia hortensis) is a robust earthworm that can tolerate much wider moisture ranges than many other spe- cies and has a preference for mild temperatures 15°C–25°C (59°F–68°F). Its life cycle can be completed in 100–150 days, and 65 days is the average time to reach sexual maturity. Mean cocoon production has been reported as 0.28 day–1, but the hatching viability seemed low (20%), and the mean period of cocoon incu- bation period is 42 days. The mean number of earthworms hatching from each viable cocoon is about 1.1 (Lofs-Holmin 1986; Viljoen et al. 1991, 1992; Muyima et al. 1994). 4 Lumbricus rubellus (Hoffmeister 1843) Lumbricus rubellus is usually found in moist soils, particularly those to which animal manures or sewage solids have been applied (Cotton and Curry 1980). In surveys of commercial earthworm farms in the United States, Europe, and Australia, earthworms sold under the name L. rubellus were usually always E. fetida or E. andrei. There are little data about its moisture and temperature requirements and pref- erences, although it is known that it clearly prefers moist conditions and can sur- vive cold temperatures well. The optimal temperature for growth is 18°C (64.4°F), and suboptimal temperatures are less harmful than supraoptimal ones. L. rubel- lus has a relatively long life cycle (120–170 days) with a slow growth rate and a long maturation time (74–91 days). The mean cocoon production rate varies from 0.07 to 0.25 cocoons earthworm –1 day–1, and hatching viability is 60%–70%. After an incubation period of 35–40 days, one single earthworm emerges from each cocoon (Cluzeau and Fayolle 1989; Elvira et al. 1996). The low maturation and reproductive rate indicate that it is not ideal for use in vermicomposting, although its size and vigor could make it of potential interest as fish bait or for land-improve- ment purposes. 5 Drawida nepalensis (Michaelsen 1907) This is a temperate earthworm species not widely utilized in vermicompost- ing although it has affinity for organic matter and shows some suitable char- acteristics for vermiculture. Although this species has a lower growth rate and produces fewer cocoons and hatchlings per cocoon than most of the other ver- micomposting species, it has a relatively short life cycle (100–120 days) and can reproduce, like E. fetida, without mating. The mean time to sexual maturity is 34–42 days, and the cocoon production is 0.15 cocoons worm –1 day–1. Its hatch- ing viability is 75%–88%, and the mean incubation time is 23.6 days. The mean number of young earthworms hatching from viable cocoons is 1.93 (Kaushal and Bisht 1992, 1995). © 2011 by Taylor & Francis Group, LLC 34 Jorge Dominguez and Clive A. Edwards B Tropical Species 1 Eudrilus eugeniae (Kinberg 1867) This earthworm species is native to Africa, but it has been bred extensively in the United States, Canada, and elsewhere for the fish bait market where it is commonly called the “African night crawler.” It is a large earthworm that grows extremely rap- idly, is reasonably prolific, and under optimum conditions can be considered as ideal for production of animal feed protein. Its main disadvantages are its narrow temper- ature tolerance and sensitivity to handling. E. eugeniae has high-reproduction rates (Bano and Kale 1988; Edwards 1988) and is capable of decomposing large quantities of organic wastes quickly and incorporating them into the topsoil (Neuhauser et al. 1979, 1988; Edwards 1988). It shows preference for high temperatures, with maximum biomass production occurring at 25oC–30oC (77°F–86°F), while the growth rates were very low at 15oC (59°F) (Loehr et al., 1985; Viljoen and Reinecke 1992; Domínguez et al. 2001). Its use in outdoor vermiculture may therefore be limited to tropical and subtropical regions, unless winter temperatures are controlled. It can tolerate moisture contents between 70% and 85%, the optimum being 80%–82%. Domínguez et al. (2001) reported that individuals continued to increase in weight with virtually no mortal- ity for 22 weeks. Reinecke et al. (1992) reported continuous growth and maximum weight up to 21 weeks at 25oC (77°F). The life cycle of E. eugeniae ranges from 50 to 70 days, and its life span can be 1–3 years. Sexual maturity is attained within 40–49 days, and a week after this period the individuals start to lay cocoons, between 0.42 and 0.51 cocoons day–1 (Viljoen and Reinecke 1989; Reinecke et al. 1992; Reinecke and Viljoen 1993; Domínguez et al. 2001). This is, together with E. fetida and E. andrei, a more rapid rate of development than for any other species of earthworm that has been reported to date and makes a very fast rate of population multiplication possible. The cocoon incubation period ranges from 12 to 16 days, and hatching suc- cess from 75% to 84%, with the mean number of earthworms per cocoon between 2 and 2.7 (Viljoen and Reinecke 1989; Reinecke et al. 1992; Reinecke and Viljoen 1993; Domínguez et al. 2001). 2 Perionyx excavatus (Perrier 1872) Perionyx excavatus is an earthworm commonly found over a large area of tropi- cal South Asia (Stephenson 1930; Gates 1972) although it has also been transported to Europe and North America. This is an epigeic species that lives solely in organic wastes, and high-moisture contents and adequate amounts of suitable organic mate- rial are required for populations to become fully established and to process organic wastes efficiently. This tropical earthworm is extremely prolific, and it is almost as easy to handle as E. fetida and very easy to harvest. Its main drawback is its inability to withstand low-temperature conditions, but for tropical conditions it seems an ideal species. Although it has a shorter maturation and incubation time than E. eugeniae, its © 2011 by Taylor & Francis Group, LLC BIOLOGY AND ECOLOGY OF EARTHWORM SPECIES USED FOR VERMICOMPOSTING 35 fecundity is higher. It is a very common species in Asia and is used in vermicul- ture in India, the Philippines, and Australia. The life cycle and the potential of this species for breaking down organic wastes have been documented by various authors under controlled conditions (Kale et al. 1982; Reinecke and Hallatt 1989; Hallatt et al. 1990; Reinecke et al. 1992; Hallat et al. 1992; Edwards et al. 1998). P. excavatus does not grow much at low temperatures although it can survive them 4°C (39.2°F), but it is less susceptible to high temperatures over 30°C (86°F) than E. eugeniae. Even in tropical areas, P. excavatus does not grow during low-winter temperatures but can survive the high-summer temperatures, whereas E. euge- niae has a much narrower tolerance range for temperature and cannot survive either the extreme low winter or the high summer temperatures. The life cycle of P. excavatus takes 40–50 days. Sexual maturity is attained within 20–28 days, and the mean cocoon production is 2.8 cocoons earthworm –1 day–1, the mean incubation time of cocoons at 25°C (77°F) is 18 days, the hatching success is high (85%–90%), and usually only one hatchling emerges from each cocoon (Kale et al. 1982; Reinecke and Hallatt 1989; Hallatt et al. 1990, 1992; Reinecke et al. 1992; Edwards et al. 1998). 3 Polypheretima elongata (Perrier 1872) This tropical earthworm species has been tested for the treatment of solid organic wastes, including municipal and slaughterhouse wastes; human, poultry, and dairy manures; and mushroom compost in India. A project in India using this species claimed to have a commercially viable facility for the “vermista- bilization” of 8 tons (7.8 t) of solid wastes day–1; they developed a “vermifilter” (packed with vermicompost and live earthworms) that produces reusable water from sewage sludge, manure slurries, and organic wastewater from food process- ing. P. elongata appears to be restricted to tropical regions and may not survive severe winters. There seem to be no detailed data about its life cycle available in the literature. III INFLUENCE OF ENVIRONMENTAL FACTORS ON SURVIVAL AND GROWTH OF EARTHWORMS Cocoon production, rates of development, and growth of earthworms are all critically affected by environmental conditions. Epigeic earthworms are relatively tolerant to the environmental conditions of the organic wastes, so quite simple low management windrow or bed systems have been used extensively to process these wastes. However, it has been demonstrated clearly that these earthworms have well- defined limits of tolerance to environmental parameters, such as moisture and tem- perature, and that the wastes are processed much more efficiently within a relatively narrow range of favorable chemical and environmental conditions. If these limits are greatly exceeded, earthworms may move to more suitable zones in the waste, leave it, or die, so that these wastes are processed very slowly. © 2011 by Taylor & Francis Group, LLC 36 Jorge Dominguez and Clive A. Edwards A Temperature Earthworms have fairly complex responses to changes in temperature. Neuhauser et al. (1988) studied the potential of several earthworm species to grow in sewage sludge, and they concluded that all these species have a range of pre- ferred temperatures for growth, ranging between 15°C (59°F) and 25°C (77°F). In their studies, cocoon production was more restricted by temperature than growth, and most of the cocoons were laid at 25°C (77°F). Edwards (1988) studied the life cycle and optimal conditions for survival and growth of E. fetida, D. veneta, E. eugeniae, and P. excavatus. Each of these four species differed considerably in terms of response and tolerance to different temperatures. The optimum tem- perature for E. fetida was 25°C (77°F), and its temperature tolerance was between 0°C (32°F) and 35°C (95°F). Dendrobaena veneta had a rather low-temperature optimum and rather less tolerance to extreme temperatures. The optimum tem- peratures for E. eugeniae and P. excavatus were around 25°C (77°F), but they died at temperatures below 9°C (48.2°F) and above 30°C (86°F). Optimal temperatures for cocoon production were much lower than those more suitable for growth for these species. Temperatures below 10°C (50°F) generally result in reduced or little feeding activity; and below 4°C (39.2°F), cocoon production and development of young earthworms ceases completely. In extreme temperature conditions earthworms tend to hibernate and migrate to deeper layers of the windrow for protection. Earthworms can also acclimate to temperature in autumn and survive the winter, but they cannot survive long periods under freezing conditions unless they are in protective cells. The unfavorable effect of high temperatures (above 30°C (86°F)) on most species of earthworms is not entirely a direct effect because these warm temperatures also pro- mote chemical and microbial activities in the substrate, and the increased microbial activity tends to consume the available oxygen, with negative effects on the survival of earthworms. B Moisture Content There are strong relationships between the moisture content of organic wastes and the growth rate of earthworms. In vermicomposting systems, the optimum range of moisture contents for most species has been reported to be between 50% and 90%. Eisenia fetida and E. andrei can survive in moisture ranges between 50% and 90%, but they grow more rapidly between 80% and 90% in organic wastes (Edwards 1988; Domínguez and Edwards 1997). C PH Most epigeic earthworms are relatively tolerant to pH and can tolerate pH levels of 5–9, but when given a choice in the pH gradient, they move toward the more acid material, with a pH preference of 5.0. © 2011 by Taylor & Francis Group, LLC BIOLOGY AND ECOLOGY OF EARTHWORM SPECIES USED FOR VERMICOMPOSTING 37 D Aeration Earthworms lack specialized respiratory organs, and oxygen and carbon dioxide diffuse through their body wall. Thus, earthworms are very sensitive to anaerobic conditions. E. fetida have been reported to migrate in high numbers from a water- saturated substrate in which oxygen has been depleted, or in which carbon dioxide or hydrogen sulfide has accumulated. E Ammonia and Salts Earthworms are very sensitive to ammonia and cannot survive in organic wastes containing high levels of this cation (e.g., fresh poultry litter). They also die in wastes with large quantities of inorganic salts. Both ammonia and inorganic salts have very sharp cutoff points between toxic and nontoxic (