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Published online 25 May 2007
Published in J Environ Qual 36:1031-1041 (2007)
DOI: 10.2134/jeq2005.0453
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Use of Wastewater and Compost Extracts as Nutrient Sources for Growing Nursery and Turfgrass Species

Robert C. Michitscha,*, Calvin Chongb, Bruce E. Holbeinc, R. Paul Voroneya and Hua-Wu Liuc

a Dep. of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
b Dep. of Plant Agriculture, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
c Super Blue Box Recycling Corporation (SUBBOR), Suite 401, 2275 Lakeshore Blvd. W., Etobicoke, ON, Canada, M8V 3Y3. Robert C. Michitsch, present address, Nova Scotia Agricultural College, c/o R. Michitsch, 20 Tower Rd., Truro, NS, Canada, B2N 5E3

* Corresponding author (rmichits{at}dal.ca)

Received for publication December 8, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nutrient salts present in liquid by-products following waste treatment are lost resources if not effectively recycled, and can cause environmental problems if improperly disposed. This research compared the growth response and mineral nutrient status of two nursery and two turfgrass species, hydroponically supplied with nutritive by-product extracts derived from anaerobically digested municipal solid waste (MSW) and aerobically composted organic wastes from the mushroom and MSW industries. Forsythia (Forsythia x intermedia ‘Lynwood’) and weigela (Weigela florida ‘Red Prince’), and creeping bentgrass (Agrostis palustris Huds.) and Kentucky bluegrass (Poa pratensis L.), were grown in nutrient solutions/extracts prepared from: (i) half-strength Hoagland's #2 solution (HH; control), (ii) Plant Products liquid fertilizer (PP; g kg–1: 180 N; 39 P; 224 K), (iii) spent mushroom compost (SMC), (iv) MSW compost (GMC), and (v) intra-process wastewater from the anaerobic digestion of MSW (ADW). Additional nutrient solutions (SMC-A, GMC-A, and ADW-A) were prepared by amending the original solutions with N, P, and/or K to concentrations in HH (mg L–1: 105 N; 15 P; 118 K). Plants receiving the SMC-A extract grew best or at least as well as those in HH, PP, and the amended GMC-A and ADW-A solutions. This study indicated that, with proper amendments of N, P, K and other nutrients, water-soluble constituents derived from organic waste treatment have potential for use as supplemental nutrient sources for plant production.

Abbreviations: -A, amended • ADW, anaerobic digestion wastewater • EC, electrical conductivity • EDTA, ethylene-diamine-tetra-acetic acid • GI, growth index • GMC, municipal compost • HH, half-strength Hoagland's #2 solution • ICP, inductively coupled plasma • INL, internode length • MSW, municipal solid waste • PP, Plant Products liquid fertilizer • SMC, spent mushroom compost • SUBBOR, Super Blue Box Recycling Corporation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ORGANIC components constitute more than 70% of the 30 x 106 metric tons of municipal solid waste (MSW) generated annually in Canada (Marshall, 2004). These nutrient-rich residuals represent a lost resource if not reused, for example, for fertilizing plants (Gill and Rainville, 1994; Mancino, 1994; Nava, 2001), and pose detrimental environmental risks if improperly disposed. Recent initiatives toward a zero-waste society (Seldman, 2004) have encouraged research to evaluate the recycling and reuse of such waste products.

In previous studies, municipal wastewaters have successfully been used to grow turfgrass (Mancino, 1994; Beltrao et al., 1999), trees, legumes, and grains (Hussain and Al-Saati, 1999), and other horticultural crops (Monnet et al., 2002). Landfill and industrial wastewaters have proven successful in the production of clover and cress (Vasseur et al., 1998), ryegrass (Revel et al., 1999), and corn and rice (Singh and Mishra, 1987). Wastewaters generated during anaerobic digestion of municipal and industrial wastes have been reported as potential nutrient sources for crops, turfgrasses, and landscaping applications (Riggle, 1996; Little and Grant, 2002). Turfgrass fertilized with compost or its extracts had longer root length and increased root density than nonfertilized plants, and exhibited less incidence of disease (Anonymous, 2001; Scheuerell and Mahaffee, 2003). Furthermore, Purvis et al. (2000) found up to 77% reduction in essential plant nutrient needs by recirculating nutrients in containerized nursery production, exemplifying the potential for agricultural and horticultural waste reuse, reduction and conservation, and overall cost savings.

The growth and nutrient status of vegetative species, grown in anaerobic digestion wastewater (ADW), spent mushroom compost (SMC), and municipal compost (GMC), were evaluated in comparison to traditional (HH) and commercial (PP) hydroponic solutions. The prepared waste solutions were independently amended to N, P, or K levels of HH (as required) as additional comparisons. Both ADW, a liquid by-product of the Super Blue Box Recycling Corporation (i.e., SUBBOR) process, which produces biogas and stabilized organic residuals from the anaerobic digestion of MSW (Vogt et al., 2002), and SMC, an abundant by-product of mushroom cultivation (Chong and Rinker, 1994), are highly nutritive. However, while both waste sources have indicated generally positive results in growing vegetables (Wang et al., 1984; Rhoads and Olson, 1995; Stewart et al., 1998a; 1998b), woody ornamentals (Chong and Rinker, 1994, McLachlan, 2001), and grasses (Guo et al., 2001), they have also caused phytotoxic responses due primarily to high salt or foreign compound contents (i.e., phenolic and volatile fatty acids, fumigants, fungicides, etc.). GMC, a nutritive extract of MSW compost, was evaluated for comparison.

This research assessed the potential for reusing by-product extracts derived from the anaerobic (ADW) or aerobic digestion of MSW (GMC), or the aerobic digestion of mushroom cultivation solid wastes (SMC), to grow selected woody and turfgrass species. Hydroponic culture was utilized due to its usefulness to rapidly identify plant growth problems/sensitivities to nutrient deficiencies (or toxicities) due to direct contact of soluble nutrients with plant roots (Voogt and Sonneveld, 1997; Hopkins, 1999; Resh, 2001). This study furthered previous research involving field and growth-room studies incorporating soil-based media (Michitsch et al., unpublished data, 2006).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nursery Species and Experimental Design
Rooted cuttings (stem height 10 cm) of forsythia (Forsythia x intermedia ‘Lynwood’) and weigela (Weigela florida ‘Red Prince’) were grown in hydroponic culture for 4 wk in a greenhouse, set to a 16 h photoperiod (supplemented with high-pressure sodium lamps during non-daylight hours), photosynthetic photon flux ranging 460–950/49–87 µmol m–2 s–1 day/night, temperature of 25°/20°C day/night, and relative humidity of 50%. Three cuttings of each species were arranged equidistantly on a styrofoam platform placed over a 3.0-L pot (17.5 cm diam.; 18.0 cm depth) with roots immersed in 2.5 L of nutrient solution, mimicking early stage commercial production (C. Chong, personal communication, 2002). This experiment consisted of eight treatment solutions (i.e., pots) of each species arranged in a randomized complete block design with three replications.

Preparation of Nutrient Solutions
Water extracts of SMC (Chong and Rinker, 1994), obtained from Money's Mushrooms (Campbellville, ON), and MSW compost (GMC; T. Barton, personal communication, 2002), obtained from the Guelph Waste Resource Innovation Center (Guelph, ON), were prepared following methods described by Wright (1986) and Jarecki et al. (2005). The extracts were prepared by pouring 21 L of distilled water through a 2-L volume of SMC or 4-L volume of GMC, and filtering the extracts through a 1.5-mm screen. Intra-process wastewater (ADW; Vogt et al., 2002) was obtained from the Super Blue Box Recycling Corporation (SUBBOR, Guelph, ON), a pilot-scale facility which anaerobically digests MSW to produce methane. The wastewater was diluted 19-fold to obtain a concentration of total N equivalent to that of HH. Batches of these nutrient stock solutions were kept at 4°C until required for use.

The treatment solutions were prepared as outlined in Table 1: two standard solutions as half-strength Hoagland's #2 nutrient solution (HH; Hoagland and Arnon, 1950) and Plant Products (g kg–1: 180 N; 39 P; 224 K) commercial liquid fertilizer (PP; Plant Products Co., Brampton, ON); three unamended wastewater or compost extract solutions (ADW, SMC, and GMC; previously described); one solution of ADW (ADW-A) supplemented with P (as H3PO4) and K (as KCl) to concentrations in the HH solution; and two solutions of SMC and GMC (SMC-A, GMC-A) similarly amended with N (as NH4NO3) and P. All nutrient solutions were prepared using distilled water and analytical grade chemicals. The concentration of nutrients in each solution were determined as follows: NO3–N, NH4–N, and Cl using specific ion electrode methods (Method #986.31, 960.14, and 969.10, respectively; AOAC, 1990); P, K, Ca, Mg, SO4, Na, Zn, Mn, Cu, Fe, and B using an inductively coupled plasma (ICP)–mass spectrometric method (Method #993.14; AOAC, 1990). Just before use and every 48 h (maximum) thereafter, the electrical conductivity (EC) of each nutrient solution was adjusted to 2.0 ± 0.2 mS cm–1 with NaCl (Jarecki et al., 2005), pH was adjusted to 6.3 ± 0.1 units with H2SO4 or NaOH (Hopkins, 1999), and solution volume was adjusted to 2.5 L with distilled water. Nutrient media were aerated continuously and changed every 2 wk.


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  		Table 1. Initial nutrient (i.e., treatment) solution composition following preparation and after adjustment to an electrical conductivity (EC) value of 2.0 ± 0.2 mS cm–1 and/or adjustment to Hoagland's half-strength #2 solution levels.

 
Visual ratings for plant quality (i.e., scale 1–5) were recorded biweekly following observation of each plant for evidence of stress (i.e., chlorosis, necrosis, wilting, etc.) in reference to available literature (Hopkins, 1999) or comparison to the best performing plants in the experiment. Plant height (platform base to plant tip) and stem diameter (2.5 cm from platform base) were measured initially and at harvest. Individual pot averages of these measurements were used to calculate growth index 1 [GI-1 = ({Delta}-height + {Delta}-diam.)/2]. At harvest, maximum leaf span (located at half plant height) and internode length (INL; generally third INL from top of plant) were measured and similarly used to calculate growth index 2 [GI-2 = (leaf span + INL)/2].

Initially (i.e., select plants only) and at harvest, plant roots, stems, and leaves for each treatment were separated, dried at 70°C for 48 h, and weighed. Duplicate tissue samples (~3 g dry weight) were analyzed for total N using the Dumas combustion method (Method #968.06; AOAC, 1990) and P, K, Ca, Mg, Na, Cl, Zn, Mn, Cu, Fe, and B using an ICP-spectroscopic method (Method #985.01; AOAC, 1990).

Turfgrass Species
Seeds of bentgrass (Agrostis palustris Huds.; No. 1 Cert. 18th Green Creeping Bentgrass) and bluegrass (Poa pratensis L.; No. 1 Cert. Minnfine Kentucky Bluegrass) were pre-germinated on moist, inert, porous plastic pads (17 cm diam., 2.5 cm thick). Following germination, shoots and protruding roots were trimmed to 2.5 cm from pad surfaces and grown hydroponically for 6 wk, as described above for the nursery species. Treatment solutions were similar to those previously described, except that the PP and amended ADW-A, SMC-A, and GMC-A solutions received additional Ca (as CaSO4·2H2O), Mg (as MgSO4), B (as H3BO3), Mn (as Mn-EDTA chelate), and/or Fe (as Fe-EDTA chelate) to concentrations in the HH solution (Table 1). Ratings for visual quality were recorded as previously described. Turfgrass clippings were collected biweekly, while root tissues were collected at final clipping only. Plant tissues were dried, weighed, and analyzed as described above.

Statistical Analyses
Analysis of variance incorporating Duncan's multiple range test was performed on visual quality, growth index, plant yield, and tissue nutrient content data using a general linear model (proc glm; SAS Institute, 2001). Correlation analysis (proc corr) was performed to identify relationships amongst the different nutrients.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nursery Species
Top and Root Biomass Production
Top biomass of forsythia (Fig. 1 ) grown in SMC-A and GMC-A solutions were significantly higher than with HH, while growth was similar in HH, PP, and both SMC and GMC solutions; growth was lowest in both ADW solutions. Root biomass (Fig. 1) was highest in the four compost extracts, intermediate in HH and PP, and generally lowest in both ADW solutions. Compared to HH, the top/root ratio (Fig. 1) was similar for PP, both ADW solutions, and SMC-A and GMC-A, but generally less in SMC and GMC.


Figure 1
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  		Fig. 1. Top and root yields, and top/root ratio, for woody species. Treatments separated by Duncan's multiple range test at P = 0.05. Treatment abbreviations: Hoagland's half-strength #2 solution (HH); Plant Products (PP); anaerobic digestion wastewater (ADW); spent mushroom compost extract (SMC); MSW compost extract (GMC); amended to HH levels (-A).

 
Top biomass of weigela (Fig. 1) was highest in SMC-A, generally intermediate in HH, SMC, GMC, and GMC-A solutions, but lowest when grown in PP and both ADW solutions. The root biomass of weigela (Fig. 1) was higher in SMC-A than with HH, similar to SMC, GMC, and GMC-A, and similarly lowest in PP and both ADW solutions. The top/root ratio of weigela (Fig. 1) was similar for HH, PP, and SMC-A, which were higher than with all other nutrient solutions.

Visual Quality Ratings and Growth Indices
Chlorotic, necrotic, and wilting symptoms were observed on nursery plants receiving unamended solutions and ADW-A. Visual quality ratings and growth indices showed a trend similar to plant biomass production (Table 2). Plants grown in HH, SMC-A, and GMC-A exhibited enhanced growth and vigor compared with SMC and GMC. Woody plants receiving the PP, ADW, and ADW-A solutions exhibited decreased vigor.


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  		Table 2. Visual quality ratings (scale 1–5, low-high) for all species following observation for stress symptoms and inter-treatment comparison. Growth index parameters (GI-1 = ({Delta}-plant height + {Delta}-stem diameter)/2; GI-2 = (maximum leaf span + third INL)/2) are included for woody species only. Comparisons between treatments separated by Duncan's multiple range test; similar letters are not significantly different at P = 0.05.

 
Tissue Analysis
Several trends were apparent for macronutrient contents (Table 3). Nitrogen contents were similar across treatments in both tops and roots, except for unamended SMC. Phosphorus contents were also similar among treatments, except for elevated contents in root tissues receiving HH and amended nutrient solutions. In both tops and roots, K contents were generally higher in HH and compost water extracts. Calcium and Mg contents were lower in both tops and roots for all treatments in comparison to initial samples, and attributed to higher levels of these elements in the initial rooting substrate. Distinct differences were observed for tissue Na and Cl contents, attributed to maintenance of pH (NaOH used) and EC (NaCl used). Micronutrient contents did not indicate any abnormal trends (Table 3); however, root Fe contents were quite variable due to inherent high levels in the wastewater and in nutrient solution preparation. Correlation analysis between individual tissue nutrient content and leaf/root yields did not indicate significant or consistent trends for the woody plant species (Table 4).


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  		Table 3. Analysis of nutrient contents in leaf and root tissues of woody species before (i.e. Init) and after harvest. Similar letters are not significantly different at P = 0.05. Mean content indicated before Duncan's multiple comparison value. Data presented as average of both species due to value similarity.{dagger}

 

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  		Table 4. Correlation coefficients (r) of leaf/shoot and root yields (g dry wt.) vs. tissue nutrient content for all species.

 
Turfgrass Species
Top and Root Biomass Production
With few exceptions, clipping yields (Fig. 2 ) of both species in the SMC-A, GMC-A, and ADW-A were comparable to those for grasses grown in the HH and PP solutions, while the SMC, GMC, and ADW solutions resulted in generally less growth. Grasses grown in ADW-A showed increased clipping yields with succeeding harvests (only third harvest data shown). Root biomass (Fig. 2) of bentgrass was highest with SMC, intermediate with GMC and GMC-A, and lowest in all other nutrient solutions. For bluegrass, root biomass was highest with ADW, SMC, GMC, and GMC-A. The top/root ratio (Fig. 2) of both grass species tended to be highest with HH and PP.


Figure 2
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  		Fig. 2. Clipping (third harvest only), root yields, and top/root ratio (i.e., third clipping harvest/root) for turfgrass species. Treatments separated by Duncan's multiple range test at P = 0.05. Treatment abbreviations: Hoagland's half-strength #2 solution (HH); Plant Products (PP); anaerobic digestion wastewater (SW); spent mushroom compost extract (SMC); MSW compost extract (GMC); amended to HH levels (-A).

 
Visual Quality Ratings
Symptoms of chlorosis, necrosis, and wilting were also exhibited by grasses receiving unamended solutions and ADW-A. The standard and three amended treatments generally exhibited enhanced growth and vigor compared with the unamended solutions (Table 2). Plants receiving the PP and both ADW solutions exhibited increased vigor than similarly treated nursery species, attributed to improved amendment and maintenance of experimental parameters.

Tissue Analysis
Nitrogen, P, Mg, Cl, and the micronutrient tissue contents in the grasses (Table 5) generally mimicked trends described for the nursery species (Table 3). However, in comparison to the nursery plants, turfgrass K contents showed less variation (Table 5), Ca contents were elevated in roots of grasses treated with compost extracts, no distinct trend was observed for Na, Mn contents were much higher, Cu root contents were elevated in PP-treated grasses, and root Fe contents remained variable due to inherently high levels in the wastewater and its amendment in this study. Correlation analysis between individual tissue nutrient content and turfgrass shoot/root yield were similarly inconsistent as for the woody plant species (Table 4). However, N, P, K, Ca, and Mg were positively correlated in shoot tissues receiving HH and PP solutions, due mainly to this relationship in bentgrass.


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  		Table 5. Analysis of nutrient contents in shoot and root tissues of turfgrass species following harvest. Similar letters are not significantly different at P = 0.05. Mean content indicated before Duncan's multiple comparison value. Data presented as average of both species due to value similarity.{dagger}

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Treatment Effects, Foliar Analysis, and Symptomology
Woody and turfgrass species growing in nutrient media prepared from SMC-A exhibited the best overall growth; interestingly, compost extracts are frequently finding a variety of uses in modern society (Scheuerell and Mahaffee, 2003). Plants grown in HH and GMC-A treatments performed just marginally less well. Improved pH adjustment and balancing of nutrient levels in the turfgrass trial allowed the wastewater-treated grasses to grow much better. In general, the unamended solutions resulted in consistently poor plant performance, exhibiting less growth, lighter tissue pigmentation, necrosis, and leaf curling.

These negative effects may be due to macro and/or micronutrient imbalances. Calcium and Mg (Table 1) were deficient in PP and both ADW solutions after preparation with distilled water. The specific role of Ca in cell turgidity and general immobility may account for the symptoms observed (Altland, 2003). Increased levels of SO4, Na, and Cl from pH (NaOH used) and EC (NaCl used) adjustment (described below) may dually have contributed to the poor plant response using these treatments. Notwithstanding, the significant and overall predominant deficiencies (i.e., negative correlations) of micronutrients, especially Fe, in woody root tissues treated with all organic waste sources (Table 4), plausibly contributed to the exhibited inferior plant growth. Competition with Mn (Plant-Prod, 2001), chelation, and/or deposition within the organic nutrient solutions (Gieling et al., 1997; C. Chong, personal communication, 2002), or exclusion due to the short experiment duration (Adriano, 1986) may also have contributed to poor plant response. This trend was significantly reversed in the turfgrass trial (i.e., enhanced amendment to HH levels; Table 1), suggesting nutrient imbalance as the primary cause of poor plant performance. Furthermore, in related field and pot-based soil experiments (Michitsch et al., unpublished data, 2006), such symptoms were not observed due to inherent nutrient levels and the natural buffering capacity of soil-based media (Hopkins,1999; Resh, 2001).

Hoagland's #2 solution (half-strength) was the chosen reference standard since it tends to resist change to alkaline levels (M.K. Jarecki, personal communication, 2002), and previous research showed that full-strength solutions were toxic (i.e., extreme chlorotic, necrotic, and etiolation effects observed) and unnecessary from a nutrient level standpoint (Purvis et al., 2000; Chong et al., 2004; Jarecki et al., 2005). While nutrient solutions were changed every 2 wk to provide sufficient nutrient levels and to avoid accumulation of root exudates, more frequent changes may decrease the observed stress effects.

Fluctuations in Electrical Conductivity (EC) and pH
Thriving plants (i.e., rapid, abundant growth) required more maintenance of water, EC, and pH levels as growth progressed. However, EC/pH fluctuations from initial solution levels are parameters requiring strict control in hydroponic culture, as shown by this research (Table 1; Fig. 3 ).


Figure 3
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  		Fig. 3. Fluctuations in pH for both woody and turfgrass species from a daily maintained level to 6.3 ± 0.1 units. Individual treatments are not separated. Unamended treatments exhibited the widest and most frequent fluctuation from 6.3 ± 0.1 units, especially following a 2 wk change of nutrient solution.

 
The EC level was maintained at 2.0 ± 0.2 mS·cm–1 in all treatment solutions due to the highly inherent level of 2.1 mS·cm–1 in SMC-A. Overall, EC levels did not vary much (i.e., 0.1 to 0.2 mS·cm–1) throughout the experiments upon daily maintenance. Interestingly, a slight increase in the EC of stored (at 4°C) compost extracts was observed, indicating continued mineralization of organic compounds contained in these solutions (Bess, 2000). However, exposure to this high level, coupled with the previously described enhanced amendments, stimulated plant growth and enhanced plant reaction to treatment solutions, causing accumulation of exudation materials on root surfaces.

Roots of plants grown in the ADW solutions, and compost extract solutions to a lesser extent, were coated with a mucous-like slime. The accumulation of root exudates, such as organic compounds (i.e., root cells, hair, debris), partially decayed organic matter from decreased decomposition rates (due to anaerobic conditions), or products of substrate and microbial activity, are known to inhibit root nutrient uptake and overall growth (Rovira, 1956; Beatrix et al., 1999). The observed slime may be an accumulation of such exudated materials since preliminary microscopic inspection (i.e., slide preparation and observation) showed no evidence of microbial growth. The slime coating may also be the primary cause of the poor response by the nursery species grown in nutrient solutions prepared from the organic sources. The grasses were less affected due to improved maintenance of nutrient, pH, and EC levels.

In contrast, the pH of treatment solutions fluctuated between 4 and 9 units throughout each experiment, tending to increase/decrease to original solution levels (Table 1; Fig. 3) following daily adjustment. The pH of each solution was maintained to 6.3 ± 0.1 units (C. Chong, personal communication, 2002; Jarecki et al., 2005) since a pH < 6.8 is necessary for proper P uptake in soil-based plants (Hopkins, 1999). In stationary hydroponic culture, short-term fluctuations in pH values of the magnitudes shown in Fig. 3 do not seem to affect overall plant performance (B. Bible, personal communication, 2004).

Fluctuations of pH predominately occurred following introduction of fresh nutrient solutions, and were attributed to physiological differences between species, nutrient source/uptake, a dynamic NO3/NH4 ratio in various solutions, and low buffering capacities of hydroponic solutions (Kirkby and Hughes, 1970; Berry et al., 1980; Bone, 1985). Furthermore, the use of NH4–based fertilizers tends to cause lower pH levels in substrate solutions due to nitrification and NH3 volatilization processes (Bernardo et al., 1984b; Kafkafi, 1990), while the use of NH4–N sources derived from anaerobic digestion processes have been shown to increase substrate solution pH due to the high overall presence of NH4–N compounds, which prevents dissociation of the more stable NH4–OH molecule (Bernardo et al., 1984b; Kafkafi, 1990; Adeli and Varco, 2001). In the ADW solutions where pH was observed to increase above 8.0 during the initial 2 to 3 h following a change in nutrient solution, plants were observed to wilt, some beyond recovery. This pH fluctuation may be the direct result of plant developmental stage or general growth, while NH4–N presence, which contributes to this pH fluctuation, acts indirectly (Moritsugu and Kawasaki, 1983; Sady et al., 1990).

Rapid uptake of NO3 and NH4 by plants conceivably contributed to the wide pH fluctuations observed in the nutrient media (Kirkby and Hughes, 1970; Bugbee, 1995), even with daily pH maintenance. Fluctuations toward alkaline pH levels in certain treatments (SMC, GMC, ADW, ADW-A) may also have contributed to P deficiencies and poor plant performance as previously discussed (Hopkins, 1999), as tissue P contents (Tables 3, 5) were generally significantly lower in plants growing in these solutions, and symptoms evident of P deficiency (i.e., very dark green foliage, thin stems, poor rooting) were observed. In addition, wide variations of pH from neutral are known to cause micronutrient deficiencies or toxicities, due to chelation and/or possible precipitation from nutrient solutions (Hoagland and Arnon, 1950; Page and Chang, 1990); however, this was not explicitly evident in our results (Tables 3, 5).

Plant reaction to a stressed environment, preventing proper exchange of external factors (i.e., light, CO2, O2, temperature) at root surfaces, may have exacerbated the above conditions (Berry et al., 1980; Bone, 1985; Erusha, 1986; Erusha et al., 2002). The presence of compounds in solution that have stimulatory or phytotoxic effects (Beatrix et al., 1999), such as phenolic and volatile fatty acids, tannins, lignins, and other humic substances (Finkle and Runeckles, 1967; Kuiters, 1989), were inherent in both ADW solutions (unpublished data, 2004) and may have further affected plant response when grown in these solutions. Although aeration of each pot in our studies was used to avoid anoxic solution conditions, this may have indirectly enhanced NH3 volatilization from solutions inherently high in NH4–N (i.e., the ADW solutions) and further stressed the plants (Cossu et al., 2001).

Biomass Production and Tissue Nutrient Contents
It is known that woody species are sensitive to salinity levels (Bernstein, 1964; Ziska et al., 1991) and that composts (such as SMC) tend to contain high levels of salts, particularly those of K and Cl (Chong and Rinker, 1994). Our results indicated high levels of these elements in the compost extract solutions (Table 1), and especially in tissue samples of the woody species (Tables 3, 5). Highly saline root-zone environments cause overall yields and the top/root ratio to decrease (Munns and Termaat, 1986; Shannon et al., 1987), as exhibited with corn (Mackay and Barber, 1985), tomato, and marigold (Jarecki et al., 2005). This emphasizes the need to maintain the EC level of all nutrient solutions at the highest value, to reduce experimental error. Top/root ratio analysis for this study (Fig. 1, 2) clearly showed that the compost extracts and wastewater sources promoted relatively greater root vs. top growth. This response was likely due to the presence of inhibitory compounds (e.g., phenolic and volatile fatty acids, tannins) in these nutrient sources, exacerbated by exposure to a consistently high EC level, thereby inhibiting top growth while roots grew uninhibited (Munns and Termaat, 1986; Shannon et al., 1987; Shahalam et al., 1998).

Overall, few trends were found for macronutrient tissue contents in these studies. Tissue analyses for Na and Cl were skewed by the addition of these elements for pH and EC adjustment. Micronutrient trends were inconclusive, perhaps due to exclusion by uptake of higher amounts of macronutrients (Wang et al., 1984; Chong et al., 2004) and varying contents in each nutrient solution. Furthermore, N, P, and K are taken up early in the growth cycle at the expense of other nutrients (especially micronutrients), which may explain our findings since the experiments were relatively short in duration (Adriano, 1986).

Nitrogen Form and Plant Response
Growing plants successfully in high NH4–N solutions has been reported (Moritsugu and Kawasaki, 1983; Bernardo et al., 1984a; Stensvand and Gislerod, 1992), emphasizing a preference to absorb NH4 due to lower energy requirements. While a 5:1 NO3/NH4 ratio has typically been suggested for optimal plant growth (Warncke and Barber, 1973; Steiner, 1984), the NO3/NH4 ratios of treatments for our studies varied from 1:20 for the ADW wastewater to 13:1 for the GMC treatments (Table 1). Our data suggest that N source did not significantly affect growth (Fig. 1, 2) and supports findings using compost extracts (including SMC) for containerized woody ornamentals and other applications (Chong and Rinker, 1994; Chong et al., 2004). Solution pH is also a factor in NH4–N fertilization of plants (previously discussed). Mengel et al. (1983) suggested that NO3 reduction, after root uptake, might not depend on the available N form or concentration, but rather on plant need to alter pH since an OH– ion is generated during reduction (i.e., NH4 has the opposite effect, releasing an H+ ion).

It is also known that plants have a strong capacity for selecting ions (e.g., K+, Ca+, Mg+) in optimal ratios (Steiner, 1984; Khalifa and Zidan, 2001), governed mainly by pH and osmotic pressures. Plants in these experiments may have selectively absorbed ions, which may further explain the fluctuating pH values observed as nitrification proceeded to provide necessary NO3 for plant uptake (Moritsugu and Kawasaki, 1983; Sady et al., 1990). It has also been reported that at high NH4–N levels, P and S tend to increase in tissues while K, Ca, and Mg decrease, due to ion competition (Stensvand and Gislerod, 1992). The results of this study indicated contrasting results as K, Ca, and Mg correlations were frequently positive. Uptake of K by turfgrass has been shown to increase with increased rates of applied N (Geber, 2000). Moreover, sufficient levels of K in the nutrient solutions used in this study may have caused luxury consumption and greater Ca and Mg uptake. These findings are similar to others using spent mushroom compost (Wang et al., 1984) and recycled wastes (Chong et al., 2004). Increased Mg/Ca and Mg/K ratios may cause nutrient disorders (Carvajal et al., 1999), but the observed luxury consumption of these cations in our experiments did not appear to affect plant growth.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of these hydroponic studies indicate that, with proper amendments of N, P, K, and other nutrients, liquid extracts derived from digested organic waste products have potential for use as supplemental nutrient sources for growing nursery stock and turfgrass species. Plants receiving the SMC-A solution performed the same or better than those in the GMC-A and intra-process wastewater solutions. Poor plant response was attributed to pH fluctuations and nutrient imbalances, and further research on these subjects is necessary. Control of the hydroponic solution environment for optimum plant growth (e.g., pH, EC, temperature) is required to promote plant production.


   ACKNOWLEDGMENTS
 
This research project was supported in part by the Super Blue Box Recycling Corp. (SUBBOR) and its parent company Eastern Power, Ltd., the Natural Sciences and Engineering Research Council of Canada (NSERC), the Ontario Ministry of Agriculture and Food (OMAF), the Canadian Mushroom Growers Assoc. (CMGA), Money's Mushrooms, and the Guelph Waste Resource Innovation Center. The research was conducted in the Dep. of Plant Agriculture (Univ. of Guelph) with the aid of Dietmar Scholz and Donna Hancock.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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