Effectiveness and Consistency of MSW Compost as a Component in
Container Growing Media
Peter R. Hicklenton
Agriculture and Agri-Food
Canada,
Atlantic Food and Horticulture Research Centre
32 Main Street, Kentville, N.S. B4N 1J5
Municipal source separated waste (MSW) compost derived from commercial scale plants in Nova Scotia has already been employed to a limited degree by local landscapers and nurseries as a soil amendment and mulch, but this barely touches the potential of the material for use in the nursery trade. Container production of ornamental trees, shrubs and perennial plants, representing over half of the total woody ornamental production in North America, depends entirely on quality, soilless media derived from both organic and inorganic constituents. For over 25 years waste products derived from biosolids (Goum, 1993) and wood waste (Lumis, 1976) have been used extensively in container production. Today an increasing proportion of the waste stream consists of MSW which can be composted. It has already shown potential as a new medium for container production (Rosen et al. 1993).
A potential problem with the use of MSW compost is variability in composition depending on the initial composition of the feedstock. While slight variations in texture, particle size and mineral composition may be of less consequence when the material is used as a landscape mulch, these factors may be of significant importance when combined with other constituents in the limited volume of a plant container.
This study was designed
to assess the potential of MSW compost derived from the same commercial
plant in two consecutive years as a component of a container medium. Comparisons
were made between various combinations of MSW compost with peat, and with
various peat:composted bark mixes which have been the standard for nursery
production over the past 3 decades. The following report deals primarily
with plant growth in relation to these combinations, and their effects on
the nutritional status of plants grown through a single growing season.
Methods
Composted MSW was obtained from a commercial composting facility operated by the Municipality of the County of Lunenburg, N.S. in 1996 and in 1997. The composted material was mixed with bark to create 4 media: 25:75, 50:50, 75:25 and 100:0 (MSW:sphagnum peat, by volume). Another 4 media were prepared using composted bark and peat (25:75, 50:50, 75:25 and 100:0 bark:sphagnum peat). In 1996, each mixed medium was divided into two and amended with either 2.5 or 5.0 g/L Nutricote controlled release fertilizer (18-6-8 with micronutrients, type 140 day release; Chisso-Ashai Fertilizer Co. Ltd.). In 1997 only 5.0 gIL Nutricote fertilizer was used. Individual media were used to fill 3.5L plastic plant containers. Three species of perennial woody nursery plants were used to test the individual media viz: Cotoneaster dammeri C.K. Schneid. 'Coral Beauty' (Coral Beauty Cotoneaster) , Juniperus horizontalis Moench. 'Plumosa Compacta' (Compact Andorra Juniper) and Vaccinium angustifolium Ait. (Lowbush Blueberry). In 1996 only Cotoneaster was used. Rooted cuttings were potted in late May each year and maintained outside under drip irrigation in a nursery area until late September. At the end of the season, leaf samples were taken from plants which were then severed at the soil line, dried to constant weight at 60C and weighed. Leaf analysis was performed to determine concentrations of N,P,K, Ca, Mg, Fe, Mn, Zn, Cu, B and Na in the foliage. Analysis was also performed on plants from selected media to determine foliar concentrations of heavy metals. The experiment was designed as a randomized complete block with 9 blocks of single plant replicates. All 9 plants were used for end of season dry weight determinations, but chemical analysis was performed on only 5 replicates. Data for 1996 and 1997 were analyzed separately.
Results and Discussion
Compost Chemical and Physical PropertiesThe 1996 and 1997 composts differed in both their physical and chemical properties. Bulk density and soluble salts were higher in 1997, whereas pH was lower (Table 1).
Table 1: Physical properties of MSW compost used in 1996 and 1997 trials
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Compost texture and appearance differed somewhat between the years. In 1996 the compost was fine-textured and flowable, but in 1997 it showed more particle aggregation. These differences were reflected in differences in percent air porosity (AP) of the formulated growing media. AP is the percent by volume of air-filled spaces in a medium at container capacity. Between 15 and 20% AP is considered ideal for containerized nursery plants (Hanan et al, 1981, Handreck and Black, 1984). AP was higher for MSW-based media in 1997 than in 1996 (Table 2), although both years showed generally favorable AP characteristics. AP declined as percentage of peat in the medium increased. AP of bark-based media differed little between years and was always lower than values for MSW-based media.
Table 2: Air porosity and water retention capacity of various media used in 1996 and 1997 trials. Values are means of 6 replicate samples shown + SEM.
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Water retention capacity (WRC) is a measure of the percentage of water held in small pores within the medium after drainage. Suggested optimum WRC for containerized nursery plants is between 55 and 65% (DeBoodt and Verdonck, 1972) although it is generally less critical to plant growth than AP. Plants can tolerate a wide range of media WRCs since adequate irrigation is always avallable in production nurseries. Low WRC and requisite high irrigation frequency can, however, lead to excessive leaching of nutrients. All media tested in this study had acceptable WRC; 100% bark and 100%MSW media showed the lowest values underlining the relatively "open" nature of these materials. When combined with peat, WRC increased significantly and was highest in the 25:75 or 50:50 bark:peat or MSW:peat media.
Shrinkage occurred progressively in all media through the season (Figure 1), and increased with the percentage of peat in the medium. This reflects the relative stability and compaction resistance of both bark and MSW compost. Media shrinkage was similar in both years and for clarity only data for MSW-based media and 75:25 bark:peat in 1997 are shown. In effect, shrinkage was of no practical significance in any of the tested media over a single growing season, although for longer production periods the impact on air porosity and hence root development could be significant (Beeson, 1996). Increasing the proportion of MSW-compost in the medium actually improved compaction resistance (decreased potential for shrinkage).
Compost Chemical Analysis
Analysis showed higher levels of nearly all plant nutrients in 1997 compost as compared with 1996 (Table 3). In many cases nutrient levels were over twice as high, and in the case of P and Ca, 3 times higher in 1997. Only the micronutrients Fe and Mn were present in similar concentrations in both 1996 and 1997 compost. Carbon content did not differ but total nitrogen did resulting in C:N ratios of 25.6 in 1996, but only 12.6 in 1997. In contrast to the plant nutrients, most non-nutrient elements were present in lower quantities in 1997 than in 1996. Generally, concentrations of all elements with the exception of Mo and Cu fell within the guidelines for Type AA compost determined by the Compost Council of Canada (Compost Council of Canada, 1998).
Electrical conductivity (Ec; a measure of soluble salt concentration) was initially high in all composts containing MSW in both years (Figure 2). Initial Ec was greater in 1997 than in 1996, as might be expected from the generally higher elemental concentrations in unamended 1997 material. Ec in mixed media measured shortly after planting were higher than values obtained in the unamended compost (Table 1). This may be due to additional soluble salts released from the fertilizer shortly after planting or to a release of salts such as chlorides and suiphates from the MSW when containers are first placed under irrigation. Ec values above 3.5 mS/cm are generally considered too high to support plant growth in containers and several authors have reported poor growth of woody ornamentals in composted media with Ec in excess of this value (Lemaire et al, 1985; Eames, 1977). Chong et al (1990) however, have reported good growth of numerous woody plants in media containing various combinations of spent mushroom compost with Ec values at planting in excess of 8 mS/cm.
Plant Growth
Different species showed quite similar growth responses to the 8 media in the 1996 trials (Figures 3,4 and 5). Generally best growth was obtained in media consisting of 25% MSW or Bark, and 75% peat. Plants in the MSW media grew as well or better than those in bark-based media, but the clear difference between the fertilizer treatments suggested that MSW contributed little to the nutrient requirements of the crops. This report deals only with the Cotoneaster results since other species showed broadly similar responses. As the proportion of bark or MSW in the medium increased, seasonal growth tended to decline, however there was little difference in growth between the three MSW:peat media. Poorest growth response was obtained from the 100% bark or 100% MSW treatments.
There was very little difference in growth between 1996 and 1997 MSW-media but growth in bark-based media was poorer in 1997, suggesting that variations in quality of wood waste may have more serious effects on growth potential than the variations in quality of MSW.
Plant Nutrition
Tissue plant nutrient analysis values for Cotoneaster in both years are shown in Figures 7 and 8. In some cases the higher nutrient content of the compost in 1997 is reflected in higher end-of-season tissue nutrient concentrations. Comparison with tissue nutrient levels from plants grown in the 75% bark: 25% peat medium are provided for comparison. They show that in only one or two cases were tissue levels in this control medium equal to or higher than those in MSW:peat media.
In some cases, notably Fe, Mn and Mg, tissue levels declined with increasing MSW compost in the medium, whereas for N, K, Cu and B, levels increased.
No species showed signs of nutrient deficiency or toxicity at any time during the experiment. Primary nutrient supply was certainly traceable to the controlled release fertilizer premixed into the medium. Nevertheless, increased levels of certain nutrients at higher levels of MSW in the media suggest that the compost enhanced the plant uptake of these elements. In some cases, e.g. copper where concentrations were quite high in the native compost elevated foliar levels may be due to a greater supply of the element, but in others where original concentrations were low (e.g. K) this may be more related to improved media conditions with higher proportions of MSW. Foliar nutrient concentrations for Cotoneaster in 1996 and 1997 were all generally in the satisfactory range for this genus as reported by Smith (1978) and in no case did any species display signs of nutrient toxicity or deficiency.
Most heavy metal concentrations in foliage at the
end of the season did not differ significantly between species or between
100% Bark and 100% MSW media. Exceptions were Co which was lower in juniper
than in cotoneaster, Cd which was inexplicably higher in bark media in
cotoneaster, but higher in MSW media in juniper, and As the only element
showing a consistently higher concentration in plants from MSW media.
Conclusions
Media which incorporate MSW compost at rates up to 75% of total volume are highly suitable for culture of woody plants in containers. In some cases (for salt sensitive species such as blueberry, Weigela and Privet), better growth may be obtained by limiting the MSW proportion to 25% in MSW:peat media. MSW compost contributes insignificant quantities of macronutrients to the plants and must be used in combination with controlled release, or liquid fertilization. It may, however, act as an important source of micronutrients.MSW composts are inherently variable in both their physical and chemical properties. However, the results of this study show that this variability has relatively little impact on plant performance when they are used in combination with peat in container media. This suggests that material obtained from commercial composting facilities can be safely used as a component of a standardized growing medium for the production of woody ornamental nursery plants.
Literature Cited
Beeson, R.C. Jr. 1996. Composted yard waste as a component of container substrates. J. Env. Hort. 14:15-121
Chong, C., R.A. Cline, D.L. Rinker and O.B. Allen. 1991. Growth and mineral nutrient status of containerized woody species in media amended with spent mushroom compost. J. Amer. Soc. Hort. Sci. 116:242-247.
Compost Council of Canada. 1998. Setting the standard: A summary of compost standards in Canada. http://www.compost.org/standard.html.
DeBoodt, M. and 0. Verdonck. 1972. The physical properties of substrates in horticulture. Acta Hort. 26:37-44.
Eames, A.G. 1977. Could spent mushroom compost be used for container shrubs? Mushroom J. 52:114 (Abstr.)
Goum, F.R. 1973. Utilization of sewage sludge compost in horticulture. HortTechnology 3:161 - 163.
Hanan, J.J., C. Olympios and C. Pittas. 1981. Bulk density, porosity, percolation and salinity control in shallow, freely draining potting soils. J. Amer. Soc. Hort. Sci. 106:742-746,
Handreck, K.A. and N.A. Black. 1984. Growing media for ornamental plants and turf New south Wales Univ. Press, Sydney.
Lemaire, F., A. Dartigues and L.M. Riviere. 1985. Properties of substrate made with spent mushroom compost. Acta Hort. 172:13-29.
Lumis, G.P. 1976. Using wood waste compost in container production. Amer. Nurseryman 163(11):10-11, 58-59.
Rosen, C.J., T.R. Halbach and B.T. Swanson. 1993. Horticultural uses of municipal solid waste components. HortTechnology 3:167-173.
Smith, E.M. 1978. Foliar
analysis survey of woody omamentals. Ohio Agr. Res. Development Ctr. Res.
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