Ornamental Plants

Annual Reports and Research Reviews

Composted Biosolids: An Ideal Organic Amendment for Container Media Supplying Both Nutrients and Natural Suppression of Root Rots

The nursery industry has used composted biosolids beneficially since the late 1970s (Hoitink, 1994). When this product first became available, it was incorporated in media at excessively high volumetric ratios, ranging from 20-60%. Many plant species produced in such mixes responded well (Logan et al., 1984). However, some plant types suffered from "salt" injury (Hoitink and Maronek, 1986), a direct result of the high rate of nutrients released by composted biosolids. High nitrogen diseases also were increased. On the other hand, Rhizoctonia and Pythium root rots were suppressed in media amended with composted biosolids (Kuter et al., 1988).

During 1994, 1995, and 1996, the utilization of composted biosolids in nursery media was revisited. This was done through a series of demonstration trials at various growers in Ohio with plant species differing in fertility needs and susceptibility to root rots. This paper reports the results of such trials and projects the best utilization strategies for composted biosolids in container media.

Materials and Methods

Composted biosolids (Technagro^TM) produced at the City of Akron composting facility were received from Kurtz Bros., Inc., Independence, Ohio. Three container media were tested at several growers and compared with the standard mix at each location.

The media contained (on a volume basis):

1. 10% Technagro (Tec), 20% composted rice hulls (CRH), 60% aged pine bark (APB), and 10% silica sand (SS);
2. 15% Tec, 20% CRH, 55% APB, 10% SS; and
3. 20% Tec, 20% CRH, 50% APB, and 10% SS.

Therefore, the pH of these mixes ranged from 5.8 to 6.1 and lime was not added. The conductivity of the mixes ranged from 1.8 to 2.2 mS, which is ideal. All other nutrients, including trace elements, were available at optimum concentrations.

Composted biosolids (Tec) release nitrogen, phosphorus, and potash for several weeks after potting and in quantities adequate for growth of most species if incorporated at 20% (v/v). Therefore, slow release fertilizer was not added to containers until four weeks after potting. The air-filled pore space after saturation and drainage in two-gallon containers for all three mixes ranged from 25-30%, which also is within the ideal range for most crops.

Plants were transplanted as quart-size liners into two-gallon containers in May 1995, irrigated as needed, and treated with standard fertility treatments used for each crop at each grower, except that fertilizer was not added until four weeks after potting. All plants were irrigated under the same system because physical properties related to drainage were similar for all mixes. Marketability, plant growth, and root rot severity for each crop was monitored for 17 months after potting.

Results

At all locations, most plants produced in the Tec mixes grew significantly (P=0.05) faster than those in the grower control mixes. Data in Tables 1 and 2 present more details for each of the crops. Evergreen azaleas tested at two growers responded well in all Tec mixes. Winterkill was not observed by either grower producing azaleas in the 20% Tec mix, even during the 1995-96 winter. However, during August 1996, Pythium root rot was evident on azalea in the 20% Tec mix, in the lowest two inches of the container. Azaleas in the 10% and 15% Tec mixes were free of root rot at all times.

Cornus alba 'Elegantissima,' Ligustrum x vicaryi 'Golden Privet,' and Spiraea x bumalda 'Gold Flame' also responded positively to all Tec amendment rates. In November 1995, at the completion of the first growing season, a significant number of plants of each of these crops was marketable (Table 1). In August 1996, most of these plants were larger than optimum for a two-gallon container crop.

Juniperus conferta 'Blue Pacific' responded well in the Tec mixes, although moderate and severe root rot was observed on these plants in the 15% and 20% Tec mixes, at the end of the 1995 growing season. Root rot severity in the 10% Tec mix was mild and not different from that in the grower mix (aged pine bark, composted hardwood bark, peat , sand). During the spring of 1996 (second growing season), roots of this juniper crop in all mixes were free of rot. In summary, Pythium root rot was observed on Juniperus conferta 'Blue Pacific' during the first six months after potting in the 20% Tec mix in particular, and plants recovered thereafter.

Blue hollies responded well in all three Tec mixes at two growers where this crop was tested. Stems were thicker and leaves larger and plants were more vigorous than those in the control mixes (aged pine bark, sphagnum peat, sand mix). Thielaviopsis black root rot was not observed on blue hollies in the Tec mixes. Controlled inoculations will need to be performed, to establish whether these mixes indeed suppress root rot caused by Thielaviopsis basicola.

Cotoneaster responded positively to all three composted biosolids amendment rates (10, 15, and 20%). Fire blight problems were encountered on this crop in trials in the early 1980s with 20% and higher rates of composted biosolids (40 and 60%). In these trials, fire blight did not present problems, probably because slow release fertilizer was not added at the time of potting.

In summary, all plant species responded positively to the 10% Tec amendment rate. Most responded positively to the 15% Tec amendment. Only Fothergilla gardenii and Juniperus conferta 'Blue Pacific' grew less rapidly in the first year in the 15% and 20% Tec mixes, but both recovered thereafter. In conclusion, composted biosolids incorporated at 10% and 15% on a volumetric basis in container media provided excellent growth and produced high-quality plants. For heavy feeders, the 20% rate provided even better results.

A 1994 publication summarizing 20 years of research and demonstration trials with composted biosolids in the United States (Clapp et al., 1994) concluded that this resource can provide fertility and many other beneficial effects. In our demonstration (1994 through 1996) trials, similar observations were made. The low incorporation rates utilized in these trials provided adequate concentrations of trace elements for a two-year crop. In addition, growers did not have to add lime. The pH of the mixes remained near 6.0 after 17 months of production.

Major nutrients (N, P, K), supplied by the compost during the first four to six weeks after potting, allow growers to apply slow release fertilizer after the crop has developed a canopy. The pellets therefore can be placed in the shade under plants, preventing excessive release rates associated with slow release pellets positioned in the open sun on pots (130ƒF), often resulting in ammonia toxicity on young plants (Inbar et al., 1990).

In early (1978-1983) trials with composted bio-solids at nurseries in Ohio (Hoitink and Maronek, 1986; Logan et al., 1984), increases in the severity of high-nitrogen induced diseases, such as fire blight of pyracantha and Phytophthora dieback of rhododendron, were observed. In these 1994-96 trials at the lower rates (10-20%), evidence for increased disease severity or excessive fertility was not obtained. Such conditions probably would have developed in the 20% mix had growers applied slow release fertilizer at the time of potting, thus providing excessively high fertility conditions and susceptibility to these foliar diseases.

The high pH of 6.0 observed in the composted biosolids-amended mixes did not result in iron deficiency on any crops tested. Even at pH 7.4 in high carbonate water regions in Ohio, iron deficiency is avoided on many crops in composted biosolids-amended mixes (Hoitink and Maronek, 1986). In bark mixes, problems would occur at that pH. Today, explanations for this "greening" effect of composted biosolids are being found. Chen et al., 1996, showed that the concentrations of dissolved micronutrients can be increased by amending mixes with composted biosolids. The soil microflora supported by these composts produces siderophores, which chelate micro nutrients such as iron, thus making it available to plants. In conclusion, composted biosolids contribute beneficial effects to the nursery industry through several mechanisms.

Acknowledgements

Salaries and research support were provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. We acknowledge grant support from Kurtz Bros., Inc., Independence, Ohio 44131. We thank Warner Nurseries, Klyn Nursery, Losely Nursery, and Roemer Nursery for performing these demonstration trials with us.

Literature Cited

Chen, L., W. A. Dick, J. G. Streeter, and H. A. J. Hoitink. 1996. Ryegrass utilization of nutrients released from composted biosolids. Compost Science & Utilization. 4:73-83.

Clapp, C. E., W. E. Larson, and R. H. Dowdy. 1994. Sewage Sludge: Land utilization and the environment. Amer. Soc. Agron. Inc., Madison, WI 258.

Hoitink, H. A. J. 1994. Beneficial effects induced by composted biosolids in horticultural crops: pg. 95-100. In: Sewage sludge: Land utilization and the environment. C. E. Clapp, W. E. Larson and R. H. Dowdy (eds). SSSA Misc. Publication. 258 Pgs.

Hoitink, H. A. J. and D. M. Maronek. 1986. Composted municipal sludge - a review of research and demonstration trials. Buckeye Nurserymen's Research Update. June 1986, pg. 1-8.

Inbar, Y., M. E. Watson, K. D. Cochran , E. M. Smith Jr., and H. A. J. Hoitink. 1990. Interactions between Subdue and slow release fertilizers at high temperatures. Ornamental Plants: A Summary of Research. In: The Ohio State University/Ohio Agricultural Research and Development Center, Research Circular 135, pg. 50-53.

Kuter, G. A., H. A. J. Hoitink , and Chen W. 1988. Effects of municipal sludge compost curing time on suppression of Pythium and Rhizoctonia diseases of ornamental plants. Plant Disease 72:751-756.

Logan, T. J., W. R. Faber, and E. M. Smith Jr. 1984. Use of composted sludge on different crops. Ohio Report. May-June: 1984 :37-40.