Ornamental Plants

Annual Reports and Research Reviews

Composted Biosolids II: Trials on The Ohio State University Campus

Summary

Four woody ornamentals were grown in three media containing composted biosolids at 10%, 15%, or 20% volume. Plant heights, diameters, and overall appearance were satisfactory in all treatments. The growth of the four species of woody ornmanentals was judged to be excellent and essentially equivalent in all three growing media. Increasing the fraction of the compost from 10% to 20% increased growing medium pH by as much as one pH unit. While roots appeared to be disease-free in the fall of 1995 and the spring of 1996, it was apparent that root growth was inhibited in all media one to two inches from the bottom of the pot. Root growth was vigorous and unrestricted throughout the rest of the container.

Introduction

This work is a companion study to another reported in this circular by Hoitink, Rose, and Zondag. The objective of both studies was to evaluate three different potting mixes containing varying proportions of Technagro^TM (TG), a composted municipal sludge product produced from the city of Akron composting facility. This study reports results from trials on the Ohio State University Columbus Campus. Other trials were carried out at commercial nurseries and are reported in "Composted Biosolids: An Ideal Organic Amendment for Container Media Supplying Both Nutrients and Natural Suppression of Root Rots."

Materials and Methods

The three mixes used in this study were the same as described by Hoitink et al.

• 10% by volume, all TG screened to 1/2" or less before addition (with fines).
• 15% TG by volume, half of the material with fines, half of the material sized between 1/2" and 1/8" (coarse).
• 20% TG by volume, all material sized between 1/2" to 1/8" (all coarse).

Thus, as the proportion of TG in the mixes increased, the proportion of fines decreased.

Four commonly grown species of woody ornamentals were potted in the three media on May 25, 1996. Quart-size Cotoneaster dammeria 'Lowfast' and Juniperus chienensis 'Hetzii Glauca' were potted into two-gallon containers; three-year Taxus x media 'Densiformis' bed liners were potted into two-gallon containers, and 6-8" rooted cuttings of Ligustrum x vicaryl were potted into one-gallon containers. Fifteen plants of each treatment were arranged in a randomized complete block design in the container holding area at Ohio State. Irrigation was supplied by overhead sprinklers. All containers were top-dressed with slow-release fertilizer (Osmocote 18-6-12) at a 2 lbs. nitrogen (N) per cubic yard rate on June 16. Occasionally, plants were pruned lightly in accordance with normal nursery practice.

Height and plant diameters were taken on July 12, August 17, and October 16. Growing medium samples were taken from all pots on July 6 (Taxus and Juniperus) or August 24 (Cotoneaster and Ligustrum). At final harvest (October 16), three soil balls from each treatment and replication (nine per treatment, total) were cut in half and the roots examined. Growing medium samples were taken from these pots. All medium samples were analyzed using the saturated media extract procedure. Soil extracts were tested for pH, soluble salts, and nitrate-nitrogen. Six plants from each treatment remained for observation in spring of 1966.

Data were analyzed using SAS statistical procedures. In Tables 1 and 2, statistical differences between treatments are indicated by the letters a, b, c. Where no letters appear, there were no differences in treatment.

Results and Discussion

Plant Growth Measurements

For all three dates, there was little difference in plant height and diameter among the three media treatments (Table 1). The only statistical difference noted between treatments was in the Ligustrum (Privet). On July 12, the 15% and 20% treatments produced somewhat more growth than the 10% medium; however, there were no differences in the later measurements. General observations of plant appearance and quality also suggested that growth was quite satisfactory in all three media. By the end of the season, juniper and cotoneaster plants had reached saleable size and were of marketable quality. Taxus and Ligustrum quality was satisfactory, but these plants had not yet reached marketable dimensions for their pot size.

Growing Media Analyses

An increase in pH was the most consistent effect that increasing TG had on the nutritional characteristics of the growing media. Increasing TG from 10% to 15% increased medium pH by as much as one full unit (Table 2). There was, however, little difference between pH values in the 15% and 20% TG media, which suggests that the fines fraction may contribute more to increasing pH. (The 20% TG treatment contained coarse TG only, no fines.)

Since pH is a critical factor in controlling nutrient availability in a growing medium, this may be a key consideration in formulating a future medium. A desirable pH range for container media is 5.5-6.5; the pH of the 15% and 20% TG media usually exceeded this range (Table 2). However, the plants chosen for this study are fairly tolerant of a wide range in pH values and did not appear to exhibit any response to differences in media pH.

In contrast to the pH data, the soluble salts (EC) and nitrate-N data were quite variable (Table 2). Although there were some statistical differences among treatments for some dates for Taxus and Cotoneaster, the differences were not great and the pattern not consistent. Overall, soluble salts and nitrate levels were in an acceptable range, with a few exceptions. Cotoneaster, the most vigorous plant, not surprisingly had the lowest nitrate concentrations in the media. Ligustrum, in contrast, had quite high nitrate and EC readings in August. These plants were quite small when planted, and root systems did not completely fill the pots until later in the season. Higher nitrate and soluble salt levels in the Ligustrum media probably reflect proportionately less uptake relative to the three larger, rapidly growing species. Only the Ligustrum media analysis revealed some potentially problematic levels of nitrates. While Ligustrum was unharmed by these levels, salt sensitive plants could be affected. The high nitrate levels in the Ligustrum media could have resulted from the compost or the slow-release fertilizer.

Observations of Roots, October 1995, May 1996

Roots in all three compost media were numerous and healthy in appearance, but in nearly all cases, roots were sparse or absent from the bottom one to two inches of medium. This bottom layer typically was saturated, and apparently the aeration was not sufficient for root growth. Several of the same species of plants were growing nearby in compost-free media; roots readily grew into the bottom of the medium in those media.

Among the three media, the 10% TRG appeared to have a slight edge over the 15% and 20% media in terms of numbers of roots in the juniper, Cotoneaster, and Ligustrum. However, the difference was not great and was not reflected in top growth. Furthermore, these visual root observations were subjective and not quantitative.

In May 1966, roots of the three compost treatments were examined once more. No signs of root rot were observed in any of the treatments.