Ornamental Plants

Annual Reports and Research Reviews

Composts in the Landscape: Effects on Soil pH and Plant Growth

Mary Ann Rose
Hao Wang

Abstract

Throughout three growing seasons, the effects of sulfur and organic soil amendments on plant growth and the pH of a calcareous soil were studied. Bedding plant growth was increased by the incorporation of peat moss, yardwaste, and biosolids compost, but was not increased by sulfur or leaf compost incorporation. Leaf and yardwaste compost increased soil pH; biosolids composts had little net effect on soil pH. Peat moss and sulfur both decreased soil pH. No soil-pH effect was observed on the growth of bedding plants. However, azaleas grown in these plots during the third year responded strongly to soil pH differences. Only those azaleas grown in plots amended with both peat moss and sulfur had good color and growth. These plots also had the lowest pH. All other treatments produced unacceptable azaleas.

Introduction

Composts are widely used to improve the physical characteristics of soil and are also valuable sources of organic matter. However, many composts have a relatively high pH, in the range of pH 7 to 8. The high pH of various types of compost is potentially a problem when these materials are used in the high pH soils that are common in central and western Ohio.

In 1995, a three-year study was started on the effects of composts and sulfur on soil pH. Objectives over the three years have been to evaluate four types of composted waste materials available in Ohio for their suitability as soil amendments and to compare these to peat moss and untreated field soil. Bedding plants were grown the first two years of the experiment. First-year results with bedding plants were reported in a previous edition of this Ornamental Plants Special Circular (Rose and Wang, 1996). Azalea, a species with an acid-soil requirement, was grown the third year. In the third year, the hypothesis that composts alter the soil environment to enable acid-soil-requiring species to grow under high pH conditions was tested.

Materials and Methods

Research plots were located in Columbus, Ohio, on a Crosby silty-clay loam soil. Initial soil pH and nutrient levels (lb/A) were pH 7.1, 156 P, 663 K, 5310 Ca, and 826 Mg. Five soil amendments were used - composted leaves, composted yardwaste, two types of biosolids compost (Technagro, Kurtz Bros., Inc., and Comtil, City of Columbus), and sphagnum peat moss. Rototilled field soil with no amendment served as the control. All treatments were replicated three times. In June 1995 and May 1996, a two-inch layer of each soil amendment was applied to the soil surface and rototilled to a six-inch depth.

Granular sulfur at 0 and 3 lb/100 square feet was incorporated with the amendments in 1995, but was not reapplied until April of 1997. Organic soil amendments were not reapplied in the third year, since soil organic matter had already reached high levels (>20%) by the preceding fall. In the spring of 1995 and 1996, geranium, zinnia, petunia, and marigold transplants were planted in all plots. In spring of 1997, six four-inch potted liners of azalea 'Rosebud' were planted in each plot. A single fertilizer application of 1.5 lb. N per 1,000 sq. ft. was made in 1995 and 1997.

Visual observations were made, and soil samples were taken throughout the growing season in all years. Soil samples were tested for soluble salts (electrical conductivity) and pH.

Results and Discussion

Effect of Soil Amendments in the First Two Years

In the first two years, all soil amendments with the exception of composted leaves improved the growth of one or more species of bedding plants. All composts increased the percentage of soil organic matter. Sulfur significantly lowered soil pH, but did not affect bedding plant growth. In 1995, the ranking of the soil amendment treatments with respect to bedding plant growth was peat moss > both biosolids composts > yardwaste compost > unamended soil = leaf compost. In 1996, in contrast to the previous year, no fertilizer was added. The biosolids composts, with the highest nutrient content, produced the best growth in this year. The 1996 ranking was Comtil > Technagro = yardwaste compost > peat moss > unamended soil = leaf compost.

The results over these two years suggested that most types of composts were quite beneficial to growth, but it is possible to obtain materials that do not improve growth. It is likely that our poor results with one commercial source of leaf compost were linked to a problem with that specific source rather than a problem with leaf compost in general. Much anecdotal evidence from growers and landscapers supports the value of leaf compost from other sources. Our lack of success with one product underscores the importance of experimenting with a new product before using it on a wide scale.

Soil pH Over a Two-Year Period

Soil pH values over a two-year period (Table 1) indicated that yardwaste and leaf compost both elevated soil pH. The leaf compost raised soil pH the most, with values on some sampling dates as high as 7.8 in these plots. Overall, the biosolids composts did not affect soil pH very much. In contrast, the pH in peat-moss amended plots was decreased by one full unit, and this effect lasted at least for the entire growing season.

Effect of Sulfur on Soil pH

Sulfur applied at 3 lb/100 sq. ft. in the spring of 1995 effectively lowered soil pH throughout that year (Table 2). Since the sulfur reaction requires time, pH minimums did not occur until October 1995 or May 1996. Since sulfur was not reapplied until 1997, soil pH values increased throughout the summer of 1996. The soil pH was lowered by sulfur addition as much as a full unit in the control and peat moss-amended plots. However, the pH did not drop nearly as much in the compost-plus-sulfur plots. It appeared that all of the composts buffered the pH against changes due to sulfur (compare differences between Tables 1 and 2 for October 1995 and May 1996, when sulfur had maximum effectiveness).

Effect of Soil Amendments and Sulfur on Azalea Establishment and Quality Ratings

A distinct growth response of azaleas to both organic amendment and sulfur incorporation was observed. In most treatments, plants were unacceptably chlorotic with poor growth. Most of these plants are not expected to survive the winter of 1997-98. However, azaleas grown in soil amended with both peat moss and sulfur had deep green foliage color, good growth, and good visual quality ratings (Table 3). Most azaleas grown in compost-amended plots, plus or minus sulfur, were unacceptable and had significantly lower ratings than the azaleas grown in the field soil plus sulfur treatment. Even when sulfur was incorporated with the composts, the growth and color of plants in these treatments were unacceptable; results suggest this was because the composts raised the soil pH (Table 2). Results indicate a very strong pH effect associated with azalea growth that was not observed with bedding plants.

Conclusions

This study was carried out in a relatively high pH, calcareous soil. Under these conditions, some composts may significantly increase soil pH for at least a full growing season. In acid soils, or with pH-tolerant species, this probably does not present a problem, but with acid-preferring species, composts may reduce the chances of successful establishment in high-pH soils. The hypothesis that some composts may alter the soil environment to overcome the effects of high pH on azaleas was not supported. Peat moss plus sulfur proved to be the best treatment for establishing azaleas in a calcareous soil. However, this work also shows that the pH effect is transitory. Future work is needed to learn how to maintain a favorable soil environment over time.

Literature Cited

Rose, Mary Ann and Hao Wang. 1996. An Evaluation of Composts for Landscape Soil Amendments. The Ohio State University, Ohio Agricultural Research and Development Center. Special Circular 152. Ornamental Plants: Annual Reports and Research Summaries. pp. 7-11.