Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityJanuary, 1996
Soil Amendments, Soil pH, and Plant Growth: a Research Project
Keywords: pH, soils, soil amendments, alternative substrates, nutrition, landscape
T here are great potential benefits from using composted organic waste materials as soil amendments in the landscape. These materials are produced throughout the country as a solution to waste disposal problems, and are economically quite attractive compared to traditional soil amendments such as peat. Several types of compost are available in Ohio. Most recently, composted yardwaste has been produced in response to curbside bans. Other products, such as composted leaves, have been available for years. Composted municipal sludge (CMS) has been investigated extensively for over a decade, and many studies have demonstrated growth enhancement of ornamental species with its use. However, CMS in some cases may lead to problems with high soluble salts and high pH.
Last summer I began a two-year study on compost amendments for landscape beds. While there have been many studies with various composts, relatively little is known about the suitability of composted yardwaste as a landscape soil amendment. Further, I was quite curious to learn what effect the composts, most of which have a pH around 8.0, would have on our central Ohio soils. In central Ohio we have heavy, calcareous soils; pH values generally range from the upper sixes to the low sevens. Finally, since I have received many calls concerning sulfur addition to reduce pH, I decided to add sulfur to half of my research plots to observe how long it took to lower soil pH, and to what degree.
In June, 1995, a two-inch layer of each amendment was applied to fertile field soil plots and rototilled six inches deep. The four composts products used in the project were 1) composted yardwaste, marketed as Earthblend, 2) CMS from the City of Akron, marketed as TechnaGro; 3) CMS from the City of Columbus, marketed as Comtil; and 4) composted leaves. Peat also was used as a soil amendment in the study, and a sixth treatment consisted of field soil only with no addition (control). Granulated sulfur was applied at 0 or 3 pounds per 100 square feet. Each four by ten foot plot was planted on July 3, 1995 with four bedding plant species : 'Orbit' geranium, 'State Fair' zinnia, 'Scarlet Sophia' marigold, and 'Dream Red' petunia. All plots were fertilized on July 10 with 18-6-12 slow release fertilizer at 60 pounds N per acre.
Results. The spring and early summer of 1995 was extremely wet in central Ohio, and soil amendments in most cases appeared to improve growth and ameliorate water-logging in the heavy field soil. Bedding plant species varied in their response to the soil amendments, but in general, there was a clear trend that all soil amendments improved growth in at least some annual species. The ranking of treatments, in terms of their benefit to plant growth, was peat > CMS (both sources) > composted yardwaste > composted leaves unamended soil. Since the CMS products have considerably more nutritive value than peat or the other materials, the results suggest that in this experiment, the improvement of soil physical characteristics was more critical than improvement of soil fertility. This is not surprising, given the very wet year we had in Columbus, and the naturally high level of fertility
present in the research field soil. Given a sandy, poor soil, we might expect to see the composts out-perform the peat.
Soil amendment effect on soil EC and pH. Soil amendments did not significantly increase soil soluble salts in any of the three sample dates (Table 1). However, soil pH was increased by as much as 1.1 unit by compost addition. In general, the pH of the treatment plots were ranked in the following order: composted leaves yardwaste > CMS-Columbus CMS-Akron > control >> peat. Soil pH was decreased by at least a full unit in the peat-amended plots compared to unamended field soil. The effects of the amendments on soil pH were stable throughout all sample dates. Bedding plant growth was not affected by soil pH or sulfur treatment; however these species are not known for soil pH preferences.
Sulfur effect on soil soluble salts and pH. Sulfur significantly increased soluble salts and decreased soil pH in all sample dates (Table 2). Elemental sulfur is oxidized to sulfuric acid by microorganisms in the soil. The sulfuric acid releases sulfate ions in the soil, which contribute to soluble salts. While the effect of sulfur on soil EC was highly significant, no detrimental effect on growth was observed. The highest soluble salt level attained was 1.0 mmhos.cm, which should not injure plants.
Sulfur reduced the soil pH gradually; this was expected, because the chemical reaction of sulfur in the soil is temperature- and time-dependent. In July, the average pH of the sulfur treatments was 0.2 units lower than the minus-sulfur treatments (Table 2). By August, the average pH was 0.6 units lower than minus-sulfur treatments.
While the soluble salt and pH effect of the composts were not critical growth factors in this experiment, the evidence that composts may significantly increase soil pH and EC may be important should we try to grow plants such as azaleas and rhododendrons in soils that are borderline for their growth. This group of plants prefers acid soils and is sensitive to soluble salts. Attempting to improve soil pH for this group of plants by adding sulfur could backfire if soluble salts were greatly increased as a result. In contrast to sulfur addition, peat incorporation produced an immediate reduction in soil pH without increasing soluble salts.
It will be very interesting to see what effect the 1995 soil amendments have on soil pH in 1996. I expect to see the natural reaction of the central Ohio soil overcome the effect of sulfur addition, causing the soil pH in the sulfur plots to increase. Soluble salts from the sulfur addition will probably be leached away by rainfall.
Table 1. The effect of compost addition on soluble salts (EC, mmhos.cm) and pH in a silty-clay-loam soil.
Treatments followed by the same letter are not significantly different
ns = no differences among treatments.
July Aug. Oct.
EC pH EC pH EC pH
Composted leaves 0.44 7.7a 0.47 7.5a 0.75 7.4a
Composted yardwaste 0.48 7.3b 0.47 7.1ab 0.78 7.2a
CMS-Columbus 0.45 7.0bc 0.38 7.0b 0.71 7.0ab
CMS -Akron 0.52 6.8cd 0.42 6.8bc 0.80 6.9ab
Control (no amendment) 0.50 6.7d 0.49 6.4c 0.70 6.7b
Peat 0.54 5.2e 0.43 5.3d 0.68 5.6c
minimum significant difference ns 0.3 ns 0.5 ns 0.5
Table 2. The effect of sulfur addition on soluble salts (EC, mmhos.cm) and pH in a silty-clay-loam soil.
July Aug. Oct.
EC pH EC pH EC pH
minus S 0.37 6.9 0.23 7.0 0.44 7.1
plus S 0.61 6.7 0.65 6.4 1.00 6.5
Compost treatments are averaged.
All differences are statistically significant.
