Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityJanuary, 1997
Soil Amendments, Soil pH, and Plant Growth: Results from a two-year study.
Keywords: pH, soils, soil amendments, alternative substrates, nutrition, landscape, new research
I n my January, 1996 L.A.N.D. column, I shared first-year results from a two-year study with composts. In this study, I hoped to build on Dr. Elton Smith's work with composted sludge in the landscape.
In the few years since that work, a greater variety of composted materials has become available. In my study, I hoped to make some meaningful comparisons between composted yardwaste, - the 'new kid on the block,' composted sludges, and composted leaves.
Further, since many composts have a rather high pH, I was interested in whether they would significantly raise the pH of the calcareous soils we have in central Ohio. These soils already have a pH that is higher than optimal for many plants (upper 6's, lower 7's). Sulfur was added to half of the plots to determine whether it would offset the effect of the composts on soil pH. Now, in January, 1997, I would like to share my conclusions from two years of work with composts in the landscape.
The experiment. In June of 1995, a two-inch layer of each amendment was applied to 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 (the control). Granulated sulfur was applied to all treatments at either 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.
In May of 1996, the soil amendments were applied again and rototilled as before. Sulfur was not added in the second year, since I wanted to see how long the sulfur was effective in lowering pH. On June 14, plots were planted with 'Red Elite' geranium, 'Ultra Red' petunia, 'Janie' marigold, and 'Cut and come again' zinnia. No fertilizer was added in 1996.
Results in 1995. As previously reported in my January, 1996 column, the first year of this study was extremely wet; 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. Peat moss improved growth of all bedding plant varieties, whereas the composted sludges and yardwaste all improved growth in at least one variety. However, plants in the leaf compost treatment were no better than controls. The ranking of treatments, in terms of their benefit to plant growth, was peat > CMS (both sources) > composted yardwaste > composted leaves = unamended soil.
Results in 1996. 1996 was another record-breaking, wet year. This time, composted sludge and yardwaste treatments came out on top - significantly improving quality and dry weights over the peat treatment. These three composts have a high nutrient content compared to peat, and since no fertilizer was added in 1996 it is not surprising that these treatments produced the best plants. Similar to 1995, plants grown in leaf compost were not significantly larger or better in quality than the control. The ranking of treatments in 1996, in terms of their benefit to plant growth, was CMS (Comtil) > CMS (TechnaGro) = yardwaste compost > peat moss > leaf compost = control. Sulfur did not significantly affect quality or fresh weights in either year.
Soil organic matter over the two-year period. All organic soil amendments significantly increased soil organic matter content. The field soil contained about 7% organic matter. In October 1995, measurable soil organic matter averaged 12.3% for all organic amendments; by September, 1996, after two years of additions, soil organic matter averaged 18.4%.
Soil pH over the two-year period. The pH of the unamended field soil (control) without sulfur addition ranged between 6.6 and 7.2 over the two year period (Table 1). The effect of organic soil amendments on soil pH was quite variable (Table 1, A & D). Plots amended with leaf compost consistently had the highest pH values, raising pH above control field soil as much as a full unit (July '95). Even 14 months later (September, 1996), the pH of soil in these plots were 0.7 unit greater than controls (Table 1, D). Perhaps the alkaline effect of the leaf compost accounts for its lack of any benefit to growth. The composted sludges, while increasing pH above the control slightly in the first season, actually decreased soil pH relative to controls for some dates in 1996. Nitrogen transformation in the soil produces residual acidity and composted sludges are a significant nitrogen source. Perhaps nitrogen transformations account for some pH reduction associated with CMS in 1996. The pH of peat-moss amended plots was as much as 1.6 units lower than the controls. The pH-lowering effect of peat was fairly consistent throughout the experiment.
Sulfur addition lowered the pH of field soil (Table 1, A,B,C) as much as 1.3 units. The full effect of sulfur addition was not apparent until August or October of 1995, depending on treatment. However, by August or September of 1996, pH values in the plots amended with sulfur began to increase. By September of 1996, the pH of sulfur-amended plots were 0.4 pH unit below the control at most (Table 1, C).
Sulfur addition to the compost treatments did not lower the soil pH as much as sulfur addition to the field-soil control plots (Table 1, C). Apparently the composts buffered the soil pH, and reduced the effectiveness of sulfur.
Conclusions.
1. All composts increased soil organic matter, but not all composts improved the growth of the bedding plants. Both composted sludge products, composted yardwaste, and peat moss improved growth relative to controls. Leaf compost did not. Probably the most important conclusion from this experiment is that not all composts are created equal. It is very important to have some experience with a product before using it extensively since compost ingredients and producers vary tremendously.
2. Composts had a variable effect on soil pH. Some increased soil pH significantly (composted leaves) whereas the effect of composted sludges on soil pH was variable.
3. Sulfur at 3 pounds per 100 square feet effectively decreased field soil pH approximately 1-1.5 unit for about one year before the pH began to rise. Compost reduced the effectiveness of sulfur in reducing soil pH.
4. Peat moss addition to the soil effectively decreased the pH 1-1.5 units throughout the entire experiment. Amending soil with peat moss thus may be a more effective long-term method of decreasing pH than sulfur addition.
Table 1. Soil pH of field soil, peat moss, and compost treatments with and without sulfur addition
(3 pounds/100 sq. ft.)
A. NO SULFUR
treatment
Jul-95
Aug-95
Oct-95
May-96
Jun-96
Aug-96
Sep-96
control
6.8
6.9
7.0
6.6
6.6
7.2
7.1
peat
5.2
5.6
6.0
5.2
5.8
6.2
5.5
comp. leaves
7.8
7.8
7.6
7.3
7.3
7.8
7.8
comp. yardwaste
7.4
7.4
7.5
6.8
7.3
7.5
7.4
CMS-Akron
6.9
7.0
7.2
6.4
6.7
7.0
6.8
CMS-Columbus
7.2
7.1
7.3
5.8
7.1
7.0
6.8
B. WITH SULFUR
treatment
Jul-95
Aug-95
Oct-95
May-96
Jun-96
Aug-96
Sep-96
control
6.4
5.7
6.2
5.3
5.4
6.3
6.7
peat
4.9
4.8
4.6
4.7
4.5
5.1
5.5
comp. leaves
7.6
7.1
7.2
7.2
7.1
7.6
7.5
comp. yardwaste
7.1
6.8
6.9
6.6
6.8
7.1
7.0
CMS-Akron
6.8
6.6
6.6
6.1
6.6
6.7
6.6
CMS-Columbus
6.9
6.7
6.7
5.7
6.6
6.6
6.5
C. DIFFERENCE IN SOIL pH DUE TO SULFUR
(part B - part A)
treatment
Jul-95
Aug-95
Oct-95
May-96
Jun-96
Aug-96
Sep-96
control
-0.4
-1.2
-0.8
-1.3
-1.2
-0.9
-0.4
peat
-0.3
-0.8
-1.4
-0.4
-1.2
-1.1
0.0
comp. leaves
-0.2
-0.7
-0.4
-0.1
-0.3
-0.2
-0.3
comp. yardwaste
-0.2
-0.6
-0.6
-0.2
-0.5
-0.4
-0.4
CMS-Akron
-0.1
-0.4
-0.6
-0.2
-0.1
-0.3
-0.2
CMS-Columbus
-0.3
-0.4
-0.6
-0.1
-0.5
-0.4
-0.4
D. DIFFERENCE IN SOIL pH DUE TO ORGANIC AMENDMENT
(from part A)
treatment
Jul-95
Aug-95
Oct-95
May-96
Jun-96
Aug-96
Sep-96
control
0.0
0.0
0.0
0.0
0.0
0.0
0.0
peat
-1.6
-1.3
-1.0
-1.4
-0.9
-1.0
-1.6
comp. leaves
1.0
0.9
0.6
0.7
0.7
0.6
0.7
comp. yardwaste
0.6
0.5
0.5
0.2
0.6
0.3
0.3
CMS-Akron
0.1
0.1
0.2
-0.2
0.1
-0.2
-0.3
CMS-Columbus
0.4
0.3
0.3
-0.8
0.5
-0.2
-0.2
