Effect of Soil pH on the Health of Transplanted Azaleas
 
 

Azalea Boys

Edward Murray
Justin Turner
Monique Koll
Eric Guitreau




Abstract

Our experiment tested the effect of soil pH on the growth of transplanted azaleas. Past studies have shown that azaleas grow optimally at a lower pH than offered in many premixed potting soils. The height, number of branches, and number of blooms were the criteria used to determine azalea health. Our hypothesis is the lower the soil pH, the better the azalea growth. Our hypothesis was rejected since the azaleas grown at pH 5.5 performed better than those at
pH 4.8. Other results were as expected.

Introduction

Azaleas require acidic soil values for optimum growth. Many prepared soil mixes have higher pH values than the values needed by azalea plants. Our hypothesis is that these acid lovers should thrive better as pH drops. Soil pH is altered and adjusted using aluminum sulfate and lime as necessary. Azalea health is assessed by recording three quantitative values: height, number of branches, and number of blooms. According to the hypothesis, the soils with the highest pH will have the lowest values for the plants in these areas. Eight azaleas divided into four pH groups are analyzed once a week for a six-week period. The pH levels are as follows: the control group averaged a pH of 7.0, the first group a pH of 6.5, the second group a pH of 5.5, and the third group a pH of 4.8.

Materials

(8) 1-gallon pots
(4) 2-gallon pots
(8) 6-inch azaleas
(8) gallons of Olivier Ap soil
(1) 5-lb. bag of aluminum sulfate
(1) 5-lb. bag of lime
(1) bag of 15-5-10 fertilizer
(1) pH meter / electrode

Methods

  1. Separate 8 gallons of soil evenly into 4 2-gallon containers.
  2. Use pH meter to measure the pH of all four soil samples.
  3. Record values.
  4. In the pot labeled "control," add 10 ml of aluminum sulfate. Record pH.  Incubate for 5 days.
  5. Repeat this process for each set of soil samples. Record pH after each amendment and
      allow to incubate for 5 days.
  6. After incubation, measure and record pH values of each pot.
  7. Repeat steps 4-6 for the pots until the following values are obtained:
      pH 7.0, 6.5, 5.5, and 4.8.
  8. In cases where pH has dropped below target value, add 10 ml of lime and incubate
      for 5 days.
  9. Measure and record pH values.
10. Divide soils into 8 1-gallon pots and label appropriately.
11. Plant one six-inch azalea into each pot.
12. Measure the height, number of branches, and number of blooms present on each plant
      weekly. Record this data.
13. Repeat step 12 every week until experiment concludes.
14. Make pH readings every 2 weeks. If deviations occur from desired values, adjust pH
      according to the procedure outlined in steps 4-7, but only allow incubation for 3 days.
15. Add 15-5-10 fertilizer to each pot every 2 weeks.

Results

This experiment began on Saturday, October 16 and concluded on Saturday, November 27, 1999.

Table 1.    pH values.
 
Week
pH 7
pH 4.8
pH 5.5
pH 6.5
2
7.0
4.9
5.6
6.6
4
7.1
5.1
5.7
6.8
6
7.1
4.9
5.6
6.5

Table 2.    Height in inches.
 
Week
pH 7.0
pH 4.8
pH 5.5
pH 6.5
1
6
6
6
6
2
6
6.5
6.5
6.3
3
6.5
6.8
6.9
6.5
4
6.7
7
7.1
6.7
5
6.8
7.2
7.3
6.9
6
7
7.5
7.7
7.2

Table 3.  Number of branches.
 
Week
pH 7
pH 4.8
pH 5.5 
pH 6.5
1
3
4
5
4
2
3
4
5
4
3
3
5
6
5
4
4
6
7
5
5
4
6
7
6
6
4
7
8
6

Table 4.  Number of blooms.
 
Week
pH 7
pH 4.8
pH 5.5
pH 6.5
1
0
0
0
0
2
0
0
0
0
3
0
1
1
1
4
0
2
2
1
5
1
2
3
2
6
1
3
4
2

Discussion

In support of our hypothesis, the azalea growth was best in acidic soil based on our growth
criteria. The plants grown at a pH of 6.5 had the lowest height, number of branches, and number
of blooms. The pH 5.5 group had the best performance, second to the lowest pH group at 4.8,
which is not in support of our hypothesis. This deviation suggests that pH 4.8 is lower than the
optimal growth range for azaleas. It is clear that a reduction in pH in the experimental groups
was beneficial and even essential for azalea growth. The addition of soil amendments such as
lime could have negative effects on such pH limited plants. In soils with high buffer capacities,
the amounts of amendments needed to adjust pH will vary and may require more frequent pH
measures to attain the optimal pH ranges for azaleas. We conclude that the optimal growth range
for azaleas is pH 5.5.


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