This study examines heart rate and heart rate fluctuation when subjects are presented with the scent of soil and their psychological changes as expressed by scores of the Profile of Mood States (POMS) and Visual Analog Scale (VAS) as indexes. In the experiment, we directed the subjects to rest in a sitting position for 5 min. (Pre.), to smell the scent for 1 min. (No Stim., Stim.), and to rest in a sitting position for 15 min. (Post 5, Post 10, Post 15). Psychological evaluation was performed before and after the experiment (Pre., Post). The scent stimulation was made by opening/closing a bottle with screw-on cap that contained the soil. In the control group, the bottle is always closed. In the stimulus group, the bottle was opened only at the time of Stim. For the physiological evaluation, although we could observe no change in the control group, in the stimulation group, the subjects’ heart rates decreased at Stim. and Post 15. For the psychological evaluation, VAS scores of “Feel relaxed” and “Feel soothed” increased and POMS scores of “Strain-Uneasiness”, “Anger-Hostility”, and “Confusion” decreased. In open-ended questions, some of the subjects recalled memories of insect-collecting, horticulture, forests and parks. There was a negative correlation between heart rate decrease (Stim.) and increase in the VAS scores of “Feel relaxed” (r = -0.896, p < 0.001) and “Feel soothed” (r = -0.684, p = 0.014). The healing effects from the scent of soil included subjects’ memories that were considered to have influenced the heart rate.
Forests constitute 30% of the total land area on earth. Forests occupy 68% or 2/3 of the total land area in Japan, making it the country with the second highest percentage of forests in the world, after Finland. The healing effect of forests has attracted wide attention in recent years and can be considered one of the benefits of forest resources. There have been studies that reported a reduction in stress [
Twelve male students (age: 24.6 ± 1.8 years old) with no olfactory disturbance received an explanation of the purpose of this study and gave their written consent to participate in accordance with the principles of the Helsinki Accords. The subjects selected one of 12 cards. Of the 12 cards, six cards were labeled “1st control group/2nd stimulation group” and the other six cards were labeled “1st stimulation group/2nd control group. The experiment was conducted from 23 November to 5 December in 2015.
The soil samples that were used for scent stimulation were collected from the green belt area at the University of Tsukuba administrative building north parking lot (36˚6'47.40''N 140˚6'14.10''E). The vegetation in the artificial forest from which the soil was collected included mainly evergreen oak trees, dotted Japanese oak, Japanese cedar, maple, and chestnut trees. The soil samples (taken from the organic sedimentary layer containing fallen leaves of evergreen oak trees to a depth of about 15 cm including topsoil) were put into a bottle measuring 15 cm in height and 10 cm in diameter with a screw-on cap.
Qualitative analysis of the soil for scent stimulation was conducted using GC/MS (GCMS-QP2010 Plus, Shimadzu Corp.) by the headspace method. The collected soil was put into a vial with 10 cc septum and 2 ml was poured from the headspace by a split trace after leaving it standing at 23˚C for 1 hour. The column (Stabilwax®-DA, Shimadzu Corp.) used was 0.25 mm i.d. × 30 m with film thickness of 0.25 μm. Ion source temperature and interface temperature were set at 200˚C; in the oven program, the temperature was first set at 40˚C for 5 min. and then increased to 200˚C at increments of 5˚C/min. and when it reached 200˚C, was then set for 10 min. at this temperature. Each component that was identified by a similar search (GCMS solution, Shimadzu Corp.) was expressed by the area percentage method (
Physiological evaluation was performed by measuring heart rate and heart rate fluctuation for which 2-lead electrocardiogram recorders were used. The R-R interval was computed with a memory heart rate meter (LRR-03, GMS), and heart rate (HR), low frequency component (LF; 0.04 - 0.15 Hz), High frequency component (HF; 0.15 - 0.4 Hz), and ratio (LF/HF) of LF component and HF component were analyzed by the heart rate fluctuation real-time analysis program (MemCalc/Tarawa, GMS) [
Component | Composition ratio (%) |
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α-Pinene | 14.96 |
3-Carene | 39.81 |
β-Myrcene | 21.27 |
D-Limonene | 23.97 |
five-grade evaluation and to determine T-Scores for the six mood scales of “Tension-Anxiety”, “Depression”, “Anger-Hostility”, “Vigor”, “Fatigue” and “Confusion”.
The experiment was conducted in a room with a room temperature of 24˚C ± 0.5˚C, and relative humidity of 31% ± 1%. The subjects sat quietly for about 10 min. after entering the room. Physiological evaluation was measured as follows: 5 min. rest (Pre.), 1 min. no stimulation or stimulation (No Stim. or Stim.), and 15 min. rest. The subjects sat with their eyes closed during measurement (Pre. to Post 15). Psychological evaluation (VAS and POMS) was conducted before and after (Pre., Post) physiological evaluation. At Post, the evaluation included the time when the scent was detected.
Before starting the experiment, the bottle with a screw-on cap for scent stimulation into which soil was put, was positioned so that the opening of the bottle was 10 cm from the nose of the subject. The scent stimulation was made by opening/closing the cap. After 5 min. rest with the cap closed (Pre.) the control group sat with the cap closed (No Stim.) and the stimulation group (Stim.) sat with the cap open so they were able to smell the scent. After that, both groups rested for 15 min. (Post 5, Post 10, Post 15) with the cap closed.
For physiological evaluation, the mean for each value of Pre., No Stim., Stim., Post 5, Post 10, and Post 15 was determined. For heart rate (HR) and heart rate fluctuation (LF, HF, LF/HF) the Fisher (LSD) multiple comparison was conducted on each group in the generalized linear model for five points (Pre., Stim., Post 5, Post 10, Post 15). Temporal changes between both groups were examined by two-way analysis of variance (interaction) in the generalized linear model. Wilcoxon signed-rank test was used for VAS and POMS. The data for the physiological/psychological evaluation was indicated by mean ± standard deviation.
Feel tired | Feel discomfort | Feel disgust | Feel depressed |
---|---|---|---|
Feel nervous | Feel excited | Feel thrilled | Feel refreshed |
Feel comfortable | Feel relaxed | Feel soothed | Feel sleepy |
Like scent of soil |
LF/HF). Heart rate significantly decreased after soil scent stimulation as indicated by the following results: Pre. level was 74.3 ± 6.5 bpm; during Stim. (p = 0.01), it was 72.0 ± 6.8 bpm and the level at Post 15 (p = 0.022) was 71.6 ± 7.8 bpm. There was also no interaction. Both control and stimulation groups showed no change in LF, HF, and LF/HF ratio and expressed no interaction.
Pre | (No) Stim. | Post 5 | Post 10 | Post 15 | |
---|---|---|---|---|---|
Control Group | |||||
HR (bpm) | 72.0 ± 10.2 | 72.7 ± 6.2 | 73.9 ± 8.3 | 73.8 ± 8.3 | 72.8 ± 7.6 |
LF (msec2) | 836.5 ± 126.6 | 737.3 ± 136.7 | 1054.1 ± 273.5 | 920.7 ± 194.3 | 974.0 ± 150.3 |
HF (msec2) | 410.1 ± 91.5 | 386.8 ± 74.3 | 351.2 ± 72.1 | 325.1 ± 75.7 | 398.4 ± 88.7 |
LF/HF ratio | 3.8 ± 1.0 | 5.3 ± 3.0 | 10.6 ± 6.7 | 8.2 ± 3.9 | 5.5 ± 2.3 |
Stimulation Group | |||||
HR (bpm) | 74.3 ± 6.5 | 72.0 ± 6.8* | 74.2 ± 7.1 | 73.0 ± 6.3 | 71.6 ± 7.8* |
LF (msec2) | 908.6 ± 157.8 | 1296.7 ± 325.4 | 855.8 ± 118.2 | 975.4 ± 130.6 | 1081.9 ± 236.2 |
HF (msec2) | 374.9 ± 134.5 | 616.2 ± 263.3 | 368.2 ± 123.0 | 399.5 ± 90.5 | 457.5 ± 113.9 |
LF/HF ratio | 8.4 ± 3.7 | 9.8 ± 6.3 | 9.1 ± 3.7 | 7.4 ± 4.0 | 6.1 ± 3.1 |
*p < 0.05; The values are mean ± SD; The control group first rested for 5 min. (Pre.), next, they smelled no scent for 1 min. (No stim.), and then they rested again for 0 to 5 min. (Post 5), for 5 to 10 min. (Post 10), and for 10 to 15 min. (Post 15). The stimulation group first rested for 5 min. (Pre.), next, they were presented with the scent of soil for 1 min. (Stim.), and then rested for 0 to 5 min. (Post 5), for 5 to 10 min. (Post 10), and for 10 to 15 min. (Post 15).
Control group | Stimulation group | |||
---|---|---|---|---|
Pre. | Post | Pre. | Post | |
Tension-Anxiety | 41.5 ± 8.6 | 38.9 ± 7.2* | 45.4 ± 14.7 | 38.1 ± 7.0* |
Depression | 43.1 ± 5.0 | 42.5 ± 4.2 | 47.7 ± 12.2 | 42.3 ± 3.3 |
Anger-Hostility | 38.7 ± 2.5 | 38.2 ± 2.4 | 41.5 ± 4.2 | 38.2 ± 1.8* |
Vigor | 37.7 ± 7.2 | 34.8 ± 5.2* | 37.2 ± 6.5 | 37.6 ± 6.1 |
Fatigue | 44.1 ± 9.5 | 45.1 ± 9.8 | 48.2 ± 10.6 | 43.5 ± 11.2 |
Confusion | 49.6 ± 9.9 | 48.2 ± 5.9 | 54.3 ± 13.9 | 47.6 ± 8.6* |
POMS: Profile of Mood States; *p < 0.05; The values are mean ± SD.
Control group | Stimulation group | |||
---|---|---|---|---|
Pre. | Post | Pre. | Post | |
Feel tired | 23.6 ± 16.9 | 31.6 ± 19.8 | 43.0 ± 25.5 | 33.1 ± 16.2 |
Feel discomfort | 18.1 ± 15.2 | 17.5 ± 14.4 | 28.1 ± 25.3 | 28.9 ± 27.7 |
Feel disgust | 8.7 ± 9.4 | 18.0 ± 16.2 | 18.3 ± 14.7 | 20.2 ± 19.7 |
Feel depressed | 9.9 ± 10.8 | 8.2 ± 7.7 | 19.5 ± 21.7 | 15.6 ± 17.5 |
Feel nervous | 19.7 ± 18.7 | 12.1 ± 10.1 | 38.3 ± 25.9 | 13.0 ± 14.1* |
Feel excited | 26.0 ± 20.8 | 19.7 ± 21.4* | 25.8 ± 20.6 | 22.4 ± 22.7 |
Feel thrilled | 26.2 ± 20.6 | 19.5 ± 20.7* | 31.3 ± 18.1 | 31.7 ± 22.8 |
Feel refreshed | 31.8 ± 0.2 | 20.3 ± 19.5 | 33.4 ± 21.7 | 32.8 ± 19.4 |
Feel comfortable | 29.8 ± 19.1 | 26.8 ± 8.5 | 33.5 ± 21.8 | 44.8 ± 21.8* |
Feel relaxed | 33.1 ± 17.6 | 28.1 ± 22.1 | 36.9 ± 23.3 | 52.7 ± 16.1* |
Feel soothed | 22.8 ± 20.3 | 22.0 ± 19.7 | 25.8 ± 23.3 | 46.2 ± 20.5* |
Feel sleepy | 24.5 ± 18.3 | 31.8 ± 15.9 | 32.5 ± 15.2 | 42.5 ± 22.5 |
Like scent of soil | 54.6 ± 6.0 |
VAS: Visual Analogue Scale; *p < 0.05; The values are mean ± SD.
38.3 ± 25.9 mm to a Post level (p = 0.012) of 13.0 ± 14.1 mm, and “Feel comfortable” significantly increased from a Pre. level of 33.5 ± 21.8 mm to a Post level (p = 0.050) of 44.8 ± 21.8 mm. “Feel relaxed” increased from a Pre. level of 36.9 ± 23.3 mm) to a Post-level (p = 0.041) of 52.7 ± 16.1 mm, “Feel soothed” increased from a Pre. level of 25.8 ± 23.3 mm to a Post level (p = 0.038) of 46.2 ± 20.5 mm.
We asked the stimulation group open-ended questions about their image of soil scent after they had smelled the soil. All the subjects gave a reply except for one. Eight subjects imagined landscapes and experiences related to soil scent, and one subject had no image. Two participants replied as to whether they liked the soil scent or not (
In this study, we compared subjects which were presented with the scent of soil (stimulation group) and those which were not (control group). Consequently, although the control group showed no physiological changes, the stimulation group showed a significant decrease in heart rate (Stim., Post 15). The heart rate is determined by the firing frequency of the sinus node (pacemaker cells). This frequency is influenced by the dominant sympathetic nerve and vagus nerve, thus leading to either excitability or inhibitory. For this reason, periodic activity of the brain stem and higher order cerebral cortex that synchronize with signals from arteries and veins and changes in blood pressure influence behavior, emotions, circadian rhythm, which increase through hyperactivity of sympathetic nerve functions, and decrease through hyperactivity of parasympathetic nerve functions [
Answers to open-ended questions: | |
---|---|
I was reminded that when I was a child I caught a beetle in a mountain. I did not feel disgust, and if anything, I felt soothed. I had a nostalgic feeling that reminded me of my early childhood when I played in the sandbox at a park. I felt nostalgic and remembered my childhood and playing with beetles. When I was a high school student, I studied horticultural therapy. I imagined rice fields and the area around my parents’ home. I imagined a forest. I was reminded of the soil in fields of rice and vegetables. (Since there are rice fields and vegetable fields around my parent’s home, I had experienced smelling soil in the past). I had thought that the soil might have an unpleasant smell; however, it was not as unpleasant as I had expected. Imagined a park. I could not imagine anything. | |
Non-response. |
and “Confusion” decreased. In addition, VAS scores of “Feel nervous” decreased and “Feel comfortable”, “Feel relaxed” and “Feel soothed” increased. From the above results, we conclude that subjects expressed improvement in their mood and were more relaxed when they were presented with the scent of soil. Some of the answers indicated that the scent of soil recalled childhood memories with subjects using such phrases as “in my early childhood”, “in my childhood” or “when I was a child”. In many cases, the scent made subjects recall memories of early childhood that were older than those created by visual or verbal cues [
HR (Stim.-Pre.) | HR (Post 15-Pre.) | ||
---|---|---|---|
VAS (Post-Pre.) | |||
Feel tired | 0.013 | −0.091 | |
Feel discomfort | −0.035 | −0.326 | |
Feel disgust | 0.164 | −0.212 | |
Feel depressed | −0.120 | −0.558 | |
Feel nervous | 0.049 | −0.489 | |
Feel excited | −0.127 | −0.571 | |
Feel thrilled | 0.588* | 0.069 | |
Feel refreshed | −0.347 | 0.145 | |
Feel comfortable | −0.511 | 0.077 | |
Feel relaxed | −0.896*** | 0.034 | |
Feel soothed | −0.684* | 0.166 | |
Feel sleepy | 0.169 | 0.320 | |
POMS (Post-Pre.) | |||
Tension-Anxiety | −0.138 | −0.277 | |
Anger-Hostility | 0.513 | 0.126 | |
Confusion | 0.355 | 0.052 |
*p < 0.05, ***p < 0.001; The values are correlation coefficient.
with the factors “feel relaxed” (r = −0.896, p < 0.001) and “feel soothed” (r = −0.684, p = 0.014) as indicated by the increase in the VAS scores. Therefore, in this study, we found that the healing effects and autobiographical memories induced from smelling the soil may lead to hyperactivity of parasympathetic nerve functions or suppression of sympathetic nerve functions which may influence the heart rate.
We would like to express our appreciation to the participants for their cooperation in this study. Also, we would like to express our gratitude to the Tsukuba University of Technology for allowing us to use their laboratories.
Morisawa, T., Hanyu, K., Mori, H. and Tamura, K. (2017) Physiological and Psychological Effects of Scent of Soil on Human Beings. Open Journal of Soil Science, 7, 235-244. https://doi.org/10.4236/ojss.2017.79017