Allelopathic effects of Eucalyptus are widely reported and are considered the major factor limiting the cultivation of agricultural crops intercropped. However, reports of the allelopathic effects of Eucalyptus are mostly based on laboratory bioassay, not on field trials. The objective of the present study was to determine the effects of different leaves powder proportions of Eucalyptus species on agricultural crops as well as change in soil reaction. This research was undertaken in a pot experiment that haricot bean and maize grown in the nursery of Hawassa University College of Agriculture campus laid in a completely randomized design. Collected leaves of Eucalyptus camaldulensis and Eucalyptus grandis were prepared powdered by air drying and grinding. Results suggested that leaves powder of each tree species induced significant inhibitory effect on germination and seedling growth of each crops compared to the control treatments. Application of low-dose leaves powder of either tree species had low effect on both crops. Eucalyptus grandis has more inhibitory effect on both crops, and the negative effect of allelochemicals is more pronounced on haricot bean. This indicates that maize and haricot bean should not be planted very close to Eucalyptus trees and crops seed rate should be increased to get the maximum germination.
Forest plantations in Ethiopia are mainly mono-culture of exotic species. Nevertheless, they are predominantly mono-culture of Eucalyptus species that are estimated to cover about 93% of the total plantation area in the country [
In the traditional agroforestry system of southern Ethiopia, people are growing several trees including Eucalyptus species in or around the agricultural fields as a boundary or woodlot [
While Eucalyptus has many benefits, of all widely used plantation species they have attracted by far the most criticism [
To discourage or promote the planting of Eucalyptus and to use them for agroforestry purposes, sufficient scientific evidences on the ecological impact, on undergrowth vegetation, soil fertility, and the quality of the product should be further investigated [
The study was conducted in the nursery site of Hawassa University College of Agriculture from December to March 2011. Eucalyptus species were considered from the plantation at Wondo-Genet College of Forestry and Natural Resources.
Randomly collecting fresh leaves of 2 to 3-year-old Eucalyptus grandis and Eucalyptus camaldulensis from the top, middle and bottom canopy and they were dried in an open air under shade for two days; the leaves were then put in oven with a temperature of 50˚C for 48 hours [
The experiment was laid down in a Complete Randomized Design (CRD) which has uniform plots or growing medias. Pots were prepared and arranged in rows for both crops independently and fairly leveled by adding local top soil collected from 0 - 30 cm depth.
Equal amount of water was added into all the plastic pots and kept for one week in the net house provided with plastic sheets to develop any possible microbial activity prior to sowing. The seeds of both crops were floated in water to eliminate the empty ones, and then, 4 seeds of haricot bean and four grains of maize were planted in each of the treated plastic pots having 42 cm depth, 30 cm top and 20 cm bottom diameters provided with drainage holes. The pots were watered every two days on the average with normal tap water. After emergence, seedlings were thinned to two plants per pot. The experiment continued over a period of 21 days in which the final measurements were recorded.
Leaves powder weighing a total of 4.08 kg of each of the two Eucalyptus species were prepared and mixed with pot soil uniformly in the following treatment proportions as: (Control, 10 g powder/pot, 100 g powder/pot, 150 g powder/pot and 200 g powder/pot) replicated four times for each crop consisting a total of 96 pots/experi- mental units.
The germination was recorded and determined using Germination (%) = (No of germinated seeds/Total No of seeds sown) × 100 [
Shoot length measurements were taken from the stem joint to the tip of the terminal leaf. Root length measurements were taken from the root joint to the tip of the tap/main root. Collar diameters of both crops were determined using a caliper. Shoot and root fresh weight, Shoot and root dry weights (g) were measured using scientific electrical balance. Shoot and root dry weights (g) were measured after dried in oven at 65˚C for 24 hours. The pH for each treatment was determined by using sample of 5 g in 25 ml of distilled water with the help of pH meter [
The analysis of variance was conducted for germination, crop seedling growth parameters and soil pH by the general linear model procedure and means were compared for all significant parameters using Least Significant Difference test at 0.05 probability level using SAS software version 9 [
The results of seed germination showed that majority of treatment levels of Eucalyptus grandis significantly (p ≤ 0.05) reduced maize and haricot bean seed germination (
Treatments | Germination (%) | Shoot length (cm) | Root length (cm) | Collar diameter (cm) | Shoot fresh weight (g) | Root fresh weight (g) | Shoot dry weight (g) | Root dry weight (g) | Soil reaction (pH) |
---|---|---|---|---|---|---|---|---|---|
Control | 91.67a | 16.10a | 17.00a | 0.41a | 3.61a | 0.19a | 1.00a | 0.13a | 8.08a |
10 g/pot | 41.67b (−54.5) | 9.98b (−38) | 10.50b (−38.2) | 0.35a (−14.5) | 1.25b (−65.4) | 0.10b (−48.1) | 0.46b (−54.5) | 0.07b (−48.8) | 7.75b (−4) |
50 g/pot | 33.33b (−63.6) | 8.50b (−47.2) | 9.25bc (−45.6) | 0.34a (−18.8) | 0.96b (−73.5) | 0.07bc (−66.2) | 0.29bc (−71.5) | 0.05bc (−65) | 7.63bc (−5.6) |
100 g/pot | 33.33b (−63.6) | 6.38bc (−60.4) | 6.88bc (−59.6) | 0.20b (−51.5) | 0.38b (−89.5) | 0.03cd (−82.3) | 0.23bc (−77.3) | 0.01c (−91.3) | 7.53bc (−6.8) |
150 g/pot | 33.33b (−63.6) | 4.25cd (−73.6) | 5.88bc (−65.4) | 0.13bc (−68.8) | 0.19b (−94.7) | 0.02cd (−88.8) | 0.16bc (−84.5) | 0.01c (−94.4) | 7.45bc (−7.7) |
200 g/pot | 33.33b (−63.6) | 2.15d (−86.6) | 3.75c (−77.9) | 0.06c (−85.2) | 0.11b (−97) | 0.012d (−95.3) | 0.09c (−90.1) | 0.01c (−95) | 7.38c (−8.7) |
LSD @ p ≤ 0.05 | 14.29 | 4.17 | 6.43 | 0.12 | 1.82 | 0.05 | 0.33 | 0.05 | 0.30 |
*Means in the same column followed by the same letter are not significantly different according to LSD at probability level of 0.05. Value in the parenthesis indicates % inhibitory (−) effects in comparison to control.
Treatments | Germination (%) | Shoot length (cm) | Root length (cm) | Collar diameter (cm) | Shoot fresh weight (g) | Root fresh weight (g) | Shoot dry weight (g) | Root dry weight (g) | Soil reaction (pH) |
---|---|---|---|---|---|---|---|---|---|
Control | 100a | 26.13a | 46.25a | 0.91a | 10.9a | 2.45a | 2.55a | 1.88a | 8.03a |
10 g/pot | 100a | 20.25b (−22.5) | 38.75a (−16.2) | 0.75a (−17.8) | 7.34ab (−32.8) | 1.09b (−55.3) | 1.55b (−39.2) | 0.72b (−61.5) | 7.85ab (−2.2) |
50 g/pot | 91.67a (−8.3) | 15.88b (−39.2) | 25.50b (−44.9) | 0.53b (−41.9) | 4.83bc (−55.8) | 0.68bc (−72.2) | 0.88c (−65.7) | 0.38c (−79.6) | 7.80ab (−2.8) |
100 g/pot | 66.67b (−33.3) | 10.35c (−60.4) | 16.88bc (−63.5) | 0.42bc (−54.5) | 1.42c (−87.1) | 0.17cd (−93.1) | 0.18d (−93.1) | 0.13cd (−93.2) | 7.63b (−4.9) |
150 g/pot | 50.00bc (−50) | 6.75cd (−74.2) | 10.38c (−77.6) | 0.33bc (−64.1) | 0.71c (−93.5) | 0.13d (−94.7) | 0.11d (−95.7) | 0.06d (−96.9) | 7.50b (-6.5) |
200 g/pot | 33.33c (−66.7) | 5.20d (−80.1) | 12.38c (−73.2) | 0.21c (−76.9) | 0.52c (−95.2) | 0.07d (−97.1) | 0.04d (−98.3) | 0.04d (−97.9) | 7.10c (−11.5) |
LSD @ p ≤ 0.05 | 22.60 | 5.08 | 12.64 | 0.22 | 4.38 | 0.51 | 0.66 | 0.27 | 0.39 |
*Means in the same column followed by the same letter are not significantly different according to LSD at probability level of 0.05. Value in the parenthesis indicates % inhibitory (−) effects in comparison to control.
The results of shoot length showed that all treatment levels of Eucalyptus grandis significantly (p ≤ 0.05) reduced both maize and haricot bean shoot length as compared to the control treatment (
The results of crops root length showed that all treatment levels of Eucalyptus grandis significantly (p ≤ 0.05) reduced both maize and haricot bean root length except for the 10 g/pot treatment in maize as compared to the control treatment (
The results on collar diameter showed that majority of treatment levels of Eucalyptus grandis significantly (p ≤ 0.05) reduced both maize and haricot bean collar diameter except for the 10, and 50 g/pot levels compared to the control treatment (
The results showed that all treatment levels of Eucalyptus grandis significantly (p ≤ 0.05) reduced both maize and haricot bean shoot, root fresh and shoot, root dry weight except for the 10 g/pot in maize shoot fresh weight as compared to the control treatment (
The results of seed germination showed that all treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced maize and haricot bean seed germination except for the 10 g/pot in haricot bean as compared to the control treatment (
The result of shoot length showed that all treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced both maize and haricot bean shoot length except for the 10 g/pot in haricot bean as compared to the control treatment (
Treatments | Germination (%) | Shoot length (cm) | Root length (cm) | Collar diameter (cm) | Shoot fresh weight (g) | Root fresh weight (g) | Shoot dry weight (g) | Root dry weight (g) | Soil reaction (pH) |
---|---|---|---|---|---|---|---|---|---|
Control | 100a | 16.50a | 22a | 0.43a | 3.69a | 0.27a | 1.08a | 0.17a | 8.08a |
10 g/pot | 91.67ab (−8.3) | 14.68ab (−11.1) | 14.38b (−34.6) | 0.38ab (−10.6) | 2.08ab (−44.9) | 0.13b (−53.3) | 0.75ab (−30.2) | 0.08b (−55.1) | 7.93ab (−1.9) |
50 g/pot | 75.00b (−25) | 12.43bc (−24.7) | 9.43bc (−57.2) | 0.32bc (−24.1) | 1.45b (−60.9) | 0.11bc (−59.9) | 0.47bc (−56.5) | 0.04b (−74.9) | 7.85b (−2.8) |
100 g/pot | 41.67c (−58.3) | 10.48cd (−36.5) | 7.98bc (−63.8) | 0.28cd (−35.3) | 1.16b (−68.7) | 0.08bc (−71.9) | 0.37bc (−66) | 0.03b (−85.5) | 7.75bc (−4) |
150 g/pot | 33.33c (−66.7) | 10.13cd (−38.6) | 7.38bc (−66.5) | 0.21d (−50) | 0.86b (−76.7) | 0.04bc (−84.9) | 0.28c (−73.7) | 0.02b (−88.1) | 7.63cd (−5.6) |
200 g/pot | 33.33c (−66.7) | 7.13d (−56.8) | 5.13c (−76.7) | 0.18d (−57.1) | 0.55b (−85.1) | 0.02c (−93.4) | 0.16c (−85.3) | 0.01b (−94.2) | 7.45d (−7.7) |
LSD @ p ≤ 0.05 | 24.06 | 4.06 | 7.41 | 0.09 | 1.69 | 0.09 | 0.42 | 0.07 | 0.19 |
*Means in the same column followed by the same letter are not significantly different according to LSD at probability level of 0.05. Value in the parenthesis indicates % inhibitory (−) effects in comparison to control.
Treatments | Germination (%) | Shoot length (cm) | Root length (cm) | Collar diameter (cm) | Shoot fresh weight (g) | Root fresh weight (g) | Shoot dry weight (g) | Root dry weight (g) | Soil reaction (pH) |
---|---|---|---|---|---|---|---|---|---|
Control | 91.67a | 30.25a | 50.75a | 1.04a | 18.62a | 2.45a | 3.30a | 1.88a | 7.85a |
10 g/pot | 91.67a (−8.3) | 25.25b (−16.5) | 40.38b (−20.4) | 0.91a (−13.2) | 11.49b (−38.3) | 1.88b (−23.1) | 2.38b (−28) | 0.98b (−48) | 7.70a (−1.9) |
50 g/pot | 83.33a (−9.1) | 20.63c (−31.8) | 40.50b (−20.2) | 0.75b (−28.3) | 7.89bc (−57.6) | 1.02c (−58.4) | 1.63bc (−50.8) | 0.65c (−65.3) | 7.50ab (−4.5) |
100 g/pot | 83.33a (−9.1) | 15.63d (−48.3) | 29.25c (−42.3) | 0.64b (−38.8) | 4.62cd (−75.2) | 0.69cd (−71.7) | 0.98cd (−70.5) | 0.45cd (−76) | 7.45ab (−5.1) |
150 g/pot | 66.67ab (−27.3) | 9.98e (−67) | 16.33d (−67.8) | 0.46c (−56.4) | 1.45d (−92.2) | 0.25e (−89.8) | 0.33d (−90.2) | 0.22de (−88.3) | 7.40ab (−5.7) |
200 g/pot | 41.67b (−54.5) | 7.25e (−76) | 12 d (−76.4) | 0.35c (−66.7) | 0.78d (−95.8) | 0.22de (−91.2) | 0.13d (−96.2) | 0.07e (−96.1) | 7.23b (−7.9) |
LSD @ p ≤ 0.05 | 29.18 | 4.39 | 8.08 | 0.14 | 4.57 | 0.46 | 0.86 | 0.29 | 0.45 |
*Means in the same column followed by the same letter are not significantly different according to LSD at probability level of 0.05. Value in the parenthesis indicates % inhibitory (−) effects in comparison to control.
The results of root length showed that all treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced both maize and haricot bean root length as compared to the control treatment (
The results on collar diameter showed that the majority of treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced both maize and haricot bean collar diameter except for 10 g/pot in both crops as compared to the control treatment (
The results showed that all treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced both maize and haricot bean shoot, root fresh and shoot, root dry weight except for the 10 g/pot in haricot bean shoot fresh weight and shoot dry weight as compared to the control treatment (
The results of soil pH showed that except treatment 10 g/pot, the rest all treatment levels of Eucalyptus camaldulensis significantly (p ≤ 0.05) reduced soil pH in haricot bean and maize pots as compared to the control treatments (
The present study revealed that no treatments of both Eucalyptus species had stimulating effect on all measured parameters of either test crops except no noticeable effect in the treatment of 10 g/pot of Eucalyptus grandis on maize germination as compared to control. Probable reasons for this could be the elevated amount of allelochemicals and inhibitory effect present in each Eucalyptus species leaf powder that affected the essential growth processes. From the mean results, the effect increased with the increase of leaf powder application. Similar studies elsewhere have found that inhibitory effect of Eucalyptus camaldulensis depended on concentration of extract and litter-fall with higher concentration of the materials, having higher effects and vice versa [
The highest reduction in the mean shoot length of haricot bean and maize was observed in the 200 g/pot of Eucalyptus grandis treated pots with values of 86.6% and 80.1%. Similarly, the highest reductions in the mean shoot length were observed in the 200 g/pot of Eucalyptus camaldulensis treated pots with 56.8% and 76% for haricot bean and maize respectively. Schumann et al. [
The utmost reduction in the mean root length of haricot bean was observed in the 200 g/pot of Eucalyptus grandis treated pots with value of 77.9%, while for maize it was observed in the 150 g/pot with value of 77.6%. At the same time the highest reduction in the men root length of haricot bean and maize with 76.7% and 76.4% respectively were observed in the 200 g/pot of Eucalyptus camaldulensis treated pots. Al-Juboory and Ahmad [
The maximum decreases in the mean collar diameter were observed in the 200 g/pot of Eucalyptus grandis treated pots with 85.2% and 76.9% for haricot bean and maize, and similarly in the 200 g/pot of Eucalyptus camaldulensis treated pots with values of 57.1% and 66.7% for haricot bean and maize respectively. These findings are confirmed by the work of Ahmed et al. [
According to the mean result a 200 g/pot treatments of both Eucalyptus species reduced the highest in shoot, root fresh and shoot, root dry weight on haricot bean and maize crops as compared to other treatments. Findings by Lisanework and Michelson [
The allelopathic compatibility of multipurpose tree species with companion agricultural crops may be crucial to determine the success of an agroforestry practice or system. Eucalyptus is claimed that it is notorious for having allelopathic effect on growth of agricultural crops growing in its vicinity. The allelopathic effect of E. camaldulensis and E. grandis is investigated on the soil reaction, seed germination, and seedling growth of maize and haricot bean growing in the nursery. It can be concluded from the results that allelochemicals present in the leaf powder of each Eucalyptus suppressed all the parameters measured in the crop species studied, and it is an increased inhibition with the increase of leaf powder application. Among the two tree species, Eucalyptus grandis has more inhibitory effect on both crops as compared to Eucalyptus camaldulensis, and the negative effect of allelochemicals is more pronounced on haricot bean than maize in both tree species.
It is recommended that maize and haricot bean should not be planted very close to or in association with Eucalyptus trees due to the likely adverse effects (allelopathy) on seed germination and other growth parameters. Thus, there is a need to provide information to farmers about where to plant or not when using the Eucalyptus species and their allelopathic effects on agricultural crops. Further studies are suggested to clarify the possible physiological mechanism related to allelopathic effects of the plants.
Rural Capacity Building Project (RCBP) and Ethiopian Ministry of Agriculture are acknowledged for financing this research project. I thank Dr. Abdu Abdulkadir and Dr. Tesfaye Abebe for their constructive suggestions and comments on this manuscript.
Wasihun Regu Gurmu, (2015) Effects of Aqueous Eucalyptus Extracts on Seed Germination and Seedling Growth of Phaseolus vulgaris L. and Zea mays L.. Open Access Library Journal,02,1-8. doi: 10.4236/oalib.1101741