Eucalypts and soil fertility
The effects of eucalypts on soils have been studied in several countries over many years [28, 50]. Most of the concerns related to effects on soil quality deal with the depletion of nutrients . Dessie and Erkossa , Kidanu et al.  and Chanie et al.  argued that eucalypts decrease soil nutrients within 20 m distance from the trees. A comparative study of eucalypts in a mixed plantation has revealed that eucalypt has three times more fine-root biomass in surface soil, which indicated that planting crops in association and adjacent to eucalypts should be avoided . However, dismissing eucalypt species as not being suitable for agroforestry misses the full scientific picture that they can extend nutrient cycling from deeper ground soil where other trees and crops do not have access . Thus, the management of eucalypts in agroforestry becomes the main issue.
A study focusing on wetland conversion has indicated that there is significant difference between wetlands and converted land to dominantly eucalypts by reducing major nutrients from the converted land . Similarly, soil nutrients and carbon pool under eucalypts were lower than the mixed plantation . Chanie et al.  also reported that the soil under eucalypts becomes water repellent, and the perceptions of the local farmers agreed with the experimental findings by reducing the crop productivity of the land. In contrast to the above, Tadele and Teketay  have found that the maize dry matter production and grain yield planted on cleared felled eucalypts stand were significantly higher than the adjacent field. According to Hailu et al. , eucalypts do not overexploit the soil than the traditional fuel usage, such as litter and cow dung collection. Similarly, the study has indicated that due to non-browsed characteristics of eucalypts than other fodder trees, they are well-fitted for soil protection purposes if they are incorporated with avoidance of litter and bark collection in places with overgrazing practices. Generally, there is lack of clear scientific evidence that shows the impacts of eucalypts on soil nutrients that lead to soil degradation. It could be legitimate to raise such concerns under poor management where there is lack of species-site matching [8, 12, 42, 43].
Eucalypts cope with such variability through a root system that has intimate contact with the large volume of soil. With the extension of their roots deep into the soil, given their high degree of adaptability, they extract nutrients outside the realm of crops feeding zone. That is why the nutrient requirements of eucalypts are significantly lower than those of many agricultural crops [39, 40]. As a result, the species flourish with sustainable high yield without fertilizer on red ash and degraded land. Further, eucalypt plantations are not like natural forests that experience little disturbance. If it were a closed system, nutrients would have been recycled from decomposing litter back to the tree and increase the nutrient bank . But, eucalypt is an open system and nutrients are removed from the site when the stem, leaves and bark are harvested for various uses [42, 43]. This means that the nutrient capital of the soil could be diminished. Therefore, the secret lies in nutrient mining. This is equally true for crops under poor management. Under viable environment, soil nutrient levels can be improved through sound management without the carrying capacity of land being overstretched.
Very few comparative studies have been made in Ethiopia on soil nutrients among plantations of different species, including eucalypts and the adjacent natural forests [1, 36, 37, 43]. These studies have shown that plantation stands of fast-growing exotic species, such as E. globulus, E. grandis, E. saligna, Cupressus lusitanica Mill. and Pinus patula Schiede ex Schltdl. & Cham. had lower nutrient contents than soils of the adjacent natural forest. This seems logical as they are fast growing, thereby drain, and consume more nutrients from the soil. Eucalypt species have high demand for nutrients, but this is incomparable with other tree species and much lower than agricultural crops. Teshome  pointed out that the nutrient consumption of fast-growing species like eucalypt species need to be well studied before conclusions and recommendations are made.
Eucalypts and soil erosion
Soil erosion is among the most important surface processes that results in land degradation in many places, especially in the tropics. Trees can influence soil erosion, mainly, through intercepting rainfall, which dissipates its kinetic energy. The rain drops that are intercepted, eventually, fall to the soil surface with reduced erosive energy, depending on the size and orientation of the leaves. Large leaves produce larger size droplets, which have greater impact on the soil. Accordingly, erosive energy of rain under the tree crowns would be least for Casuarina spp. with Acacia spp. (e.g. A. auriculiformis) and narrow-leaved eucalypts (e.g. E. camaldulensis) occupying the mid-range, and the broad-leaved eucalypts (e.g. E. globulus) at the top of the range for the eucalypts .
Jagger and Pender  reported that there is no evidence to single out the eucalypts for special criticism with regard to soil erosion. It has been hypothesized, however, that-long term exposure to allelo-chemicals from the leaves of eucalypts may result in increased risk of soil erosion, which may have implications for sustainable land use over time .
Eucalypts have been found to impact on topsoil retention and soil erosion [10, 50, 58]. Some studies have concluded that eucalypts can worsen soil erosion as an indirect result of frequent disturbance from repeated harvesting . Others argue that eucalypt plantations can help control soil erosion on sloped or degraded sites, but their efficacy depends on environmental factors, such as intensity of rainfall, soil condition, slope and the presence of ground vegetation and litter cover. Though few Ethiopia-specific case studies exist, the limited evidence available suggests that eucalypts may be ineffective choices for erosion control . Rather, eucalypt trees are generally expected to lead to an increase in soil loss due to the reduced understory cover in densely planted eucalypt areas .
The litter, which accumulates under most eucalypt plantations, can help to form a protective barrier against erosion, but in many places the litter is collected for fuel or removed to reduce fire hazard. For instance, the depth of the accumulated litter under eucalypt stands in Munessa-Shashemene Forest Project area was, on average, 20–30 cm . However, under eucalypt stands around Addis Ababa and very big towns, the accumulation of litter is very low as a result of human and livestock disturbances. People take away most of the litter and cattle and foot traffic compact the soil. If the litter had been left on the site, it would have been incorporated into the soil system to slow down runoff and improve infiltration, and a substantial amount of nutrients would, then, have been able to pass to the soil system, thereby, improving soil fertility . However, as a result of litter collection, the ground under the trees is left bare, and the soil is exposed to erosion. Therefore, litter should be allowed to accumulate where possible, particularly on sites that are easily eroded.
A common theme in eucalypt reforestation outlined in this paper is that scientific management of the forest can achieve much to alleviate problems that have started to prevent further reforestation initiatives using eucalypts. A realistic assessment of each area to be planted is needed to decide whether erosion will be aserious problem, and if so, whether it can be controlled. Some places may not be suitable for plantation establishment. Eucalypt plantations on steep slopes can provide effective erosion control if careful techniques, such as contour planting are used . The root systems of selected species for catchment protection influence the soil binding capacity and as a result reduce erosion. Eucalyptus globulus, for instance, has a strong tap root and good lateral root system that makes it a very reputable species for catchment protection [60, 62].
With regard to soil erosion by water under trees, there is no evidence to single out eucalypts for special criticism. Erosive resistance (physical characteristics) of soils is more important than crop management and crop management is more important than the type of tree crop. Since, in nearly every example where the litter is removed, erosion increases substantially, it is important to focus more on ground cover and ground-level activities (cultivation, compaction by foot traffic, livestock grazing, trampling and harvesting/logging damage) rather than on the species of trees planted. On erosion-prone slopes, it is better to use a periodic, partial harvesting system based on cutting of trees along lines around the contour or removal of small patches in a mosaic pattern [8, 23, 60].
Allelopathy and eucalypts
Allelopathy is the release of chemicals from leaves or litter that inhibits the germination or growth of other plant species and, consequently, reduces the output of crops . Allelopathic effects of eucalypts are among the issues dominating the agroforestry literature. Allelopathic exudates from eucalypt tree components have shown an inhibiting effect on undergrowth vegetation regeneration and growth . Therefore, the issue of allelopathic impacts of eucalypts needs to be discussed.
Most of the studies put forward as evidence for eucalypts being strongly allelopathic involve laboratory studies of extracts on germination of seeds or early growth of potted plants, which may not accurately represent field conditions. Soil bioassay studies have been carried out with three agricultural crops: chickpea (Cicer arietinum L.), tef [Eragrostis tef (Zucc.) Trotter] and durum wheat (Triticum turgidum L.) under laboratory and field conditions in the Ethiopian highlands. According to the findings, bioactive compounds from the decomposing litter of E. globulus did not affect the test crop seed germination nor root growth. However, a litter extract with 5% dry matter concentration significantly hindered germination and root growth of the tested agricultural crops. On a farm field experiment, declining barley yield was observed near E. globulus plantation .
Results evidently vary across a wide spectrum of conditions from humid, fertile sites to dry, infertile ones. The magnitude of the negative effects is likely to be influenced by rainfall. Although it is likely that allelo-chemicals do accumulate in the soil, they are highly soluble and rainfall is likely to leach them out, and the effects of allelopathy are, thus, likely to be negatively correlated with rainfall. It has been noted that allelopathic effects are more severe in low rainfall regions prone to soil erosion than in drier regions. However, the hampering effect on growth of understory or adjacent intercropped crops may more often be the result of strong competition for water and nutrients than allelopathy. Farmers in the highlands of Ethiopia linked this effect to competition for water and nutrients .
The potential allelopathic effect of E. camaldulensis, Cupressus lusitanica, E. globulus and E. saligna on seed germination and seedling growth was investigated with four crops: chickpea, maize, pea and tef . The results revealed that aqueous leaf extracts of all the tree species significantly reduced both germination and radical growth of the majority of the crops. It has been shown that the shoot and root dry weight increase of the crops was significantly reduced after ten weeks treatment with leaf extracts.
Allelo-chemicals can affect germination and growth of plants through interference in cell division, energy metabolism, nutrient uptake and possibly other factors [16, 42, 62, 68]. In this regard, eucalypt has toxic allelo-chemicals that consist of phenolic acids, tannins and flavonoids . When released into the soil, these inhibit other plants and play a role in shaping plant communities. For instance, leaf decomposition product from eucalypts is shown to suppress germination and growth of chickpea, field pea, maize, and tef  while it exerted an antibiotic effect on soil microorganisms .
However, concentration matters. For instance, allelo-chemicals from decomposed eucalypt litter in high rainfall areas did not accumulate in sufficient concentration to affect seed germination and root growth of crops. Different strengths of water extract from leaves of eucalypts did not delay the onset of germination and seedling growth of Olea [30, 32, 68]. In fact, positive results have also been reported concerning the interaction of eucalypts with other plants . The lack of susceptibility of certain crops and the regeneration of other species suggest that eucalypts provide some benefit rather than harm. Again, it is not only eucalypts, but other exotic tree species, such as Grevillea robusta A.Cunn. ex R.Br. showed allelopathic effects on most agricultural crops .
However, in Ethiopia, little attention has been given to allelopathy as a determinant of crop production and productivity [4, 26, 27] and plant community structure [29, 39, 51, 67]. Therefore, empirical information is needed to resolve such negative effects. Until then, eucalypt allelopathy can be minimized with sound management through compatible crops based on proper site selection of eucalypt species.