Does eucalyptus tree reduce soil fertility?

Cloned eucalyptus trees may be ready for harvest after 10 years. PHOTO/MICHAEL J SSALI

What you need to know:

  • Eucalyptus seems to be the most commonly available form of wood to ordinary people for housing construction, fuel for cooking, and saw dust used as bedding material in livestock shades and houses.

There is an ongoing debate among farmers and environmentalists about whether or not planting of eucalyptus trees should be discouraged with one side saying that the trees deplete soil nutrients and another claiming that the eucalyptus tree leaves are very good organic manure. 

Eucalyptus tree farming is said to be quite lucrative and, according to some studies, it can grow quite well across all the agro-ecological zones in Uganda, ranging from lowland to the mountainous highlands. 

Currently a mature eucalyptus pole may cost up to Shs200,000. Some really tall and well developed eucalyptus trees may cost up to Shs1m especially if they are to be sawed into timber for housing construction.  

The poles are used to support electric wire lines all over the country and without eucalyptus poles it would almost be impossible for most people to construct their own houses in Uganda. 

A simple Google search indicated that Uganda Electricity Distribution Company Limited (UEDCL) spends Shs10b annually to purchase mature eucalyptus trees from local farmers. It would appear therefore that eucalyptus tree growing is here to stay given its economic importance and it seems to be a sector in which farmers ought to be given some expert guidance regarding the tree’s sustainable production and its impact on soil fertility. 

Common wood 
Eucalyptus seems to be the most commonly available form of wood to ordinary people for housing construction, fuel for cooking, and saw dust used bedding material in livestock shades and houses. Potting eucalyptus tree seedlings in nurseries is an important form of employment.

Eucalyptus tree farming has greatly reduced the rate at which our natural forests are being depleted. People can grow their own trees on their land and keep away from National Forest Authority forests. 

Cloned eucalyptus trees can be mature and ready for harvest after just after 10 years but the farmer may do some thinning which involves cutting down some of the trees or their branches which may be used as fire wood or construction material. Some farmers sell off trees as old as just two years, commonly referred to as “Kampala” and it is not uncommon to see trucks loaded with “Kampala” eucalyptus poles heading for Kampala and other towns.  Eucalyptus leaves are often used as herbal medicine for coughs, flu and other illnesses. Some eucalyptus tree varieties are said to be mosquito repellant.

John Bosco Bwankya, a coffee farmer at Ngereko Village, Kisekka Sub-county, Lwengo District says that the coffee growing in an area on his farm that used to be under eucalyptus does a lot better than the coffee growing elsewhere on the farm. Many other coffee farmers in the area have chosen to use eucalyptus tree branches as mulch in their gardens and they all report great improvement in the vigour of their crop’s growth.

Yet it is also true that complaints abound in many farming communities that eucalyptus trees are not good neighbours because they are said to have far extending roots that consume a lot of soil moisture from nearby gardens and cause crop failure.

There are also complaints about eucalyptus trees planted in wetlands because of their high water consumption. It is widely believed that the trees degrade the wetlands by causing water shortage. 

Dr Eseri Nankya a soil scientist attached to National Agricultural Research Organisation (Naro) recently concluded a study titled: “Assessing the Impact of Eucalyptus Cultivars on Soil Properties in Three Districts of Uganda.” It investigated the influence of eucalyptus cultivation on soil properties across the three districts in Uganda: Kabarole, Mpigi, and Rakai which are part of Western Highlands, Lake Victoria Crescent, and South Western Agro ecological zones respectively. 

The study according to the abstract is aimed at evaluating the effects of two dominant eucalyptus cultivars GU7 (Improved) and E grandis (Local) within these regions.

Composite soil samples were picked from all the selected districts and taken to the soil laboratory for analysis of major soil nutrients which included total nitrogen available, phosphorus, potassium, exchangeable magnesium, calcium, soil organic carbon, and HP.

“Results showed a general decline in major soil nutrients (N, P and K) across the eucalyptus strands in all three districts with more significant decline observed under GU7 cultivar plantation,” reads the extract. 

“Additionally, results showed that magnesium and calcium were inherently low and the most deficient nutrients in both Kabarole and Rakai while PH was significantly higher in Mpigi compared to the other two districts in the control plot but also significantly reduced under GU7 plantations in the same district compared to the two districts. The study also revealed that phosphorus is the most deficient in Rakai District for eucalyptus production while Nitrogen and SOM are the most deficient in Mpigi.

Potassium was significantly low in both Kabalore and Rakai soils compared to Mpigi District.”
Conclusively, according to Dr Nankya, this study shows a significant negative effect on major soil nutrients (Nitrogen, Phosphorus, and Potassium) associated with eucalyptus GU7 cultivar compared to E.grandis across the three districts.

Eucalyptus too require fertilising  
Dr Nankya who also spoke to Seeds of Gold in a separate interview revealed that since GU7 (improved or cloned) cultivar grows quickly and vigorously its consumption of soil nutrients is quite high. Yet most eucalyptus tree farmers don’t care about applying fertilisers on the crop.

“They plant the trees and leave everything else to nature. When the trees are harvested the bark is peeled off and it is often used as fuel for cooking instead of being left in the forest to decompose and turn into manure,” says Dr Nankya.  

She also said that the E grandis (Local type) grows rather slowly and its nutrients intake is also low. She therefore thinks that farmers ought to carry out practices that protect and replenish the soil on which eucalyptus is growing. “I found that the local variety ---Egrandis----, much as it takes long to mature, doesn’t significantly affect soil nutrients,” says Dr Nankya.

“While the improved types, especially GU7, grows so fast, it extracts more soil nutrients. Unfortunately, eucalyptus farmers do not apply fertiliser according to the survey that I made, yet the majority of farmers, especially those growing eucalyptus on a large scale do venture into growing the improved types. The question now is: Where is the sustainability of the eucalyptus systems in the country when there is more nutrient mining rather than input?”

Eden Kamugisha, a eucalyptus tree farmer at Kisagazi Village, Mukungwe Sub-county, Masaka District told Seeds of Gold, “Most eucalyptus tree farmers apply fertilisers only during planting to hasten root formation so that when the dry season begins the young tree will be in a position to support itself. After that hardly anybody worries about fertiliser application.”

Cloned eucalyptus trees can be mature and ready for harvest after just 10 years, but the farmer may do some thinning which involves cutting down some of the trees or their branches which may be used as firewood or construction material.