Abstract:

A study to determine the effects of soil substrate and nitrogen fertilizer on the
growth rate and biomass production of Acacia senega! and A. sieberiana in North
Eastern Uganda was conducted between February and June, 2012. The objectives
of the study were to determine the effects of soil substrate and nitrogen
fertilizers on the growth rate and biomass allocation below-ground and
above-ground of A. senega! and A. sieberiana seedlings. A multi-factorial
experiment design was set up for collecting the data to address the two
objectives of this study. There was a significant effect (P<0.05) of soil
substrate and the species- soil interaction on growth rate between two acacia
species. The mean relative leaf length of A. sieberiana (0.013 mm mm'1 d ')
in unfertilized (NO) soil A was higher compared to those in the soils treated
with N fertilizer. Analysis of variance shows that while there was no
significant effect (P<0. 05) of species, soil substrate and species- soil
interaction on the relative stem and leaf biomass growth, relative root
biomass growth, and the root biomass allocation and stem and leaf biomass
allocation at final harvest, there was a significant species effect (P<0. 05) on
leaf N concentration A. senega! seedlings had a higher stem and leaf
biomass allocation (83%) in soil substrate A with NO compared to the N
treatments. A. sieberiana and A. Senegal planted in the unfertilized soil B
produced higher relative stem and leaf biomass growth (0.034 g g'1 d'1)
compared to those treated with N fertilizer. A. senega! had a higher leaf N
concentration (7.1%, 6.6%, and 6.3%) in soil B treated with N50, N100 and
N150 mg / plant, respectively, compared to the NO treatments. Analysis of
Pearson correlation showed that there was no statistically significant effect
(P<0. 05) of the leaf N concentration on the growth traits of the two acacia
species in all treatments. It can be concluded that unfertilised soil substrates
(NO) supported higher growth rate of both A. Senegal and A. sieberiana
seedlings than fertilization with a pure ammonium nitrate solution lacking
other nutrients important for growth. It is recommended that further
investigations using a complete and a balanced nutrient solution with small
quantities of N fertilizers less than the rates used in this study be carried out.
Application of the N treatment produced less stems and leaves biomass
allocation compared to NO treatments. Therefore, N fertilizers may not be
used to enhance biomass production of two acacia species at the age 2
months. Further research can be conducted on the effects of N fertilizer on
two acacia species for a long period of experiment. The two soil substrates
affected the growth traits of the two acacia species differently while that on
biomass allocation in the same way. The leaf N concentration did not
enhance the growth traits of the two acacia species.