| dc.description.abstract | To supplement high cost of inorganic fertilizers, smallholder farmers in the tropics
are likely to increase the use of plant residues as a suitable source of plant nutrients
especially nitrogen (N) and phosphorus (P). Management of these organic N - sources
demand that their N - release patterns coupled with synchronization of the released N
with crop growth be fully understood. Consequently this study was undertaken to evaluate
the effect of chemical composition of various residues on N - release, improve the N release
pattern of low quality organic materials by mixing them with those of high quality
and synchronize the N released with crop production.
These objectives were achieved through a series of controlled experiments. The
experiments involved incubation, in the laboratory for 12 weeks, of six selected plant
residues which involved leaves of Leucaena leucocephala, Croton macrostachyus,
Calliandra calothrysus, Tithonia diversifolia, Sorghum bieolor and husks of Oryza
sativa. Parallel to this, mixtures .of C macrostachyus (Cm) and O. sativa (Os)in various
ratios were also incubated. Finally maize .wasgrown in the glasshouse in pots whose soil
had been amended with C macrostaehyus, O. sativa, T. diversifolia(Ts) and Liz: LI:!
mixture of Croton maerostaehyus and Oryza sativa. In the laboratory incubation, soil
samples were taken after every 2 weeks for analysis of ammonium nitrogen (NH4"" -N) and
nitrate nitrogen (N03·-N), while in the glasshouse experiment, harvesting of maize tops
was done at 2 week intervals 8 weeks and the shoots- dry matter as well as N content
determined.
Two patterns of N (N03--N + NH4+-N) release were observed during the 12 week
incubation period: Leaves of C macrostachyus, L. leucocephala, T. diversifolia and C
calothrysus had a net release throughout the incubation period while S. bicolor leaves and
O. sativa husks showed significant net immobilization. Due to concurrent nitrification
over the 12 week incubation period, 65 - 80% of the accumulated mineral N was in
nitrate form.
The dynamics of N - mineralization of the vanous mixture of Croton
macrostachyus (em) and Oryza sativa (as) were in general not significantly (p ~ 0.05)
different from those predicted from the isolates of Oryza sativa (Os) and Croton
macrostachyus (Cm) alone with the exception of the 3/4 Cm + 1/40s having depressed
the incubated N - release in weeks 6 - 8 significantly while the 1I4Cm + 3/40s
stimulated the incubated N-release at weeks 2 and 12 weeks of incubation respectively.
Addition of plant residues increased maize biomass in the glasshouse significantly
(P < 0.05) throughout the .growth period. However, the study showed a sharp contrast of
maize response from the results of the incubation. Mixture of plant residues in the pots
had the highest contribution to maize dry matter yield and N uptake. It could be
speculated that high C:N ratio Oryza sativa material in the mixture stimulated microbial
activity in the rhizosphere leading to high organic material decomposition.
Of the chemical variables studied, initial contents of N and P as well as C:N and
polyphenol:N ratios were significantly correlated with cumulative N mineralized.
Nitrogen- release was best correlated with C:N ratio having r = - 0.84 to -0.90 for most
of the sampling periods. Polyphenol: N ratio also gave high correlation with cumulative
N mineralized with correlation coefficient (r) ranging from -0.65 to -0.95. Initial N and
P contents showed positive significant correlation but not as high as those of the two
ratios. These results show that the best predictors for N mineralization were residue CN
and Polyphenol.N ratios. | en |
