| dc.description.abstract | Nitrogen fertilizers have assumed a prominent role in
the cultivation of maize especially in pure stands in Kenya.
However, knowledge on N utilization in the maize - legume
cropping systems which are commonly used in Kenya is lacking.
A field study was therefore conducted between 1987 and
1990 at two sites, Kimutwa and Masii in Machakos District, a
semi-arid region in Eastern Province of Kenya to determine the
effects of N, bean residue and maize-bean cropping systems on
nutrient uptake, yields and soil chemical properties. Soil
types used were sandy clay loam (dystric Nitisol) for Kimutwa
and sand (Acrisol) for Masii. A split-plot design was used in
which N levels (0, 25, 50 and 100 kg N/ha) formed the mainplots
and cropping systems formed the sub-plots. Single
superphosphate at 40 kg P20s/ha was applied uniformly to all
plots.
Fertilizer N significantly increased dry matter yields,
yield components, seed yields and uptake of N, P, K, Mg and Ca
by maize and beans in sole, intercropping and rotation systems
and also improved growth and development of maize in the three
cropping systems. Response to N by both crops depended on
.cropping system, site and season. Bean response to N occurred
where soil mineral N prior to planting was 18 and 23 ug N/g or
below on the Acrisol and dystric Nitisol, respectively. For
maize, response occurred where mineral N was 36 and 60 ug N/g
or below on the two soil types, respectively. Generally, the
highest response to N by both crops in continuous sole
cropping, rotation and those intercropped in the alternate rows
was at 50 kg N/ha while that of crops intercropped in the same
row was at 100 kg N/ha. Regression analysis indicated that
cropping systems with highest response to N at 50 kg N/ha could
respond to N rate of 75 kg N/ha while crops intercropped in the
same rows could have maximum response at either 75 or 100 kg
N/~a·depending on the season and site. Effects of nitrogen
rates of 50 and 100 kg N/ha were often not significantly
different, and therefore, 50 kg N/ha could be used for the
production of both crops. But, 25 kg N/ha rate was not adequate
although it was better than zero N.
Intercropping was a superior cropping system in both
low and high input agriculture with respect to land equivalent
ratio (LER) and total grain yields, respectively. Fertilizer
N improved soil total N, maintained organicC and increased
residual mineral N which consequently improved dry matter, seed
yield and nutrient uptake of a test sole maize. Residual
mineral N and uptake of nutrients by a test ~ol~ -m~ize were
higher in plots previously under continuous beans than
continuous sole maize at both sites; higher in intercropping
and rotation than in continuous sole maize in Masii while the
reverse was true in Kimutwa. However, N lowered soil pH, CEC
and resulted in greater depletion of soil extractable P and
exchangeable K, Mg and Ca relative to zero N treatments. The
levels of these chemical properties in the soil were influenced
by cropping systems.
Competition between maize and bean intercrops was
manifested as early as 20 days after seedling emergence upto
maturity, through reduction of total dry matter, growth and
development, ..yield components, seed yield, nutrient uptake and
response to N by the intercrops. Intercropping systems,
however, resulted in high LERs of 51-176% on average, which
depended on bean spatial arrangements and site. competition
was influenced by bean spatial arrangements and-generally, the
same row intercrops individually or combined out performed
those in alternate rows. Rotation benefited both crops,
improved maize response to N and had a supplementary effect to
N fertilizer on maize of an equivalent of 1-56 kg N/ha
fertilizer. Bean residue returned to plots had negative effects
on beans but improved LERs, maize yields, response to N by
maize in rotation and that in the same row while the effect on
nutrient uptake by maize was either posi tive or negative
depending on cropping system, season and site. The returned
bean residue also lowered soil pH, extractable P, exchangeable
K, Mg, and Ca: it had no effect on total N but had effects on
mineral N which were seasonal and dependent on the site. In
Masii, soil pH, organic c, mineral N, exchangeable K, Mg and
Ca had posi tive relationships and significant correlations with
total N uptake and total grain yield. In Kimutwa, there were
negative correlations between soil pH and extractable P,
total N uptake ~nd total grain yield. In Kimutwa, positive
correlations were found between total grain yield and organic
C and N uptake: between mineral N and exchangeable Mg | en |
