| dc.description.abstract | The relationship between plant density and radiation,
soil temperature and soil moisture was studied i~ a maize field
during the long rains season of 1975. The maize (Hybrid 512)
was planted at three densities, viz: the normal density -
N(5l 667 plants/ha), Higher than normal density - H (110 741
plants/ha) and Lower than normal density - L(24 630 plants/ha).
Measurements of incident global radiation, reflected global
radiation and net (total) radiation were made within and above
the maize canopy. Soil temperature and soil moisture wereI
measured at different depths. The measurements were made during
the vegetative, flowering and ripening phases of the crop.
Measurements were made for five consecutive days during each
phase unless this sequence was interrupted by a breakdown of
equipment. From the data obtained, five-day averages were
calculated. The averages of the data of each five-day period
were considered to be representative of a growth phase.
A consistent relationship between the reflection coefficient
of global radiation and plant density (21 ± 4%, 19 ± 4% and 17 ± 3%
over the canopies of densities H, N and L respectively) vas
observed only during the flowering phase.
The t.r-ansrn i ssi.on coefficient of global radiation
increased as plant density decreased. On the average, the
transmission coefficient waS a minimum during the flowering
phase. The leaf area ind ices during the flowering phase were
10.2 ± 0.9, 4.2 ± 0.7 and 1.9 ± 0.0 in densities H, N and L
respectively. In each of densities H and N the highest leaf
area index was recorded during the flowering phase. The
value of le.if area index for density L during this phase
was lower than during the vegetative phase. The lowest
(13 ± 4%) and the highest (48 ± 17%) values of transmission
coefficient were obtained in densities H (during the flowering
phase) and L (during the ripening phase) respectively.
The absorption coefficient of global radiation for the
maize crop decreased as plant density decreased; variations
from phase to phase were not sufficiently precise. The
lowest (38 :t 21%) and the highest (67 ± 10%) values of
absorption coefficient were observed during the vegetative
phase in densities L and B respectively.
The derived net long-wave radiation values were too
small compared to the corresponding errors and no valid
conclusions could be made from the data.
The pattern of transmission and absorption of net (total)
radiation vas similar to that of global radiation. The net
radiation below canopy expressed as a percentage of net
radiation above canopy increased as plant density decreased.
The lowest (13 ± 4%) and the highest (45 Z10%) values were
obtained during the vegetative phase in densities H and L
respectively.
At 5 and 10 em depths, the rate of increase of soil
temperature with time increased as plant density decreased. At
the last of the depths where soil temperature was measured i.e.
30 em, the variations of soil temperature with time were very small
in comparison with the other two depths.
The soil temperature was a minimum at 10 em depth in
densities N and L during the three growth phases. In density at
it was a minimum at this depth only during the ripening phase.
During the vegetative and flowering phases no differences in soil
temperature were observed at 5 and 10 em depths in density H.
In general, no consistent relationship between soil moisture
depletion and plant density was observed; except that the soil
profile below 20 em depth under the lowest plant density tended to
be slightly more moist than the corresponding profiles under
the two higher plant densities.
During the vegetative phase, the soil moisture content was
lowest at a depth of 40 em in densities H and L and between
40 and 60 em in density N. During the last two phases, the soil
moisture content increased with depth. | en |
