Influence of maize plant density on radiation, soil temperature and soil moisture distribution

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.