Row spacing, one of the most important agricultural practices, is found to affect soil water use by changing not only the evapotranspiration (ET) but also the ratio of evaporation (E) and transpiration (T) to ET (Loomis, 1983).
When plants are grown in wide row (WR) with unlimited water, soil evaporation will be large when the leaf area index (LAI) is small. Under wide rows the ratio of E/ET is larger than T/ET and this results in a lower water use efficiency (WUE). Plants grown in narrow row (NR) use most water for transpiration and this results in a higher WUE. Micro meteorological factors such as wind speed, soil and air temperature and solar radiation regime are believed to be the main factors which affect T, E and hence ET.
This experiment was conducted to study the effect of row spacing on sorghum water use, in particular how row spacing modifies the plant environment and how this modification
influences ET and plant growth.
The experiment was conducted from 12th December 1989 to 6th February 1990 at Redland Bay Research Farm of University of Queensland. It consisted of two treatments, WR and NR(lxl m2, 0.33x1 m2 respectively) with four replications arranged in a completely randomized block design under a rainout shelter. Soil evaporation was measured using minilysimeters and plant transpiration was estimated from transpiration efficiency (TE) method using plants in sealed pots. Total soil water use was determined using a neutron moisture meter and soil core sampling.
Micrometeorological measurements included radiation interception, windspeed, air and soil temperature, relative humidity, leaf temperature, and stomatal conductance.
It was found that when plants were young, crop growth rates (CGR) of NR plants were significantly higher than WR plants, 24.88 g/m2/day and 13.77 g/m2/day respectively.
However, when they were older, there were no significant differences in CGR between the two row spacings. This was related to the amount of PAR intercepted by the plants. Radiation use efficiency (RUE) was high in NR when plant were young, but RUE increased in WR as plants aged. Total water use was significantly higher in WR when plant were young, but when older, water use was not different. Water use efficiency (WUE) was significantly higher in NR, when plants were young and there was no significant difference in WUE in older plants.
All micrometeorological measurements indicated that WR plants were exposed to a drier environment than NR plants as higher soil and air temperature, higher windspeed and vapour pressure deficit and lower relative humidity were found in WR. Although only soil temperature and windspeed were significantly different. Vapour pressure differences between plant leaf and air were higher in the WR and resulted in more transpiration as
shown by the stomatal conductance data, stomatal conductance was higher in WR (significant on several measurement occasions).
This experiment shows that wider spacing modified the plant environment, and resulted in a hotter and drier conditions for plant growth. As water was not limited in this experiment, plant growth was determined mainly by radiation interception and not water use.