Two experiments were carried-out to study the effect of irrigation frequency and
timing on root developments, tomato yield (var. First power) and soil water content at
JIRCAS Okinawa Subtropical Station, Ishigaki, Japan. The first experiment was
conducted in root containers (31 x 15 x 60 cm) with one transparent side for visual
viewing of the root development. Sandy clay loam soil mixed with CaCO3 and P2O5
fertilizers was packed in the containers to 50 cm height with bulk density of 1.5 g/cm3.
Three irrigation frequencies, 1, 3 and 5 days were investigated. The soil water content
in the containers was kept at field capacity by compensating the loss in weight by
adding water. The second experiment was conducted in a greenhouse with two
irrigation frequencies, 1 and 3 days and three irrigation timings, early morning (8:00h),
afternoon (14:00h) and night (20:00h). Soil water content at 0-60 cm and soil
temperature at 15 cm depth were measured at 15 cm distance far from dripper by
installing TDR sensors and thermocouples, respectively. The supplied amount of water
was the same for all irrigation frequencies and gradually increased to cover the crop
water requirements during each growing stage.
The root containers results indicated that increasing water supply increased the root
development and root biomass. The 1-day irrigation frequency produced the highest
root biomass while the least root biomass was obtained from the 5-days irrigation
frequency, indicating that water stress promoted the development of root system in the
deeper layer where available soil moisture content was higher than the top layers. The
5-days irrigation frequency saved 18% and 12 % of water at early growth stage
compared to 1 and 3 days frequencies.
The results of greenhouse experiment showed that the best irrigation frequency was 3-
days. The average yield in 3-days frequency was 70 ton/ha while 63 ton/ha in 1-day
frequency. The effect of irrigation timing varied with irrigation frequency. For 3-days
frequency, irrigation at early morning was better than afternoon and night irrigations.
The average yield for irrigation at early morning was increased by 15% and 14% than
irrigation at afternoon and night, respectively. For 1-day frequency irrigation at night increased the yield by 11% and 3% than irrigation at early morning and afternoon
correspondingly. The lowest soil water content and soil temperature were displayed by
the treatment, which produced the highest yield. With the same amount of water, the
early morning irrigation after every 3-days increased tomato yield by 11- 20 %
compared to night and early morning irrigation of 1-day frequency. A similar increase
in water use efficiency in the early morning irrigation every 3-days was also recorded.
Therefore, a selection of the proper irrigation frequency and timing led to a higher
yield and high water use efficiency.