Due to the environmental changes, especially global warming, water scarcity will be more serious in the water shortage areas. Efficient and judicious husbandry of the limited water resources is very important. Usually irrigation water is applied considering only the plant water need whereas the impact of the excess water on the soil air O2 is ignored. Applied water fills the soil pores, replaces the soil air and reduces the amount of soil air O2. Lack of or insufficient soil air O2 reduce root growth, nutrients availability and photosynthesis rate, result in sever reduction in crop yield.
The study was conducted in a greenhouse at the Okinawa Subtropical Station, JIRCAS. Five wooden boxes (180cmX90cmX30cm) were connected with each other. Six galvanized batteries along with a T-type thermocouple, was set at the entrance of the first box and at the end of the each box. Nine tomato (var. First Power) seedlings were transplanted in each box on February 18, 2005, and grown as a normal crop. At the flowering stage, soil surface in the boxes was covered by white polyethylene sheets and closed tightly to stop entry of air into the soil through the soil surface. Tomato was irrigated as and when required with a PVC pipe inserted in the surface of the boxes.
We successfully developed a soil air O2 creation technique connecting wooden boxes with each other and in situ soil air O2 measurement using the galvanized battery. The results revealed that the amount of soil air O2 gradually reduced from the first box to the last one. The available soil air O2 concentration in the first box was 20.1% while it was 12.8 % in the last box. Due to the reduction in soil air O2, chlorophyll content and root dry matter were drastically reduced causing reduction of tomato yield from 2 to 18%. The tomato yield was 28.5 ton/ha in the first box, while 23.4 ton/ha in the last box. The seasonal mean soil air O2 indicated that by decreasing the available soil air O2 content by 1%, the tomato yield would reduce by 0.88 t/ha. Therefore, consideration of the balance between soil air O2 and soil water while deciding amount of irrigation water may increase crop yield, water savings and water use efficiency.