Research for Tropical Agriculture
Online ISSN : 2187-2414
Print ISSN : 1882-8434
ISSN-L : 1882-8434
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Original Article
  • Ken OKAMOTO, Shinkichi GOTO, Toshihiko ANZAI, Shotaro ANDO
    2020 Volume 13 Issue 2 Pages 57-67
    Published: 2020
    Released: April 23, 2021

    Nitrogen leaching from agricultural fields is a major source of nutrient load into groundwater on tropical and sub-tropical islands, where limestone is widely distributed. To clarify the effects of reducing the nitrogen application rate on the growth of sugarcane, a major crop on Okinawa, and the leaching of nitrate nitrogen, we carried out a sugarcane cultivation experiment and monitored the leaching using a lysimeter. The experimental design was a randomized block in a 3 × 2 factorial design and control (without nitrogen) with two replications, including the nitrogen rate at the 1st application (0, 35 or 70 kg ha-1) and the amount of the 2nd and 3rd nitrogen rates (80 or 160 kg ha-1). Different rates did not affect crop growth in the early growth stages and leaching of nitrate nitrogen occurred mainly at that time. These results suggest that the current standard of nitrogen application for sugarcane is excessive in the early growth stages. When the first nitrogen application was reduced by 50%, the cane yield did not differ significantly from that with the recommended nitrogen application rate. Therefore, to decrease the nitrogen load in groundwater while maintaining the current level of sugarcane yield, the amount of the first fertilizer application could be reduced in order to decrease the leaching of nitrate nitrogen in the early growth stages.

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  • Haruyuki DAN, Yoko OKI, Shinji HIROUCHI
    2020 Volume 13 Issue 2 Pages 68-74
    Published: 2020
    Released: April 23, 2021

    Paddy rice requires relatively large amounts of water. Its field irrigation provides a stable supply of water in rice fields and contributes in increasing the rice yield. In the inland valleys of Ghana, a reinforcement technology for irrigation facilities in paddy fields is being developed by planting native plant species on the ground surface of irrigation and drainage canals and levees. In order to sustain long term reinforcement, it is important to maintain and manage the vegetation appropriately. Concerning the characterization of native plant communities, the bare canal crowns were invaded in succeeding order by Cynodon dactylon, Chrysopogon aciculatus, and Stenotaphrum secundatum. Concerning the community structures, Stenotaphrum secundatum showed the highest above-ground plant fresh weight and Chrysopogon aciculatus the highest root fresh weight. Moreover, the root systems of the three plants were concentrated within the 10 cm soil surface layer. In an established community which was left unmanaged for one year, there was an invasion of weeds with the Cynodon dactylon communities being fewest and their total fresh weight the lowest as compared with the other two communities. These results demonstrate the validity of the maintenance and management schedule of plant communities such as the timing of planting, replanting, manual weeding, cutlass slashing and so on, and the clarifying of the characteristics of the established plant communities in terms of structure and stability.

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  • Satoru IHA, Atsushi AJITOMI, Hirotsugu INOUE
    2020 Volume 13 Issue 2 Pages 75-80
    Published: 2020
    Released: April 23, 2021

    Mango ‘Lippens’ has the problem that black scratch disorder occurs during the harvest. In order to develop the technology to reduce this disorder, the effect of calcium formate was investigated. Results of field test indicated that, the incidence of black scratch disorder was lower in the calcium formate sprayed plot than in the control plot. In 2017, the incidence of the calcium formate sprayed plot was 2.1% and that of the control plot was 33.4%. In 2018, the incidence of the calcium formate sprayed plot was 8.0% and that of the control plot was 37.5%. The effect of calcium formate spraying on the quality was not obvious. Flesh and peel hardness were measured to investigate the effect of calcium formate spraying on the peel. As a result, the flesh and peel hardness of the calcium formate sprayed plot showed significantly higher values. Analysis of the peel pectin content showed that the content of insoluble pectin combined with calcium was significantly higher in the calcium formate sprayed plot, and the calcium ion content in the extract was also higher. Spraying calcium formate on ‘Lippens’ was thought to form insoluble pectin bound to pectinic acid present in the cell wall, to enhance peel hardness and to reduce black scratch disorder.

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