代表的沖縄本島土壌層の乾燥特性(農業工学科)
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概要
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一般に畑地や斜面において石灰岩風化土は乾燥しやすく, 泥岩風化土は保水力に富み, 受食性の国頭礫層土は両者の中間に位するといわれているが, 一連のガラスブロックによる水分測定試験結果から各試料の乾燥特性についての正しい認識のために役立つ2,3の解答が得られた。(1)恒率乾燥から減率乾燥への変位点(限界含水比)は30%前後にみられるが, その含水状態への到達時間は各試料・各深さによって著しく異なる。(2)土層の水分分布は時間の経過と共に, 表面から深さ7∿10cm位までの含水比差が大くなるが, 10cm以上の深部ではその差が小さい。(3)国頭礫層土の上表面にPF5以上の薄い乾燥膜が短期間(400cal/(cm)^2/dayとして連続7日間)に形成される。これは下層土の水分保持に寄与する反面, 水食にたいし弱点となる。(4)干ばつ性の石灰岩風化土は土層箱試験結果からすれば必ずしも乾燥速度の高い土とは言い難い。(その土の干ばつ性にたいしては地質学的な層序の影響が大だと考える)。(5)島尻泥岩風化土においてはキ裂の発達が著しく, 下層部分の乾燥を促進する。(6)PF-含水比曲線を作成し, Glass-beads block soil moisture instrumentと併用することによってPF値を知ることができる。例えば積算日射量9.5kcal/(cm)^2下における乾燥状態は国頭礫層土層でPF4.2∿5.0,石灰岩風化土層でPF3.6∿5.0,泥岩風化土層でPF3.8∿5.0となる。In order to know the drying characteristics of the erosive soil from Kunigami gravels (sample : Yaka, Kin-son), the soil from Ryukyu limestone (sample : komesu, Itoman-city) and the soil from Shimajiri mudstone (sample : Tomigusuku, Tomigusuku-son), a glass-beads block soil moisture instrument was used. Before the instrument was put in practical use, fundamental experiments were carried out on the operation of the instrument and its calibration. The sample was packed uniformly in drying box, and was tested in two cases : the thermostatic drying and the out-door drying. The experimental results are summarized as follows : 1. The boundaries transiting from drying in constant rate to drying in decreasing rate of three samples are seen respectively in water constant of about thirty percents, and the drying time required to nearly reach this soil moisture differs greatly with each sample and each depth. 2. Water content profile in soil layer varies with time. In this profile, the difference in water content so far 7∿10cm depth from upper surface becomes very large with time, but there is not so much difference in water content in the parts deeper than 10cm depth. 3. Thin film of drying above PF 5 is formed on the upper surface of erosive soil from Kunigami gravels in the short period. 4. From the results of drying box tests, it doesn't always follow that drying velocity of soil from Ryukyu limestone droughting frequently is large. 5. Cracks developed in the soil from Shimajiri mudstone cause the evaporation of surface to increase markedly and accelerate drying of the lower parts of layer. 6. PF value is to be known by using the glass block soil moisture instrument along with the PF- water content curve. For instant, under the cumulative global radiation of 9.5kcal/(cm)^2 that the soil layers recived in the drying test period, soil from Kunigami gravels was dried up to the PF 4.2∿5.0 of soil moisture, soil from Ryukyu limestone up to the PF 3.6∿5.0,and soil from Shimajiri mudstone up to the PF 3.8∿5.0.
- 琉球大学農学部の論文
- 1976-12-01
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