THE MOISTURE CONDITIONS OF AUTO-IRRIGATED SOILS AND THE PLANT GROWTH IN THEM AS RELATED TO THE SOIL STRUCTURE

1. In order to find how the moisture conditions and plant growth vary as the structure of the soil of the same texture varies, the experiments presented here were carried out at the Agronomical Laboratory of the Kyushu Imperial University during the period of 1930-1931. 2. Soils o f different structure were prepared by adding different amounts of water to a air-dried and pulverized soil and rubbing by hands; the greater the amount of water added, the coarser the granulation of the soil was. The soils prepared in such a way showed a greater volume weight and hence smaller pore space as clods became larger and the granulation coarser (Table III). Hence it may be assumed that, particles constituting clods were arranged very densely, although the pore space between the clods was rather larger. 3. Filling porcelain containers with these soils respectively and auto-irrigating them after LIVINGSTON\u27s method, broad-bean plants were grown and the moisture conditions of the soils and the growth of the plants were investigated. 4. Even when the soils were auto-irrigated under s ame hydrostatic pull, the soil contained more water and showed a higher water-supplying power determined by LIVINGSTON\u27s soil point, as the granulation became coarser. The higher watersupplying power, however, appeared to be not only due to the high water content, but also due to the soil structure itself. Because the soils containing the same amount of water showed different water-supplying power, as the structure varied (Fig. 1.). 5. The facts stated above may be explained as follows : In clods particles being arranged densely, such thick capillary films cannot be parformed between them that the hydrostatic pull of the auto-irrigator allows, and hence water molecules in the clods are attracted very strongly. Not only for this reason, water in the clod is resisted to move by the pressure exerted by the surface tension at concave menisci performed between particles on the clod surface, which become more concave and hence exert a pull to the water in the clod when an external force attracts it. In addition to this, the clods themselves being arranged rather sparcely and hence the actual effective surface and the number of angles possible for capillary concentration being less, the amount of water in the films on the clod surface is less. For these reasons the soils of coarse structure prepared in such a way as started above contain less water and show a lower water-supplying power (Tables I and IV.). 6. As the time proceeded, the water-supplying power of t h e soil increased very markedly, and such an increase was more conspicuous when the soil was autoirrigated under a lower hydrostatic pull and hence the soil contained more water (Tables I and IV.). This is probably due to the fact that the pressure of water in the clod widened the arrangement of particles gradually and broke down the clods. 7. The water-supplying power determined by the soil point appeared to express pretty well the water-supplying power of the soil to the plant root. Because the plant transpired more water and showed a large water-requirement as the watersupplying power measured by the soil point was higher although the yields of plants, which are controIled not only by the moisture conditions of the soil, did not vary pallarel to the water-supplying power (Tables II, V and VI.). 8. The results would be somewhat different when the preparation method of the material varies and the soil texture is different1. 質(Texture)を同じくした土壤を風乾し粉碎せるものに, ,種々の量の水を加へて手で力張く攪拌することにより組織(Structure)の異なりたるものたらしめ, 此等を夫々ポツトに填め, LIVINGSTON 氏の方法により自働灌水して?豆を栽培し, 土壤の水濕状態と植物の成育状態とに就て觀察した。2. 斯る方法により調製したる土壤は團粒の大なるに從て容積重大に, 從て團粒内に於ては粒子の極めて密に配列されてゐることを示した。3. 灌水器に仕掛けた靜水壓が同じでも, 換言すれば粒子の表面に形成される水膜の毛管力が同じ場合でも, 團粒の大なるに從ひ含水量は小に, Soil point で測た給水能は低かった。而してかゝる給水能の差は含水量の差よりのみ來るものでなく, 同じ含水量でも團粒の大なるに從ひ給水能は低かつた。4. 斯る含水量及給水能の差は團粒内に於て粒子が密に配列され, 之が爲に水膜に働く靜水壓に相當する丈の厚さの水膜の形成を許さぬこと, 從て團粒内部の水は強く保持されてゐること, 更に團粒の外部に形成された水膜が團粒の表面をなす粒子と粒子との間に於て作る凹面メニスカスに於ける壓力が内部の水の移動に對して強く抵抗すること, 竝に團粒そのものゝ配列は粗なるが爲に毛管現象の起り得る表面は小に又メニスカスを作る角の少きこと等により説明し得るものの樣であつた。5. 時日の經過に伴ふ含水量の變化は實驗により一致を缺いだが, 給水能は例外なく著しく増加した。而して此の増加は灌水器の靜水壓低く含水量大なる時に著しかつた。之によりかゝる給水能の増加は水の壓力により粒子間隔の増大乃至團粒の崩壞による土壤組織の變化といふことで説明し得る樣であつた。6. 斯る給水能の大小は植物の成育状態, 就中蒸散量及要水量(Water requirement)の大小とよく一致した。故に組織の細粗による Soil point の吸水量に現れる給水能指數の差は克く植物に對する土壤の給水能の大小を示すものと考へてよい樣であつた