Solute transport in clay systems is strongly governed by ion exchange phenomena. We carried out two kinds of experiments in order to investigate the effect of Ca+—Na2+ ion exchange on the transport of solutes. One was a leaching experiment of Na-bentonite with CaCl2 solutions. The other was an evaporation experi-ment of Ca-bentonite having groundwater consisting of NaCl solution. Since Ca2+—Na+ exchange is nonlinear, two types of ion exchange can take place: favorable exchange due to a high affinity for Ca2+ and non-favorable exchange due to a low affinity for Na+. Favorable exchange could be observed in the leaching experiment. Penetrating Ca2+ exchanged with almost all adsorbed Na+ at the moving front of the Ca2+ concentration profile. Then, the Ca2+ front became steep. On the other hand, non-favorable exchange could be observed in the evaporation experiment. The penetrating front of Na+ concentration profile became flat. Ca2+—Na+ ion exchange affects the hydraulic properties of soils. In the leaching experiment, the water flux was suddenly increased when the whole clay from the surface to the bottom was transformed into a Ca-clay system having higher hydraulic conductivity. Water retention also varies significantly depending on the clay systems. For a given water suction, the water content of Ca-bentonite leached with CaCl2 solution is less than that of Na-bentonite, whereas it is much greater than that of Ca-saturated-bentonite mixed with CaCl2 solution. Predicting the changes in the hydraulic properties caused by ion exchange is important to quantify the transport of exchanging solutes in soils.
This study investigated the shrinkage behavior of strongly clayey soil with the soil texture of LiC on SiC and the part it played in the formation of soil structure in the Nakaumi reclamation. The results obtained were as follows:
1.The strongly clayey soil in the Nakaumi reclamation, especially the muddy sediment forming the subsoil, showed the anisotropy that the shrinkage length of the vertical direction to the depositional surface of this soil changed little; the change in shrinkage length of the horizontal direction was always fairly larger than that of the vertical one in the shrinkage behavior of soil during the progress of drying.
2. It was supposed that the anisotropy in the shrinkage behavior of this soil was due to the flat faces of platelike soil particles with the low swelling being horizontally oriented and the vertical distance between the platelike soil particles, i.e., the distances between their flat faces, being shortened under the normal consolidation caused by the covering soil pressure; and consequently the horizontal ones between them, i.e., the ones between their edges, become rather larger than the vertical ones.
3. The orienting structure of this soil was also clarified by the result of an unconfined compression test. The compression stress of the vertical direction to the depositional surface of this soil was large and that of the horizontal one to it was small.
4. It was supposed that in this soil, because of the anisotropy in its shrinkage behavior, the occurrence of primary cracks was greater, the development of secondary cracks easier, and consequently the formation of the prismatic and angular blocky structure was smoother than that in the soil consisting mainly of platelike montmorillonites with a high swelling from the Hachirohgata reclamation.
We examined methods of soil improvement by accelerating the clay slate weathering in new reclaimed upland field at Hujisawa-cho Higashi-Iwai-gun Iwate-Prefecture. It showed that:
1.The subsoil improvement method, or soil treatment by the ripper dozer accelerated clay slate weathering. It is considered that this effect was retained more than for 4 years at the lower layer (15—30cm). However at the upper layer (0—15cm) it was lost about 4 years after reclamation.
2 . Soil treatment by the ripper dozer in winter, when the rock and soil was frozen, accelerated the clay slate weathering. In particular, big rocks, over 30mm in diameter, were broken. The effect was greater in the lower layer than the upper layer.
3 . The cultivation of pasture, wheat and fruit trees delayed clay slate weathering.
To discuss the interrelation among the water potentials of soil, plant and atmosphere and their relation to evapotranspiration, the water potential of atmosphere, plant and soil were measured and the evapotranspira-tion rate was estimated using a heat balance method. The observations were conducted in wheat fields of a Pseudogley (PSG), an Ordinary Andosol (OAS), and a Sand-Dune Regosol (SRS) in the neighborhood of Sapporo for the period from early June to harvest in 1986,1987 and 1988, respectively. The distribution of roots and the dry weight of the above-ground biomass were also determined.
(1) The dry weight of above-ground biomass of wheat was in the order of SRS > PSG > OAS.
(2) In PSG, with the least available water capacity of the three fields, the amount of rainfall was low during the vegetative growth and flowering stages. However, the average daily evapotranspiration (ET) had a high value of about 3.3 mm/day, and the daily ratio of latent heat flux to net radiation (IE Rn) was found to be higher than 0.7. In spite of a remarkable decrease in the water potential of atmosphere (PA), plant water potential (PP) was maintained to be high, ranging within — 6 to —1.2 bar. This suggests that water supply from soil to plant roots was satisfied. We ascribed this to the fact that the roots took up the stagnant water directly. This was supported by the following facts 1) lowering of stagnant water level with wheat growth, and 2 ) some wheat roots having touched at the stagnant water pool.
(3)The available water capacity of OAS is 5 times greater than that of PSG. The pattern of rainfall was similar as in PSG. However, the average daily ET was a low value of about 2.7 mm.z day, and the lE/Rn ratio varied with growth stage and rainfall event, and has low values, ranging from 0.4 to 0.6 dur-ing a dry spell in the flowering stage. PP decreased remarkably with decreasing PA in this stage, and the value converged into a narrow range of -11 to - 7 bar. These facts suggest that plant might have suffered from water stress, and explain why the dry weight was the least among three fields.
(4)In SRS, the available water capacity is 12 times greater than that of PSG. Total amounts of ET during vegetative growth and flowering stages corresponded to total amounts of rainfall in the vegetative stage. Therefore a higher ET of about 3. 65 mm day was found during the dry spell in the flowering stages. The lE/Rn ratios in the growth stages were maintained within a range of 0.6 to 0.8, and also PP values less than ― 8 bar were not found. These facts explain why the good wheat growth resulted.
In the rice/znon-rice cropping systems on heavy paddy soil, adequate drainage, pulverizability, and traf-ficability during the non-rice phase are necessary. Effects of tillage method on physical properties of heavy paddy soil were investigated. In anticipation of conversion of puddled ricefield to non-rice cropping, it is desirable that land preparation for the final rice crop should not destroy all soil structure. One non-puddling land preparation method is to rotovate only those strips of soil along which rice shall be transplanted. The grain yield of rice using this method was equal to that from conventional puddling. After rice harvest, permiability of strip-tilled soil was higher than that of puddled soil and rotovation was relatively easy. Seedling-emergence rate for a succeeding wheat crop and the yield thereof was higher than from soil previously puddled. In studies of wheat-soybean systems on heavy clay soil, emergence, rooting and yield of soybean were higher with non-tillage than with conventional tillage. In rice-wheat-soybean sequences on heavy clay, soil, the suitable tillage methods gave high production.
An investigation was made on the applicability of the Fall Cone Test for determining the plastic limit of cohesive soils. The test was carried out by using 2a=60。,Q=ION cone (60°,ION cone) where a and Q are semi-angle of the cone tip and the cone weight, respectively, on 11 cohesive soils. The test results were compared with those obtained previously using 2a=60°,Q= 0.6N cone (60°, 0.6N cone). According to the results, the penetration depths hp
read at the Casagrande plastic limit on the logarithmic penetration (h) vs logarithmic water content (w) straight lines became constant and took the mean value of hp= 5.4 mm. The accuracy of the plastic limits determined by using 60。,0.6N cone and 60°, ION cone for those by the Casagrande method was found to be almost same. However, 60°,10N cone should be preferably used rather than 60°, 0. 6N cone in case that the Fall Cone Test was carried out to determine only the plastic limit.