The ability of roots to penetrate into consolidated subsoil was examined by the field and pot experiments using various kind of tropical leguminous plants. The results revealed that Pigeon pea (Cajanus cajan L.) was the most penetrative and effective in improving the compacted subsoil. Its characteristics are summarized as follows:
1) The growth rate of Pigeon pea was so high, and the roots could penetrate deeply into compacted soil within such a short period of 3 months.
2) For a physically inadequate soil with the high mechanical impedance and low gas diffusion rate, the root of Pigeon pea could penetrate well as far as the soil was not highly acidic.
3) It was expected that the biopores in soil produced by the decomposition of roots would improve the drainage of soil.
4) Dry weight of the plant top of Pigeon pea was estimated as much as 10 t/ha, which carbon-nitrogen ratio was 24 or so, making it possible to utilize as a green manure.
To determine the applicability of subirrigation in clayey paddy fields converted to upland fields, water movement during subirrigation and the effect of subirrigation on wheat yields were investigated in field tests. Subirrigation supplies water to drain pipes by gravity from the irrigation canals. The results were :
① Where the subsoil had well-formed cracks, irrigation water flowed laterally in the subsoil through the cracks, and penetrated the plowed layer by gravity from the subsoil near the drain pipes after the cracks in the subsoil were filled.
② When supplying irrigation water up to the plowed in this manner, it was necessary to pay attention to the ability of crop to endure wet conditions as the plowed was extremely wet for some time even when drainage took place after irrigation. Supplying water only to subsoil with roots growing in cracks, not up to the plowed layar, was found to be suitable for wheat.
③ Little leakage of irrigation water and high permeability in the subsoil by well-formed cracks were necessary to use subirrigation. However, it is difficult to establish objective criteria that will determine fields which satisfy these requirements.
The soil sample was impregnated with the unsaturated polyester resin with 100cm3 made of metal a cylinder core for the actual-volumenometric method. The sample of each metal, which did was impregnated, was cut out by using CBN cutting blade with slicing machine. A consecutive soil thin section was easily obtained. There was a very high correlation with gaseouse phase ratio of the actual-volumenometric method and void ratio of thin section. The gaseouse phase ratio was able to be presumed from the void ratio of the soil thin section with a lot of large gravel.
Actual soil erosion and conservation practices were observed over two years on two differently shaped farm
fields which had been reclaimed into two types, one had a slightly graded side, the other had two graded sides,
but the same slope gradient and the same soil of decomposed granite.
The field observations showed that soil erosion occurred along the flow ends of furrows in a field and that
soil loss was decreased after proper conservation practices leading the flow to a stable waterway directly. On
the fields of the two graded sides type, however, furrow erosion was accelerated by ridging and so conservation
practices became harder because the flow was obstructed by sedimentation and thick sod along a waterway.
Sustainable agriculture in reclaimed land of decomposed granite soil needs feasible conservation practices
and stable fields easy to control soil erosion year by year. From these viewpoints, farm fields with a slightly
graded side are suggested to be superior to the others.
The soil of the paddy field of Uso-river basin in Kotoh town, Shiga prefecture generates the very stable suspension when the paddy field is puddled. That causes a serious environment problem. The auther investigated the reason why the puddled suspension is so stable and contains so much suspended substances from the stand point of particle size distribution and dispersion-flocculation condition of the suspension. The two factors are considered to be the reasons. One is that this suspension is in a chemically dispersive condition. The other is that the soil increases clay size particles when it is mechanically disturbed. In order to flocculate the dispersed puddled water, the chemical condition of flocculation was estimated using the hetero coagulation theory. In the calculation, the particles are supposed to be spherical, because the main clay mineral of the soil is Halloysite. The entimated flocculation condition was in good agreement with the experimental results. The Calcium ion concentration higher than 0.001mol/dm3 was found to flocculate and clarify the suspension.
In order to clarify the puddled water by sedimentation, flocculation aids were tested by the flocculation test in a laboratory. Eight flocculation aids were selected to test among the soil amendment inputs and the feritilizers which contain relatively high percentage of calcium. They are considered to be used safely, because they are not special but are frequently used in usual farming. Among the eight flocculation aids tested, gypsum showed the most remarkable effect. The addition of 25kg/10a gypsum to the puddled water caused a rapid flocclation and clarified the water in a short time. The way of addition was also examined. A simple mixing method, spreading the gypsum on the puddled water and stirring them slowly for a short time, resulted a successful flocculation.