Abstract
This study was undertaken to clarify the effect of, soil on the seed production of onion. Mother bulbs were planted in large bottomless frames filled with soils differing in their origin and texture. The soils used were fluvial sand, Shirasu sandy loam, volcanic ash loam, fluvial clay loam, basalt clay and polder clay.
Plant growth was vigorous in fluvial clay loam and polder clay, medium in fluvial sand and basalt clay, and poor in Shirasu sandy loam and volcanic ash loam. Leaves in the volcanic ash loam plot were abnormal and showed die back in winter.
Upper portion of seed stalks turned yellow as seeds matured. This change occurred quickly and abruptly in the seed stalks growing on sandy soils, while in clay and clay loam soils, it proceeded gradually, and seed stalks retained green until harvesting date.
Heavy infection of the black stalk rot disease (Macrosporium porri) was observed in sandy loam, sand and volcanic ash loam. Severe injury was induced by the Botriotinia allii in volcanic ash loam, sandy loam and polder clay.
Number of florets per plant was found tc be closely correlated to the plant growth in spring (r=+0.82), when the plant growth was expressed in cm of leaf length in March.
The yield of seeds was highest in fluvial clay loam, followed by basalt clay, polder clay and fluvial sand. The yield was poor in Shirasu sandy loam, and especially in volcanic ash loam, in which it was about one third as small as that in clay loam.
It was found that the seed weight was depending on the conditions of seed stalks on which seeds were produced. Stalks remaining healthy and green until harvest produced heavy seeds, while those turned yellow quickly or affected by diseases produced light seeds with low germination rates.
