In order to effectively use vermiculite materials from different sources in substrates used for the growth of vegetable plug seedlings, we examined the chemical constitution and mineral composition of these materials, and their influence on NH_4^+ and K^+ fixation. Dominance of black mica and vermiculite in the materials from South Africa and Australia was characterized by high contents of magnesium, whereas dominance of black mica in the material from China and Japan was characterized by high contents of silicic acid, iron, and aluminum. These differences in chemical constitution and mineral composition are believed to have an effect on NH_4^+ or K^+ fixation by plants. There was almost no difference in the physical properties of the different vermiculite materials used in substrates. In storage tests, substrates containing Chinese vermiculite material caused a mark increase in NH_4^+ fixation. When cabbage seedlings were raised using these substrates, the fresh weight of the above-ground parts and nitrogen absorption decreased, and, consequently, overall seedling performance declined. On the basis of these observations, we conclude that when using Chinese vermiculite in substrates for the growth of vegetable plug seedlings, there is a marked increase in NH_4^+ fixation, and therefore it is necessary to take into consideration the amount of added nitrogen.

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We grew taro (Colocasia esculenta) under black mulch or biodegradable plastic mulch with slow-release fertilizer. Under biodegradable plastic mulch, the leaf area index (LAI) quickly reached its maximum. This maximized the storage of photosynthate in the leafstalks and thus dry weight, and allowed immediate translocation of photosynthate to support enlargement of grandchild tubers. Therefore, it is possible to maintain a high crop growth rate (CGR) throughout the growth period while maximizing yield. Under black mulch, in contrast, the child tuber leafstalks did not extend; therefore, the child tuber and pro-tuber leaf blades competed for photosynthetic product, the net assimilation rate of the pro-tuber blades decreased, and LAI decreased early. As a result, photosynthetic product was limited, the child tubers were small, and the yield was lowered. By maintaining a high CGR, we hasten the growth of the leaf blade, lengthen its life, and thus improve the yield of the taro.

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Tillage pans in soils often restrict root access to subsoil moisture and nutrients. As part of research to reduce the nitrate load from upland fields to groundwater, we evaluated tillage pan breaking treatments in the Okhotsk region, one of the major onion (Allium cepa L.)-producing regions in Japan. For 4 years on a total of 7 sites on various soil types, we grew onions on plots subsoiled with a wide vibrator (W), subsoiled (S), or not subsoiled (NS). The W treatment reduced the penetration resistance to <1.5 MPa, reduced the bulk density, and increased the gaseous phase in the 20〜40-cm zone compared with the S and NS treatments. The rooting depth in the W treatment was almost twice that in the S and NS treatments. The W treatment increased N uptake, total dry matter, and yield of onion relative to the S and NS treatments. In addition, higher yields and N uptake in the W treatment following a dry period during the vegetative growth stage are attributable to the use of moisture below the broken tillage pan. The W treatment reduced the surplus in the N budget by an estimated 15 kg ha^<-1> owing to the higher N uptake, and reduced the residual inorganic N in the 0〜80-cm zone by 11 kg ha^<-1> at harvest. In 9 cases out of 10, the W treatment with less N fertilizer than recommended by Hokkaido Prefecture (150 kg ha^<-1>) reduced the surplus in the N budget to within the acceptable amount of residual N in the soil profile. We conclude that breaking the tillage pan in onion fields can reduce the nitrate load to groundwater by improving the N budget.

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The red soils of the coastal region of Hokkaido on the Sea of Okhotsk were believed to have been formed during the Diluvial epoch under a humid climate. However, no stratum with accumulated gibbsite or iron has been found at five sites within the Hidaka belt, all of which vary in stratigraphy, thickness, and weathering. Plant fossils, spores, and paleoclimatology confirm that the red soils were not formed during the Diluvial epoch. The results of boring, analysis of primary minerals, and the chemical forms of iron show that the red soils were formed by weathering of andesite and tuff and became red via high-temperature oxidation from the middle to the end of the Miocene epoch. In the Tokoro belt, by contrast, two types of non-volcanic red soils formed during the Mesozoic era were found, one a mechanically crushed red chert and shale, the other a hematite derived from long-term weathering of schalstein. The present distribution of the red soils in both belts was created by translocation and redeposition due to periglacial phenomena from the Piacenzian stage.

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