Lowland Rice Cultivation Research Articles

Possibility and Problem for the Expansion of the Lowland Rice Cultivation Area in Africa

Possibility and Problem for the Expansion of the Lowland Rice Cultivation Area in Africa

Soil properties, C fractions and their dynamics in land use conversion from native forests to croplands in northern Iran

Land use conversion from forests to agricultural lands may have a major influence on soil organic carbon (SOC) and total nitrogen (TN) stocks and dynamics in northern Iran. The objectives of this research were to investigate the effects of lowland and upland farming in forest ecosystems on bulk soil chemical and physical properties, SOC and TN fractions, and their dynamics in northern Iran. Soil samples (0–0.2 m and 0.2–0.4 m depths) from adjacent cultivated and forest lands were obtained at Paresar site with lowland rice cultivation and at Gorgan site with upland cultivation. The study sites differed in rainfall, annual tillage frequency and cultivation practices. Land use changes from forests to agricultural fields resulted in a significant alteration in soil properties, and SOC fractions and their dynamics at both study sites. However, the magnitude and direction of changes depended upon the land use type replacing forests and the soil depth. Results indicate that after 50–70 years of cultivation, SOC and TN declined by 29–66% in the 0–40 cm layer, and that 23–62% of the decline occurred in the sand-size fraction C, particularly in the 0–20 cm layer. Forest clearing and subsequently intensive tillage practices resulted in soil degradation by declining SOC and TN, decreasing aggregate stability, increasing dispersible clay (DC) and stimulating SOC mineralization in northern Iran. Our study shows that losses in SOC and soil structure are more evident in forest ecosystems with low rainfall and more frequent tillage practices (i.e., upland crops) than in forest ecosystems with high rainfall and less tillage practices (i.e., lowland rice). Therefore, the natural forest ecosystems of northern Iran may represent a significant carbon (C) source after conversion to agricultural cropping systems. In brief, significant losses in SOC, TN and soil structure following land use conversion would affect CO 2 emission from the soil and nutrient dynamics in the natural forests of northern Iran.

Research on yam production, marketing and consumption of Nupe farmers of Niger State, central Nigeria

Questionnaire survey was conducted to obtain information from lowland rice cultivation based Nupe farmers in Niger State, central Nigeria regarding the current practices of yam production, marketing and consumption. In order to intensify yam production in Africa, scientists have developed various technologies to control growth cycle of yams for dry season production on wetlands. With appropriate natural environment and tradition of wetland utilization, the Nupe farmers in Niger State are targeted for introducing the new technologies to initiate dry season yam cultivation. Farmers can harvest ware yams during months when market prices of yam tubers are high, which would improve their incomes. Cassava as the current major dry season crop in the region may be replaced by yams which have far higher market and nutrition values. This research thus attempts to obtain basic information for future project planning. The findings suggest that, although in small scale, yams have been incorporated into the cropping system of Nupe farmers. Yams were mainly cultivated for self-consumption, so it was not market-oriented and resources inputs were limited. There were discrepancies between complex villages and upland villages in terms of production scale, variety, cultivating calendar and time pattern for sale and consumption. Key words: Yams, Nupe, farming system, inland valley utilization, off-season production.

Assessing phenotypic diversity of interspecific rice varieties using agro-morphological characterization

The aim of this study is to identify the phenotypic variability of 60 lowland NERICA varieties and 18 promising lines for better management of their genetic inheritance. The experimental design used was an augmented randomized complete block. Data on 23 agro-morphological traits were collected and analyzed. The analysis of variance showed highly significant differences (P<0.0001) between the variables and revealed the structure of the different genotypes from 11 quantitative discriminate traits. Cluster analysis and principal component analysis were carried out. The entries were grouped into three clusters irrespective of the level of backcrossing of genotypes. Eighty percent of lowland NERICA containing the desired characters adapted to lowland rice cultivation were grouped in cluster 1. The second cluster includes lowland NERICA varieties that can be grown in both upland and lowland ecologies, while the last group comprised only varieties close to their African parent and mainly adapted to the upland. Tillering at 60 days after sowing (DAS), plant height at maturity, maturity and spikelet fertility were the most discriminate and could therefore supplement a wide range of variables for genetic diversity study. Key words: Lowland NERICA, promising lines, agro-morphological descriptors, cluster

&lt;strong&gt;INCREASING WATER PRODUCTIVITY OF LOWLAND RICE THROUGH THE WATER SAVING TECHNIQUES AND CROP MANAGEMENT IN RESPONSE TO DROUGHT&lt;/strong&gt;

The water saving technology for lowland rice cultivation was very crucial because of in the future irrigation water become scarce and competed with other sectors. The lowering of the availability of irrigation water had the impact for sustainability of rice production. The review of the paper described the pattern of basic water requirement, yield responses of several lowland rice varieties to moisture stress, days interval irrigation and the alternatives of water saving techniques for improving yield and water productivity. The pattern of the actual water requirement (ET+P&amp;S) showed the maximum value of 8.8 mm/day (1.02 lt/sec/ha) for high yielding varieties (HYV) that occurred from heading to 50% flowering. Under limited water supply, irrigation water should be applied that period to prevent yield loss. Soil moisture stress at moderate level (- 0.5 bar) from heading to full flowering was significantly decreased yield about 30% when compared with the yield of continuously flooded 3 cm depth. This period was a critical period of HYV to soil moisture stress. For rotational irrigation purposes, information of the optimum days interval irrigation was important. It was found that 3 days irrigation interval was a critical limit for HYV to achieve higher yield. The SRI model of rice cultivation had the lowest rice yield in the lowland soil, poor drainage, clay soil texture and low permeability. The modified irrigation of the SRI plus fertilizer N based on LCC readings gave a greater yield as well as water productivity. The hybrid and NPT line rice varieties had higher yield components and grain yield than Ciherang variety. Ciherang variety was not favor to grown for the wet season, it was more productive when grown in dry season even with AWD irrigation model. The plant spacing of 25 cm x 25 cm gave higher number of panicle/hill and number of spikelet/panicle under both AWD and continuously flooded 3 cm depth for dry and wet season consistently. The fertilizer N management based on SSNM with low and high rates for the early vegetative stage were not significantly affected all yield components and grain yield. The AWD irrigation could save irrigation water about 18% when compared to the continuously flooded conditions.The grain yield of the hybrid, inbred and NPT line rice varieties was higher for the dry season than wet season under both AWD irrigation and continuous flooding consistently.

Malaria and Agricultural Production: Are There Bidirectional Effects? The Case of Coffee and Cocoa in Côte d'Ivoire

The sectors of coffee and cocoa in Cote d’Ivoire represented, before the political crisis, approximately 15% of the GDP and 40% of exports. The production zone of these two crops is the forest, which is a malaria endemic area. The cultivation of these crops is less constraining than that of the food crops such as rice or yam, which need to be replanted each year. However, the maintenance of the ground and trees and pest management are major tasks contributing to high yields. But, by increasing the work time in fields, they also expose farmers to mosquito bites and, as a consequence, to the risk of malaria. Farmers in this area also grow food crops, and more specifically rice, as rain cultivation crops, but some also cultivate these crops in irrigated lowland. The aim of this paper is twofold: first of all, to evaluate the role of malaria in coffee and cocoa production; secondly, to assess the role of the rice production scheme on malaria transmission. Three functions are therefore estimated: the production of coffee, the production of cocoa, and the production of health. Data were collected during a survey carried out on 750 households (21 villages) in the forest area. The main results show that malaria has no effect on coffee and cocoa production and that lowland irrigated rice cultivation does not constitute a risk factor for malaria transmission.JEL Classification: I12, O13.

Soil in Suitability Evaluation for Irrigated Lowland Rice Culture in Southwestern Nigeria: Management Implications for Sustainability

Rice grows in all agro‐ecological zones in Nigeria and it is largely grown by small‐scale farmers. An investigation was conducted to assess the representative pedons used for rice cultivation in the southwestern part of Nigeria. The aims were to assess how suitable these lands are for rice cultivation and what constraints to continuous and sustained rice production occurs on these soils. Data (climate, soil, water, etc.) were collected from rain‐forest agro‐ecological zones (AEZ) in which these pedons are located. After laboratory analyses, the land qualities/characteristics (LQ/LC) of these pedons were compared with the land‐use requirements (LUR) for cultivating lowland rice using two methods of evaluation (conventional/FAO approach and the parametric method). Results showed that the two methods rated the aggregate suitability of these pedons for rice between marginal (S3) and unsuitable (N1). Major constraints identified in these pedons are poor soil texture, which translates to poor water management coupled with suboptimal nutrient contents [i.e., available phosphorus (P), exchangeable potassium (K), and cation exchange capacity (CEC)], which may predispose rice plants to excessive iron (Fe2+) uptake (or bronzing or yellowing symptoms). Grain yields (GYs) collected from the farmers' field (1994 and 1995 cropping seasons) showed that in the current state of two of the soil series (Apomu and Matako), the GY of the two rice cultivars ranged between 0.61 and 2.13 t/ha and declines progressively across years in the two cropping seasons. The result suggests that in spite of good climate for irrigated lowland rice cultivation in SW Nigeria and indeed all Nigeria, special attention must be paid to the soils on which this crop is grown in terms of the soil texture, appropriate water management, and optimum contents of soil nutrients, especially available P, exchangeable K, and CEC. Thus for a sustained and continuous rice production on these soils, it is important that nutrient contents of the soil be augmented with fertilizer (organic and inorganic) coupled with appropriate water management.

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance.

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 μmol g (root FW)−1 h−1], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only ∼50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional ‘leak-pump’ model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes.

Management-induced structural dynamics in paddy soils of south east China simulated in microcosms

The puddling of paddy soils is the most common method of lowland rice cultivation in Asian countries and results in a complete distortion of the soil structure of the puddled layer and the formation of a distinct plough pan. In addition, the soil structure and the interrelated chemical and biological parameters in these soils are influenced by the alternating of flooding and drying stages. Since physical properties are basic features for the availability of water and nutrients characteristics such as soil structure and soil moisture have been recorded at the different stages of rice growth in detail. For a better understanding these results have been compared with micropedological investigations. For these purposes undisturbed microcosms filled with paddy soils of three different sites from south east China were taken and planted with rice in a climate chamber. Management-induced dynamics have been reflected very prominently as far as bulk density, pore size distribution and soil moisture of the puddled layer are concerned. The properties of the undisturbed plough pan did not change during all stages of the experiment. Most significant effects of soil puddling have been found in the soil with a clay content of more than 40%. Comparing the different sampling stages after puddling the soils with high clay contents have shown the greatest variety in pore size distribution. Micropedological investigations demonstrated the different texture-influenced aspects of pore size distribution and the arrangement of soil structure in the topsoil in detail. Micrographs verified the changes of microhabitats caused by soil structure dynamics. The structural alteration consequences different living-conditions of microorganisms and, therefore, population dynamics related to the management of paddy soils which has been reflected in this study.

Soil Fertility Advantages of Submerged Rice Cropping Systems

Wetland rice production systems in Asia are making a major contribution to the global rice supply. Wetland rice cultivation is often cited as an example of a sustainable cropping system. Wetland or paddy rice growing involves land preparation by cultivating in the flooded or saturated state (puddling), followed by transplanting of seedlings in soils under submerged condition and growing of the crop until two to three weeks before harvest. In other rice-based cropping systems, the land is either dry—or flood-fallowed during the period between two crops. Afterwards, two or three crops of rice are grown in submerged soil condition. However, shortage of freshwater is becoming critical for this traditional lowland rice cultivation. Obviously, there is high potential in exploring rice cultivation under moisture regimes that save water and also increase productivity. Such a situation provides an opportunity to critically analyze the fertility benefits of submerged rice cropping systems that would help facilitate in making a practical and right choice for growing of rice in future. The objective of this paper is to highlight the underlying principles, which govern the fertility advantages to submerged rice cropping systems. The advantages of growing rice in submerged soils include a general amelioration of chemical fertility, preferential accumulation of organic matter and improved availability of major, secondary and selected micronutrients. These soil fertility advantages benefit the long-term maintenance of soil fertility and sustainability of wetland rice systems. The paper emphasizes the potential of growing wetland rice in monsoon Asia, specifically in poorly drained, waterlogging-prone areas where the water table is shallow (within 30 cm of the soil surface).

Modeling the dependence of the crop calendar for rain-fed rice on precipitation in Northeast Thailand

Rain-fed lowlands are major agricultural ecosystems used for rice production in Northeast Thailand. Developing a tool to assess the effects of variable water conditions on the regional scale yield is also important to predict the effects of climate change on food supply. To estimate regional yields, we need a simple but accurate measure of the crop calendar (i.e., the distribution of planting dates), which has a strong influence on grain yield. In this article, we modeled the dependence of the crop calendar on rainfall patterns based on a survey of the region’s farmers as a part of an effort to provide a stronger basis for regional yield estimates. Our survey, conducted in 11 provinces for 2 years, confirmed the existence of large windows for sowing and transplanting versus narrow windows for heading and harvesting for rain-fed lowland rice culture in all the provinces. Variable water, soil, and toposequential conditions in the paddy fields were responsible for the large sowing and transplanting windows, whereas the use of photoperiod-sensitive varieties explained the narrow windows for heading and harvesting. The crop calendar was well expressed as a function of cumulative precipitation from June onward. When the crop calendar model was combined with a simple phenology-based model that uses growing degree-days adjusted by a day-length factor, we could estimate the rice crop calendar under rain-fed lowland conditions with acceptable accuracy. The model described in this article will be combined with a crop growth model to improve regional yield estimates for rain-fed lowland rice.

INTENSIFICATION OF RAINFED LOWLAND RICE PRODUCTION IN WEST AFRICA: PRESENT STATUS AND POTENTIAL GREEN REVOLUTION

The objective of the current paper is to explore the factors influencing the expansion and intensification of rice production in rainfed lowland sites. The village/lowland-level data show the following: (1) the expansion of lowland rice cultivation has been driven by population pressure and accessibility to the market; and (2) the adoption of water control technologies is enhanced by the existence of immigrants and accessibility to the market. Rice cultivator data show that investment in water supply canals is influenced by land tenure security and that the canals enhance yield. This suggests that investment in water control technologies in rainfed lowland is necessary to realize a rice Green Revolution. Considering the fairly high average yield already achieved with water control technologies and the vast area of lowlands without water control technologies in rural area, there is a high potential of a rice Green Revolution in rainfed lowland in West Africa.

Long‐term effects of lantana residue additions on water retention and transmission properties of a medium‐textured soil under rice–wheat cropping in northwest India

Abstract. Hydraulic properties of soils after rice cropping are generally unfavourable for wheat cultivation. Poor drainage, delayed planting and oxygen stress in the root zone may adversely affect the wheat crop after lowland rice cultivation. We studied long‐term effects of lantana (Lantana spp. L.) residue additions at 10, 20 and 30 t ha−1 yr−1 (fresh biomass) on physical properties of a silty clay loam soil under rice–wheat cropping in northwest India. At the end of ten cropping cycles, soil water retention, infiltration rate, saturated hydraulic conductivity and drying rate of soil increased significantly with lantana additions. The available water capacity (AWC), on volume basis, declined at rice harvest (from 22.0 to 18.8–20.9%), but increased at wheat harvest (from 12.9 to 13.4–15.0%) after lantana treatment. The volumes of water transmission (&gt;50 μm) and storage pores (0.5–50 μm) were greater, while the volume of residual pores (&lt;0.5 μm) was smaller in lantana‐treated plots than in controls at both rice and wheat harvest. Infiltration rate in the lantana‐treated soil was 1.6–7.9 times that of the control (61 mm d−1) at rice harvest, and 2–4.1 times that of the control (1879 mm d−1) at wheat harvest. Thus lantana addition improved soil hydraulic properties to the benefit of the wheat crop in a rice–wheat cropping sequence.

The paddy, the vector and the caregiver: lessons from an ecosystem approach to irrigation and malaria in Northern Côte d’Ivoire

Malaria is one of the most serious public health problems in the world. For the last few decades, numerous studies have focused on the potential links between environmental transformations (such as the expansion of irrigation) and malaria occurrence. Most of these studies have been based on relatively simple models outlining the interactions of the host–vector–parasite triad. In this paper, we investigate the links between the intensification of irrigated rice cultivation and malaria. In an attempt to complement biomedical and entomological approaches we propose a model that recognises the influence of human–vector contacts on transmission processes, but stresses the importance of taking into consideration socio-economic and cultural factors in the management of disease episodes, and how these can be affected by transformations of natural resource management strategies. Using a case study in Northern Côte d’Ivoire, we investigated the complex mechanisms by which agriculture-generated changes in ecosystems and socio-economic organisation influence disease risks and produce new scenarios in the management of disease. Our results show that the socio-economic transformation and gender repositioning induced, or facilitated, by the intensification of lowland irrigated rice cultivation influence the health care system for malaria in the study area. They lead to a reduction of the capacity of women to manage malaria episodes among children and influence their vulnerability to the disease. We argue that these elements contribute to higher malaria prevalence in villages involved in double cropping of rice annually.

Toposequential Variation in Soil Fertility and Rice Productivity of Rainfed Lowland Paddy Fields in Mini-Watershed (Nong)in Northeast Thailand

Mini-watersheds called Nongin Thai are geographical components of rainfed lowland rice culture in Northeast Thailand, and constitute distinct units in understanding environmental constraints for low and unstable rainfed rice production there. The toposequential variation of soil fertility and its relation to rice productivity within mini-watersheds, was examined by phytometry of sampled soils and field measurements of rice growth and yield. The phytometry experiment with irrigated potted rice using soils sampled from various rice fields within each mini-watershed, revealed that soil fertility as evaluated by rice dry matter production showed a 5 times difference among the fields at most. The difference in the soil fertility was ascribed primarily to that in nitrogen (N) supply capacity, which itself had a strong correlation with soil organic carbon (SOC) content. Accordingly, the biomass production of pot-grown rice was proportional to SOC. content, which suggested the usefulness of SOC as an index for soil fertility evaluation. The effect of clay on the soil fertility was much less than that of SOC. The actual rice yield in each field also showed quite large field-to-field variation, most of which was explained by the SOC content, rice growth duration and fertilizer application rate even though water availability also affected the yield. The yield positively correlated with growth duration and hence with earlier transplanting. Both SOC and clay contents of fields showed steep gradients with ascending field elevation within mini-watersheds, resulting in a marked toposequential distribution of rice yield. The toposequential distributions of SOC and clay contents imply that rice culture after deforestation accelerated soil erosion from upper to lower fields. The large toposequential gradient in soil fertility requires different resource and crop management for each toposequential position, in order to improve rice productivity of the mini-watershed as a whole.

Effect of calcium peroxide coating, soil source, and genotype on rice (Oryza sativa L.) seedling establishment under hypoxic conditions

Unstable seedling establishment is still noticed when calcium peroxide-coated seeds are used for uniform seedling establishment in direct-seeded lowland rice culture. To observe the genotype response of calcium peroxide-coated seeds, Oryza sativa L. var. Haenuki and Sasanishiki, were grown in test tubes and in plastic pots containing soils from: Oami hill (Oami B) and Akita Prefecture (Akita calcareous) (Experiments 1 and 2). The soils collected from upland and lowland areas from the Shonai Agricultural Experiment Station (Shonai alluvium), Yamagata University farm (University alluvium), and Shinjo Agricultural Experiment Station (Shinjo volcanic), Yamagata Prefecture, Japan were used to observe the response of soil types and soil sources in Haenuki in the case of calcium peroxide-coated seeds (Experiment 3). The seedlings were grown under hypoxic conditions at 30°C. The growth duration for in vitro culture was 7 d in the dark and 10 d for pot culture in the phytotron under natural day and night conditions. The Akita calcareous soil failed to accumulate dissolved oxygen even when calcium peroxide-coated seeds were sown. Seedling establishment of Haenuki was slightly affected regardless of whether coated or non-coated seeds were used in the Oami B and Akita calcareous soils. The coating of seeds of Sasanishiki, led to poor seedling establishment in these soils, whereas in the case of Sasanishiki, seedling establishment was as good as that of Haenuki when the seeds were not coated in the Oami B soil. In Haenuki, plant height increased when coated seeds were used in in vitro culture but in pot culture, plant height was similar between the coated and non-coated seeds in both soils (Experiments 1 and 2). Haenuki also showed a similar seedling establishment between coated and non-coated seeds in the Shonai alluvium upland and lowland, University alluvium upland, and Shinjo volcanic lowland soils. However, in the case of the calcium peroxide-coated seeds, plant height was adversely affected in the Shonai and University alluvium upland soils. In the University alluvium lowland soil, seedling establishment was markedly reduced when coated seeds were used. As the degree of reduction in this soil was not conspicuous, the worst performance of Haenuki might be attributed to factors other than anaerobic toxins like volatile fatty acids or hydrogen sulfide (Experiment 3). This study revealed that the efficacy of calcium peroxide coating of seeds varied with the genotypes, soil types, and soil sources.

Impact of weather on lowland rice cultivation and planning decisions over humid tropical India

The present study investigates the effect of the climatic environment on three different varieties of paddy. Crop coefficient in different stages of growth, the consumptive uses and radiation use efficiency has been determined and discussed in each case. Ideal date which could give optimum yield, has been determined in two varieties. The yield was correlated with weather parameters for each of the phases of the crop growth by forward ranking method and a statistical prediction model developed. Path analysis was applied to the predictors thus selected and direct and indirect contribution of the predictors to yield determined and discussed.

Yield of no-tillage direct-seeded lowland rice as influenced by different sources and application methods of fertilizer nitrogen

Due to high the labor cost, direct seeding is being gradually applied for crop establishment throughout Asia. Recently, the development of the so-called, “controlled release fertilizers” (CRFs) which requires only “one shot dose” of N, has enabled to reduce the cost of labor for the application of topdressed N. Thus, in order to limit pollution and cost, no-tillage direct-seeded lowland rice cultivation accompanied by the use of CRF is being recommended. To evaluate the effects of CRFs on the growth and yield of no-tillage direct-seeded rice, a field experiment was carried out in the north-eastern region of Japan in 1993 and 1994. Five N treatments were used: (1) ammonium sulfate (AS) band placement application with seeds at the time of seeding (ASl); (2) AS broadcast application after irrigation (AS2); (3) CRF (LPI00) co-situs application at the time of seeding (Ll); (4) CRF (LPS100) co-situs application at the time of seeding (L2); and (5) minus N (MN). The rate of basal N was 60 kg N ha-1. Topdressed N in the AS1 and AS2 treatments was applied at 25 d before heading at the rate of 20 kg N ha-1. The highest yield was obtained in the Ll treatment in both years. The number of grains per unit area was affected by the amount of N in the plants at heading. The percentage of filled grains in the L2 treatment was the lowest by about 15 to 20% among the treatments. The recovery percentages of basal N on July 20 were about 20 to 30% in the ASl, AS2, Ll treatments, only about 14% in the L2 treatment. About 75 to 80% of basal N was absorbed by the plants at harvest in the Ll and L2 treatments. On the other hand, the plants had absorbed only about 25 to 30% of the applied basal N in the ASI and AS2 treatments at harvest. There was no difference in the N-recovery percentage (based on the amount of released N) between the Ll and L2 treatments. Nitrogen-use efficiency was not significantly different among the treatments. However, higher N -use efficiency was obtained in this experiment than in early studies in the tropics. The agronomic efficiency of CRFs was higher than that of ammonium sulfate.

EFFECT OF REDUCING CONDITIONS ON THE WEATHERING OF Fe3+-RICH BIOTITE IN THE NEW LAHAR DEPOSIT FROM MT. PINATUBO, PHILIPPINES

Biotite in the new lahar (volcanic mud flow) deposit from Mt. Pinatubo, Philippines (hereafter referred to as P-Bt) was easily converted to vermiculite during lowland rice-cultivation in pots. The K 2 O content of the P-Bt gradually decreased, and the peak intensity of the X-ray diffraction (XRD) at 1.4 nm increased during seven cultivations of the paddy rice. The amount of K + release from the P-Bt with Na 2 S 2 O 4 treatment was about 1.2 times greater than that released with a NaCl treatment when they were compared at the same pH and Na + concentration. This result was considered to be a characteristics of biotite containing a large amount of Fe 3+ because the amount of K + released from two reference biotites having a Fe 3+ /total Fe ratio of < 0.1 was smaller after the Na 2 S 2 O 4 treatment than after the NaCl treatment. Fe 3+ at the edge site of the P-Bt might react with the reducing agent, and released Fe 2+ and other polyvalent cations might replace the interlayer K + because a 1.4-nm reflection peak appeared after the Na 2 S 2 O 4 treatment. The reducing conditions during lowland rice cultivation could facilitate vermiculitization of the P-Bt and contribute to the K + supply for the rice plants. The characteristics of the P-Bt were confirmed by XRD, infrared absorption (IR) spectroscopy, and elemental analysis. The trioctahedral type was also confirmed from the (060) reflection peak at 0.153 nm. IR bands of P-Bt were observed in the 400-1400-cm -1 and 3400-3700-cm -1 region. The IR bands of P-Bt from 400 to 1400 cm -1 were very similar to those for the two reference biotites whereas the IR bands in the 3400 to 3700 cm -1 region of the P-Bt differed from those for the reference biotites. The atomic Mg/Fe ratio of the P-Bt was 1.56, which was in the range of biotite. The atomic Fe 2+ /total Fe ratio of the P-Bt was 0.47, and its average structural formula was K 0.74 ,Na 0.09 (Si 2.80 ,Al 1.20 )(Al 0.06 ,Fe 3+ 0.50 ,Fe 2+ 0.45 ,Mg 1.48 , Ti 0.21 )O 10 (OH) 2 .

Performance of Azolla hybrids in lowland rice culture

Performance of Azolla hybrids in lowland rice culture