Papers by Michelle Quimbo
The catastrophic 1991 eruption of Mt. Pinatubo in the Philippines inflicted severe damages to agr... more The catastrophic 1991 eruption of Mt. Pinatubo in the Philippines inflicted severe damages to agriculture. It blanketed the landscape with a thick layer of unconsolidated silica-rich tephra. The remobilization of this material by rain produced debris flows (or lahars) that flooded and buried the lowland soils. Nearly three decades after this catastrophic event, agriculture has not yet fully recovered and adapted, making the livelihoods of farmers more vulnerable to a future shock. The objectives of this Ph.D. research were to assess the impacts of the eruption on soil properties and development. We investigated (i) the weathering of the pyroclastic (tephra and lahar) deposits from which a new soil is being formed; (ii) the accumulation of biogenic silicon (phytolith) and (iii) the status of phosphorus, a typically limiting nutrient, in the newly formed soils under different vegetation covers. These analyses were compared to the properties of thẽ 500 years old soils buried by the 1991 volcanic products. Finally, we interviewed the small-scale farmers in order to identify the main problems that they face while reestablishing agriculture in the tephra-and lahar-impacted area. We show that very limited weathering has taken place in the recent tephra and lahar deposits. However, in most cases, the ~500 years buried soil exhibits roughly similar properties, suggesting slow weathering reactions despite a tropical climate. The early weathering of the fresh pyroclasts seems to promote biogenic silicon accumulation in the newly formed soil. Phosphorus is not limiting plant growth...
Proper method and timing of incorporating farm biomass such as rice straw into the soil is vital ... more Proper method and timing of incorporating farm biomass such as rice straw into the soil is vital to ensure soil organic matter (SOM) build-up. Two field experiments were conducted in two sites with different levels of soil organic matter (SOM) to determine the appropriate means of managing the biomass left after harvest of rice crops during fallow period. The effect of mowing rice stubbles and incorporation of rice straws left after harvest during the fallow period were determined. The effects of different land preparation method and fertilizer manage ment were also assessed, based on the physical and chemical properties of the soil. In general, mowing of rice stubbles and methods of land preparation have no effect on carbon-nitrogen (C:N) ratio of soil organic matter, bulk density, depth of mud, and on certain soil chemical properties. About 70% of the rice straw incorporated is composed of stubbles left after harvest and 30% from threshed rice straw. Increased C:N ratio 2 months before transplanting up to harvest time implies that the incorporated straw has not been thoroughly decomposed. Mineralizable or available nitrogen (N) tend to be lower with higher SOM level. This is due to the recalcitrant nature of SOM when conditions allow them to accumulate, such as under reduced condition in poorly drained soils. Additional inorganic N must be supplied even in soils with high SOM to offset immobilized N. The amount of straw to be incorporated and its respective N content are critical in SOM accumulation. Higher amount of straw incorporated resulted to higher SOM accumulation. Incorporation of 4-5 t ha-1 straw, which is equivalent to one season straw biomass production per hectare, was not enough to significantly change the level of SOM, as well as the physical and che mical properties of the soil. Nevertheless, continuous recycling and incorporation of rice straw in soil during fallow period may lead to the improvement of soil properties over the long-term.
The study was conducted in a Lipa clay loam soil at Los Baños, Laguna, Philippines during the 201... more The study was conducted in a Lipa clay loam soil at Los Baños, Laguna, Philippines during the 2012 and 2013 wet cropping seasons to compare the growth and yield performance of upland rice (Oryza sativa L.) varieties under organic and inorganic fertilization. The parameters considered include periodical changes in soil nutrient status, agronomic efficiency of nitrogen (AEN), grain yield and return on investment (ROI). The treatments were arranged in split-plot design fitted into a randomized complete block design, with varieties as the main plots and varying fertilizer management treatments using organic (Pureganic and Vermicompost) and inorganic fertilizers as the subplots. NSIC Rc9 variety performed better in terms of growth and grain yield than Inipot-Ibon, a traditional variety; it had higher harvest index, number of spikelets, filled grains and AEN. Generally, inorganic fertilizers improved the grain and straw yields as well as the AEN of NSIC Rc9 compared with Inipot-ibon that received organic fertilizers. Applying organic fertilizer alone tended to increase the grain yield but its effect was not significant. The grain yield of Inipot-Ibon was not significantly improved by organic or inorganic fertilization in two consecutive croppings. No significant effect of varieties or fertilizer management was observed on the soil organic matter (SOM), total available nitrogen, available phosphorus and exchangeable potassium but the effect tended to decline every season after cropping of upland rice. In general, the ROI obtained from upland rice production was either break-even or had negative returns, making upland rice production less profitable.
Organic farming is being promoted by advocates to sustain and improve soil fertility compared to ... more Organic farming is being promoted by advocates to sustain and improve soil fertility compared to conventional farming that uses inorganic fertilizers and pesticides. The study was conducted to determine the effect of pesticide and fertilizer management practices on grain and straw yields, yield components, and soil fertility of irrigated lowland rice. The experiment was conducted for three consecutive seasons (2009-2010) at Los Baños, Laguna, Philippines. It was laid out in a Split Plot design consisting of two main plots: with pesticides (M1) and without pesticides (M2), and four subplots: control (T1), organic fertilizer only (T2), inorganic fertilizer only (T3), and one-half (½) rate of inorganic plus ½ rate of organic fertilizers (T4), with four replications. Generally, pesticide application did not affect the yield and yield components. Plots applied with inorganic and one-half rate of combined organic and inorganic fertilizers had significantly higher grain yields than the control and organic only fertilizer treatments. Application of limiting nutrients through inorganic fertilizers helps to correct most of soil deficiencies such as phosphorus and sulfur compared to the use of organic fertilizer alone. Combining organic materials such as compost and rice straw with inorganic fertilizers at one-half rates is an effective strategy to attain yields comparable to pure inorganic application to sustain soil fertility.
Two consecutive upland rice wet season croppings (2012-2013) were conducted in Lipa clay loam soi... more Two consecutive upland rice wet season croppings (2012-2013) were conducted in Lipa clay loam soil at Los Baños, Laguna, Philippines. This was done to determine which among NSIC Rc11 (modern variety) and Inipot-Ibon (traditional variety) applied with six fertilizer rates [kg/ha of nitrogen (N), phosphorus (P) and potassium (K)] with increasing N levels: control (zero fertilizer), 0-0
Application of organic and inorganic fertilizer and pesticides to the rice crop had no consistent... more Application of organic and inorganic fertilizer and pesticides to the rice crop had no consistent effect on the content of α-tocopherol (vitamin E), α-tocotrienol, γ-tocopherol, γ-tocotrienol, total tocols and γ-oryzanol of NSIC Rc146 and NSIC Rc160 brown rice and NSIC Rc146 milled rice. Brown rice had more tocols and γ-oryzanol than milled rice. Total phenolics of NSIC Rc160 brown rice were lower in the control and organic rice than in rice treated with inorganic fertilizer and pesticide. The 1 st and 2 nd crops of organic milled rice had lower protein content than milled rice treated with inorganic fertilizer, and subsequently tended to have softer Instron cooked rice hardness value and higher Rapid Visco-Analyser (RVA) peak viscosity. Milling quality, color, apparent amylose content and alkali spreading value of rice in the various treatments were similar.
Biofertilizers have been claimed as an alternative source of nutrients for plants in order to inc... more Biofertilizers have been claimed as an alternative source of nutrients for plants in order to increase yield and reduce the inorganic fertilizer use by 50%. Sixteen (16) trials were undertaken nationwide covering the provinces of Ilocos Norte, Isabela, Nueva Ecija, Laguna, Camarines Sur, Negros Occidental, Leyte, Samar, Bohol, Agusan, and North Cotabato during wet and dry seasons of 2008-2010 to verify the effectiveness of biofertilizers (Bio N, Vital N, BioCon) under lowland or anaerobic ecosystem. The experiment consisted of 9 treatments which include control, and biofertilizers applied alone or in combination with inorganic fertilizers either at one-half or full recommended rate. Generally, the biofertilizers evaluated showed no positive effect on grain and straw yields of lowland rice as well as on yield components such as plant height, tiller count, number of productive tillers, spikelet count, number of filled grains and unfilled grains, and weight of 100 grains. The positive effect of BioCon, Vital N and Bio N on root length at seedling stage may be attributed to the microorganisms producing plant growth hormones which might have enhanced root growth under " dapog " seedbed condition. This is probably because the condition in the seedbed is not totally anaerobic. However, the beneficial effect on root growth was not sustained and reflected on the yield when the seedlings were transplanted in the field. It is quite evident that biofertilizers evaluated were not effective under anaerobic condition.
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Papers by Michelle Quimbo