Ph.D. - Agronomy and Soil Science

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    Effects of Calcium Silicate on Yield and Nutrient Uptake and Mechanism of Silicon Transport in Plants
    ( 1971) Thiagalingam, Kandiah
    The effect of calcium silicate on yield and nutrient uptake by plants and the mechanism of Si uptake were studied in a series of three experiments. First, response to Si was measured on 22 plant species grown on two soils with four levels of calcium silicate in a greenhouse; second, corn was grown in the field to determine the magnitude of response to residual Si with variable P and pH levels; and third, five plant species were grown in culture solutions at varying transpiration rates to study the mechanism of Si uptake. Response to calcium silicate differed with species and the same species grown on different soils had variable amounts of Si and P depending on the Si contents of the soils. In general the 2.2 T Si/ha application produced maximum yields in both soils and in some species it produced yields comparable to those with 8.8 T Si/ha at lower cost. Silicon concentrations in different plant groups were in the following order: grains > grasses > vegetables and fruits > legumes except for those of the two Desmodiums in which concentrations were similar to those of grasses. Plant Ca concentrations generally increased whereas Mg, Mn, A1 and Fe concentrations generally decreased when calcium silicate was applied. Silicon concentration was greatest in papaya and pineapple leaves and in sugarcane sheaths and was lowest in stems. Hawaiian Cement Corporation (HCC) and Tennessee Valley Authority (TVA) calcium silicates generally produced higher yields than Technical grade (TG) calcium silicate, especially at low rates and in the third and fourth harvests. Plant Si concentrations in both HCC and TG calcium silicate were generally higher than in the TVA material indicating greater Si availability in these two materials. Ear corn yields in the field were significantly increased by P applications but were not significantly affected by residual Si or pH. Highest stover yields were obtained at pH 5.5 suggesting increased Si solubility at this pH may have increased stover yields by increasing mechanical strength and P availability. The application of 280 kg P/ha with Si produced yields nearly equal (98%) to those of 1120 kg P/ha without Si suggesting that comparable yields at lower cost may be obtained with the combination of high Si and low P, than with high P alone. Corn leaf Si values of 0.5 to 0.6% at silking appeared adequate for corn growth. Silicon taken up by the plant is more closely related to soil Si extracted with water than with sulphuric acid. Multiple regression analysis with yield and leaf nutrients indicated that Si, P and Fe are especially important for stover production while P, Ca, K, A1 and Fe are important for ear production. A statistically significant increase in the amount of water transpired per gram of dry weight with decreasing relative humidity was found in all species except D. intortum in solution cultures, but no significant increase in Si transport was obtained with increasing transpiration in any species. This suggests that transport of Si in plants is not related to transpiration. In continuous dark, plants accumulated Si in the roots and only sugarcane translocated large amounts of Si to the tops suggesting that metabolic energy is required for Si transport. Additional evidence of active Si transport was provided by the transpiration stream concentration factor (TSCF) values which were above or below one. Silicon concentrations in xylem exudates of D. Intortum, corn and sugarcane were greater than those of external solutions suggesting Si movement by active transport rather than by mass flow. In tomato and alfalfa Si concentrations were lower in exudates than in external solutions suggesting a selectivity mechanism in the root. These experiments demonstrated that both Si and P transport require metabolic energy.
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    Evaluation of Phosphorus Fertilizer Materials on Two Benchmark Soils of the Tropics
    ( 1983) Harris, David J.
    One possible strategy for improving the economic attractiveness of P fertilization on acid P-deficient soils in the tropics was investigated agronomically in two field experiments on sites of the Benchmark Soils Project in the Philippines and Indonesia. The specific strategy is the use of phosphate rock for direct application or partially acidulated or thermally altered phosphate rock in place of acidulated phosphates, e.g., superphosphate. The experiments were conducted on Hydric Dystr'andept soil on a site near Naga City, Camarines Sur, Philippines and on a Typic Paleudult on a site located near Kotabumi, Lampung on the island of Sumatra, Indonesia. The experiments compared crop response (primarily maize) to one highly reactive phosphate rock (North Carolina, NC) and one moderately reactive phosphate rock (Central Florida, CF), which were used because they are well characterized with respect Co reactivity. These two P-sources were used in finely-ground form and also each was used in a different minimally altered form. For the North Carolina rock, a minigranular form was used form, Co determine if the minigranulation process can be used Co overcome the handling and transportation problems of the dusty, finely-ground form without reducing agronomic effectiveness. For the Central Florida rock a minigranular form which was 20% partially acidulated with phosphoric acid was used as the minimally altered form to determine if partial acidualcion of moderately reactive phosphate rocks could adequately improve agronomic effectiveness on acid soils. Comparing the results for the two soils sites in the first crop, a large difference between sources is seem on the Hydric Dystrandept, while no significant differences are seen between sources on the Typic Paleudult. In the second crop, fresh applications of superphosphate were made on former control plots on both sites and the other treatments were left as residuals. On the Hydric Dystrandept significant differences between sources are seen in the residual effects. On the Typic Paleudult, soybeans were planted, and no significant differences are seen between residual effects of the sources. The higher P requirement evidenced by the Hydric Dystrandept as compared to the Typic Paleudult, made reapplication necessary at all rates for the third crop on the Hydric Dystrandept, whereas for the Typic Paleudult, reapplication of all sources was made only on the LO kg/ha plots, on which 70 kg P/ha were applied to permit comparison with the residual 80 kg P/ha treatments. The third season yield results again show no significant differences between sources on the Typic Paleudult, even for the reapplications of 70 kg P/ha, whereas some significant differences are seem for the reapplication on the Hydric Dystrandept. With respect to rock modifications on the Hydric Dystrandept, where differences between sources are detected, minigranulation of the NC rock did not reduce its effectivesness, thus confirming the agronomic viability of the process. Also on the Hydric Dystrandept, partial acidulation did improve the performance of the OF rock, but not sufficiently to make it equal Co superphosphate or the NC forms. In summary, these results indicate that the Typic Paleudult is well suited for direct application of phosphate rocks of moderate and possibly lower reactivity, whereas the Hydric Dystrandept requires phosphate rocks of high reactivity to obtain yields comparable to those of superphosphate. This difference in performance of phoshpate rock between the two soils can be related to the slightly lower pH of the Typic Paleudult and also, its lower P-sorption capacity, in comparison to the Hydric Dystrandept. The results from the post-harvest P soil test analyses indicate that neither the modified Truog nor the Bray P I procedure can be satisfactorily used to predict residual P in the Hydric Dystrandept when P-sources varying in solubility are used. In the Typic Paleudult there is some indication that the Bray I procedure underestimates residual P from the CF sources; however, it appears that minor modification of the procedure by increasing the solution to soil ratio could overcome this problem. This is considered to be a worthwhile area for research since direct application of phosphate rock over the complete range of solubility appears to be an agronomically advisable practice for the Typic Paleudult and similar soils.
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    Soil Nitrogen Mineralization as Affected By Drying, Liming and Sewage Sludge Addition
    ( 1980) Ahmad, Nazir
    This investigation was undertaken to determine the effect of drying and liming on soil nitrogen mineralization, to measure in N-supplying power of soils and to assess the effect of soil type on mineralization of nitrogen from sewage sludge. Laboratory, greenhouse and field experiments were conducted on a number of soils from the Hawaiian Islands. Air and oven-drying for 12 weeks increased the amount of mineral-N in five soils studied. Oven-drying released about 10-40 times more N than air-drying. Most of the mineral-N was released as NH^-N. Cultivation of undisturbed soil in the field resulted in more N being mineralized than from soil which was left undisturbed. Dry matter yield of corn was also affected when grown on air and oven-dried soils. Air-drying was more beneficial to corn yield in the Kaiwiki (Typic Hydrandepts). However, oven-drying had harmful effects on the growth of corn in this soil. Air-drying had little effect on the growth of corn in the Maile 7 (Hydric Dystrendept) soil. But in the oven-dried soil plants were much more healthy and more dry matter yield of corn was produced, compared to the control. The effect of lime application on N mineralization was studied on the Paaloa (Humoxic Tropohumult) and Wahiawa (Typic Eutrustox) soils in the field. The Paaloa soil had never previously been cultivated or limed. The Wahiawa soil had been limed twice before. Liming at 2 and 4 tons/ ha resulted in greater N mineralization than the control in the Paaloa soil. However, liming from a pH of 4.7 to 7.1 had little effect on the amount of N mineralized in the Wahiawa soil. An evaluation of the N supplying power of seven soils was made. Two chemical extraction methods as well as aerobic incubation procedures at 25°C and 35°C were used to obtain an index of N availability. Nitrogen mineralized was correlated with N-uptake by corn in the greenhouse. Aerobic incubation at 25°C was significantly correlated (r=0.96) with the N-uptake by corn. Incubation at 35°C was also highly correlated (r=0.92). The chemical methods employed were, by comparison, unsuitable in evaluating the N status of these soils. The amount of N mineralized from anaerobically-dried sewage sludge when mixed with two soils was very small, and depended on the soil type and the sludge rate. In the Waimea soil (Typic Eutrandept), 3-4% more N was mineralized than in the Wahiawa soil (Humoxic Tropohumult) when the sludge was applied at 22.4 and 44.8 tons/ha rates. N minrealization potential (Nq ) did not adequately predict the actual N mineralized from the soil: sludge mixtures. A two-function equation with two values of N q may be necessary to adequately describe N mineralization in sludge amended soil, because of two different N mineralization rates. One function describes the release of N from a readily-mineralizeable fraction in early incubation (0-4 weeks), while the other function describes the later r e lease of N from the more stable fraction in sewage sludge.
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    Calibration and Validation of the Ibsnat/Ceres Rice Model
    ( 1987) Jang, Li-Ling Lin
    Two rice varieties were subjected to two nitrogen rates and three temperature regimes in the greenhouse and growth chambers to study the effects of temperature, variety and N fertilization on N uptake, development and growth of rice. Nitrogen fertilization had a significant effect on grain and straw yields for both varieties. High nitrogen application resulted in high grain yield and N stress reduced biomass production but had no effect on the timing of phonological events of variety Starbonnet, but delayed panicle initiation in variety K-C-A. Temperature affected grain yield and nitrogen uptake during the grain filling stage. High day and night temperature hastened maturation and resulted in lower filled grain percentage, lower 1,000-grain weight and lower overall grain yield. Nitrogen concentration and N uptake were higher in the higher temperature. However, the persistence of green color and a low ratio of grain N to straw N indicate that nitrogen translocation from straw to grain was diminished by the high temperature. The IBSNAT/CERES Rice Model was calibrated and validated with data collected from field experiments under a wide range of agroenvironments. The model was able to adequately predict phenological development for a wide range of agroenvironments. Model prediction of final biomass was also acceptable. The model is sensitive to seasonal variation and altitudinal difference and is able to mimic the high sensitivity of rice to temperature and solar radiation.