Graduate Thesis Or Dissertation
 

Molybdenum status of subclover (Trifolium subterraneum) as affected by Mo and S fertilization and soil type

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  • Field studies designed to study residual effects from fertilizer molybdenum were initiated in September 1971 on established grass-clover pastures at ten sites in Western Oregon. Treatments were 0.0 and 1.12 kg molybdenum /ha on plots receiving 0-35-0-20S fertilizer (elemental sulfur-S) and 0.0, 0.56, 1.12 and 2.24 kg molybdenum /ha on plots receiving single superphosphate. Treatments were replicated four times in a randomized block design. Forage yields were measured during the spring in 1972 and 1973. Subterranean clover samples were analyzed for nitrogen, sulfur, copper and molybdenum. Yield response to applied molybdenum was noted on three harvests of seventeen; these responses were all on plots receiving single superphosphate. Clover nitrogen and copper levels were not significantly affected by molybdenum application. Form of sulfur fertilizer had a profound effect on molybdenum uptake by clover. On plots which received 1.12 kg Mo /ha, application of elemental sulfur (0-35-0-20S) produced plants much lower in molybdenum than did single superphosphate application. It was hypothesized that depression of Mo uptake resulted from one or a combination of the following phenomena: (1) a decrease in soil pH, (2) increased competition between sulfate and molybdate for absorption by the plant, and (3) increased leaching of molybdate due to the sulfate release pattern of the elemental sulfur material. Samples of the surface 10 cm of soil from each treatment were analyzed for anion exchange resin extractable molybdenum. Correlation between plant molybdenum and soil molybdenum was significant where all harvests were considered together, but the correlation was improved by separating soils into two groups. The soils in one of these groups were found to sorb large amounts of molybdenum, to release high amounts of hydroxyl anions on treatment with sodium fluoride, and to contain large amounts of amorphous and exchangeable iron compared with soils in the other group. Laboratory studies were carried on to investigate the molybdenum adsorption patterns peculiar to these soils. Surface samples were taken from eight of the field sites which showed a large range of molybdenum responses. Molybdenum solutions (0-20 ppm Mo in 0.01 M CaCl₂) were allowed to equilibrate with soil at 25° C and the supernatant molybdenum concentration measured. Molybdate adsorption by soils was found to conform to the Freundlich adsorption isotherm in all instances. Four of the eight soils showed sorption patterns which followed the Langmuir adsorption isotherm at low equilibrium concentrations. The increase in solution hydroxyl concentration measured two minutes after mixing soil with 1 N NaF correlated significantly with molybdate adsorption at an equilibrium concentration of 0.03 ppm Mo. The fluoride ions may be reacting directly or indirectly with soil constituents responsible for molybdate retention. Iron extracted in ammonium acetate, pH 4.8, and ammonium oxalate, pH 3.3, correlated significantly with molybdate sorbed at an equilibrium concentration of 0.03 ppm, and with hydroxyls measured by the above sodium fluoride technique. Neither citrate-dithionite extractable iron nor aluminum in ammonium acetate and ammonium oxalate extracts were significantly correlated with molybdate sorption. Oxalate-extractable aluminum was significantly related to hydroxyls released with sodium fluoride. No correlation was found between laboratory measurements of molybdate retention by soils and the increases in clover molybdenum following molybdenum fertilization. It is suggested that several parameters not investigated in the present study such as soil moisture content, phosphate and sulfate levels, and the soil organic regime may affect rates of molybdate uptake into and translocation through the plant. Results indicate that it is difficult to predict the effect of molybdenum application on plant uptake of molybdenum in the field. A larger number of samples representing a vast range of soils appears desirable to reliably predict residual effects from applied fertilizer molybdenum.
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