Effects of Molybdenum and Interacting Elements on the Preformance of Tropical Pasture/Forage

Abstract

This investigation was to study the effects of Mo, Sand lime on yield, biological nitrogen fixation and nutrient concentration of tropical pasture legumes; to determine the external and internal Mo requirements of tropical pasture legumes; and to evaluate the mycorrhiza-rhizobium-legume association as related to Mo status in acid tropical soils. In order to fulfill these objectives, the study was conducted in three parts. In the first part, the experiments were conducted in the greenhouse and in the field. In the greenhouse experiment, Desmodium intortum was grown in the Wahiawa (Tropeptic Eutrustox) and Paaloa (Humoxic Tropohumult) soils and Centrosema pubescens was grown in the Wahiawa soil. Treatments were composed of five levels of Mo (none added, seed-applied Mo at 0.1 and 0.5 kg Mo/ha, and soil-applied Mo at 2.0 and 4.0 kg Mo/ha); two levels of S (none added and 50 kg S/ha); and two levels of lime (none added and lime rate so that a pH of soil is raised to 6.0). In the field experiment, the treatments were composed of two legume species (D. intortum and C. pubescens) and ten fertilizer treatments (none added; lime; seed-applied Mo at 0.1, 0.3, 0.5 and 0.5 kg Mo/ha plus lime; soil-applied Mo at 1.0, 2.0, 4.0 and 4.0 kg Mo/ha plus lime). In the second part, D. intortum was grown in the Wahiawa and Paaloa soils in the greenhouse. Seven rates of Mo (o, 0.5, 1.0, 2.0, 4.0, 8.o, and 16.0 kg Mo/ha) were added to the soil. Molybdenum adsorption and desorption curves were also studied. In the third part, treatments were composed of three legume species (D. intortum, C. pubescens and Stylosanthes humilis); three Mo levels (none added, seed-applied Mo at 0.3 kg Mo/ha and soil-applied Mo at 2.0 kg Mo/ha); and two mycorrhizal inoculation rates (non-inoculation and inoculation). Mycorrhiza used was Glomus mosseae. Soil was fumigated with methyl bromide one month before planting. Molybdenum increased dry matter yield of desmodium and centrosema grown in the Wahiawa and Paaloa soils. Soil-applied Mo at 2.0 kg Mo/ha was adequate for legumes grown for one cutting in the greenhouse. However, soil-applied Mo at 4.0 kg Mo/ha should be used for excellent stands of legumes harvested five times a year. Seed-applied Mo at 0.5 kg Mo/ha was as effective as soil-applied Mo at 2.0 and 4.0 kg Mo/ha. Seed-applied Mo at 0.1 kg Mo/ha was less effective than seed-applied Mo at 0.5 kg Mo/ha and soil-applied Mo at 2.0 and 4.0 kg Mo/ha. Molybdenum did not affect nodule fresh and dry weights of desmodium grown in the Wahiawa soil. However, seed-applied Mo at 0.5 kg Mo/ha decreased nodule fresh and dry weights of desmodium grown in the Paaloa soil. Molybdenum did not affect nodule number and n6clule fresh weight of centrosema grown in Wahiawa soil. However, soil-applied Mo at 2.0 and 4.0 kg Mo/ha increased nodule dry weight of the legume. Nitrogenase activity of desmodium grown in the Wahiawa soil tended to increase when Mo was applied. Seed-applied Mo at 0.5 kg Mo/ha, soil-applied Mo at 2.0 and 4.0 kg Mo/ha increased nitrogenase activity by 29.1%, 37.0% and 31.8;'&, respectively, above the no-Mo treatment. Molybdenum also increased nitrogena.se activity of centrosema grown in the Wahiawa soil. Molybdenum increased N concentration in the tops of desmodium and centrosema grown in Wahiawa soil. Seed-applied Mo at 0.3 and 0.5 kg Mo/ha and soil-applied Mo at 1.0, 2.0 and 4.0 kg Mo/ha increased N concentration from the first to fifth cutting. At the fifth cutting, plant N concentration of legumes receiving seed-applied Mo at 0.3 kg Mo/ha was lower than those of legumes receiving seed-applied Mo at 0.5 kg Mo/ha and the three soil-applied Mo rates. Seed-applied Mo at 0.1 kg Mo/ha did not increase plant N concentration, The residual effect of seed-applied Mo at 0.5 kg Mo/ha appeared to be less than that of soil-applied Mo at 4.0 kg Mo/ha. Molybdenum application increased Mo concentration in the tops of legumes grown in the Wahiawa and Paaloa soils, Plant Mo concentrations in the later cuttings decreased when compared to those in the earlier cuttings. Critical Mo concentrations in plant top associated with 80% and 95% maximum yield of desmodium grown in the Wahiawa soil were 0.13-0.20 and 0.22-0.30 ppm, respectively. Comparable Mo concentrations of desmodium with the Paaloa soil were 0.40 and 1.30 ppm, respectively. Critical Mo concentration in the top associated with 80% maximum yield of centrosema grown in the Wahiawa soil was 0.20 ppm. Molybdenum application also increased Mo concentration in the nodule of legumes. Critical Mo concentrations in nodule associated with 80% and 95% maximum yield of desmodium grown in the Wahiawa soil were 2.2-3.5 and 7.85 ppm, respectively. Comparable Mo concentrations of desmodium with the Paaloa soil were 4.0 and 13.2 ppm, respectively. Critical Mo concentration associated with 80% maximum yield of centrosema grown in the Wahiawa soil was 4.2 ppm. Molybdenum application increased available Mo in soil. The available Mo associated with 95fo maximum yield of desmodium grown in the Wahiawa and Paaloa soils were 0.09 and 0.10 ug Mo/ml, respectively. Predicted quantity of applied Mo required to establish 0.09 ug Mo/ml available Mo for the Wahiawa soil was 4.4 kg Mo/ha. For the Paaloa soil, predicted quantity of applied Mo to established 0.1 ug Mo/ml was 4.7 kg Mo/ha. Molybdenum had no effect on S concentration in the tops of desmodium and centrosema but tended to increase Cu concentrations. The potential harmful effect of Mo regarding Mo-induced Cu deficiency in animals should not occur since Cu/Mo ratios in plant tops were well above 3, which is the ratio under which Mo-induced Cu deficiency usually occurs. Sulfur did not increase dry matter yield, nodule number, nodule fresh and dry weight~,nitrogena.se activity, plant N concentration and other nutrient concentrations of desmodium and centrosema grown in the Wahiawa soil. Sulfur increased yield of desmodium grown in the Paaloa soil. Nodule fresh and dry weights of desmodiwn grown in limed Paaloa soil and receiving seed-applied Mo with S was higher than that of desmodium grown in the same treatment without S. Lime increased dry matter yields of desmodium and centrosema grown in the Wahiawa soil. Lime increased plant N, Mo but decreased Cu concentrations of desmodium grown in the Paaloa soil. However, lime increased Cu concentrations in the tops of desmodium and centrosema grown in the Wahiawa soil. Lime increased nodule number, nodule fresh and dry weights, nitrogenase activity and plant N concentration of centrosema grown in the Wahiawa soil. Lime did not increase Mo and S concentrations in the tops of desmodium and centrosema grown in the Wahiawa soil. Mycorrhizae enhanced plant growth and increased yields of desmodium, centrosema and stylosanthes. Mycorrhizae strongly stimulated nodulation of legumes. Nonmycorrhizal plants did not nodulate. Mycorrhizae did not affect Mo concentration in the tops of desmodium, centrosema and stylosanthes. Seed-applied Mo at 0.3 kg Mo/ha increased the percentage of mycorrhizal infection, suggesting that there is a relationship between Mo concentration and mycorrhizal infection.