Nutrition and nitrogen-fixation in Malaysian Pterocarpus Indicus willd

Pterocarpus indicus is a promising tropical woody legume for the establishment of forest plantations in Malaysia. Woody legumes that form symbiotic relationships with nitrogen-fixing bacteria also play an important role for forest restoration on degraded land. Although P. indicus has been widely planted as an amenity tree in SE Asia, its silvicultural requirements have not been determined. There are no recommendations for fertilizer or for inoculation with nitrogen-fixing bacteria. This thesis explores the phosphorus (P) and nitrogen (N) requirements of seedlings and identifies a range of nitrogen-fixing bacteria capable of forming root nodules under glasshouse conditions. Four glasshouse experiments were undertaken on two soil types: Yalanbee sandy gravel (YB) and yellow sand (YS) to determine the P and N concentration ranges in the foliage of deficient and healthy plants and to define critical nutrient concentrations for the diagnosis of deficiency. There was a narrow range in rates of P fertilizer, supplied as aerophos, Ca(H2PO4)2.H2O, between deficiency and toxicity in both soil types. The relationship between yield and P concentration in the youngest fully expanded leaf (YFEL) enabled critical P concentrations for the diagnosis of deficiency (0.17%) and toxicity (0.41%) to be determined at 90% maximum yield from linear regressions fitted to the data. The foliar P concentration ranges for deficiency and toxicity were similar to other nitrogen-fixing trees. Only plants in YS responded to inorganic N fertilizer, and soil analysis suggested that seedlings may take up ammonium-N in preference to nitrate-N at luxury supply. A critical concentration for the diagnosis of N deficiency was not able to be determined due to the lack of data points. Plants with adequate N fertilizer had YFEL N concentrations of 2-3.5% dry weight. To determine whether P. indicus is more sensitive to luxury soil P supply than other fast-growing legume trees, the P response of P. indicus in YB was compared with three other woody legumes, Pterocarpus macrocarpus, Acacia mangium and Sesbania formosa. The sensitivity of P. indicus to high P was confirmed and the response shown to be similar to P. macrocarpus. Both species showed severe symptoms of P toxicity, namely leaf necrosis and stunted growth. In contrast, shoot and root yields of A. mangium and S. formosa were not reduced at luxury P supply and yield x fertilizer relationships were able to be fitted to the Mitscherlich model. Critical P concentrations for the diagnosis of P deficiency in A. mangium and S. formosa, derived using the Mitscherlich model, were estimated to be 0.2-0.3% dry weight for the YFELs. Two glasshouse inoculation trials were carried out using diverse strains of root nodule bacteria in order to identify strains suitable for inoculation in the nursery. There were eight strains from Bradyrhizobium, five from Rhizobium, three from Sinorhizobium and two Mesorhizobium strains. P. indicus formed nodules with strains from Bradyrhizobium, Rhizobium, Sinorhizobium and Mesorhizobium, which suggests it is a promiscuous host. Nodules formed were globose, single and of aeschynomenoid type. In the first trial, Bradyrhizobium strain WSM 2096, promoted shoot growth while in the second trial, Bradyrhizobium strain WSM 3712 promoted shoot growth. Growth stimulation was similar to the uninoculated control supplied with inorganic N as KNO3 but was inferior to plants given (NH4)2SO4. The response of P. indicus to low soil P in inoculated and uninoculated plants was studied in a pot trial comprising two P treatments (nil, just adequate). Plants were grown for 3 and 6 weeks. At nil fertilizer P, uninoculated P. indicus seedlings had higher total root length and root dry weight than those with adequate P. Inoculation with WSM 3712 suppressed root growth relative to uninoculated plants. Information gathered in this thesis has application for the production of planting stock in forest nurseries. Firstly, care needs to be taken to ensure that rates of P supplied as hard or liquid fertilizer are not in the range likely to cause toxicity. Secondly, if any starter inorganic N fertilizer is to be used then it should be supplied either as ammonium-N or as urea. Thirdly, more research is required in order to identify effective strains of rhizobia for widespread commercial application. Fourthly, the critical foliar P concentrations identified for the diagnosis of P deficiency and P toxicity can be used to help interpret foliar analysis of seedlings in the future.