Micronutrient requirementsfor biological nitrogen fixation and growth of legumes

• Universiti Pertanian Malaysia, Malaysia
• Department of Agriculture, Thailand
• Khon Kaen University, Thailand
• Chiang Mai University, Thailand

Nutrient deficiencies limit nitrogen fixation and legume growth in many areas, and accurate diagnosis and correction of those deficiencies can greatly improve legume production. Most diagnostic standards for legumes have been developed under temperate conditions, and their validity must be checked for the tropics. Additional problems arise for nutrients that vary in their behaviour in plants according to symbiotic and environmental conditions.
This project concerns development of simple techniques for diagnosing nutrient deficiencies in two important grain legumessoybeans and groundnutsbeginning with the most important ones limiting growth in the soils of Thailand. It comprises four sub-projects.
At Murdoch University, scientists will grow both crops in soil or water cultures in the glasshouse, and develop diagnostic procedures for individual nutrients by examining the relation of nutrient supply to plant growth, critical nutrient concentration, functional nutrient requirement and, where useful, biochemical activities in specific organs. They will begin with phosphorus, sulfur and potassiumall know to be deficient in many areas of Thailandand the potentially important boron, zinc and copper, which have produced responses in some crops on some Thai soils. All the techniques will be checked and standardised for soybean and groundnuts, and procedures will be checked in the Thai field trails as part of the third sub-project.
Because a legume's requirements for some nutrients, and hence its standards for deficiency diagnosis, vary with the state of the nitrogen-fixing symbiosis and also with environmental conditions, the second sub-project will study these two aspects. For example, some legumes need higher concentrations of molybdenum, calcium and copper when they are fixing nitrogen than when they have an adequate supply of soil nitrogen. Morever, almost nothing is known of the mineral nutrition of Rhizobium species in general. Initial trials at Murdoch will cover zinc and especially molybdenum requirements in R. japonicum, and will compare nutrient requirements for bacterial growth with those for nitrogen fixation processes in different bacterial strains. These values will in turn be related to those for host legume metabolism and to deficiency diagnosis. Both soybean and groundnuts have responded to boron and Zinc at Chiang Mai, but the responses have varied markedly with time of year, suggesting interactions with environmental factors such as light and temperature. Using the diagnostic standards developed in the first sub-project, the team will carry out glasshouse studies to determine the effect of varying light intensity, duration and quality on the legume responses to boron and zinc and on their critical values for deficiency diagnosis.
The third sub-project will define the nutrient deficiencies in selected soils in Thailand for soybean and groundnut growth. Grey podsolic soils cover extensive groundnut-growing areas, and field and glasshouse omission-type trials at Khon Kaen with various combinations of fertilisers will identify the nutrients limiting production there. Plant diagnostic tests in field and pot trials at Chiang Mai will study the crops' growth and production response on the low humic gley soilsto nutrients generally and to boron specifically. The team will also examine boron sorption by this soil and its relation to fertiliser use and agronomic practice. On reddish brown lateritic and brown forest soils, field trials will identify the nutrient deficiencies for the two crops in both limed and unlimed soils.
Scientist in the fourth sub-program will seek to develop physiological and biochemical diagnostic tests for the copper status of legumes in Malaysia. Glasshouse experiments at the University Pertanian Malaysia will study copper enzymes, with emphasis on ascorbic acid oxidase, in groundnuts, soybeans and possibly mung beans, using nutrient cultures or specific problem soils such as peat. Field trials will follows in different parts of the country on different soils.
In addition to their immediate application, the techniques should apply to the diagnosis of deficiencies in soybeans and groundnuts grown anywhere, and should be adaptable for other legumes.