Nitrogen fixation 34 nitrogen fixation 10 Nitrogen 6 Nitrogen Fixation 6 NITROGEN FIXATION 5 Soil Biology 5 Agricultural microbiology 4 Fixation 4 Nitrification 4 Plants 4 Biofertilizers 3 Biofertilizers-congresses 3 Nitrogen fixing bacteria 3 Nitrogen-Fixation 3 Plant Nutrition 3 Trees 3 Woody plants 3 biological nitrogen fixation-congresses 3 hydraulic conductivity 3 Agroforestry 2 Agroforestry and Multipurpose Trees: Community, Farm and Social Forestry 2 Biochemistry and Physiology of Microorganisms, (New March 2000) 2 Biotechnology 2 Biotechnology-agriculture 2 Cropping systems 2 Crops 2 Denitrification 2 Ecological pest management 2 Field Crops, (New March 2000) 2 Forage and Fodder Crops, (New March 2000) 2