Nitrogen Phosphorus 9 Fixation 6 Nitrogen-fixing microorganisms 6 Fertilizers and other Amendments 5 Nitrogen-fixing plants 5 Plant Production 5 Plants 5 Pollution and Degradation 5 Soil Biology 5 Soil Chemistry and Mineralogy 5 Soil Fertility 5 Legumes 4 Plant Nutrition 4 Soil Physics 4 Agriculture 3 Chemical composition 3 Forage and Fodder Crops, (New March 2000) 3 Manures 3 Nitrogen fertilizers 3 Nutrient availability 3 Plant nutrition 3 Potassium 3 Soil fertility 3 Soil organic matter 3 Sustainability 3 Water quality 3 Analysis 2 Animal Wastes 2 Biochemistry and Physiology of Microorganisms, (New March 2000) 2