Economic aspects Agriculture 1 170 History 178 Education 169 Economic conditions 165 Agriculture and state 164 Congresses 123 Social aspects 98 World War, 1939-1945 86 Rural conditions 84 Land tenure 83 Technological innovations 74 World War, 1914-1918 68 Economic policy 61 Periodicals 57 Pollution 56 Finance 54 Mathematical models 54 Economic development 53 Forests and forestry 50 Food supply 48 Industries 47 Automation 46 Research 46 Roads 46 Environmental aspects 45 Government policy 45 Environmental policy 41 Case studies 40 Licenses 38