8 Mineral Nutrition of Plants 



significant increases in the average yields of several of our major crops 

 in the last few years, aside from climatic effects. 



As applied to the United States, these effects of technology are much 

 more clear when considered in terms of efficiency (2). For example, 

 between 1800 and 1940 the number of man hours neede.d to produce 

 100 bushels of wheat dropped from 373 to 47. Similar improvements 

 were made with the other major crops. In 1820, after science had al- 

 ready had some effect, one farm worker supported about four and one- 

 half other people. In 1946 the figure was fourteen and one-half. 



Modern science has already increased our efficiency and continues to 

 do so at an accelerated rate. This has happened in the temperate re- 

 gions. There is every reason to believe that similar potentialities for the 

 application of science to agriculture exist in the far north and especially 

 in the tropics. The tropics have some handicaps but they also have many 

 advantages. Our guess would be that science will be even more influ- 

 ential in the tropics than it has been in the temperate regions. The great 

 need there is for research institutes devoted to fundamental investiga- 

 tions. The soils are so different from those in the temperate regions 

 that technology can be transferred only to a very limited extent. 



SOIL FERTILITY OF THE GREAT SOIL GROUPS 



Following this general view of the potentialities for food production, 

 let us look more specifically at soil fertility. Plant nutrients tend to be- 

 come limiting wherever man uses soils for crops, but the fertility prob- 

 lems vary in both kind and intensity from place to place. Different 

 crops have unlike nutrient requirements, and different soil types vary 

 in their capacity to provide nutrients. Since many thousands of unique 

 soil types exist in the world, exceptions must be permitted in any gen- 

 eralization. Yet, a broad view of the fertility status of the principal great 

 groups of soils may help us measure the fertility problems in world food 

 production. 



with what we know now. Of course, we do need to learn these relationships 

 specifically, farm by farm, to get on with the job of preparing recommendations 

 for sustained production. 



