1 6 Mineral Nutrition of Plants 



is severe and exchange capacities low, this difference may be quite 

 important. 



Further, the balance among nutrients, important everywhere, is espe- 

 cially so in tropical soils. Much more research is needed to establish the 

 correct fertilizer ratios to use on the many thousands of local soil types. 

 The level of all nutrients is so low, in absolute terms, that an excess of 

 one, caused by liming or fertilization, can easily upset the balance. 

 Thus, in our ignorance, the use of compost has an advantage. Where 

 a mixture of normal plants is used, one is bound to get a reasonably 

 balanced supply of plant nutrients with decomposition. Someday we 

 shall know from research the proper chemical mixtures to use. Yet, 

 these mixtures can never substitute entirely for the good effects of 

 mulch and the effects of organic matter in maintaining soil structure. 



The large yields obtained with heavy fertilization on tropical soils 

 in Hawaii, northern Queensland, and other places, where the products 

 of industry are available in the tropics, give us a glimpse of enormous 

 potentialities, provided agriculture and industry are developed together. 



In the meantime, much can be done in the tropics to improve fertility 

 and increase production without the use of mineral fertilizers. Our 

 guess would be that scientific agriculture will increase yields even more 

 in the tropics than it has on the Gray-Brown Podzolic soils of western 

 Europe, and only a part of this will be due to fertilizers. 



Besides the four major groups of soils discussed here, there are many 

 others, each with unique fertility problems. But most of these soils are 

 intergrades among the four broad groups. 



SOIL DISTRIBUTION AND FERTILITY PROBLEMS 



The provisional soil map shown in the accompanying figure gives a 

 broad view of how the various kinds of fertility problems are distri- 

 buted. This map is schematic and highly generalized; it is not detailed 

 enough for local predictions of land-use potentialities. 



Gray-Brown Podzolic soils and their associates are shown in the 

 northeastern part of the United States, north-central Europe, and 

 China. In relation to the total land area of the world, the extent of 

 these soils is small. Yet, on them, both food production and fertilizer 

 consumption are high, especially in Europe. In the prewar years, 



