4 Mineral Nutrition of Plants 



modern methods of soil classification, irrigation, drainage, and fertili- 

 zation. 



The other one-half of the world's land is only partly arable. Some 

 soils are too stony, too sandy, too hilly, too salty, or too wet for culti- 

 vation. A sharp line cannot be drawn between those which are arable 

 and those which are not. First, any estimate must be based on economic 

 conditions, either consciously or unconsciously assumed. It is physically 

 possible to grow crops almost anywhere: mountainsides can be terraced; 

 stones can be removed; dikes can be built; and water can be carried 

 long distances. Secondly, we do not have a good soil map of the world. 

 Soil maps for several large areas do not exist. In these we can only fall 

 back on informed opinion. Thirdly, the potential use of soil depends 

 upon the associated industrial facilities and transportation. 



Even estimates of the land now cultivated in the world vary widely 

 around ten per cent. This is because we may start with intensive culti- 

 vation and pass gradually through general farming and extensive farm- 

 ing to nearly wild land with no sharp breaks. 



Western civilization — our civilization — has grown up mainly in the 

 temperate regions, first in Europe, then in America, and later in Aus- 

 tralia, New Zealand, South Africa, and similar places. It began on the 

 well-watered forested soils near the oceans and great rivers. With the 

 development of technology, it has spread to the interior of continents 

 along the railroads. Now most, although by no means all, of the good 

 land in temperate regions is occupied. We could expand considerably 

 in the United States, probably to a total of around 450 million acres, or 

 possibly even 500 million under conditions of reasonably full employ- 

 ment. This does not count some of the poor soil now in farms that 

 should be used for ranching and forestry. 



North of the temperate region in the region of Podzol soils, only 

 about one per cent of the land is cultivated. If we increased the per- 

 centage to ten, about 300 million acres of new soil would be available. 

 On the basis of experience in Finland and Scandinavia this seems 

 reasonable, provided transportation and industry are developed along 

 with the agriculture. At least when first cultivated, these soils would 

 not be so fertile as most of those in the temperate region, but experience 

 has demonstrated that they are responsive to management and can be 



