282 DRY-FARMING 



A clear distinction must be made between the 

 total and available plant-food. The essential plant- 

 foods often occur in insoluble combinations, value- 

 less to plants ; onl}' the plant-foods that are soluble 

 in the soil-water or in the juices of plant roots are 

 of value to plants. It is true that practically all 

 soils contain all the indispensable plant-foods; it 

 is also true, however, that in most soils they are 

 present, as available plant-foods, in comparatively 

 small cjuantities. When crojxs are removed from 

 the land year after year, without any return being 

 made, it naturally follows that under ordinary con- 

 ditions the amount of available plant-food is dimin- 

 ished, with a strong probability of a corresponding 

 diminution in crop-producing power. In fact, the 

 soils of many of the older countries have been per- 

 manently injured by continuous crt)pping, with 

 nothing returned, ])racticed thnjugh centui'ies. Even 

 in many of the younger states, continuous cropping 

 to wheat or other crops for a generation or less has 

 resulted in a large decrease in the crop yield. 



Practice and experiment have shown that such 

 diminishing fertility may I)e retarded or wholly 

 avoided, first, by so working or cultivating the soil 

 as to set free much of the insoluble plant-food and, 

 secondly, by returning to the soil all or part of the 

 plant-food taken away. The recent development 

 of the commercial fertilizer industrj- is a response to 

 this truth. It may be said that, so far as the agri- 



