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; only 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 quantities. When crops 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 cropping, with 
nothing returned, practiced through centuries. 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 be 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 industry is a response to 
this truth. It may be said that, so far as the agri- 
