PRINCIPLES OF PERMANENT AGRICULTURE 23 



in the mixed-farming districts is absolutely essen- 

 tial to intelligent farm management. Yet none of 

 these in itself restores anything to the soil from 

 which vast quantities of material have been taken. 

 The mere rotation of crops, because it tends to 

 larger production, actually wears out the soil faster, 

 the large crops thus grown taking away more of 

 the available fertility than a small crop would do. 

 Rotation in itself tends to exhaust the soil faster 

 than would otherwise be done. 



PRINCIPLES OF SOIL MAINTENANCE 



There are certain principles and practices, how- 

 ever, which so far have stood the test of time and 

 which are now believed to make possible a per- 

 manent agriculture, while at the same time produc- 

 ing constantly increasing crops annually, and by 

 means of which the fertility of most land may not 

 only be maintained, but constantly increased. The 

 three elements which are most likely to become 

 deficient in the soil and which are absolutely neces- 

 sary to the growth of the plants are nitrogen, phos- 

 phoric acid and potash. Several other elements are 

 used in the growth of the plant in small quantities, 

 but they usually occur in sufficient amounts in all 

 soils, with the exception of calcium, and this can be 

 added in the form of land plaster or common lime. 

 Most American soils contain sufficient potash to last 

 for several hundred years, although occasionally 

 there are circumstances which justify the addition 

 of this element to the soil. Generally speaking, the 

 fertility problem narrows itself down to the main- 

 tenance of the supply of nitrogen and phosphorus. 

 } Humus is decayed organic matter, the function of 

 t which is partly chemical and partly mechanical. 



