PRINCIPLES OF PERMANENT AGRICULTURE 21 



longer be made to pay dividends on a reasonable 

 valuation. 



SOIL MAINTENANCE AND MANAGEMENT 



The questions pei taining to soil management and 

 soil fertility are the most unsatisfactory to discuss 

 of any phase of agricultural science. They offer 

 the most complex problems and are influenced by 

 the most widely varying factors of any phase of 

 agriculture. These questions have occupied the at- 

 tention of writers and scientists since the days of 

 the Roman republic. The earliest agricultural in- 

 vestigators and writers gave especial attention to 

 this phase of the science, and various infallible 

 formulas have been promulgated to perpetuate 

 maximum production. One of the oldest theories 

 for the maintenance of maximum production is the 

 doctrine of continuous culture advocated by Jethro 

 Tull of England. By deep plowing and continuous 

 intensive cultivation of the soil this gentleman was 

 able to produce large crops, without rotation and 

 without the addition of any outside material for a 

 considerable number of years, upon the same plot 

 of land, and he concluded from this, and widely pub- 

 lished the conclusion, that "tillage is manure," and 

 that thorough cultivation is all that is needed to 

 produce maximum crops indefinitely. 



The fact is as true today as it was then that till- 

 age of this sort is an absolutely correct agricultural 

 practice, but the idea that this alone suffices to 

 maintain fertility was long ago abandoned. With 

 the advance of science, it became possible to 

 analyze the different soils and to determine accu- 

 rately not only the different elements contained 

 therein, but to compute exactly in what proper- 



