AGRICULTURAL CHEMISTRY. 181 



a quantity of potash, lime, phosphoric acid, and other fixed mineral 

 matters, and likewise more or less ammonia and nitric acid. With 

 every crop the field yields, its own stores of fertility arc drawn upon, 

 and, in fact, lessened, and after a certain number of crops are gath- 

 ered, the available food of most soils is so far diminished that the 

 succeeding crops fail of full development; in other words, the soil is 

 exhausted. By exhaustion in a practical sense is meant, be it noticed, 

 no absolute removal of plan! (nod. but such a relative diminution as 

 causes the harvests to fall below a medium or standard yield. 



It is the business of culture to replace this spent material, to restore 

 the capacity of the soil, to keep it up year after year to a remunera- 

 tive degree of productiveness. 



Jethro Tub, a. distinguished Englishman, who worked and wrote in 

 the last century, was led to adopt the theory — not at all improbable, 

 viewed from the scientific stand-point of his day — that the impalpably 

 fine particles of earth are the real food of vegetation, and accordingly 

 he sought to fit the soil for a more rapid and perfect nutrition by 

 pulverizing it. He introduced the horse-hoe, or cultivator, into 

 English husbandry, and actually succeeded, by the diligent use of his 

 improved implements, and by a. peculiar mode of occupying his field, 

 in obviating the necessity of any manures and in raising successive 

 crops on the same field uninterruptedly for twelve years. He failed, 

 however, in maintaining this system for a longer time, having adopted 

 one fatal rule, "never plough below the staple." It is but just to 

 the memory of this eminent agricultural philosopher to explain why 

 he adhered to a notion to us so absurd. Tull was aware of the im- 

 portant part played by the atmosphere in the nutrition of plants. 

 The use of stirring and pulverizing the soil was to enable the parti- 

 cles of earth to attract from the atmosphere "the nitre or acid spirit 

 of the air," which, in his view, further dissolves and prepares the 

 soil to support vegetation. He had no chemistry to teach him that 

 the indispensable mineral matters of the soil exist in it in such 

 minute quantity, and are therefore liable to exhaustion. He had no 

 analytical data to reveal the difference between the chemical statics 

 of the vineyard — from the sagacious observation of which his theory 

 originated — and the wheat field, which more largely robs the soil of 

 alkalies and phosphates, and so he found it reasonable to use only 

 that portion of the soil — the staple or usual tilth — to which the atmos- 

 phere has obvious access. 



The system of Tull has, however, been revived, and, with the modi- 

 fications suggested by. modern science, has been eminently successful 

 in the hand of its ingenious advocate, the Rev. S. Smith, of Lois 

 Weedon, Northamptonshire, England. Mr. Smith has produced large 

 wdieat crops continuously on the same soil for a series of years by 

 simply laying off his fields in strips five feet wide, and -rowing his 

 crops in drills, with frequent and deep hoeing, on alternate strips in 

 successive years. The tillage of the vacant strip this year prepares 

 it to sustain a crop next year — enables the solution and absorption of 

 'food enough to feed a full crop. 



By this" plan of culture Mr. Smith raised the yield of his wheat 



