4 THE SCIENTIFIC PRINCIPLES 



Professor Daubeny, in his lectures on Roman 

 Agriculture, quotes Cato as having said — 



' If I am asked, what is the first point in good hus- 

 bandry, I answer, good ploughing ; what the second, 

 ploughing of any kind ; what the third, manuring/ 



We have thus evidence of the relatively little 

 esteem in which manuring was held in Italy two 

 thousand years ago ; and in the fact that so much 

 more value was set upon the mechanical operations 

 we have evidence that the resources of the soil itself 

 were far from being exhausted, and only required 

 such means to be taken to render them available. 



There is nevertheless evidence that, long before the 

 time of Cato, it was sought to restore fertility to the 

 soil by practices the efficacy of which is still fully 

 recognised ; though the explanation of some of the 

 phenomena involved is still a matter of controversy. 

 Such, for example, was the growth of various crops of 

 the Leguminous family, sometimes to be in great part 

 removed, but in others to be at once ploughed into 

 the land, by either of which methods the growth of 

 succeeding crops was enhanced. This subject will 

 receive detailed consideration further on. 



But although manuring, in various ways, has thus 

 been so long recognised as desirable, it is in fact only 

 in quite recent times that the rationale of such prac- 

 tices could be at all satisfactorily explained. To this 

 end it was obviously essential not only to know the 

 composition of the vegetable products grown, but 

 something of the sources of their various constituents 

 — whether these must be derived from the soil, or 

 whether from the atmosphere, or from water % 



What then is the composition, and what are the 

 sources of the constituents of vegetable products \ 



When a vegetable (or animal) substance is burnt, 



