THEORIES CONCERNING SOIL FERTILITY 301 



"Water, from Van Helmont's experiment, was by some great philosophers 

 thought to be it. But these were deceived, in not observing that water has 

 always in its intervals a charge of earth, from which no art can free it. 



"Air, because its spring, etc., is as necessary to the life of vegetables as the 

 vehicle of water is, some modern virtuosi have affirmed, from the same and 

 worse arguments than those of the water philosophers, that air is the food of 

 plants. . . . 



"Fire. No plant can live without heat, though different degrees of it be 

 necessary to different sorts of plants. Some are almost capable of keeping 

 company with the salamander, and do live in the hottest exposures of the hot 

 countries. Others have their abode with fishes under water, in cold climates; 

 for the sun has his influence, though weaker, upon the earth covered with 

 water, at a considerable depth, which appears by the effect the vicissitudes of 

 winter and summer have upon the subterraqueous vegetables. 



" But that fire is the food of plants, I do not know any author has affirmed, 

 except Mr. Lawrence, who says: ' They are true fire-eaters '; and even he does 

 not seem to intend that this expression of his should be taken literally." 



"Earth. That which nourishes and augments a plant, is the true food of it. 



"Every plant is earth, and the growth and true increase of a plant is the 

 addition of more earth." 



"Too much earth, or too fine,. can never possibly be given to roots . . . and 

 earth is so surely the food of all plants, that with the proper share of the other 

 elements, which each species of plants requires, I do not find but that any 

 common earth will nourish any plant." 



"The mouths, or lacteals, being situate, and opening in the convex super- 

 ficies of roots, they take their pabulum, being fine particles of earth, from the 

 superficies of the pores, or cavities, wherein the roots are included. . . . These 

 particles, which are the pabulum of plants, are so very minute and light, as not 

 to be singly attracted to the earth, if separated from those parts to which they 

 adhere, or with which they are in contact (like dust to a looking glass, turn it 

 upwards, or downwards, it will remain affixed to it), as these particles do to 

 those parts, until from thence removed by some agent. 



"A plant cannot separate these particles from the parts to which they adhere, 

 without the assistance of water, which helps to loosen them. 



"As to the fineness of the pabulum of plants, it is not unlikely that roots may 

 insume no grosser particles than those on which the colors of bodies depend; 

 but to discover the greatness of those corpuscles, Sir Isaac Newton thinks, will 

 require a microscope that with sufficient distinctness can represent objects five 

 or six hundred times bigger than at a foot distance they appear to the naked 

 eye." 



In general, Jet.hro Tull taught that the soil particles are the food 

 (pabulum) of plants, and that, if the soil were made sufficiently 

 fine by cultivation, the plants could then absorb these fine particles 

 of earth and produce large crops continuously. In answer to the 



