HISTORY OF COMMERCIAL FERTILIZERS 321 



crops was published, in 1669, by Sir Kenelm Digby. He 

 says, "By the help of plain salt petre, diluted in water, 

 and mingled with some other fit earthly substance, 

 that may familiarize it a little with the corn into which 

 I endeavored to introduce it, I have made the barrenest 

 ground far outgo the richest in giving a prodigiously 

 plentiful harvest." His dissertation does not, however, 

 show any true conception of the reason for the increase 

 in the crop through the use of this fertilizer. In fact, 

 the want of any real knowledge at that time of the com- 

 position of the plant would have made this impossible. 



In 1804, Theodore de Sausure published his chemical 

 researches upon plants, in which he, for the first time, 

 called attention to the significance of the ash ingredients 

 of plants, and pointed out that without them plant-life 

 is impossible, and further, that only the ash of the plant 

 tissue is derived from the soil. 



Justus von Liebig, in his writings published about 

 1840, emphasized still more strongly the importance 

 of mineral matter in the plant, and its extraction from 

 the soil. He refuted the theory, at that time popular, 

 that plants absorb their carbon from humus, but made 

 the mistake of attaching little importance to the pres- 

 ence of humus in the soil. He showed the importance 

 of potassium and phosphorus in manures, but, in his 

 later expressions, failed to appreciate the value of 

 nitrogenous manures, holding that a sufficient amount 

 is washed from the atmosphere in the form of ammonia. 



A true conception of the necessity for a supply of 

 combined nitrogen in the soil was even at that time enter- 

 tained by Boussingault and by Sir John Lawes, although 



u 



