490 SOILS: PROPERTIES AND MANAGEMENT 



idea probably originated through the observation that 

 farm manure, which was the only manure in use at that 

 time, made the soil less cloddy. 



It has been argued, notably by Jethro Tull, 1 that since 

 tillage pulverizes the soil it may be used as a substitute 

 for manures. There are, however, conditions aside from 

 tilth that are influenced by manures, and good tilth alone 

 will not suffice to maintain a permanently intensive agri- 

 culture. It is true in the United States, as it is in Europe, 

 that a large consumption of manures goes hand in hand 

 with a highly developed and intensive system of farming. 



411. Development of the idea of the nutrient function 

 of manures. — While the use of animal excrement on cul- 

 tivated soils was practiced as far back as systematic agri- 

 culture can be definitely traced, the earliest record of 

 the use of mineral salts for increasing the yield of crops 

 was published in 1669 by Sir Kenelm Digby. 2 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 har- 

 vest." 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 composition of 

 the plant would have made this impossible. 



In 1804, Theodore de Saussure 3 published his chemical 



1 Tull, Jethro. Horse-Hoeing Husbandry. London. 1829. 



2 Digby, Kenelm. A Discourse Concerning the Vegetation 

 of Plants. London. 1669. 



3 Saussure, Theodore de. Recherches Chimiques sur la 

 Vegetation. Paris. 1804. 



