MANUKES AND FERTILISERS. 21 



Pliny (62 A.D.) has told us that lime, marl, and 

 wood ashes were found to have a good effect on land. 

 These records show clearly that some at least of the 

 ancient cultivators knew of the fertilising value of 

 vegetable and animal refuse, and probably even 

 these early writers were not the first pioneers of the 

 subject of manures and manuring. An old Roman 

 maxim says that " the more you give to the land the 

 more it returns to you," and this presumably had 

 reference to the materials already indicated, for it is 

 not until the seventeenth century, when saltpetre 

 (nitrate of potash) was advised as a stimulant to the 

 growth of crops, that we find any fresh substance 

 being brought to notice as manure. 



No clear conception of the elements of plant food 

 was held until about the middle of last century, 

 when by analyses and experiments conducted along 

 scientific lines much valuable information was ob- 

 tained and the foundations laid upon which a rational 

 system of manuring has been gradually built up. 

 The requirements of crops became known, and many 

 substances now familiar under the name of 

 " artificial " manures were brought freely into use. 

 These substances have grown enormously in pop- 

 ularity as their properties and the remarkable 

 results to be achieved by their use have become 

 more generally recognised. 



The substances required by crops are known from 

 the composition of the ash of the various plants, and 

 their relative importance in plant nutrition has been 

 determined by minute and careful tests. These 

 substances may be divided into : 



1. Essential or necessary, which include 



