124 OF FERTILIZERS. 



398. It is probable that plants sometimes obtain min 

 eral elements which they need from the rocks them 

 selves ; and there are some facts which make it certain 

 that they do so. We frequently find, in meadows, smooth 

 lime-stones with their surfaces covered with a net work 

 of small furrows ; and we find that each furrow corres 

 ponds to a rootlet, which appears as if it had eaten into 

 the stone. So, lichens grow upon the surface of bare 

 rocks ; and forest trees form vast trunks, full of potash 

 and other salts, on the rocky soils of hills from which all 

 the loose soil has been washed. It seems probable that 

 their rootlets have the power of decomposing the rock 

 and taking potash from the felspar or mica they find in 

 them. Liebig. 



399. Is it necessary that each particular clement of 

 plants should be present in the soil ? Or, if one be 

 wanting, cannot plants be sustained by the others ? 



The agriculturist requires eight substances in his soil, 

 that all the plants may flourish luxuriently, and his fields 

 produce the largest crops. These eight substances are like 

 eight links of a chain round a wheel. If one is weak, 

 the chain is soon broken, and the missing link is always 

 the most important, without which the machine cannot be 

 put in motion by the wheel. The strength of the chain 

 depends on the weakest of the links. Liebig. 



Those eight are phosphoric acid, potash, silicic acid, 

 sulphuric acid, lime, magnesia, iron, and chloride of sodi 

 um. All these are essential to the growth of plants. 

 Still more essential are oxygen, hydrogen, nitrogen and 

 carbon ; but these are always supplied by the atmosphere, 

 in the form of water, ammonia and carbonic acid. 



400. If we cannot obtain stable manure or other ani 

 mal manure, how is the want to be supplied ? Chemists 



