342 



PLANTS WITHOUT CHLOROPHYLL 



first crop. For this reason, clover is often grown on land in which 

 it is proposed to plant corn later, the nitrates left in the soil giving 

 nourishment to the young corn plants. In scientifically managed 

 farms, different crops are planted in succession in a given field in 

 different years so that one crop may replace some of the elements 

 taken from the soil by the previous crop. This is known as rota- 

 tion of crops. ^ The an- 



Firsf /ezr 



^^Second year 



Third/ear 



Fourth 



nual yield of the average 

 farm may thus be greatly 

 increased. 



Five of the elements 

 necessary to the life of 

 the plant which may be 

 taken out of the soil by 

 constant use are calcium, 

 nitrogen, phosphorus, po- 

 tassium, and sulphur. 

 Several methods are used 

 by the farmer to prevent 

 the exhaustion of these 

 and other raw food mate- 

 rials from the soil. One 

 method known as fallow- 

 in this rotation of crops, notice that clover is ifig [q tO allow the SOil tO 

 used once in three years. This helps to maintain . . ,, ., , 



the supply of nitrates in the soil. remam idle Until bacteria 



and oxidation have re- 

 newed the chemical materials used by the plants. This is an expen- 

 sive method, if land is high-priced. The more common method 

 of enriching soil is by means of fertilizers or materials rich in plant 

 food. Manure is most frequently used, but many artificial fertiliz- 

 ers, most of which contain nitrogen in the form of some nitrate, are 

 used because they can be more easily transported and sold. Such 

 are ground bone, guano (bird manure), nitrate of soda, and many 

 others. Most fertilizers contain other important raw food ma- 

 terials for plants, especially potash and phosphoric acid. Both 

 of these substances are made soluble by the action of the carbon 



1 Crop rotation is not only a process to conserve the fertility of the soil, but 

 also a sanitary measure to prevent infection of the soil. 



