Food from the Soil 1 1 r 



entirely lose it; that is to say, no one of our senses is 

 keen enough to detect it. We can neither see, nor taste, 

 nor smell it. But of course it is there, and we can find it 

 again by adding starch, which is turned to a brilliant blue 

 by coming in contact with even this minute quantity. 



But the iodine contained in sea-water is less even than 

 this — it is less even than the hundredth part of this infini- 

 tesimal amount. And yet the sea-weed manages to 

 extract it. And although plants take their mineral food 

 in such weak dilutions that we cannot detect its presence 

 either by taste or smell, and might be inclined to think 

 that it can matter very little what it is, yet they are dis- 

 criminating; and their roots have to some extent the 

 power of choosing what they will or will not take up. 



This is evident from the fact that plants growing side 

 by side will take up different food, or take it in very differ- 

 ent proportions. 



There is, for instance, the common reed and the com- 

 mon species of moss, which both grow in bogs. The soil 

 is dissolved by water and gases equally for both, and both 

 take up a good deal of dissolved flint, or silica; but the 

 reed takes up also a very small quantity of salt, a little 

 more, but still a very small quantity, of iron, no soda, a 

 little magnesia, and a great deal of phosphoric acid; 

 whereas the moss, which grows close by, takes very little 

 either of phosphoric acid, magnesia, or salt, but some soda, 

 and much iron. 



The same thing is also true of the farmer's crops, and 

 it is for this reason that he varies them, not growing the 

 same crop year after year, or even two years running, on 

 the same soil, lest it should be exhausted and unable to 

 feed them. 



