WHAT WATER DOES IN SOILS 57 



water then gets an " inky " taste from dissolved iron. Cop- 

 per, zinc, lead, and other metals rust in the same way. 

 Well waters commonly contain in 10,000 parts 3 or more parts 

 of solid matter. River waters contain about half as much. 



Water dissolves plant food. Among the substances 

 we find in water that has filtered through soil, the most n 

 abundant is usually lime, which is also one of the larg- 

 est ingredients of most plant ashes. Soil water also 

 contains some common salt, and if you boil a pint^ 1 

 of water until only a few teaspoonfuls are left, you can 

 taste the salt. There is also a little potash, the ' 

 substance that forms the chief part of the lye from 1 ; 

 wood ashes. All natural water also carries a little 

 phosphoric acid, which is a very important part of plant 

 ashes, as well as of animal bones. There is always 

 some carbonic and sulfuric acid, as well as some mag- 

 nesia and iron, and often much silica, the substance 

 which forms flint, quartz, and most sand. All these 

 substances (with some others) are found in plant ashes, 

 and, of course, have been taken up by the plant with the 

 water it takes from the soil. 



Since all plants take up these substances, we must 

 find out whether they need them for their growth; and 

 all experiments in trying to make plants grow without 

 them have shown that, with the exception of silica, 

 they are absolutely needed by plants. Hence we con- 

 sider and call them plant food, together with nitrogen, 

 which, as already stated, is mostly taken from humus by 



