PLANT FOODS 27 



loss. Tobacco and root crops, being so much heavier, 

 exhaust the soil faster than small grains. 



But worn-out soil does not mean soil in which all 

 the different kinds of plant foods are used up. In 

 fact, soil usually contains all plant foods in inexhaust- 

 ible quantities with but three exceptions, namely: 

 potash, phosphoric acid and the nitrogen found in 

 ammonia. To restore the fertility of the soil means 

 only to restore these three substances. The general 

 rule for fertilizing soils will be taken up later. 



Experimental Study of Plant Foods. 



1. Make a collection of the following plant foods: 

 Lime, iron-rust, soda, ammonia, magnesia, acid, sul- 

 phur, and sand. Place each specimen in a small bottle 

 and label properly. 



2. Potash may be easily prepared from wood 

 ashes. Place about two quarts of wood ashes in a pan, 

 cover with water, and let stand for a few hours, stir- 

 ring frequently. Then allow the ashes to settle and 

 pour off the clear liquid into another tin dish. Place 

 this lye on the stove and evaporate all the water. The 

 dry powder found on the bottom of the dish is potash. 

 A little potash dissolved in water makes it feel soapy. 

 This "soapy feel" is the test for potash. Put some of 

 the potash in a small bottle, label, and add to the 

 collection. 



3. It is very difficult to obtain uncombined phos- 

 phoric acid. The easiest way to procure it is in com- 

 bination with lime. Burn a bone to whiteness, crumble 



