PLANT FOODS 147 



its plant food, and it will soon begin to show evidence 

 of being ' ' worn out. ' ' Plant foods are being continually 

 used up by the growing plants, and removed with them, 

 and none are returned to take their place. The heavier 

 the crop the greater will be the 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: pot- 

 ash, phosphoric acid and the nitrogen found in am- 

 monia. To restore the fertility of the soil means only 

 to restore these three substances. 



Practical Exercises 

 1, Collection of Plant Foods 



Place specimens of the following plant foods in small 

 bottles and label properly: Lime, muriate or sulphate 

 of potash, phosphoric acid, nitrate of soda, iron-rust, 

 magnesia, sulphur, and pure sand. 



2. Experiment with Plant Food Tablets 



A plant food tablet has been prepared which contains 

 the food essential to plant growth. Each tablet is com- 

 posed of the following ingredients : 



Common salt (sodium chloride), 2y 2 grains. 



Plaster of Paris (calcium sulphate), 2 l / 2 grains. 



Epsom salts (magnesium sulphate), 2y 2 grains. 



Phosphate of lime (calcium phosphate), 2y 2 grains. 



Salt-petre (postassium nitrate), 5 grains. 



Compounds of iron and chlorine (ferric chloride), 

 1-10 grain. 



