24 ' ELEMENTARY AGRICULTURE 



spoonful of it in a cup about one-fourth full of water 

 without stirring. Add a little vinegar and notice what 

 takes place. The soda disappears because the acid 

 acts on it. Gas is given off very rapidly, causing it 

 to bubble and "foam." This gas is carbon dioxide, 

 one of the four acids named in the last chapter. These 

 acids help the water to dissolve the plant foods in the 

 soil and are themselves taken in as plant foods. 



*Silica. Silica needs no discussion. By some it is 

 believed to be the food that gives stiffness to the stalks 

 of barley, oats and other grains, which, when grown 

 on rich bottom land, usually "lodge" partly because 

 they are unable to get sufficient sand from the soil. 



Ammonia. Ammonia is known by its odor. It is 

 used for cleaning clothing and windows. If you go 

 into the barn on a warm morning when the barn has 

 been closed during the night you will get a strong 

 odor of ammonia from the horse manure. Ammonia 

 is always given off to the air when animal matter 

 decays. It contains the element, nitrogen, so essential 

 to plant growth. 



Carbon Dioxide. Carbon dioxide is a plant food 

 and it is also found in the air. You will remember it as 

 the gas that came off when you put vinegar on soda. 

 This gas is always given off to the air when vegetable 

 matter burns or decays. You are throwing it off from 

 your lungs with every breath that you breathe. So, 

 too, are all animals. Here is a simple test for it that 

 any child can easily make. Put a piece of fresh lime in 

 some water, shake well and let it stand until it settles 

 * Sand. 



