EXPERIMENTS IN SOIL CHEMISTRY 271 



A simple method of testing soil is as follows: Make a 

 paste out of soil and water. Place one end of a small 

 strip of litmus paper in the paste. Allow it to stand for an 

 hour or more, and then note whether or not the color has 

 changed. This should be tried several times before a con- 

 clusion is reached. Have the children bring in small 

 quantities of soil from fields and gardens. Test these 

 different kinds for acidity and alkalinity. 



If a soil is acid, mix a small quantity of lime with it and 

 test again. If a sufficient quantity of lime is applied the 

 acid is neutralized and the soil is sweetened. 



Let us now look over the remaining substances given 

 to plants by the soil and try to get some idea of how they 

 exist in the soil. Iron is found chiefly in the form of iron 

 oxides. What are these combinations of? Silicon is 

 largely in sand. Some plants seem to be able to get along 

 without this. Calcium is in the limestones. Burn a piece 

 of lime rock. It now becomes quicklime. Is it easily 

 broken ? Put it in water to see if it will dissolve. Potas- 

 sium is in the form of potassium carbonate. Chlorine is in 

 combination with sodium, while the sodium is frequently 

 obtained from another compound, sodium bicarbonate. 

 Phosphorus is in the form of phosphates and phosphoric 

 acid. 



It is easy to see that sodium cannot exist in the soil as 

 an element. Place a small piece on the surface of water in 

 a pan or dish. What does it do? It unites at once with 

 the water. In the laboratory it must be kept under kero- 

 sene or naphtha. 



It may be shown also that phosphorus cannot exist as 

 an element in the soil. In the laboratory it is kept under 



