38 SOIL AND SOIL PREPARATION 



Suggestion. Mix the last residue with water and filter 

 it. Evaporate the filtrate to drive off the water. The re- 

 sulting material will show the amount of soluble mineral 

 contained in the soil. The soluble parts are of course the 

 plant foods. The insoluble residue will be left in the filter. 



Compare different soils as in Experiment 24 to determine 

 which are richest in soluble mineral salts. The relative 

 fertility of a given soil can thus be determined. 



For comparison, the following table may be made : 



1. First weight 16 oz. 



2. Loss through drying 



3. Amount of water 



4. Loss through burning 



5. Amount of organic material 



6. Final weight 



(mineral matter) 



7. (a) Insoluble 



8. (6) Soluble (plant food) 



Note. See action of roots in producing acids which dissolve 

 minerals not soluble in water (Ex. 84), thus changing them to liquid 

 form so that they may be absorbed. 



26. CONTENTS OF SOILS 

 Object. To show that there is air in soil. 



Apparatus. Straight glass vessels such as battery jars or grad- 

 uate glasses, soils of various kinds, and water. 



Method. Accurately mark with a rubber band or paster 

 a level one half the height of the jars. Fill the jars with 

 water to the mark and then add equal volume of soil slowly 

 to prevent slopping. Both soil and water should be meas- 

 ured first to insure accuracy. If the volume of the mixture 



