274 NATURE STUDY AND AGRICULTURE 



There are a number of agencies that tend to make soils 

 sour, especially soils that are constantly undergoing cultiva- 

 tion. In our study of fungi we discussed the bacteria that 

 cause organic matter in the soil to decay. Now as this 

 matter decays, acids of various kinds are given off. An- 

 other acid in the soil is carbon dioxide, some of which is 

 carried into the soil by the rain. Sulphuric and nitric acids 

 are also found in soils. Some of the nitric acid is formed 

 in the following way: When stable manure is plowed into 

 the soil certain bacteria feed upon it, making it decay, and 

 at the same time making part of it into ammonia. Other 

 bacteria feed upon the ammonia, making part of it into 

 nitric acid. Some of this acid remains in the soil, but some 

 of it unites with potassium or magnesium and forms 

 nitrates. It is in the form of these nitrates that plants 

 obtain the nitrogen from the soil. 



From our study of clover and other legumes we know 

 that nitrogen may be also supplied by growing those plants 

 that produce root tubercles. 



If possible, add to your collection samples of the com- 

 mercial fertilizers. In connection with this work a num- 

 ber of experiments in pot culture should be started. The 

 following is a list that has been tried successfully. Others 

 may suggest themselves to you. 



Have the children bring in some soil from a field or 

 garden near the schoolhouse. If you have a school garden 

 choose part of the soil from that. Use flower pots seven 

 or eight inches in diameter or small tin buckets or boxes. 

 If the latter are used be sure to provide for drainage. 

 Plant oats, or spring wheat, or some- flowering plants like 

 nasturtium or zinnia. 



