CHEMICAL CHANGES 2IQ 



organisms are essential to the process, but they cannot thrive un- 

 less the conditions are favorable to their activity, whether they 

 are already present in the soil, or introduced in the manure. Such 

 conditions are, either a diminished supply of oxygen, as by con- 

 solidation of the soil or by saturation with water, or a very large 

 quantity of oxidizable organic matter. 



Lipman 1 determines the "denitrifying power" of a soil by seed- 

 ing 10 grams, of it into 100 cc. of a neutral solution containing 

 definite amounts of nitrates, dextrose, citric acid, and nutrient 

 salts. When the nitrates have disappeared (in about 10 days); 

 the total nitrogen is estimated in the solution. The percentage of 

 nitrogen lost from the various cultures is taken as a measure of 

 denitrifying power of the soil. The solution used is as follows : 



1,000.0 cc. water 



2.0 grams magnesium sulphate 



2.0 " potassium phosphate 



i.o " potassium nitrate 



0.2 " calcium chloride 



5.0 " citric acid 



2.0 drops 10 per cent, ferric chloride. 



Neutralize while boiling with sodium hydroxide and add 2 

 grams dextrose. 



The soil may also be used as a medium in which to grow the 

 bacteria for the study of denitrification. 



Assimilation of Nitrates and Ammonia. By growing plants in the 

 solution of a nitrate, it is easily proved that they have the power to 

 absorb it, and as the plant grows vigorously (when other necessary 

 elements are present), the nitrogen must be in a form to be 

 utilized. The nitrate ion may be completely removed from solu- 

 tion when the other nutrients are present. Plants have also the 

 power to take up an excess of nitrates. Small amounts of nitrates 

 are often found in plants. An instance is on record in which corn 

 took up so much that crystals of potassium nitrate would fall out 

 when the stalk was rapped on a table. Nitrates appear to be a 

 form of nourishment most eminently fitted to all cultivated 

 plants. They are easily and rapidly taken up by plants. All 

 1 Report New Jersey Exp. Sta., 1907, p. 179. 



