REVIEW QUESTIONS 331 



ments ? 118. In what ways were the conditions of field experi- 

 ments different from those conducted in the laboratory ? 119. Give 

 the results of HellriegePs and Wilfarth's experiments. 120. What 

 is noticeable regarding the composition of clover-root nodules ? 121. 

 Of what agricultural value are the results of Hellriegel ? 122. What 

 is the source of the soil's nitrogen ? 123. How may the organic 

 nitrogen compounds of the soil vary as to complexity? 124. To 

 what extent may the nitrogen in soils vary ? 125. To what extent 

 is nitrogen removed in crops? 126. To what extent are nitrates, 

 nitrites, and ammonium compounds found in soils ? 127. To what 

 extent is nitrogen returned to the soil in rain water ? 128. How 

 may the ratio of nitrogen to carbon vary in soils ? Of what agricul- 

 tural value is this ratio ? 1 29. Under what conditions do soils gain 

 in nitrogen content ? 130. What methods of cultivation cause the 

 most rapid decline in the nitrogen content of soils, and how can a 

 nitrogen equilibrium be maintained in the soil? 131. What is 

 nitrification ? 132. What are the conditions necessary for nitrifica- 

 tion, and what are the food requirements of the nitrifying organ- 

 ism ? 133. Why is oxygen necessary for nitrification ? 134. How 

 do temperature, moisture, and sunlight influence this process ? 

 135. What part do calcium carbonate and other basic compounds 

 take in nitrification ? 136. How is nitrous acid produced ? 137. 

 What is denitrification ? 138. What other organisms are present in 

 soils besides those which produce nitrates, nitrites, and ammonia ? 



139. What chemical products do these various organisms produce ? 



140. Why are soils sometimes inoculated ,with organisms? When 

 is this necessary and when is it not ? 141. Why does summer 

 fallowing of rich land cause a loss of humus and nitrogen ? 142. 

 What influence have deep and shallow plowing, and spring and fall 

 plowing, upon the available soil nitrogen ? 143. Into what three 

 classes are nitrogenous fertilizers divided ? 144. How is dried blood 

 obtained ? What is its composition, and how is it used ? 145. 

 What is tankage ? How is it used, and how does it differ in com- 

 position from dried blood ? 146. What is flesh meal ? 147. What 



