REVIEW QUESTIONS 277 



value of a soil analysis? 83. What are some of the important facts 

 to observe in interpreting results of soil analysis ? 84. Under what 

 conditions are the results most valuable? 85. Do the terms volatile 

 matter and organic matter mean the same ? 86. How may organic 

 acids be employed in soil analysis? 87. Why are dilute organic 

 acids used ? 88. Is the plant food equally distributed in both sur- 

 face and subsoil ? 89. Do different grades of soil particles, from 

 the same soil, have the same composition? 90. What are "alkali 

 soils " ? 91. Why is the alkali sometimes in the form of a crust? 

 92. Are all soils with white coating strongly alkaline? 93. Give 

 the treatment for improving an alkali soil. 94. How may a small 

 " alkali spot " be treated ? 95. What are the sources of the organic 

 compounds of soils ? 96. How may the organic compounds of the 

 soil be classified? 97. Explain the term humus. 98. How is the 

 humus of the soil obtained ? 99. What is humification ? What is 

 a humate ? How are humates produced in the soil ? 100. Explain 

 how different materials produce humates of different value. 101. 

 Arrange in order of agricultural value the humates produced from 

 the following materials : Oat straw, sawdust, meat scraps, sugar, 

 clover. 102. Of what value are the humates as plant food ? 103. 

 How much plant food is present in soils in humate forms ? 104. 

 What agencies cause a decrease of the humus content of soils? 

 105. To what extent does humus influence the physical properties 

 of soils? 106. What is humic acid? 107. What soils are most 

 liable to be in need of humus? When are soils not in need of 

 humus? 108. In what ways does the humus of long-cultivated 

 soils differ from that of new soils ? 109. How many different 

 methods of farming influence the humus content of soils? 



CHAPTER IV. 



no. What may be said regarding the importance of nitrogen as 

 plant food? in. What are the functions of nitrogen in plant nu- 

 trition ? 112. How may the foliage indicate a lack or an excess of 

 this element? 113. In what three ways did Boussingault conduct 

 experiments relating to the assimilation of the free nitrogen of the 

 air? 114. What were his results? 115. What conclusions did 

 Ville reach ? 116. Give the results of Lawes and Gilbert's experi- 

 ments. 117. How did field results compare with laboratory exper- 

 iments? 1 1 8. In what ways were the conditions of field experi- 

 ments different from those conducted in the laboratory? 119. 

 Give the results of Hellriegel's and Wilfarth's experiments. 120. 

 What is noticeable regarding the composition of clover root 

 nodules? 121. Of what agricultural value are the results of Hell- 

 riegel ? 122. What is the source of the soil's nitrogen ? 123. How 

 may the organic nitrogen compounds of the soil vary as to com- 

 plexity ? 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 



