REVIEW QUESTIONS 267 



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

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

 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 may different methods of 

 farming influence the humus content of soils ? no. What may be 

 said regarding the importance of nitrogen as plant food? in. 

 What are the functions of nitrogen in plant nutrition? 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 experiments. 117. How did 

 field results compare with laboratory experiments? 118. In what 

 w r ays were the conditions of field experiments 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 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 ma}- the 

 nitrogen in soils vary ? 125. To what extent is nitrogen removed 

 in crops ? 126. To w r hat extent are nitrates, nitrites, and ammo- 

 nium 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 agricultural value is this 

 ratio ? 129. Under what conditions do soils gain in nitrogen con- 

 tent? 130. What methods of cultivation cause the most rapid de- 



