33O SOILS AND FERTILIZERS 



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? 

 What is black alkali ? 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 ? What are acid 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 ob- 

 tained ? 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 ? 



CHAPTER TV 



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

 plant food ? 1 1 1 . 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 experi- 



