ELEMENTS OF AGRICULTURE. 
37 
than those grown upon stiff lands, because they are less 
watery. 
23. Silica is found in the ashes of almost all vegetables, 
but principally in those of the cereals. This is why it 
has been supposed that it concurs in the nutrition of plants, 
although from its nature it does not seem fit for this func» 
tion. In all cases, its principal function is to act mechan¬ 
ically upon the soil. 
QUESTIONS. 
1. Of what is silica composed? 
2. How is a silicious soil recognised ? 
3. Is it widely diffused throughout the earth ? 
4. Is it always injurious to land ? 
5. To what are crops growing in sandy land exposed ? 
6. How does it combine with manures ? 
7. Hoes sandy land require much work 1 
8. What is done to prevent the roots from being exposed? 
9. Under what circumstances ought sandy lands to be converted into 
meadows ? 
10. What cereals can be cultivated on them ? 
11. What crops thrive best on them ? 
12. In what bodies is silica found ? 
LESSON IV. 
CLAY. 
24. Clay is composed of silica and another body called 
alumine , or alumina. This last, when obtained in a pure 
state, is a powder of a whitish color ; it is distinguished 
by the facility with which it absorbs water. 
25. Land in which clay predominates is not everywhere 
of the same color. It is often of a reddish hue; this is 
owing to the oxyde of iron it contains. If it contains a 
large proportion of humus, it becomes black, and loses its 
color by calcination. 
26. Clayey soils have, as their distinguishing character 
4 
