56 
ELEMENTS OF AGRICULTURE. 
than benefit the soil. This is the case when the sub-sou 
contains principles,such as the oxyde of iron, that may be 
fatal to vegetation. But generally in a few years these 
injurious properties disappear, after the elements brought 
to the surface have been acted upon by atmospheric in¬ 
fluences, and mixed with humus. 
91. The farmer sometimes contents himself (and per¬ 
haps it is the best plan) with merely stirring the sub-soil, 
without bringing it to the surface. It is then acted upon 
by the atmosphere, and gradually mixed with fertilizing 
influences. This operation is performed by a plough made 
for the purpose, called a sub-soil plough, which follows in 
the furrow immediately behind the ordinary plough. 
QUESTIONS. 
1 Are the mineralogical elements of the soil and sub-soil of the same 
nature ? 
2. How many kinds of sub-soil are there ? 
3. Is a clay sub-soil favorable to a clay soil ? 
4. What is the remedy for too much moisture in a clay soil ? 
5. Is a sandy soil favored by a clay sub-soil? 
6 Do frequent workings preserve moisture in the soil ? 
7. Is a sandy sub-soil favorable to a clay soil ? 
8. When the soil and sub-soil are both sandy,is it favorable ? 
9. Can a calcareous sub-soil improve the surface ? 
10. Which soils are most benefited by mixture with carbonate of lime ? 
11. In what state should the carbonate of-lime be to produce an effect 
sipon the soil ? 
12. In what circumstances would deep ploughing be disadvantageous ? 
13. How is the soil deepened without bringing the sub-soil to the surface ? 
LESSON XIII. 
THE EFFECT OF CLIMATE ON VEGETATION. 
92. The temperature requires great attention on the 
part of the cultivator ; for it is well known what influence 
it exercises on vegetation, and that it varies according tu 
