ELEMENTS OF AGRICULTURE. 
43 
confirmed by facts. It is probable that carbonate of mag¬ 
nesia acts upon the soil like carbonated lime. 
45. Iron is found in the soil in the state of an oxyde; 
that is to say, united with oxygen. It is sometimes, also, 
met with as a carbonate. It rarely exists in large quantity 
in the arable soil; if it existed in proportions rather strong, 
it would render the last entirely barren. 
46. Iron, through its property of coloring soils, causes 
them to absorb more heat; for it is well known that white 
substances are not so readily warmed by the solar rays as 
black ones. 
47. Iron is more or less injurious according to the de¬ 
gree of oxydation. When it does not contain the whole 
of the oxygen with which it is capable of combining, it 
injures vegetation. 
QUESTIONS. 
1. What substances besides silica and clay are sometimes to be met 
with in the soil ? 
2. Is plaster of importance to agriculture 1 
3. Is it generally disseminated throughout nature ? 
4. Of what is marl composed ? 
5. Which are the richest marls ? 
6. To what use are they put ? 
7. How do we detect the presence of calcareous principles in a soil? 
8. What is magnesia 1 
9. In what state is iron found in the soil T 
10. Is a ferruginous soil fertile ? 
11. How can iron render the earth warmer? 
LESSON VII. 
THE ORGANIC PARTS OF THE SOIL. 
48. That a soil should be productive, it is not alone 
sufficient that it should contain, in suitable proportions, 
the mineral substances that we have studied in the pre* 
