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SECOND BOOK. 
3. But the ease is very different when, year after 
year, we carry the crops away from the soil on 
which they have been raised. Whether we take 
the tubers of yams, the stems of sugar-cane, the 
trash and ears of corn, the fruits of oranges and 
bananas, or any other produce, we thereby carry 
off what the plant worked up from the soil. And 
of course this leaves the soil so much the poorer. 
So also, in grazing, sheep and cattle carry off in 
their flesh and bones the material which the grass 
drew up from the soil. 
4. Thus, little by little, the soil may be robbed, 
until it happens that some of the kinds of plant- 
food begin to run short. Then the plants grov 
weak and sickly, and are no longer able to yield 
good crops. When this happens we say the soil is 
exhausted. 
5. In Jamaica, when the ground was first cleared 
for growing coffee or sugar, fine crops were raised 
year after year for a time. But some of the plant- 
foods were thus used up faster than a fresh supply 
could be formed; so that the crops grew poor, and 
had to be given up for a time. 
6. Every plant requires several kinds of food, and 
if any one of these runs short the plant cannot 
thrive, although all the other kinds may be within 
reach in abundance. 
7 . Suppose you were amusing yourself by threading 
a large number of blue, red, white, and green beads 
