62 HINTS ON SOILS AND FERTILIZERS 
again as a crop to illustrate the point. Most 
of us are quite familiar with the potato dis¬ 
ease known as potato scab. This is a disease 
that attacks the skin of the potato, causing a 
rough appearance of the skin, and lowering the 
value of the potato materially. This is a 
fungus disease, and, we might say, “lives” in 
the ground over winter, and it is usually spread 
by planting potatoes that are affected with 
scab. If we raise one crop of potatoes on a 
certain piece of land one year, and the result¬ 
ing potatoes are “scabby,” we should not raise 
another crop of potatoes on this same piece for 
about four years afterwards, if we wish to eradi¬ 
cate the disease, as the next crop of potatoes, 
if planted on this land the following year, will 
acquire this scab from the ground, whether 
we plant potatoes free from scab, or not. Hence, 
we can see that rotation of crops helps in more 
ways than one to produce better crops. 
It is a well-known fact among farmers that 
one of the most effective ways of killing out 
weeds, is to keep the field in cultivated crops 
for a few years. By rotating the crops in a 
certain field that is unusually infested with 
some noxious weed, such as quack grass, the 
weeds can soon be eradicated, if an earnest ef¬ 
fort is put forth, not only with the cultivators, 
but by hoeing between the plants in the same 
row. If, however, a forage crop, or a crop that 
is not capable of being cultivated, is allowed to 
grow on a weedy piece for a number of years, 
then the weeds are liable to finally take pos¬ 
session of the field, unless an unusually vigor¬ 
ous stand of the forage crop is maintained. 
