14 BULLETIN 784, U. S. DEPARTMENT OF AGRICULTURE. 
soil. The thorough culture, late digging, and winter exposure of soil, 
especially in the Northern States, rapidly use up the soil organic 
matter. The cheapest and easiest way of replacement is by rotation 
with grass, clover, and other hay crops which have a money value 
as food for animals, and also leave large amounts of organic matter 
in the soil from their roots and stubble, as well as that in the manure 
from the hay feed. 
6. Different plants draw on the plant food in the soil in varying 
proportions, consequently a rotation of crops utilizes the plant food 
to better advantage than a single crop grown continuously. 
7. Though the potato sends its roots deeper than some other crops, 
others, like clover and alfalfa, root much deeper. These bring up 
fertility from the subsoil, and the roots of the potatoes following are 
able to grow lower, even in hard clay soils. 1 
8. The theory has been advanced that plants in their growth throw 
off in the soil toxic substances which, by accumulation, become inju- 
rious to succeeding crops of the same plant, but not to others. Rota- 
tion avoids this possible danger. 
9. The physical condition of the soil for succeeding crops is im- 
proved by the thorough culture given the potato. Examples of this 
are the use of wheat following potatoes without plowing in the fall, 
and oats without plowing in the spring. 
The nef return of the rotation as a whole must be the deciding 
factor in the choice of crops. The potato gives such large cash 
returns to the acre and responds so well to the use of fertilizers, 
that usually it is the most important crop of the rotation in which 
it is grown. Wherever this is true, the choice of other crops to go 
with it is largely governed by the effect on the soil for the production 
of the potato. 
Some suggested rotations are: 
1. Potatoes — oats — grass _and clover — grass and clover — potatoes. 
2. Potatoes — wheat — clover — potatoes. 
3. Clover or alfalfa — a small grain — potatoes. 
4. Potatoes — wheat — clover — potatoes. 
5. Potatoes — small grain — clover. 
6. Potatoes — corn — cowpeas. 
7. Winter rye — clover — potatoes. 
8. Small grain two years — clover — potatoes. 
Practical exercises. — Have the class make a study of the district 
farm practice in potato growing. What are the common rotations? 
What examples can be found of the continuous cropping? What dif- 
ferences in yields may be noticed ? Does the acreage in potatoes 
on the farms of the district justify any plan of rotation of crops? 
(See potato survey. If possible, visit one or more farms where rota- 
1 Colorado Sta. Bui. 216. 
