PROPER ROTATION INCREASES ORGANIC MATTER 265 



Some potato growers have experienced the destructive effect 

 of potato scab on fields growing potatoes continuously (Fig. 168). 

 This disease can be prevented through seed disinfection; but 

 when the soil is allowed to become infested by planting untreated, 

 scabby potatoes, no remedy is so effective as crop rotation in 

 eliminating the disease. The unfortunate experience of the pioneer 

 farmer in growing wheat continuously on the same land might 

 well be remembered as a warning against the one-crop system. 



Proper Rotation Increases Organic Matter. It is a striking 

 fact that the growing of intertilled crops greatly hastens the 

 destruction, and hence the loss, of the soil organic matter. It is 

 not wise, therefore, especially on soils low in organic matter, to 



FIG. 168. Scabby potatoes the result of no clover or green manuring in rotation 

 (sugar beets, cabbage, potatoes). This condition developed in six years. No scab on pota- 

 toes grown in rotation with clover. (Clover, potatoes, corn, oats.) 



practice a rotation consisting of all intertilled crops, without the 

 introduction of clover, other legumes, and green-manuring crops to 

 maintain the supply of this very necessary soil constituent. A 

 rotation including clover or pasture, and in which manure and 

 green-manuring, or catch and cover crops are systematically used, 

 increases the soil supply of organic matter. The introduction of 

 non-cultivated crops such as grain, hay and pasture, lessens its 

 destruction, and the use of manure, the development of good sod 

 (Fig. 169), and the plowing under of green crops are ways to add 

 it to the soil. A carefully planned crop rotation is the most con- 

 venient method whereby something definite may be done to main- 

 tain the soil organic matter. It is entirely possible for practically 

 every farmer to plan a rotation or rotations which will enable him 

 to accomplish this. 



