MAINTAINING SOIL FERTILITY 303 



the clover is cut they immediately throw up their 

 flower stalks. These may be mowed a few weeks 

 after the clover is removed, but a better way is to 

 plow the clover stubble soon after the first cutting, 

 in preparation for winter wheat or rye. 



The value of a rotation of crops for killing weeds 

 depends largely upon the fact that different crops 

 receive different kinds of tillage. Different tools 

 are used. Weeds that escape destruction under the 

 system of tillage given one crop are caught by the 

 tillage of another. Sown crops, especially grains, 

 should be rotated with tilled crops. Grams cover 

 the ground sparsely and there is no inter- tillage, 

 so weeds like the "paint-brush," Canada thistle, 

 dock, and Russian thistle, find an excellent oppor- 

 tunity to gain a foothold. Some crops that grow 

 very rapidly and quickly shade the ground, as 

 potatoes, are not as apt to be weedy as slow-growing 

 and sparse-foliage crops. This should be re- 

 membered when planning a rotation. 



Insect and Disease Injury Lessened by Rotation. 

 Each crop has its own peculiar troubles. Some 

 of these are fungous diseases. These are spread 

 mostly by seed-like bodies called spores; each 

 disease has a different kind of spore, which can 

 cause the disease only upon a certain crop. Thus 

 the spore of potato scao can make scao on po- 

 tatoes, but no other disease, either on potatoes or 

 any other plant. The longer a certain kind of 

 crop is grown upon the same piece of land the more 

 the land becomes infected with parts of diseased 

 plants and with spores, and the greater is the like- 

 lihood that the crop will be injured by the disease. 

 This is particularly true of such common diseases 

 as potato scab, and the club root of cabbages and 

 turnips, which increase very rapidly on crops grown 



