SOIL STRUCTURE AND ITS MODIFICATION 147 



effective puddling agent when the soil is wet. Therefore, care 

 in the judging of optimum conditions for plowing is a most 

 important feature in the maintenance and encouragement 

 of soil granulation. A careful study of the moisture con- 

 ditions in a clay soil is especially necessary in order to de- 

 termine just what is the correct moisture content for good 

 plowing. That this condition must be gauged carefully and 

 immediate use made of the advantages it offers is shown by 

 its narrow limits. A few days may suffice for the moisture to 

 pass through and beyond such a condition. A clay soil is so 



Fig. 26. — A six-shovel cultivator. 



difficult to handle at best that no opportunities such as are 

 offered by optimum moisture conditions should be lost. More- 

 over, a heavy soil plowed too dry or too wet does not regain 

 its normal granular condition for several seasons. Such care 

 is unnecessary with a sandy soil. 



82. Cultivators, packers and crushers. — The many 

 types of cultivators may be grouped under three heads: (1) 

 cultivators proper, (2) levelers and harrows, and (3) seeder 

 cultivators. The action of all these implements is the same 

 in that they stir the soil, at the same time loosening the struc- 

 ture and cutting off weeds. While the action is much shal- 

 lower than with the plow, the same attention should be paid 

 to moisture conditions. Particularly is this true in pulveriza- 



