THE ROTATION OF CROPS 29 



balanced removal of plant food; (11) systematizes farming; and (12) may- 

 control toxic substances. 



Maintain Good Physical Condition of Soil. — Deep-rooted plants, 

 such as alfalfa and the clovers, improve the physical condition of the 

 subsoil as a result of root penetration. The cultivation given to inter- 

 tilled crops, such as corn, potatoes, beets and the truck crops, improves 

 the physical condition of the surface soil. Such frequent cultivation may 

 tend to reduce the organic matter of the soil, but this will be largely over- 

 come by the stubble and roots of the grasses and clovers that follow the 

 grain crops. 



Conserve Organic Matter and Nitrogen. — Extensive rotation experi- 

 ments at the Minnesota Experiment Station show that standard rotations, 



Dangers of Continuous Cropping. 1 



On the left is corn growing on land that has grown corn continuously for 

 19 years. On the right is corn in a five-year rotation. Both fields were 

 planted on the same day to the same kind of corn. The yield on the field 

 to the left is 27.5 bushels to the acre. The field on the right gives 61.3 

 bushels an acre. These are the average yields for ten years. 



which include an inter-tilled crop, small grains and grasses with clover, 

 all gave net profits. A four-years' rotation of millet, barley, corn and oats 

 was no better than four years of continuous growing of wheat. All of 

 these are classified as exhaustive crops. They cause a reduction in both 

 the organic matter and nitrogen supply of the soil. Land cropped 

 continuously to wheat, corn, potatoes or mangels for a period of ten years, 

 showed a loss of 1100 pounds of nitrogen and 20,000 pounds of carbon 

 per acre. In twelve standard rotations covering the same period of time, 

 there was a gain of 300 pounds of nitrogen per acre, while the carbon and 

 humus in the soil was maintained and in some cases increased. In the 

 standard rotations eight tons of manure per acre were applied once during 

 the rotation. 



Provide for Extermination of Weeds. — Noxious weeds often cause 

 a serious loss in farming. Weeds not only rob the crops of plant food and 



» From " Farm Management" by Boss, Courtesy of Lyons and Carnahan, Chicago. 



