378 SOIL PHYSICS AND MANAGEMENT 



These are the exceptions. England's farms are models of scientific 

 rotation. 



4. Variation in Depth of Root Systems. By rotation differ- 

 ent crops with root systems varying in depth are brought succes- 

 sively upon the land. Some especially deep-rooting crops, such as 

 clovers and alfalfa, should be grown. These obtain much of their 

 food below the zone from which the ordinary shallow-rooting crops 

 take their food. More than this, they bring much plant food to 

 where it may be reached by other crops. In this way the plant food 

 from a deeper zone is utilized and soil exhaustion will not occur so 

 soon. These deep-rooting crops have a very favorable effect in open- 

 ing up the subsoil for better aeration and drainage. 



5. Helps Maintain Good Tilth. At the University of Illinois 

 one plot has been growing corn for thirty -seven years ; another has 

 had a two-crop system of corn and oats ; a third has had a three-crop 

 system of corn, oats, and clover for the same time. The soil of the 

 first is compact, "runs together" badly, and erodes considerably. 

 A heavy rain packs it and forms a smooth, solid surface. The second 

 acts somewhat similarly to the first but is not so bad. The third is 

 mellow, granular, and even heavy rains do not cause the surface to 

 run together. This difference is due to the legume crop grown. 

 No crop is of more benefit to the tilth of a soil than a deep-rooting 

 legume. 



6. Helps to Maintain the Organic Matter. The part rota- 

 tion plays in maintaining organic matter has been discussed in 

 Chapter XI. As previously shown, a legume crop is essential in 

 every scientific rotation. The manner in which the legume is dis- 

 posed of is of the utmost importance. Very little in the way of 

 permanent improvement is accomplished unless the legume is turned 

 back into the soil either directly or as manure. 



7. Rendering Toxic Substances Less Harmful. Soils some- 

 times contain organic substances that are harmful to plants. The 

 same substance is not equally injurious to all crops, but is espe- 

 cially detrimental to the growth of the kind of plants that gave rise 

 to the toxic material. Changing the crop renders this less harmful. 



8. Produces Larger Yields. From what has been said it will 

 be seen that rotated crops have a decided advantage over those of 

 the single-crop system. 



Many experiments have been carried on at different stations 



that prove definitely the great value of rotation in increased yields. 



Iowa gives results that show a nine-year average for continuous 



