CROP ROTATION 89 



of the latter in the form of decaying roots. When it is remembered 

 that roots of clover, for instance, represent nearly one-half of the 

 weight of the crop, the importance of this source of humus is apparent. 

 The principal benefits of rotations are derived from the legumes 

 included. Without a legume, a rotation is hardly worthy of the name. 

 The members of the legume family of crops, alfalfa, the clovers, soy 

 beans, cowpeas, vetch, etc., possess the power of utilizing the nitro- 

 gen of the air through the medium of bacteria which grow and form 

 nodules upon the roots. The decaying roots help to replenish the sup- 

 ply of nitrogen and humus. By use of legumes in the rotation, 

 the nitrogen and humus supplies of the soil can be very cheaply and 

 profitably maintained or increased. Leguminous catch crops should 

 be frequently grown and if the soil is especially deficient in humus or 

 nitrogen these crops should be plowed under. If the land is subject 

 to washing or blowing, the catch crops should be left on the ground 

 during the winter. 



*At the Indiana Experiment Station for 24 years, corn and wheat 

 were grown in rotation with each other in comparison with the same 

 crops rotated with oats, roots, grass and clover. No manure or fer- 

 tilizer was used. In 1909 after 20 years of cropping with the legume 

 i-otation, the yield per acre was 61.1 bushels as compared with an 

 average yield of 25.4 bushels with the corn and wheat alone. 



At the Illinois Experiment Station continuous corn growing has 

 been compared with rotations of corn and oats; and corn, oats and 

 clover with the following results. 



TABLE NO. 17 

 YIELDS FROM FIELD AT URBANA, TYPICAL CORN BELT PRAIRIE SOIL** 



Crop Years Crop System ExpeVYme'nts Experiments" 



1905-6-7 Corn every year | 35 Bushels | 27 Bushels 



1903-5-7 Corn and oats 1 62 " j 4t) 



19U1-4-7 Corn, oats and clover 66 " I 58 



The lesson of these experiments is that 12 years of cropping, where 

 corn follows corn every year, reduces the yield from more than 70 

 bushels to 35 bushels per acre, after which the decrease is much less rapid, 

 amounting to only 8 bushels reduction during the next 16 years. Un- 

 doubtedly the rapid reduction during the first 12 years of continuous corn 

 growing is due in large part to the destruction of the more active decaying 

 organic matter, resulting ultimately in insufficient liberation of plant 

 food within the feeding range of the corn roots. In addition to this, 

 the development of corn insects in soil on which their favorite crop 

 is grown every year, is sometimes an important factor in reducing the 

 yield. 



'Indiana Bulletin. Nos. 55 and 64. 

 '♦Illinois Bulletin No. 125 (May 1908.) 



