CONSERVATION OF ORGANIC MATTER. 



55 



ROTATIONS FOR THE CONSERVATION OF ORGANIC MATTER IN 



THE SOIL. 



The conservation of the organic matter in the soil is probably the 

 most important problem in crop rotation. In the various 3-year 

 rotations which have been discussed in the foregoing pages no atten- 

 tion has been paid to this important subject for the reasons already 

 given, that its introduction in these experiments would tend to 

 obscure the effects of sequence and methods of soil preparation. A 

 great deal of work has, however, been done at these same stations 

 with various rotations calculated to conserve the organic matter in 

 the soil. There are several reasons why the results from these experi- 

 ments have not yet reached the stage where any very safe conclusion 

 can be drawn. Among the most important of these reasons are the 

 following : 



(1) Rotations planned for this purpose must be of at least four and, 

 preferably, six years' duration; and a complete cycle of the rotation 

 should be completed before results will be at all marked. 



(2) The plowing under of sod or green manure is frequently less 

 beneficial to the crops immediately following than to the subsequent 

 crops. Under certain conditions of soil and climate the first effect 

 may be even to reduce rather than to increase the following crop. 

 This is especially true where a green-manure crop is allowed to become 

 too mature before plowing under, where a grass or clover sod is broken 

 when it is not in the best condition for plowing, or where the soil is 

 not properly treated after plowing. 



(3) The great diversity of soil and climatic conditions encountered 

 at the several stations has made it impossible always to foresee just 

 how the best results can be obtained. Some experiments and some 

 mistakes had to be made before the best methods of investigation 

 could be worked out. 



(4) The growing of most of the cultivated perennial grasses, the 

 biennial and annual legumes, and the winter grains is still in the early 

 experimental stages at many of the stations. The failure to secure 

 a catch of seed and the injury or total loss of the crop by winterkill- 

 ing has in some instances seriously interfered with carrying out the 

 plans of the experiments. 



(5) Experience is enabling us to gradually reduce these sources of 

 error, but they can never be entirely eliminated. It will therefore 

 require a much longer period of time to reach the same degree of 

 accuracy in these investigations as that attained with the simple 

 3-year rotations and the continuous cropping and summer tillage 

 experiments already described. 



(6) Another important consideration in this connection is that the 

 effects of all these methods are cumulative. Continuous cropping or 



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