CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 65 
GENERAL SUMMARY AND CONCLUSIONS 
This bulletin reviews a study of the effects of crop rotation and the 
Tise of fertilizers on the yields of crops, the primary objects being (1) 
to determine some definite measure of the value of crop rotation in 
•crop production and (2) to compare the beneficial effects of rotation 
with those of the use of manure and complete chemical fertilizers in 
maintaining and increasing soil productivity. 
The maintenance of soil productivity depends in a large measure on 
three factors commonly referred to as farm practices: (1) Cultivation 
of the soil, (2) rotation of crops, and (3) the use of fertilizers. 
Three methods of study are suggested. One is discarded because 
of the assumptions involved; the other two methods, which are 
accepted, are fully explained (pp. 3-10). 
The effects of crop rotation and the use of fertilizers on crop 
yields, as determined by the first mehod, in which evaluations are 
based on increases over the yields in continuous culture and rotation 
alone, are as follows : 
Results of 72 years on wheat at Rotharnsted, England, show that, 
under the conditions of these experiments, crop rotation without 
fertilizers is somewhat more effective than the use of a heavy applica- 
tion of complete chemical fertilizer; whereas, in case of barley, 
rotation is a little less than 28 per cent as effective as the use of fer- 
tilizer. And further, rotation, when added to the use of fertilizer, is 
105.6 per cent as effective as the use of fertilizer when added to 
rotation; in other words, the relative value of rotation is 105.6 per 
cent. When rotation and the use of fertilizer are conjoined, the 
effects on the yields of wheat are somewhat less than fully additive; 
in the case of barley their conjoint effects are not additive (pp. 10-18) . 
Under the conditions of the experiment at Columbia, Mo. (results 
for 30 years), the relative values of rotation alone, as based on the 
effectiveness of the use of fertilizer when practiced in the absence 
of rotation, are as follows: On wheat, corn, and oats, when fertilized 
with farm manure, 100 per cent, 102.7 per cent, and 53 per cent, 
respectively; and on wheat fertilized with complete chemical fer- 
tilizer, 54.7 per cent. When one practice is added to the other, the 
relative values for rotation in the manure series are as follows : Wheat, 
100 per cent; corn, 103.9 per cent; and oats, 33.7 per cent; and on 
wheat fertilized with chemical fertilizer, 36.4 per cent. When rotation 
-and the use of fertilizer are conjoined their combined effects on crop 
yields in each case are somewhat less than fully additive (pp. 18-29). 
The tests at Wooster, Ohio, as reported for 25 years, show the 
relative values for rotation alone, as follows: On wheat, corn, and 
oats in the chemical-fertilizer series, 31.5 per cent, 54.5 per cent, and 
60.4 per cent, respectively; and on the same crops in the manure 
series, 37 per cent, 83.1 per cent, and 123.7 per cent, respectively. 
When one practice is added to the other, the relative values for rota- 
tion are as follows: On wheat, corn, and oats in the chemical-fertilizer 
series, 50.5 per cent, 55.4 per cent, and 62.5 per cent, respectively, 
and on the same crops in the manure series, 37.2 per cent, 80.8 per 
cent, and 124.9 per cent, respectively. When rotation and the use of 
fertilizers are conjoined the effects on crop yields are practically 
fully additive, except in case of the use of chemical fertilizer on wheat. 
where the conjoint effects are more than fully additive (pp. 29-38). 
