CROP ROTATION IN RELATION TO SOIL PRODUCTIVITY 67 
ing to the second method of study, results show that both crop rota- 
tion without the use of fertilizers and the use of fertilizers without 
rotation maintained the producing power of the soil, except in a 
few cases (pp. 57-58). 
In 5 of the 13 experiments considered, crop rotation without fertiliz- 
ers is equally or more effective than the use of fertilizer without 
rotation in increasing soil productivity. In the other 8 experiments 
it is shown that the use oi fertilizer is the more effective. And fur- 
ther, in all cases, except the barley experiment at Rothamsted, the 
conjoint effects of rotation and the use of fertilizer are additive in 
effecting increases in soil productivity — in 10 of the experiments the 
effects being more than fully additive (pp. 59-60). 
The following facts concerning the value of crop rotation sum- 
marize all the results arrived at from both methods of study: 
(1) Rotation of crops, when practiced with and without the use 
of fertilizer, averages 75 per cent as effective as the use of fertilizer, 
in effecting increases in crop yields, or 90 per cent as effective 
as fertilizers when the results on wheat, corn, and oats, only are 
averaged. 
(2) In most cases, as determined by the conditions of the experi- 
ments considered, it has been found that rotation is 91.5 per cent as 
effective as the use of fertilizer in maintaining the producing power of 
the soil. 
(3) In increasing soil productivity, as measured from the mainte- 
nance yields, the effects of rotation alone have been found, at times, to 
equal or exceed the effects of the use of fertilizers. 
(4) The conjoint effects of rotation and the use of fertilizers are 
additive, as effecting increases in yields over the check plots in con- 
tinuous culture and rotation; or as effecting increases in soil pro- 
ductivity, when measured by increases above the maintenance yields. 
(5) Altering or changing the acid reaction of a soil by liming 
increases the relative effectiveness of crop rotation. 
(6) On soils long under cultivation, highest yields are possible only 
when rotation of crops and the use of fertilizers are conjoined. 
The above facts point to the following principles of permanent soil 
productivity : 
(1) Crop rotation is so important a farm practice, especially in 
maintaining and increasing the yields of cereal crops, that its effective- 
ness may often equal or even exceed the effectiveness of the use of 
complete chemical fertilizer or farm manure. 
(2) The conjoint effects of crop rotation and the use of fertilizers 
are additive, as effecting increases in crop yields. 
(3) The relative efficiency of crop rotation is greater on soils 
naturally supplied with lime or on soils whose reactions have been 
altered or changed by liming than on soils that are acid in character. 
