16 BULLETIN 1377, U. S. DEPARTMENT OF AGRICULTURE 
It is to be observed that the increase (R) effected in the yield of 
wheat by adding rotation to the use of fertilizer is about equal to 
the increase (F) effected when the use of chemical fertilizer is com- 
bined with the practice of rotation. This is indicated by the rela- 
tive value of 105.6 per cent for R. In case of barley, R has an abso- 
lute negative value of — 6.62 bushels, or a negative relative value of 
— 49.2 per cent. These negative values for R may be explained, 
at least in part, by the fact that the fertilizer treatments given to 
barley in rotation and continuous culture are not exactly compara- 
ble, since no fertilizer is applied directly to the barley crop in rota- 
tion, whereas in continuous culture a liberal direct application is 
made annually. (See Table 3.) 
Combining the rotation and fertilization practices effected a total 
increase over the check plot in continuous culture, of 20.16 bushels 
of wheat, which increase is greater than the gain resulting from either 
rotation or the use of fertilizer alone. This fact defines and illus- 
trates the meaning of the expression " additive effects of rotation and 
the use of fertilizers" as it is used in this bulletin. This definition 
is in harmony with the meaning " tending to increase." That is to 
say, the yield of wheat, for example, is increased when to rotation is 
added the use of fertilizers, or wnen to the use of fertilizers is con- 
joined rotation. 
Three possibilities may result from conjoining the practices of 
rotation and of fertilization: The total increase resulting may be 
equal to, less than, or greater than the sum of the increases effected 
by rotation and fertilization when practiced independently of each 
other. In describing these additive effects the following expressions 
are used: "Fully additive," "less than fully additive, and "more 
than fully additive," respectively. The data in Table 7 show that, 
on wheat, the effects produced in conjoining rotation and the use of 
chemical fertilizer are somewhat less than fully additive; while in 
case of barley, the combined effects are not additive, reflecting, no 
doubt, the difference in fertilizer treatments and the different habits 
of the barley plant as compared with wheat. 
In discussing the yields of barley grown in continuous culture, 
Lawes and Gilbert stated that results, as compared with wheat, 
were dependent on the differences in the habits of the two plants. 
Wheat, because of its greater root system, gains possession of a much 
greater range of soil, especially in depth, than barley; barley, on the 
other hand, is a surface feeder and hence relies in a much greater 
degree on the nutrients within the soil near the surface. Accord- 
ingly, barley is found to be more benefited by direct applications of 
fertilizers than is wheat when sown under equal soil conditions (9, 
p. 100). 
DIAGRAMMATIC SUMMARY OF ROTHAMSTED RESULTS 
Figure 2 represents in diagrammatic form the average wheat and 
barley yields that have been obtained on the four Rotnamsted plots 
herein considered, summarizing the following points: The portion 
of each yield (in bushels) that is credited to cultivation alone (c), 
that is, the yield obtained in continuous culture without fertilizers; 
the increase effected by combining rotation or the use of chemical 
fertilizer with cultivation; and the total increase effected, over culti- 
vation alone, by conjoining rotation and the use of fertilizer. 
