CROP ROTATION AND CULTURAL METHODS AT AKRON, COLO. 23 
GREEN MANURES 
The green-manured plats may be considered as modified fallows. 
The cost of the seed and seeding of the crop plowed under makes 
them more expensive than bare fallows, and in addition they require 
the sacrifice of a crop already produced. For a green manure to be 
relatively profitable the crop or crops following it must yield con- 
siderably more than those following bare fallow, but they have 
actually yielded less. The yields following them decrease in com- 
parison to those following fallow as the date of plowing becomes 
later. The same effect is noted in experiments on the time of plow- 
ing for fallow. The beneficial effects of the fallow decrease as the 
date of plowing is delayed and weeds are permitted to grow. The 
same result follows the growth of weeds after plowing and emphasizes 
the necessity of keeping fallow land free from weed growth. 
There is little to choose between allowing weeds to grow until July 
17 and plowing them under, and allowing peas to grow until the same 
date. The only observed difference between them is that the latter 
is a legume and capable of adding nitrogen to the soil, whereas the 
former is a heterogenous nonleguminous crop. This difference, how- 
ever, can not manifest itself in the results because of the overpowering 
control of the moisture factor. Both crops more or less completely 
exhaust the available water from the soil before they are plowed 
under. The greater moisture supply in fallow land, it is thought, 
accounts almost entirely for the greater yields. 
The 15-year average yield of winter wheat following fallow plowed 
on the average date of June 3 was 19.1 bushels per acre; following 
rye plowed under for green manure on the average date of June 12 
the average yield was 14.2 bushels per acre; and following peas 
plowed under for green manure on the average date of July 17 the 
average yield was only 10.8 bushels per acre. The latter was prac- 
tically the same as the yield v on land from which a crop of wheat 
was harvested. With such late plowing the water in the soil is 
exhausted, and the season of heaviest rainfall being past there is 
little opportunity to store more. 
The difference between fallow, rye for green manure, and peas 
for green manure, in their effect upon the immediately succeeding 
crop, seems to be primarily one of the length of the fallow season 
and the consequent differences in the quantity of water stored in 
the soil at seeding time. The first crop restores the land to uniformity 
in this respect. The second and third crops following the green 
manures do not show any increases over crops in corresponding 
positions following fallow. Neither do successive repetitions of the 
treatments as the rotations are repeated show any cumulative 
advantage from the green manures. The evidence inclines to favor 
the bare fallow. 
Sweet clover is used in the experiments as a green manure, and the 
same general effect is noted as with rye and peas. Any difference 
in fertility that might result from the green manure can not manifest 
itself in the subsequent crop yields because of limited moisture. 
