8 
COLORADO EXPERIMENT STATION 
all, when planted to wheat the yield per acre was far greater than when 
the land was first subdued. 
Now what had the alfalfa done to the soil? It had actually added 
some nitrogen which by the aid of the nitrogen fixing bacteria it is able 
to gather from the air. It also brought up from the lower depths of 
soil, ten or more feet below the surface, potassium and phosphorus. 
These two important elements of plant food were deposited in the 
surface soil in the shattered leaves and stems as crop after crop was 
removed from the soil and when the sod was broken the decaying roots 
and stubble added their quota. But more important than all these, 
decaying vegetable matter and its products had been added to the soil. 
Without decaying vegetable matter the physical condition of the soil 
is almost ruined for agricultural purposes and the host of bacteria 
which perform many essential activities are prevented from developing. 
Our orchards have, for the most part, been planted on desert 
land and in most cases the land was cleared of the native growth and 
planted directly to trees. Thus there was little or no vegetable matter 
in the soil and, since our growers have been very insistent on clean 
cultivation and stable manure is scarce, but little has been added. 
Now does it not stand to reason that continued croping to apples or 
to peaches will bring the same disastrous results that befell the wheat 
growers mentioned above? The following table complied by Dr. 
Roberts* is valuable in this connection. Table I shows the amounts 
of plant food elements which are removed in the grain and straw in 
twenty years of continuous cropping to wheat. It is assumed that an 
average of 15 bushels of grain and 35 pounds of straw are removed 
from an acre each year. 
Table I. Amounts of Plant Food Removed From an Acre in 20 Years 
Continuous Cropping to Wheat. 
Nitrogen Phos. Acid Potash Value 
lbs. lbs. lbs. 
Grain . 424.80 160.20 . 109.80 $79.86 
Straw . 234.78 50.40 214.20 48.37 
In contrast with this the same author gives similar figures of the 
amounts of plant food which may be expected to be carried away in 20 
years in fruit and leaves from an acre of bearing apple orchard. The 
figures represent 20 years of the productive life of a New York apple 
orchard between the ages of 13 and 33 years and does not include the 
materials stored in the wood of the tree. 
Table II. Amounts of Plant Food Removed From an Acre of Apple 
Orchard in Twenty Years. 
Nitrogen Phos. Acid Potash Value 
lbs. lbs. lbs. 
Apples . 498.60 38.25 728.55 $110.26 
Leaves . 456.75 126. 441. 97-17 
Total value in wheat, grain and straw for 20 years .$128.23 
Total value'in apple, fruit and leaves for 20 years. 207.45 
These figures show that an apple crop takes more fertility from 
* 1 . P. Roberts, Bull. 103, Cornell Experiment Station. 
