PLANT INDUSTRIES 325 
the average yield; and in the third period of twenty years 
the average yield was 36.4 bushels per acre. In the second 
twenty-year period one year of general wheat failure materially 
reduced the average for that period. 
The benefits that are derived in plant industries from rota- 
tion of crops are universally recognized by scientific students 
of the matter, but the explanation of the results secured fur- 
nishes material for active discussion. Within the last decade 
it has been shown ! that at least some cereals excrete and leave 
in the soil substances that are injurious to the kinds of plants 
which produced them. These plant poisons may be the cause 
of loss of fertility of soil, or loss of fertility may be due to the 
fact that the soil has been exhausted of the available materials 
needed in the growth of plants. Soil replenishment through 
use of fertilizers may possibly bring in a new supply of avail- 
able materials, may neutralize or counteract the injurious excre- 
tions within the soil, or may merely change the physical nature 
of the soil so as to make it more favorable to plants. What- 
ever the scientific explanation, the great value of natural fer- 
tilizers and of rotation of crops in replenishing soils is evident 
to any practical student of soils. 
306. Loss of soils. In some places the chief danger is not 
primarily that of loss of fertility, but loss of the soil itself. It 
may be washed or blown away or removed by fire. If a pane 
of glass that has been moistened with oil is exposed for a time 
to the wind on a dry day, and then examined with a strong 
magnifying glass, it will furnish a good demonstration of 
the dust-carrying power of moving air. Windowpanes in the 
houses near the end of Cape Cod finally become translucent, 
like ground glass, from the action of sand driven by the wind. 
When cultivated fields become dry, the wind may carry away 
large quantities of soil. 
10. Schreiner, and H. 8. Reed, '*Some Factors influencing Soil Fertil- 
ity,’’ Bulletin 40, Bureau of Soils, U.S. Dept. Agr., 1907; also ‘' The Pro- 
duction of Deleterious Excretions by Roots,”’ Bulletin 34, Torrey Botanical 
Club, 1907. 
