62 Journal of the Mitchell Society. [ June 
moving- things, that helped the crops to get their nitrogen 
from the atmosphere. Long ago it was discovered that cer- 
tain plants, as the beans, clovers, peas, vetch, alfalfa, and 
the like, form upon their roots little bunches of tubercules, as 
they are called. When science sought out the meaning of 
these tubercules, why they formed on these particular plants, 
what purpose they served, it was seen that they were not 
abnormal, but necessary, and that plants that had them 
were more thrifty than those that had them not. It was dis- 
covered that their task was to take nitrogen from the air and 
transform it into nitrogen suitable to be taken up by the 
plant. 
Having learned, then, the soil conditions necessary for 
plant growth, the next thing is to apply them. 
Residual soils, those found upon the rocks from which they 
are derived, have certain definite characters determined by 
the characters of the rocks beneath, and they are not apt to 
deteriorate, since their source of food-supply is immediately 
at hand, unless the fine particles are carried away by erosion 
faster than the rock beneath can rot into soil. Transported 
soils, on the other hand, are very readily exhausted, since 
they are far removed from the parent rock, and they need to 
have their supply of plant-food constantly replenished by the 
use of fertilizers. One way of keeping up the fertility of the 
soil is by rotation of crops requiring different plant-foods. ; 
The best way to farm is to plant in each field the crop to 
which the soil of that field is by nature best adapted. 
But we often desire, or actually need, crops to which the 
soil of a given district is ill-adapted. Since we cannot j 
change the soil materially, the difficulty is met by breeding j 
plants to suit the soil, and what has been accomplished in J 
this direction is little short of miraculous. 
The wasting of soils where serials are grown, and the jj 
gradual reduction, year by year, in the yield of these crops, j| 
has led more than one thoughtful student of human condi- j; 
