152 NATURE STUDY AND AGRICULTURE 
grass for a few years and then plow the field. Another farm 
practice sometimes adopted for restoring humus to the 
soil is to plow under a green crop. Any plants serve the 
purpose, even the weeds that are plowed under in summer 
fallowing, but plants with deep roots and heavy foliage are 
most helpful. By far the best of all are the legumes, as 
clover and alfalfa, for they not only restore humus to the 
soil, but are rich in nitrogen, which is the most valuable of 
all the plant foods and the supply of which is most liable to 
run short. 
How Soils Are Impoverished. — When people began culti- 
vating the rich prairies of the Northwest many of them made 
the mistake of assuming that the marvelous fertility of the 
virgin soil could not be exhausted, but experience in all parts 
of the country shows that 30 to 60 years of grain farming  . . 
will ‘ run down ” the best land to such an extent that it will 
cease to pay. It is common to grow grain every year without 
replenishing the organic matter in the soil, which is therefore 
reduced to lower and lower proportions. Exclusive grain 
farming always implies that very few cattle are kept and but 
little farm manure produced. Even that which is formed is 
either allowed to remain in the barnyard or at least left there 
until half the plant food has been washed out of it by the rains. 
The straw and even the stubble are burned. Plant material 
is taken off the land every year, and none is returned to it. 
The causes of the loss of fertility under these conditions 
may be summarized as follows: (1) Constant cropping 
diminishes the supply of available plant food. This does 
not mean that there is really no more plant food in the soil, 
but that it is not “ available,” because it is not in soluble form 
so that plants can use it. To render it soluble is a very slow 
process even under favorable conditions. Commercial fer- 
