38 Nebraska Experiment Station Research Bui . 15 
slight. It is even possible that the western soils, being a 
little more open and more level than the eastern soils, give 
a slight advantage to the western farms. With effective vari- 
ation in both heat and soil largely eliminated, the study along 
this line can be more positively centered upon the size of 
farm in relation to rainfall and economic conditions. In the 
graph (Fig. 16), rainfall tends to give similarity to the shape 
of the curves while economic conditions are largely responsible 
for their position relative to one another. Each of these curves 
shows that as soon as rainfall is decreased below a certain point 
the size of farm increases rapidly. This point in rainfall is 
sometimes designated as critical rainfall. Along the line -of 
the graph, critical rainfall varies with economic conditions. 
Thus in 1880, 1890, 1900, and 1910, critical rainfall stood 
at approximately 25.0, 19.5, 22, and 21 inches respectively. 
It is a rather striking fact that between 1880 and 1910 the 
critical point in rainfall along this line had been lowered 
only about 4 inches. When cross-sectional studies of this kind 
are made across the northern part of the Great Plains, the 
curves reach into lower rainfall and are less sharp. On the 
other hand, cross-sectional studies to the south of the 240- 
month-degrees line give very sharp curves that do not reach 
into extremely low rainfall. 
SIZE OF FARM IN RELATION TO PLANT GROWTH 
The connecting link between size of farm and moisture 
conditions in the Great Plains is plant growth. It stands to 
reason that where rainfall is normally low and as a result crop 
yields are low and pasture growth is short, a farm must of 
necessity occupy enough land to bring the total annual growth 
of usable plants to a level that will insure the average farm 
family a living. Figure 17 gives a view of the size of farm in 
relation to the total annual growth of crops and pasture. As 
shown by the key map accompanying the figure, the heat and 
soil conditions in the areas chosen are much the same as 
those described in the preceding cross-sectional study. In 
the accompanying diagram the blocks placed adjoining one 
