32 BULLETIN Hi, U. S. DEPARTMENT OF AGRICULTURE. 
tractor, the investment cost per acre is of great importance. Table 
XIX shows that on the smaller farms of approximately 300 acres the 
cost per acre for mechanical power is about $7.60, while on the larger 
farms, averaging about 1,400 acres, the cost per acre is less than $2. 
Similarly, while the small farms show an investment of about $2 per 
acre for special equipment, the large farms have only one-fourth this' 
amount. 
In this connection, the value of work horses per acre should also be 
noted. For the 300-acre farms the cost for work stock is about $3 
per acre, while for the 1,400-acre farms it is only $2 per acre. 
Especial attention is invited to the difference in the ratio of the 
investment cost per acre for the two kinds of power. For mechanical 
power the investment per acre for the small farms is more than 3 J 
times as great as for the large farms, while for animal power it is only 
1J times as great. 
The reason for this difference is probably the fact that a stable 
of horses, consisting of a number of individual units, can be regu- 
lated in size to meet actual requirements, the price per unit being 
practically uniform no matter in what number purchased. On the 
other hand, the tractor is a complete unit and must be of sufficient 
power to fulfill the maximum demands which may be made upon it, 
while the cost per horsepower is greater in the small sizes than in the 
large ones. In other words, the owner of a 600-acre farm who pur- 
chases a 30-horsepower tractor will have a lower investment per 
acre for power than the owner of a 300-acre farm who purchases a 
15-horsepower tractor, because the 15-horsepower tractor costs more 
per horsepower than the 30-horsepower outfit; while the owner of a 
600-acre farm who purchases one work horse for each 30 acres of 
land, or 20 horses, will have the same investment charge per acre as 
the owner of a 300-acre farm who purchases one work horse for each 
30 acres of land, or 10 horses, the cost per horse being nearly the same, 
no matter in what number purchased. 
From Table XIX it will be seen that the total investment per acre 
for power on the 300-acre farms is about $10, while for the 1,400-acre 
farms it is only $4 per acre, although the 300-acre farms have a unit 
of power for every 12 acres, while the 1,400-acre farms have one unit 
for every 32 acres. It is evident, therefore, that either the 300-acre 
farms have more power per acre than is necessary and economical 
or that the 1,400-acre farms have an inadequate amount of power. 
From a careful study of the data shown, in conjunction with other 
information available, it is believed that the large farms have a normal 
acreage per unit of power and that farms of the grain type which have 
a smaller acreage per horsepower are overequipped and therefore less 
economically equipped. The owner of a 300-acre farm who has an 
invested capital of $10 per acre for power and one unit of power for 
