HINTS ON SOILS AND FERTILIZERS 61 
of available nitrogen present in the soil become* 
the limiting factor. If there is only about half 
enough nitrogen in this piece of land the fifth 
year to produce a normal corn crop, then, as 
stated in an earlier chapter, we will get but 
half a corn crop, the yield being determined 
by the limiting factor, which in this case is 
nitrogen. 
But what happens if we plant different crops 
each succeeding year? We will find that we 
get about a normal crop in each case, as the 
next crop, planted after corn, is selected be¬ 
cause it is not such a heavy feeder on nitro¬ 
gen, but uses more than a normal supply of 
either phosphorus or potash. One very com¬ 
mon rotation in the Middle West is corn one 
year, oats one year, and clover two or more 
years, the clover being seeded with the oats 
the second year. This is done because the 
clover does not make any appreciable growth 
the first year, so farmers always sow this with 
a grain crop the year before they plan to use 
the clover crop. There are different rotations 
for practically every part of the country, so 
that no hard and fast rule can be laid down as 
to the best rotation to follow. The climate, na¬ 
ture of the crops raised, and various other fac¬ 
tors all enter into the matter of selecting the 
best rotation for any particular locality. 
There is another decided advantage in rotat¬ 
ing crops besides those already mentioned, and 
this is the control of insect pests and diseases. 
By changing the land every year for each crop, 
the insects and especially the diseases are more 
easily controlled. Let us take the potato 
