46 HINTS ON SOILS AND FERTILIZERS 
is known as mixed fertilizers. A mixed fer¬ 
tilizer is a combination of the three fertilizers 
that we have been dscussing, namely, nitro¬ 
gen, phosphorus and potassium. In such a 
case, there is no serious lack in the soil of 
just one element, but the conclusion is that 
all three are more or less needed. 
Another element that is sometimes needed 
by soils is that known as calcium, or lime. 
While lime is regarded chiefly as a soil neu¬ 
tralizer, it is a fact that it is often used by 
plants as food. Alfalfa and tobacco are good 
examples of heavy lime feeders. When it is 
found that lime is needed, this can be applied 
the same as the other fertilizers, about a ton 
to three tons to the acre. We will take this 
matter of lime more in detail in the follow¬ 
ing chapter. 
SOIL ACIDITY 
It seems to be a common experience of 
many farmers that their land fails to grow cer¬ 
tain crops as well as in previous years. Of 
course, there may be various reasons for this 
state of affairs, but one of the most frequent 
and important, in the humid climates, at least, 
is the absence of lime in the soil. Crops like 
alfalfa and clover, for instance, are often fail¬ 
ures in humid climates, for no other reason 
than a lack of lime. These particular crops 
do not succeed because the bacteria (which 
we have mentioned before as working in the 
roots of these plants and converting the nitro¬ 
gen of the soil air into nitrogen that the plant 
<can utilize) cannot work in a soil that is not 
