Excepting soil acidity, lack of inoculation probably is responsible
for more failures with sweet clover than any other one cause. When
sweet-clover plants are not inoculated they must depend upon the
available nitrogen in the soil for their supply, and as the crop is
grown for the most part on soils low in nitrogen the plants can not be
expected to make more than a small growth. (Fig. 9.)
[Illustration: Fig. 9.--White sweet clover at Arlington, Va., showing
the effect of inoculation upon their growth. The plants at the left
represent the average growth on the inoculated plats; those at the
right the average growth on the plats not inoculated. The plats had
been previously limed and were seeded on the same date.]
Arny and Thatcher, at the Minnesota Agricultural Experiment Station,
obtained 10 times as much dry matter in the tops and seven times as
much in the roots of sweet-clover plants which had been grown on
thoroughly inoculated soil as from plants Which had been grown on soil
not inoculated. Moreover, the plants grown on the inoculated soil
contained 117 pounds more nitrogen to the acre than those grown on the
uninoculated soil.
Experiments in many other sections of the country, and especially in
the northeastern quarter of the United States, where but little sweet
clover or alfalfa has thus far been grown, show that inoculation
is very essential to success. Ordinarily it is not necessary to
inoculate sweet clover when it is to be planted on land where alfalfa,
bur-clover, or black medic thrives, because the same strain of
inoculating germs inoculates all of these plants. However, when this
closer is to be planted on land where none of the plants inoculated
by this strain of the organism have been grown, inoculation should be
provided. In localities where sweet clover or other plants inoculated
by the same strain of bacteria thrive, the early growth has at times
been made much more vigorous by inoculating the soil or seed thoroughly.
It is not safe to assume that a certain piece of soil is inoculated
because any one of the plants inoculated by the same strain of the
organism is growing or has been grown on other fields in the same
vicinity. Many fields have come under observation where sweet clover
was a failure because the plants were not inoculated, when plats or
fields of alfalfa growing near by were abundantly inoculated.
There are several methods of inoculating sweet clover, any one of which
when properly applied should give good results.