Unlike many legumes, sweet clover will make a good growth on soils too
depleted in humus for profitable crop production. In addition to its
ability to grow and to produce a considerable quantity of forage on
such soils, it will add much humus to them. The extensive root systems
do much toward breaking up the subsoil, thereby providing better
aeration and drainage. The effect of the large, deep roots in opening
up the subsoil and providing better drainage is often very noticeable
in the spring, as the land upon which sweet clover has grown for
several years will be in a condition to plow earlier than the adjacent
fields where it has not been grown. The roots are often one-eighth of
an inch in diameter at a depth of 3 feet, and they decay in five to
eight weeks after the plants die. (Figs. 12 and 13.) The holes made by
the roots are left partly filled with a fibrous substance which permits
rapid drainage. Sandy soils are benefited materially by the addition
of humus and nitrogen, while hardpan often is broken up so completely
that alfalfa or other crops will readily grow on the land. The roots
add much organic matter to the layers of soil below the usual depth of
plowing, while those in the surface soil, together with the stubble
and stems, when the crop is plowed under, add more humus than possibly
any other legume which may be grown in short rotations. Not only does
this crop add organic matter to the soil, but in common with other
legumes it has the power of fixing atmospheric nitrogen by means of the
nitrogen-gathering bacteria in the nodules on the roots.
[Illustration: Fig. 12.--A portion of a root of sweet clover, collected
30 days after the seed crop had been cut. The cortex was so decayed
that it remained in the ground when the root was removed. Note that the
pith has largely disappeared and that the half-rotten central cylinder
is all that remains.]
[Illustration: Fig. 13.--The same root shown in figure 12 after being
crushed between the thumb and forefinger. Illustrating how rapidly
sweet-clover roots decay after the plants die. The holes left in the
ground by the rapid decay of the roots facilitate drainage.]
The ability of sweet clover to reclaim abandoned, run-down land has
been demonstrated in northern Kentucky and in Alabama. In these regions
many farms were so depleted in nitrogen and humus by continuous
cropping with nonleguminous crops that profitable yields could be
obtained no longer, Through the use of this crop many of these farms
have been brought back to a fair state of fertility. Tests at the
Alabama Agricultural Experiment Station show that the increased yield
of corn following sweet clover which had occupied the land for two
years was 6-3/4 bushels per acre. The cotton grown on the land the
second year showed an increase of 56 pounds per acre. The combined
value of the increased yields of corn and cotton for the two years was
estimated at $9.75. The total yield of hay for the two preceding years
was 6.8 tons per acre. In another experiment at this station cotton was
planted on land that had grown sweet clover the two previous years and
on land that had received an application of 18 tons of stable manure
per acre. The sweet-clover plat produced 280 pounds of seed cotton the
first year and 120 pounds of seed cotton the second year more than the
plat which received the heavy application of manure.
Land on which sweet clover had been grown for four years at the Ohio
Agricultural Experiment Station yielded 26.9 bushels of wheat per acre
as compared with 18.6 bushels on the check plat. Sweet clover was
seeded at the Tennessee Agricultural Experiment Station in the spring
of 1912. One cutting of hay was removed that year and the following
spring the field was plowed and planted to corn. The corn yielded
58.8 bushels per acre as compared with 41.1 bushels per acre for an
adjoining plat where rye was turned under. A number of tests have been
conducted in southeastern Kansas which show clearly the value of sweet
clover as a soil-improving crop for that section. The yield of wheat
has been increased as much as 7 bushels per acre and that of corn as
much as 22 bushels per acre by plowing under the second-year growth of
clover.
Annual yellow sweet clover is rapidly gaining in favor as a
green-manure crop for orchards in the Southwest. In Arizona two plats
seeded in October and plowed under in April yielded, respectively, 16
and 17 tons of green matter to the acre. At the Arizona Agricultural
Experiment Station annual yellow sweet clover, lupines, and alfalfa
were tested as green-manure crops for orchards. In this experiment the
sweet clover clearly showed its superiority to lupines or alfalfa for
this purpose, as it yielded from 21 to 26 tons of green matter per
acre, whereas the highest yield for the lupines was 10 tons and for the
alfalfa 15 tons per acre.
The use of annual sweet clover as a green-manure crop in southern
California has increased very rapidly in recent years, and this
increased use apparently has been justified by the results obtained
with it. One of the most interesting green-manure tests thus far noted
was conducted at the California Citrus Experiment Station. In this
experiment nine legume plats and eight nonlegume plats alternated
with each other. The 4-year average weight of green matter produced
on the sweet-clover plat was 14-3/4 tons per acre, whereas the 5-year
average weight of green matter produced by common vetch and Canada
field peas was 12 tons and 9 tons, respectively, per acre. On one
series of these plats corn was planted in rotation with the clover.
The average yield of shelled corn for four years was 46 bushels to
the acre on the sweet-clover plat, as compared with 35 bushels to the
acre on the common-vetch plat and 40 bushels per acre on the field-pea
plat. One barley plat receiving each year an application of 1,080
pounds of nitrate of soda gave an average yield of 41 bushels per acre.
The 2-year average yield of potatoes following sweet clover was 252
bushels per acre, as compared with 171 bushels following common vetch
and 234 bushels following field peas. Sweet clover has proved to be an
excellent plant to grow in rotation with sugar beets, as the 2-year
average for the beets following it was 19.8 tons per acre, as compared
with 15.3 tons following common vetch, and 17.6 tons following field
peas.
Annual yellow sweet clover makes a profitable growth only in the South
and Southwest and therefore should not be planted in any other section
of the country.
In those sections of the United States where the soils are low in humus
it is to be strongly recommended that sweet clover be grown for green
manure. This method is being practiced in some sections of the country
with excellent results.
It should be remembered that sweet clover will not make a satisfactory
growth on acid soils and that it is very essential to provide
inoculation if the soil is not inoculated already.