Sweet clover, like many other legumes, requires a soil containing
an abundance of limestone if a maximum growth is to be expected.
Throughout the world it makes a luxuriant growth only on calcareous
soils. On the black prairie limestone soils of Alabama and Mississippi
it grows luxuriantly, although in this region it is very seldom found
on the outcroppings of red clay, which are acid. The distribution
corresponds sharply with the line of demarcation between the black
prairie soils and other soil types. In some places sweet clover makes a
vigorous growth on the Selma chalk (rotten-limestone) soils, while none
is to be found on red post-oak clay but a few yards away; yet sweet
clover will grow on the red post-oak clay after the soil has received
an application of lime. It will thrive on the bald lime-rock spots
and rotten-limestone hills of Mississippi, which are so barren that
practically no other plants will survive. Thus it appears that lime is
essential for the maximum growth of sweet clover in this Region.
The reason for the exceptional growth of sweet clover in north-central
Kentucky is undoubtedly the fact that these soils contain an abundant
supply of limestone. The Kentucky Agricultural Experiment Station
states that this area is the only portion of the State where sweet
clover is being grown with general success without applying lime.
Soils on which sweet clover is aggressive are almost invariably
alkaline or but slightly acid. This plant is often found in valleys of
streams in localities where the soils are supposedly acid, but such
streams generally have their origin in limestone areas or flow through
limestone regions, and calcium carbonate is thus deposited in these
valleys during flood periods with the sedimentary deposits from flood
waters. Sweet clover often appears in deep cuts along highways or
railroads in localities where the soil is known to be acid and where
sweet clover has not previously grown. In many of these cuts the acid
soil has been removed and neutral or alkaline subsoil exposed, or
limestone has been used in ballasting or road making and the dust has
blown on the exposed soil. It is a very common occurrence to find sweet
clover making an abundant growth along macadamized roads from which the
wind has scattered the finely pulverized limestone.
An application of burnt lime or finely ground limestone has made the
difference between success and failure in most experiments which have
thus far been conducted on decidedly acid soils. (Fig. 7.)
A number of sweet-clover experiments were performed on acid soils and
on adjacent plats or fields of the same type of soil that had received
applications of limestone varying from 1 to 4 tons to the acre. There
was a marked difference in the stands obtained and in the growth of the
plants on the limed and unlimed areas. In some cases the difference in
growth was so marked that the last round of the lime spreader could be
distinguished at some distance from the plats. The stands were much
heavier on the limed areas and the plants made from two to three times
more growth than those on the unlimed plats. Yields of hay were doubled
on soils that received only sufficient limestone to neutralize the
acids in the surface soil, although the yields were further increased
when more limestone was added.
Mr. W. E. Watkins, county agent of Allen County, Kans., made counts of
the number of plants which winterkilled during the winter of 1914-15
on given areas of limed and unlimed soil. It was found that from 15
to 35 per cent more plants winterkilled on the unlimed soil than on
the limed areas. That portion of the unlimed field on which the fewest
plants winterkilled was found to have the lowest lime requirement.
On the unlimed areas with a low lime requirement 15 per cent more
plants winterkilled than on the limed areas; on those with a high lime
requirement the increase in winterkilling was 33 per cent. In the fall
of 1914 the hay cut from the limed areas exceeded that from the areas
with a low lime requirement by 600 pounds per acre and exceeded that
from the areas of high lime requirement by 4,000 pounds per acre. In
July, 1915, the increase in hay yield on the limed areas over that from
the areas with a low and with a high lime requirement was 2,300 and
9,400 pounds per acre, respectively. The area of high lime requirement
returned a small yield in 1914 and no hay in 1915.
In spite of the fact that sweet clover is as sensitive to soil acidity
as red clover or alfalfa, a large percentage of the acreage thus far
seeded in the eastern half of the United States has been composed of
acid soils, and this soil acidity undoubtedly is responsible for a very
large percentage of the failures with sweet clover in this section.
Where sweet clover is to be sown on acid soils a sufficient quantity of
lime should first be applied to at least neutralize the fields in the
soil to a depth of 6 inches. An application of 1 ton of burnt lime or
2 tons of finely ground limestone will usually be sufficient for this
purpose.
[Illustration: Fig. 7.--Sweet-clover plants, showing the effect of lime
upon their growth. The plants at the left represent the average growth
on the unlimed portion of a field; the plants at the right show the
average growth on the limed part of the same field.]
Fields have been noted where sweet clover was making a fair growth
on apparently acid soils. Such fields usually are rich in humus or
phosphorus and are exceptional cases rather than the rule.
Soil types which have slightly acid surface soils and alkaline subsoils
will grow sweet clover successfully, provided the acid soil is not more
than 6 to 12 inches in depth.