Owing to the fact that sweet clover thrives on the barren Selma
chalk (rotten-limestone) hills of Alabama and Mississippi and grows
abundantly on worn-out, abandoned land in north-central Kentucky, it
is often assumed that it will grow on soils too depleted in plant
food to produce other crops. These regions represent soils which have
become exhausted primarily in nitrogen and humus as the result of
continuous cropping with nonleguminous plants. Some of these soils
contain sufficient phosphorus and potassium for fair crop production,
although this supply may be in such a condition that it will not
become available fast enough to supply the needs of most crops. Sweet
clover, like all legumes, has the power to extract nitrogen from the
atmosphere, and on account of its extensive root system it is able to
obtain phosphorus and potassium from a larger area than most plants.
The large roots not only add a quantity of humus and nitrogen to the
soil but they also open it up to a considerable depth, thus providing
better aeration and improving its physical condition. Improved physical
condition causes the bacterial flora to increase and thereby indirectly
causes a larger quantity of unavailable phosphorus and potassium to be
made available for plant use.
On soils which are known to be low in phosphorus or potassium an
application of fertilizer containing the necessary element should be
made when sweet clover is sown without a nurse crop. However, when
it is sown with a nurse crop or in the late summer or early fall on
grain stubble, the residues left in the soil from fertilizers applied
to the nurse crop will, under ordinary conditions, be sufficient for
the plants. That sweet clover will respond readily to applications of
phosphorus on soils low in this element has been well demonstrated by
the farmers of Livingston County, Ill. In this county finely ground
rock phosphate was applied to a portion of a number of fields at the
rate of 1,500 to 2,000 pounds per acre. The phosphate was thoroughly
incorporated with the soil just before seeding oats and sweet clover.
In the growth of sweet clover there was a marked difference the
following year between the treated and untreated portions of the
fields. Those portions of the fields which received an application of
phosphate not only contained many more plants on a given area, but the
vigor and growth of the plants were most marked. On June 1 the plants
on the treated areas were 12 to 15 inches taller than those on the
untreated parts of the fields. This difference in the thickness of
stand and the height of plants was so striking that the last round of
the phosphate spreader was plainly distinguishable.
Yields of sweet-clover hay have been increased as much as 2 tons per
acre from applications of barnyard manure. Such an increased yield
would be equal approximately to 8 tons of green-manure. Some people
may consider it poor farm practice to apply manure to such crops as
sweet clover, but it is very probable that the cumulative effect of the
increased yields of the following crops, especially on soils low in
organic matter, will be greater than if the manure is applied to other
crops. Heavy applications of manure to the preceding crop should also
greatly benefit sweet clover.