NEED OF LIME ON ACID SOILS.

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.

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.

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.

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.

FERTILIZERS.

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