Sweet clover thrives on the adobe and granitic soils of the Pacific coast; upon the gumbo, hardpan, prairie, and sandy soils of the western North-Central States; and upon the heavy clay, loam, limestone, and sandy soils of the South and East. In fact, it has been grown successfully on all the principal soil types of the United States where the soils were not acid and were well inoculated. It grows luxuriantly on the Selma chalk (rotten-limestone) soils of Alabama and upon soils rich in calcium carbonate in many parts of the country where the lack of nitrogen and humus has caused large numbers of farms to be abandoned. The plants thrive on newly exposed heavy clay soils and upon steep embankments where little else will grow. Sweet clover is more tolerant of poor drainage, overflow, and seepage conditions than alfalfa. In irrigated sections, especially where the reservoir system is in use, large bodies of land are likely to become useless for the growth of alfalfa because of the rising of the water table. On such areas sweet clover will make a vigorous growth. However, maximum growth is to be expected only on well-drained soil.
Sweet clover will do well on many soils which are not fertile enough to grow red clover or alfalfa, and it is on these soils that it will prove most valuable. Like many other plants, it makes its best growth on fertile soils rich in calcium carbonate, although it will make sufficient growth on poor soils which are not acid to warrant planting it on them. Many hilly pastures may profitably be seeded to sweet clover. It will not only make a valuable addition to the forage of these pastures but will improve the soil so that grasses will grow more abundantly. Some of the best pastures in the Middle West are composed of bluegrass, timothy, and sweet clover.
Sweet clover grows successfully on soils in the West which apparently are too alkaline for grains or alfalfa. The Wyoming Agricultural Experiment Station reports that it has obtained good yields of sweet clover on seepage land which is so strongly alkaline that no other plants except some of the native grasses will survive, while the California Agricultural Experiment Station found that sweet clover will withstand alkali to a remarkable degree. Prof. F. S. Harris, agronomist of the Utah Agricultural Experiment Station at Logan, claims that it is one of the most alkali-resistant crops grown in Utah, and that in and 1913 and 1914 quite an industry developed in some parts of that State in growing sweet clover for hay and seed on land too alkaline for other crops.
In reply to a circular letter on the culture of sweet clover, approximately 100 county agents and extensive growers of this crop located in many parts of the West state that this plant is one of the most alkali-resistant plants grown in their respective districts. In Crook County, Oreg., a good stand was obtained from April seeding in 1915 on a 20-acre demonstration field of sandy loam bottom land so strongly alkaline from black alkali that only salt grass was growing on it before it was planted to sweet clover. This field pastured from 18 to 28 head of calves, cows, and horses from June 1 to October 1 without being irrigated. Sweet clover generally will grow on soils where salt grass[6] will survive, and it is very much superior to this grass as pasture. After the drainage of water-logged land on which there is a surface accumulation of alkali, it is the common practice in parts of Utah to grow sweet clover for several years before planting alfalfa. It is often stated that alkali land will grow less tolerant crops after sweet clover has been grown on it for a few years. The long roots will open up the subsoil and cause better drainage, thereby affording an excellent means for removing the salts from the soil, as they are readily soluble in water.
[6] Distichlis spicata.
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.