CLOVER. Figs. 333-343.
The word clover is popularly used to designate herbaceous forage plants of several genera of the family Leguminosze, but by botanists it is re stricted to species of the genus Trifolium. In this article, the clovers are considered to be Trifo hums. The Florida clover will be found under the article Beggarweed, the Japan clover under Les pedeza, the bur and hop clovers under Medicago, the Sweet, Bokhara or tree clover under Melilotus. Related plants are alfalfa, serradella, sulla, sain foin, vetch, lupine.
The genus Trifolium comprises probably two hun dred or more species and marked natural varieties, most frequent in the temperate parts of the north ern hemisphere, but occuring also on mountains in tropical countries, and to some extent in South Africa. They are annual, biennial or perennial, usually with compound leaves of three leaflets (whence the name trifolium), but in some species of five or seven leaflets, and papilionaceous (pea like) small flowers usually in dense heads ; stamens ten, nine of them united by their filaments ; fruit a very small and usually indehiscent pod contain ing few nearly spherical seeds. The flowers are white or in shades of red, red-purple or yellow. Several of the clovers are sometimes grown for ornament [see Cyclopedia of American Horticul ture], but the great value of the plants lies in their usefulness for green-manuring [see Vol. I, page 504] and for forage [see, also, Forage, Meadows and Pastures, in this volume]. The important agricul tural clovers are Trifolium pratense, T. hybridum, T. repens, T. incarnatum and T. Alexandrinum; sev eral other species are more or less weedy plants along roadsides and in waste places. The impor tant forage clovers and also most of the weedy kinds are native of the Old World.
The ability to grow clover successfully and uniformly is one of the marks of a good farmer in the northern states and Canada. Clover of some kind is almost a necessary part of self-sustaining rotations in these regions. In the very short rota tions in which clover occurs, the land is likely to refuse to produce clover after a few courses.
In that case, other crops may be substituted for a time. it is not known just why clover will not grow in certain cases. In Europe much is said about "clover sickness," but it is doubtful whether the same cause or condition is present in this country, at least to any great extent. Experi ments at Rothamsted, as reported in 1901, "seem to exclude the supposition that the primary cause of failure (`clover sickness') is either destruction by parasitic plants or insects, injury from excreted matter, or shade of a corn crop, and to indicate that it must be looked for in exhaustion of some kind within the range of the roots." It has been asserted by others that lack of available potash in the subsoil is the cause. Giissow, reporting to the Royal Agricultural Society of England in 1903, considers the fungus Sclerotinia ciborioides to be the real cause of clover sickness. The refusal of lands in America to produce clover is probably due to various causes. It is frequently attributed to soil acidity. Lack of the nitrogen-gathering bacteria may sometimes be a cause. Inoculation of soils with artificial cultures has been tried, but not with uniform or very important results, although the nitragin culture has given promising returns in Europe. Inoculation with soil from an inoculated field has given good results in this country, and its value seems to be fully demonstrated.
Group L The forage clovers.
(1) Red clover, medium red clover (Trifoliuni pratense, Linn.) Fig. 333, is one of the most important of hay plants.
It is variable in size, habit and other charac teristics, suggesting that it offers a promis ing field for the plant breeder. It is usually perennial, although tending to run out after the third year, and some times even after the second year. It is a spreading, hairy plant, bearing purplish (or sometimes rarely white) heads on the summits of branching, leafy stems, the upper leaf be ing nearly or quite sessile and borne close under the head ; leaflets oval or oblong-ovate, sometimes notched at the end, very short -stalked, marked with a prominent whitish spot.