Used as a Green Manure.—Where dependence must be placed upon clover as a fertilizer, little or no manure being returned to the land, at least one of the two clover crops within the year should be left on the land. The maximum benefit from clover, when left on the land, can be obtained by clipping it before it is sufficiently heavy to smother the plants, leaving it as a mulch. When the cutter-bar of the mower is tilted upward, the danger of smothering is reduced. Truckers, remote from supplies of manure, have found it profitable to make two such clippings just prior to blossoming stage, securing a third heavy growth. The amount of humus thus obtained is large, and the benefit of the mulch is an important item.
Some growers clip the first crop for a mulch, and later secure a seed crop. The early clipping and the mulch cause increase in yield of seed.
A common practice is to take one crop off for hay, and to leave the second for plowing down the following spring. Early harvesting of the clover for hay favors the second crop.
When to turn Down.—When the maximum benefit is desired for the soil from a crop of clover, the first growth should not be plowed down. Its office should be that of a mulch. In its decay all the mineral plant-food and most of the nitrogen go into the soil. The second crop should come to maturity, or near it. As a rule, there is gain, and not loss, by letting the second crop lie on the ground until spring if a spring-planted crop is to follow. Some fall growth, and the protection from leaching, should equal any advantage arising from rotting the bulky growth in the soil. In some regions it is not good practice to plow down a heavy green crop on account of the excessive amount of acid produced. When this has been done, the only corrective is a liberal application of lime.
Mammoth Clover.—When clover is grown with timothy for hay, some farmers prefer to use mammoth clover in place of the medium red. It may be known as sapling clover, and is accounted a perennial, though it is little more so than the red. It is a strong grower and makes a coarse stalk but, when grown with timothy, it has the advantage over the red in that the period of ripening is more nearly that of the timothy. It inclines to lodge badly, and should be seeded thinly with timothy when wanted for hay. The roots run deep into the soil, and this variety of clover compares favorably with the medium red in point of fertilizing power, the total root-growth being heavier. While its yield of hay, when seeded alone, is greater than the first crop of the red, its inclination to lodge and its coarseness are offsets. It produces its seed in the first crop, and the after-growth is small, while red clover may make a heavy second crop. Its use should become more general on thin soils, its strong root-growth enabling it to thrive better than the red, and the lack of fertility preventing the stalks from becoming unduly coarse for hay. The amount of seed used per acre, when grown by itself, should be the same as that of red clover.
Alsike Clover.—A variety of clover that may have gained more popularity than its merit warrants is alsike clover. It is more nearly perennial than the mammoth. The roots do not go deep into the subsoil like those of the red or the mammoth, and therefore it is better adapted to wet land. It remains several years in the ground when grazed, and is usually found in seed mixtures for pastures. It is decumbent, and difficult to harvest for hay when seeded alone. It is credited with higher yields than the red by most authorities, but this is not in accord with observation in some regions, and it is markedly inferior to the red in the organic matter and the nitrogen supplied the soil in the roots.
The popularity of this clover is due to its ability to withstand some soil acidity and bad physical conditions. In regions where red clover is declining on account of lack of lime, one may see some alsike. The rule is to mix alsike with the red at the rate of one or two bushels of the former to six bushels of the latter. As the seed of the alsike is hardly half as large as that of the red, the proportion in the mixture is greater than some farmers realize. The practice is an excellent one where the red will not grow, and the alsike adds fertility, but when the soil has been made alkaline, the red clover should have nearly all the room. Alsike is a heavy producer of seed.
Crimson Clover.—Wherever crimson clover is sufficiently hardy to withstand the winter, as in Delaware and New Jersey, it is a valuable aid in maintaining and increasing soil fertility. It is a winter annual, like winter wheat, and should be seeded in the latter half of summer, according to latitude. It comes into bloom in late spring. The plant has a tap-root of good length, but in total weight of roots is much inferior to the red. This clover, however, compares favorably with red clover in the total amount of nitrogen added to the soil by the entire plant when grown under favorable conditions. It is peculiarly fitted for a cover crop in orchards and wherever spring crops are removed as early as August, or a seeding can be made in them, as is the case with corn. Even when winter kills the plants, a successful fall growth is highly profitable, adding more nitrogen before winter than red clover seeded at the same time. Where the plants do not winter-kill, they are plowed down for green manure when in bloom in May, or earlier in the spring to save soil moisture and permit early planting, although a good hay for livestock can be made, and the yield is about the same as that of the first crop of red clover.
In the northern states a large amount of money has been wasted in experimental seedings with crimson clover, and it is only in exceptional cases that it continues to be grown. There is reason to believe that many of these failures were due to lack of soil inoculation. The Pennsylvania experiment station is located in a mountain valley where winters are severe. Crimson clover is under test with other cover crops for an experimental orchard, and success with it has increased as the soil has become fully inoculated. This view is supported by the experience of various growers in the North, and while crimson clover can never make the success in a cold climate that it does in Delaware, there is a much wider field of usefulness for it than is now occupied. Experiments should be made with it under favorable conditions respecting moisture and soil tilth. Fifteen pounds of seed should be used, and the seed should be well covered, as is the case with all seeds sown in mid-summer.