Clover has for centuries been a most valuable forage crop; and for eons it has been the special partner of the bees, giving them honey for their service in carrying its pollen; and in recent years it has been discovered that it has also formed a mysterious and undoubtedly an ancient partnership with bacteria below ground, which, moreover, brings fertility to the soil. The making of a collection of the clovers of a region is a sure way of enlisting the pupils' interest in these valuable plants. The species have some similarities and differences, which give opportunity for much observation in comparing them. There may be found in most localities the white and yellow sweet clovers, the black and spotted medics and their relative the alfalfa; while of the true clovers there are the red, the zigzag, the buffalo, the rabbit's foot, the white, the alsike, the crimson, and two yellow or hop clovers.
In all the clovers, those blossoms which are lowest, or on the outside of the head, blossom first, and all of them have upon their roots the little swellings, or nodules, which are the houses in which the beneficent bacteria grow.
If we pull up or dig out the roots of alfalfa, or of the true clovers or vetches, we find upon the rootlets little swellings which are called nodules, or root-tubercles. Although these tubercles look so uninteresting, no fairy story was ever more wonderful than is theirs. They are, in fact, the home of the clover brownies, which help the plants to do their work. Each nodule is a nestful of living beings, so small that it would take twenty-five thousand of them end to end to reach an inch; therefore, even a little swelling can hold many of these minute organisms, which are called bacteria. For many years people thought that these swellings were injurious to the roots of the clover, but now we know that the bacteria which live in them are simply underground partners of these plants. The clover roots give the bacteria homes and place to grow, and in return these are able to extract a very valuable chemical fertilizer from the air, and to change its form so that the clovers can absorb it. The name of this substance is nitrogen, and it makes up more than three-fourths of the air we breathe. Other plants are unable to take the nitrogen from the air and use it for food, but these little bacteria extract it from the air which fills every little space between every two grains of soil and then change it to a form which the clovers can use. After the clover crop is harvested, the roots remain in the ground, their little storehouses filled with this precious substance, and the soil falls heir to it.
Nitrogen in the form of commercial fertilizer is the most expensive which the farmer has to buy. So when he plants clover or alfalfa on his land, he is bringing to the soil this expensive element of plant growth, and it costs him nothing. This is why a good farmer practices the rotation of crops and puts clover upon his land every three or four years.
Alfalfa is so dependent on its little underground partners, that it cannot grow without them; and so the farmer plants, with the alfalfa seed, some of the soil from an old alfalfa field, which is rich in these bacteria. On a farm I know, the bacterial soil gave out before all of the seed was planted; and when the crop was ready to cut it was easy to see just where the seed without the inoculated soil had been planted, for the plants that grew there were small and poor, while the remainder of the field showed a luxurious growth.
It is because of the great quantity of nitrogen absorbed from the air through the bacteria on its roots that the alfalfa is such a valuable fodder; for it contains the nitrogen which otherwise would have to be furnished to cattle in expensive grain or cotton-seed meal. The farmer who gives his stock alfalfa does not need to pay such large bills for grain. Other plants belonging to the same family as the clovers—like the vetches and cow-peas—also have bacteria on their roots. But each species of legume has its own species of bacteria; although in some cases soil inoculated with bacteria from one species of legume will grow it on roots of another species. Thus, the bacteria on the roots of sweet clover will grow on the roots of alfalfa and many farmers use the soil inoculated by sweet clover to start their alfalfa crops.
In addition to the enriching of the soil, clover roots, which penetrate very deeply, protect land from being washed away by freshets and heavy rains; and since clover foliage makes a thick carpet over the surface of the soil, it prevents evaporation and thus keeps the soil moist. Crimson clover is used extensively as a cover crop; it is sowed in the fall, especially where clean culture is practiced in orchards, and spreads its leaves above and its roots within the soil, keeping out weeds and protecting the land. In the spring it may be plowed under, and thus add again to the fertility. This is also an aesthetic crop, for a field of crimson clover in bloom is one of the most beautiful sights in our rural landscape.
Red clover has such deep florets that, of all our bees, only the bumblebees have sufficiently long tongues to reach the nectar. It is, therefore, dependent upon this bee for developing its seed, and the enlightened farmer of to-day looks upon the bumblebees as his best friends. The export of clover seed from the United States has sometimes reached the value of two million dollars per year, and this great industry can only be carried on with the aid of the bumblebee. There are sections of New York State where the growing of clover seed was once a most profitable business, but where now, owing to the dearth of bumblebees, no clover seed whatever is produced.
Leading thought—The clovers enrich with nitrogen the soil in which they are planted. They are very valuable as food for stock; and their flowers are pollenated by bees.
Method—Each pupil should dig up a root of red clover or alfalfa to use for the lesson on the nodules. The flowers should be studied in the field, and also in detail in the schoolroom.
1. How many kinds of clover do you know? How many of the medics?
2. In all clovers, which flowers of the head blossom first, those on the lower or outside, or those on the upper or inside?
3. Take up a root of red clover or alfalfa, noting how deep it grows. Wash the root free from soil, and find the little swellings on it. Write the story of what these swellings do for the clover, and incidentally for the soil.
4. How must the soil be prepared so that alfalfa may grow successfully? What does the farmer gain by feeding alfalfa, and why?
5. How do clover roots protect the land from being washed by heavy rains?
6. How do clovers keep the soil moist? How does this aid the farmer?
7. What is a cover crop, and what are its uses?
8. Upon what insects does the red clover depend for carrying pollen? Can it produce seed without the aid of these valuable bees? Why not?