Imágenes de páginas

Clover Sickness. — On certain of the soils of Great Britain and probably on those of other countries in Europe, where clover has been grown quite frequently and for a long period, as good crops cannot be grown as previously, and in some instances the crop is virtually a failure. The plants will start from seed in the early spring and grow with sufficient vigor for a time, after which they will show signs of wilting and finally they die. Various theories were advanced for a time as to the cause before it was ascertained by experiment what produced these results. Some thought they arose from lack of water in the soil, others claimed that they were due to the presence of parasites, which in some way, preyed upon the roots, others again attributed them to improper soil conditions. It is now just about certain that they arose from a deficiency of soluble potash in the subsoil. Such, at least, was the conclusion reached by Kutzleb as the result of experiments conducted with a view to ascertain the cause of clover wilt.

The cause being known, the remedy is not difficult. It is to grow. clover less frequently on such soils. Sufficient time must be given to enable more of the inert potash in the subsoil to become available. Another way would be to apply potash somewhat freely to these soils, and subsoil them where this may be necessary.

It is thought that' clover sickness is as yet unknown in the United States and Canada, although its presence had sometimes been suspected in some sections where clover has been much grown. This

does not mean that it may not yet come to this country. Should the symptoms given above appear on soils on which clover has been grown frequently and for a long period, it would be the part of wisdom to take such indications as a hint to grow clover less frequently in the rotation.

Possible Improvement in Clovers. Some close observers have noticed that there is much lack of uniformity in the plants found growing in an ordinary field of clover, especially of the medium red and mammoth varieties. Many of the plants vary in characteristics of stem, leaf, flower and seed; in the size and vigor of the plants; in the rapidity with which they grow; and in earliness or lateness in maturing. So great are these differences that it may be said they run all the way from almost valueless to high excellence. Here, then, is a wide-open door of opportunity for improving clover plants through selection. This question has not been given that attention in the past which its importance demands.

There may be a difference in view as to all the essential features of improvement that are to be sought for, but there will probably be agreement with reference to the following in desirable varieties: 1. They will have the power to grow quickly and continuously under average conditions. This power will render them valuable as pasture plants in proportion as they possess it. 2. They will produce many stems not too coarse in character. This will affect favorably the character of the hay and will also have a bearing on increase in the production of

seed. 3. There should be an abundance of leaves. Such production will affect favorably palatability in the pasture and also in the hay. 4. The blossoms should be so short that the honey which they contain may be accessible to the ordinary honey bee. The importance of this characteristic cannot be easily overestimated. It would not only tend to a great increase in seed production through the favorable influence which it would have on fertilization, but it would greatly increase the honey harvest that would be gathered every year, and 5. They should be possessed of much vigor and hardihood; that is, they should have much power to grow under adverse conditions, as of drought and cold. The person who will furnish a variety of red clover possessed of these characteristics will confer a boon on American agriculture.

Bacteria and Clovers.—The fact has long been known, even as long ago as the days of Pliny, and probably much before those days, that clover, when grown in the rotation, had the power to bring fertility to the soil. This fact was generally recognized in modern agriculture and to the extent, in some instances, of giving it a place even in the short rotations. But until recent decades, it was only partially known how clover accomplished such fertilization. It was thought it thus gathered fertility by feeding deeply in the subsoil, and through the plant food thus gathered, the root system of the plants were so strengthened in the cultivated surface section of soil as to account for the increased production in the plants that followed clover. According

to this view, the stems and leaves of the plants were thus equally benefited and, consequently, when these were plowed under where they had grown these also added plant food to the cultivated portion of the soil, in addition to what it possessed when the clover seed which produced the plants was sown upon it. In brief, this theory claimed that fertility was added by the clover plants gathering fertility in the subsoil and depositing it so near the surface that it became easily accessible to the roots of other plants sown after the clover and which had not the same power of feeding so deeply. This theory was true in part. The three important elements of plant food, nitrogen, phosphoric acid and potash, were and are thus increased in the soil, but this does not account for the source from which the greater portion of the nitrogen thus deposited in the soil was drawn, as will be shown below.

It was also noticed that when the seed of any variety of clover was sown on certain soils, the plants would grow with more or less vigor for a time and then they would fail to make progress, and in some instances would perish. It was further noticed that if farmyard manure was applied freely to such land, the growth made was more vigorous. Yet, again, it was noticed that by sowing clover at short intervals on such soils, the improvement in the growth of the plants was constant. But it was not understood why clover plants behaved thus under the conditions named. It is now known that ill success at the first was owing to the lack of certain microorganisms, more commonly termed bacteria, in the

soil, the presence of which are essential to enable clover plants to secure additional nitrogen to that found in the soil and subsoil on which to feed. When manure was applied, as stated above, the clover plants secured much or all of their nitrogen from the manure. Bacteria were introduced in very limited numbers at first, it may be through the medium of the seed or in some other way, and because of an inherent power which they possess to increase rapidly in connection with continued sowing of clover at short intervals, they came at length to be so numerous in the soil as to make possible the growth of good crops of clover where these could not be thus. grown a few years previously.

Careful observers had noticed that certain wartylike substances were found attached to the roots of clover plants, and that the more vigorously the plants grew, the larger and more numerous were these substances, as a rule. It was thought by many that these warty substances, now spoken of as nodules, were caused by worms biting the roots or because of some unfavorable climatic influence or abnormal condition of soil. It is now known that they are owing to the presence of bacteria, whose special function is the assimilation of free, nitrogen obtained in the air found in the interstices; that is, the air spaces between the particles of soil. This they store up in the nodules for the use of the clover plants and also the crops that shall follow them.

The nodules in clover plants vary in size, from a pin head to that of a pea, and they are frequently present in large numbers. Bacteria are present

« AnteriorContinuar »