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 micro-organisms, 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 warty-like 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 within them in countless myriads. They gain an entrance into

the plant through the root hairs. The exact way in which benefit thus

comes to the clover plants is not fully understood, but it is now quite

generally conceded that the nitrogen taken in by these minute forms of

life is converted into soluble compounds, which are stored in the

tissues of the roots, stems and leaves of the plants, thus furnishing an

explanation to the increased vigor. It cannot be definitely ascertained

at present, if, indeed, ever, what proportion of the nitrogen in clover

is taken from the air and from the soil, respectively, since it will

vary with conditions, but when these are normal, it is almost certain

that by far the larger proportion comes from the air. But it has been

noticed that when soil is freely supplied with nitrogen, as in liberal

applications of farmyard manure, the plants do not form nodules so

freely as when nitrogen is less plentiful in the soil. The inference

would, therefore, seem to be correct, that when plants are well supplied

with nitrogen in the soil they are less diligent, so to speak, in

gathering it from the air. In other words, clover plants will take more

nitrogen from the air when the soil is more or less nitrogen hungry than

when nitrogen abounds in the soil. And yet the plants should be able to

get some nitrogen from the soil in addition to what the seed furnishes

to give them a vigorous start.

This power to form tubercles, and thus to store up nitrogen, is by no

means confined to clovers. It is possessed by all legumes, as peas,

beans and vetches. It is claimed that some of these, as soy beans, cow

peas and velvet beans, have even greater power to gather nitrogen from

the air and store it in the soil than clover, since the nodules formed

on the roots of these are frequently larger. In some instances, on the

roots of the velvet bean they grow in clusters as large as an ordinary

potato. With reference to all these leguminous plants it has been

demonstrated that under proper conditions good crops may be grown and

removed from the soil and leave it much richer in nitrogen than when the

seed was sown. It is thus possible by sowing these crops at suitable

intervals to keep the soil sufficiently supplied with nitrogen to grow

good crops other than legumes, adapted to the locality, without the

necessity for purchasing the nitrogen of commerce in any of its forms.

They may be made to more than maintain the supply of nitrogen,

notwithstanding the constant loss of the same by leeching down into the

subsoil in the form of nitrates, and through the more or less constant

escape of the same into the air in the form of ammonia, during those

portions of the year when the ground is not frozen.

They will do this in addition to the food supplies which they furnish,

hence they may be made to supply this most important element of

fertility, and by far the most costly when purchased in the market,

virtually without cost. The favorable influences which these plants thus

exert upon crop production is invaluable to the farmer. They make it

possible for him to be almost entirely independent of the nitrogen of

commerce, which, at the rate of consumption during recent years, will

soon be so far reduced as to be a comparatively insignificant factor in

its relation to crop production. It is possible, however, and not

altogether improbable, that by the aid of electricity a manufactured

nitrate of soda or of potash may be put upon the market at a price which

will put it within reach of the farmer. The power of legumes to increase

the nitrogen content in the soil should allay apprehension with

reference to the possible exhaustion of the world's supply of nitrogen,

notwithstanding the enormous waste of the same in various ways.

The more common sources of loss in nitrogen are, first, through the

leeching of nitrates into the drainage water; second, through oxidation;

third, through the use of explosives in war; and fourth, through the

waste of the sewerage of cities. When plant and animal products are

changed into soluble nitrates, they are usually soon lost to the soil,

unless taken up by the roots of plants. When vegetable matter on or near

the surface of the ground is broken down and decomposed, in the process

of oxidation, there is frequently much loss of nitrogen, as in the rapid

decomposition of farmyard manure in the absence of some material, as

land plaster, to arrest and hold the escaping ammonia. Through

explosives used in war there is an enormous vegetable loss of nitrogen,

as nitrate salts, which should rather be used to preserve and sustain

life than to destroy it. The waste of nitrogen through the loss of

sewerage is enormous, nor does there seem to be any practicable way of

saving the bulk of it.

In many soils the germs which produce nodules are present when clovers

are first grown on them. But where they are not present, the clover

plants have no more power to gather nitrogen than wheat or other

non-leguminous crops. But since in other soils they are almost entirely

absent, how shall they be introduced? The process of introducing them is

generally referred to as a process of inoculation, and soils when

treated successfully are said to be inoculated.

Three methods have been adopted. By the first, as previously indicated,

the grower perseveres in sowing clover at short intervals in the

rotation. He may also add farmyard manure occasionally, and thus,

through the inherent power of multiplication in the bacteria, they

increase sufficiently to enable the land to grow good crops. By the

second method, inoculating is effected through soil which is possessed

of the requisite bacteria; and by the third, it is effected through the

aid of a prepared product named nitragin.

When fields are to be inoculated by using soil it is obtained from areas

which have grown clovers successfully quite recently, and which are,

therefore, likely to be well filled with the desired bacteria. In some

instances the seed is mixed with the soil and these are sown together.

To thus mix the seed with the soil and then sow both together broadcast

or with a seed drill is usually effective, and it is practicable when

minimum quantities of soil well laden with germs are used. In other

instances the soil containing germs is scattered broadcast before or

soon after the seed is sown. Considerable quantities of earth must needs

be applied by this method.

It should be remembered that each class of legumes has its own proper

bacteria. Because of this, inoculation can only, or at least chiefly, be

effected through the use of soils on which that particular class of

legumes have grown, or which are possessed of bacteria proper to that

particular species. In other words, bacteria necessary to the growth of

vetches will not answer for the growth of clovers, and vice versa. Nor

will the bacteria requisite to grow medium red clover answer for growing

alfalfa. In other words, the bacteria proper to the growth of one member

of even a family of plants will not always answer for the growth of

another member of the same. But in some instances it is thought that it

will answer. The study of this phase of the question has not yet

progressed far enough to reflect as much light upon it as could be

desired. It is certainly known, however, that alfalfa will grow on soils

that grow burr clover (Medicago maculata) and sweet clover (Melilotus

alba), hence the inference that soil from fields of either will

inoculate for alfalfa.

Nitragin is the name given by certain German investigators to a

commercial product put upon the market, which claims to be a pure

culture of the root tubercle organism. These cultures were sold in the

liquid form, and it was customary when using them to treat the seed with

them before it was planted. Their use has been largely abandoned,

because of the few successes which followed their use compared with the

many failures. But it is now believed that these cultures can be

prepared and used so as to be generally effective and without excessive

cost to the grower.

In preparing cultures it has been found that by gradually reducing the

amount of nitrogen in the culture of media, it is possible to increase

the nitrogen fixing power in these germs from five to ten times as much

as usually occurs in nature. It is now known that the bacteria thus

grown upon nitrogen free media retain high activity if carefully dried

and then revived in liquid media at the end of the varying lengths of

time. Some absorbent is used to soak up the tubercle-forming organisms.

The cultures are then allowed to dry, and when in that condition they

can be safely sent to any part of the country without losing their

efficacy. It is necessary to revive the dry germs by immersing them in

water. By adding certain nutrient salts the bacteria are greatly

increased if allowed to stand for a limited time--as short, in some

instances, as 24 hours. The culture thus sent out in a dry form, and no

larger than a yeast cake, may thus be made to furnish bacteria

sufficient to inoculate not less than an acre of land. It is stated that

the amount of inoculating material thus obtained is only limited by the

quantity of the nutrient water solution used in increasing the germs, so

that the cost of inoculating land by this process is not large. The

culture may be applied by simply soaking the seed in it, by spraying the

soil, or by first mixing the culture into earth, spreading it over the

field and then harrowing it. Inoculations thus tried under the

supervision of the United States Department of Agriculture have proved

quite successful.

Where any legume is extensively grown surrounding soils come to be

inoculated through the agency of winds and water. The increase brought

to the yield of plants on various soils runs all the way from a slight

gain to 1000-fold. And when soil is once inoculated it remains so for a

long time, even though the proper legume should not be grown again on

the same soil.

The amount of nitrogen that may thus be brought to many soils by growing

clover and other legumes upon them is only hedged in practically by the

nature of the rotation fixed upon. An acre of clover when matured will

sometimes add 200 to 300 pounds of nitrogen to the soil under favorable

conditions. Where the soil contains the requisite bacteria, the young

plants begin to form tubercles when but a few weeks old, and continue to

do so while the plant is active until mature. That the plants use much

of the nitrogen while growing would seem to be clear, from the fact that

toward the close of the growing season the tubercles become more or less

broken down and shrunken.

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