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VII. Gains of Nitrogen from the Air by Means Other than Legumes

VII. Gains of Nitrogen from the Air by Means Other than Legumes

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1. Some Reported Nitrogen Gains under Field Conditions



In the early years of the science of soil microbiology it was determined by many investigators that uncropped soils may sometimes show

gains in nitrogen. The isolation of the anaerobic nitrogen-fixing bacterium, Clostridiurn pasteurianum, by Winogradsky in 1895, and of

two species of the aerobic Azotobacter by Beijerinck in 1901, served to

explain, at least partially, these nitrogen gains. An intensive and enthusiastic study of the free-living organisms followed. I n the laboratory,

where both pure cultures of the bacteria and soils were studied, there

was usually no difficulty in demonstrating nitrogen fixation if sugar

or other suitable energy sources were supplied. Under field conditions,

however, negative results were predominant, and interest in these organisms from the practical standpoint waned. Most of these early

workers seemed convinced that in ordinary soils the fixation was too

small to be measured accurately, especially under conditions where

leaching and gaseous losses of nitrogen were occurring simultaneously.

Many reports of appreciable nitrogen fixation in field plots or

lysimeters, usually attributed to nonsymbiotic bacteria, have, however,

been reported. Examples of these are the publications of Chapman

et al. (1949), Hall (1905, 1912), Lohnis (1909), Morse (1936), Smitb

(1944), and Smith et al. (1954). Reports of nitrogen fixation in grasslands (Karraker et al. 1950; Lyon and Wilson, 1928; Miller, 1947;

Richardson, 1938; White et al., 1945; Whitt, 1941) are especially common, considering the number of such studies made. Numerous reports

of fixation have also come from studies in tropical and Far Eastern soils.

How is the reader t o evaluate claims of fixations of 20 t o 40 and

more pounds of nitrogen per acre annually in cultivated and grassland

soils in the Temperate Zone? It should be emphasized first that for every

positive result there are many, many negative ones. Furthermore,

when a large fixation is obtained it is likely to be emphasized, whereas

a loss or small gain may scarcely be mentioned. From our knowledge

of the physiology of the nonsymbiotic nitrogen-fixing organisms, as

well as of the accuracy with which field experiments can be conducted,

it would seem that reported nitrogen gains in grassland soils are much

more likely to be reliable than are those for cultivated soils. But even

nitrogen increases in grass sods that may seem to be real are very likely

to be only apparent, for a variety of reasons including: (1) errors of

soil sampling and analysis, especially where organic matter accumulations are involved; (2) inadequate allowance for additions of nitrogen

in the rainfall; ( 3 ) failure to keep the grass free from all leguaes; (4)



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insufficient protection against the collection of extraneous material,

especially due to winds; ( 5 ) lack of certainty that nitrogen is not obtained from the subsoil or ground waters; and (6) inadequate replication and lack of statistical treatment of the data. Frequently a grass

plot is located along side of a cultivated plot, and the marked difference

in nitrogen content after several years is emphasized. Obviously such

data furnish no reliable information on the importance of nonsymbiotic

fixation if the quantities of nitrogen lost by leaching and volatilization

are not known accurately, and they seldom are.



2. T h e Organisms Involved

Another and, it is believed, more accurate way of evaluating the importance of nonsymbiotic nitrogen fixation in soils is to consider the

organisms involved and their physiology so far as it relates specifically

to nitrogen fixation. The microorganisms of chief importance are the

anaerobic butyric acid bacteria, such as Clostridium; the aerobes of

which Azotobacter is the chief representative; and certain blue-green

algae, chiefly Nostoc.

The anaerobic nitrogen-fixing bacteria are widely distributed in

soils and are apparently able to grow, or at least to live, under rather

widely varying environmental conditions. Often they are the predominant, or even the only, nitrogen-fixing organisms present, although

they are likely to occur chiefly as spores (Swaby, 1939). Some workers

have expressed the opinion that the anaerobes are probably of more

economic importance than are Azotobacter, but present information

does not justify definite conclusions on this matter. Although anaerobic,

these organisms can grow in soils that are aerobic, possibly owing in

part to their association with other species of organisms that are removing oxygen from their immediate environment. So far as their nitrogenfixing ability is concerned, it has been rather generally agreed that the

chief limitation to growth in soils is the small supply of suitable energy

sources, and the inefficiency with which they utilize these sources.

Normally they convert sugars to organic acids, especially butyric, but

cannot utilize the energy in these products. Most workers have obtained

less nitrogen fixation per unit of sugar utilized by these anaerobes than

by Azotobacter, although the fixation per unit of energy utilized may

be higher. These long-accepted views have, however, been challenged

recently by Parker (1954). He found that when the necessary growth

factors were supplied to a liquid medium, Clostridium butyricum fixed

in 10 days an average (6 analyses) of 27 mg. nitrogen per gram of

glucose supplied. Using such an improved medium he also obtained

much larger numbers of viable cells in soils than most other investiga-



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tors have observed. These newer findings are very interesting, but it

is still necessary to know if any appreciable percentage of the cells

found in soils occur in other than the spore form. Data are also much

needed on the quantities of nitrogen fixed by these anaerobes in soils

under normal conditions.

Azotobacter are widely distributed in soils that have a pH value of

6 or above, but not in more acid soils. One strain, Azotobacter indicun,

will tolerate marked acidity, but this organism has been reported from

only a few locations. Although a fairly common soil organism, Azotobacter seldom occurs in large numbers. Its energy requirements are

high. Under the most ideal conditions, and in experiments lasting less

than 24 hours, Wilson and Burris (1953) state that in pure culture

fixation of 15 to 20 mg. of nitrogen per gram of sugar is common; the

efficiency decreases with age of culture. The organism can utilize only

simple energy sources. Crop residues become available only as a result

of attack by other organisms that are likely to use most of the food

supply themselves. It sometimes cooperates (Imshenetskii and Solntseva,

1940; Jensen and Swaby, 1941; Vartiovaara, 1938) with cellulosedecomposing organisms to obtain its energy supply, but information on

this is still rather meager. In the presence of much available nitrogen,

or in the absence of an adequate supply of the essential catalytic element molybdenum, the quantity of nitrogen fixed is usually low. Some

workers have thought that Azotobacter lives in the rhizosphere of higher

plants and obtains its energy supply from root excretions, but recent

studies (Clark, 1948; Jensen, 1940) have not shown that the organism

grows in large numbers in association with roots. Even if they could

thrive in this environment, it is not likely that they could obtain much

utilizable carbohydrate from the roots.

Many attempts (Allison, 1947; Allison et al., 1947; Bukatsch and

Heitzer, 1952; FBhraeus et al., 1948; Gainey, 1949; Schmidt, 1948-49;

Wichtman, 1952) have been made to demonstrate that Azotobacter

inoculation of soils increases crop production, but nearly all of these

attempts, except in the U.S.S.R., have given negative results. Many

large increases in yields have been reported from the U.S.S.R., however,

and at the present time millions of acres of nonlegumes are being inoculated annually in that country. ApparentIy Soviet scientists are convinced that such a practice is profitable. There is, however, an increasing tendency for workers in the Soviet Union, and also in Germany, to

attribute any benefits obtained to the production of auxins and other

plant growth factors rather than to nitrogen fixation. Considerable work

(Bukatsch and Heitzer, 1952; Spicher, 1954) is also being done in Germany and in the U.S.S.R. in an attempt to get strains of Azotobacter to



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adapt themselves to the roots of nonlegumes. There is little indication

of success in any of these studies.

A few genera of blue-green algae are able to utilize atmospheric

nitrogen if they are grown in a suitable inorganic medium with light as

the source of energy. Under some special conditions they are probably

of far greater importance as nitrogen gatherers than are free-living

bacteria. In an environment such as a rice field, for example, several

studies (De, 1936; De and Sulaiman, 1950; Okuda and Yamaguchi,

1952; Singh, 1942; Watanabe, 1950; Watanabe et al., 1951) show that

blue-green algae are commonly abundant and active in nitrogen fixation. Although not limited by lack of energy supply if grown in light,

they do require much moisture, a neutral medium, and a fairly warm

climate. Even under optimum conditions their rate of growth is very

much slower than that of bacteria. These facts would indicate that they

are not of great importance in normal soils, but quantitative data in

support of this viewpoint are lacking.

Much interest is now being manifested in the photosynthetic bacteria which were only recently shown to be able to utilize atmospheric

nitrogen. The work to date, which is reviewed by Wilson and Burris

(1953), is concerned with the physiology of the organisms and the

mechanisms of the fixation process. There is no evidence that these

bacteria are of any appreciable importance in soils.

Certain higher nonlegume plants, such as A h u s and Casuarina,

also fix nitrogen just as do legumes, when they are inoculated with the

proper organisms and have developed root nodules. Other plants have

nodules in the leaves. Most of these plants are trees or shrubs and hence

of no importance in general agriculture. Many claims have also been

made that various common nonlegume crops can fix nitrogen when

they are grown in a deficiency of nitrogen. These claims have not been

substantiated and hence need not be considered further in this discussion.



3 . Nonbiological Nitrogen Gains

Aside from the nitrogen brought down in rainfall, two other suggested sources of soil nitrogen gain by other than biological agencies

need to be mentioned. These are nitrogen fixation by sunlight and

absorption of ammonia from the air by soils and plants.

Photochemical fixation of nitrogen in soils has been much emphasized in India by various workers, especially Dhar (Dhar, 1946; Dhar

and Shesharcharyulu, 1941; Dhar et al., 1941). These workers report

comparatively large fixations under laboratory conditions when sterile

soil is exposed to light if suitable energy sources are available for oxida-



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tion. Until such claims have been verified by workers elsewhere, serious

consideration cannot be given to such a nonbiological fixation process,

especially in nontropical countries.

Ingham (1940, 1950) was not impressed with the importance of

free-living nitrogen-fixing bacteria in soils and suggested that the

restoration of nitrogen to soils, commonly attributed to microorganisms,

is more likely due to the absorption of ammonia from the air by cellulose and related materials, and by organic and inorganic colloids in the

soil. He was of the opinion that the quantity of ammonia so absorbed

each year may be adequate for a good growth of many crops. At a

much earlier date, Hall and Miller (1911) studied this same phenomenon, using sulfuric acid as the absorbing agent, and concluded that

such absorption would be less than a pound of nitrogen per acre per

year. If comparatively large quantities of ammonia are constantly

being absorbed from the air by living plants, soil humus, and inorganic

soil colloids, as Ingham suggests, it is difficult to see why nitrogen should

ever be very deficient in any cropped or uncropped soil. Certainly more

evidence is required before much importance can be attached to this

method of soil nitrogen gain.



4 . Practical Importance-A General Statement

The above brief review of the chief known facts regarding nitrogen

gains from the air through channels other than legumes emphasizes how

little actual quantitative information is available that applies to field

conditions. There is, of course, little doubt but that nonsymbiotic fixation of nitrogen is of some importance in agriculture, but just how important under various soil and climatic conditions cannot be stated with

accuracy. Most workers (Jensen, 1950; Norman, 1946), at least those

outside of Russia and India, consider that the quantity of nitrogen fixed

in the average soil by free-living organisms is small. The writer is in

agreement with this rather generally accepted view. However, it does

seem barely possible that as much as 20 pounds per acre of nitrogen

may be fixed annually in an occasional well-limed grassland, where

conditions are especially favorable for biological activity. Likewise,

fixations of this magnitude might possibly occur where very large applications of carbonaceous crop residues are made to certain cultivated

soils. Although such nitrogen gains may be possible, there is little

justification for a positive statement that they occur. As has been

emphasized above, methods do not permit the accurate measurement of

such quantities of nitrogen under field conditions. Even if there were no

losses of nitrogen through volatilization and leaching, the sampling and

analytical errors would be as large or larger in most experiments as the



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maximum gains expected. Researches during the past 20 years have not

supplied information that would seem to justify any substantial increase

in the estimate made by Lipman and Conybeare in 1936 that an average

of 6 pounds of nitrogen is fixed per acre per year by nonsymbiotic organisms. This estimate, when made, was based on wholly inadequate

data, and any estimate made today would also be little more than a

guess. Our present knowledge of nonsymbiotic bacteria might indicate

that the 6-pound figure is high. On the other hand, the contribution of

blue-green algae, which had barely been discovered in 1936, might be

appreciable. Although their main contribution is doubtless in rice paddies and similar locations, it is not unlikely that they may also make

some small contribution to the nitrogen supply in nonacid grassland

soils and other places where moisture is favorable at certain seasons of

the year. In addition, there are good reasons to believe that soils may

absorb traces, at least, of ammonia and oxides of nitrogen from the

atmosphere.

VIII. CONCLUDING

STATEMENT

This evaluation of the pertinent data that relate to soil nitrogen

balance sheets has, in general, emphasized the ease with which nitrogen

is lost from soils. Mineral nitrogen, whether added as fertilizer or

formed from soil organic matter, will not long remain in the soil in this

form. If it is not assimilated by higher plants it will usually either be

leached from the soil or be lost by volatilization. The extent of loss by

leaching depends chiefly upon soil texture and on the amount of rain

that penetrates the soil before the crop can assimilate the nitrogen.

Leaching losses can be minimized to some extent by providing sufficient

carbonaceous materials to combine with it. But this fixing of the nitrogen by carbon is only temporary unless there is a permanent change in

the agronomic system that will permit the establishment of a new equilibrium, and a higher organic matter level. Losses by volatilization,

which may be large under some conditions described above, can be

kept to a minimum by using a few precautions. It is especially important that ammonia either not be applied to alkaline soils or that it be

well incorporated with them; otherwise some of it may escape as gas.

Bacterial denitrification can be minimized by applying nitrate-nitrogen

only at the time the crop is ready to assimilate it. The longer such

nitrogen is present in soils, especially if conditions of aeration are not

good, the greater the probability of loss as oxides of nitrogen and as free

nitrogen gas.

From the facts presented it is obvious that, regardless of years of

research, an accurate soil nitrogen balance sheet for a field soil can



SOIL NITROGEN BALANCES



24 7



seldom be drawn up. This is, of course, because we usually lack quantitative data for some of the major items that are known to be involved in

the calculations. The data are lacking because of the experimental difficulties encountered in obtaining them. It is impossible, or at least impractical, to account for all soil gains and losses in a single experiment

conducted under reasonably normal growth conditions. In order to

obtain accurate values for some sources of gains and losses it is necessary to make the experimental conditions more and more artificial. The

experimenter then wonders how closely the data obtained apply under

field conditions.

It would seem, however, that the enigma of soil nitrogen balance

sheets is much less of an enigma now than it was a few years ago.

Enough facts have been established with regard to both losses and gains

to permit rather satisfactory explanations for most observed unsatisfactory balances, provided the soil conditions, cropping system, and fertilizer practices used, are known. Although the main mechanisms of loss

are probably known, quantitative data relating to each type of loss are

certainly inadequate. Even the leaching losses under widely different

soil and topographic conditions are not known accurately, and these

are far easier to measure quantitatively than are the gaseous losses. On

the other side of the balance sheet, there is need for convincing experimental support, established statistically, for the claims of large fixations

of nitrogen in tropical soils and in grasslands through channels other

than legumes.



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Weed Control in Principal Crops of the Southern United States

W . B . ENNIS. JR .

U . S . Department of Agriculture, State College. Mississippi

CONTENTS



I. General Nature of Problem. . .

11. Cotton . . . . . . . . . .



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1. Nature of Problem . . . . . . . . . . . .

2. Cultural Weed Control Methods . . . . . . . .

a . Rotary Hoe . . . . . . . . . . . . .

b . Cross-Plowing . . . . . . . . . . . .

c. Sweep Cultivation . . . . . . . . . . .

3. Chemical Control in Combination with Cultural Practices

a . Residue Removal and Land Preparation . . . .

b. Pre-Emergence Treatments . . . . . . . .

c. Post-Emergence Treatments . . . . . . . .

d. Economics of Weed Control in Cotton . . . . .

4 . Chemical Weed Control without Cultivation . . . .

5. Problems Requiring Further Work . . . . . . .

. . . . . . . . . . . . . . . . . .

1II.Corn

1 . Nature of Problem . . . . . . . . . . . .

2 . Herbicidal Usage . . . . . . . . . . . .

a . Early-Season Weeds . . . . . . . . . .

b . Late-Season Weeds . . . . . . . . . .

3. New Herbicides and Techniques . . . . . . . .

4. Work Needed . . . . . . . . . . . . .

IV. Soybeans . . . . . . . . . . . . . . . .

1. Nature of Problem . . . . . . . . . . . .

2 . Herbicidal Usage . . . . . . . . . . . .

3. Work Needed . . . . . . . . . . . . .

V . Sugar Cane . . . . . . . . . . . . . . . .

1. Nature of Problem . . . . . . . . . . . .

2 . Broadleaf Weeds . . . . . . . . . . . .

3. Control of Johnson Grass . . . . . . . . . .

a . Plant Cane . . . . . . . . . . . . .

b . Stubble Cane . . . . . . . . . . . .

4. Economics of Chemical Weed Control . . . . . .

5. New Herbicides . . . . . . . . . . . . .

VI . Peanuts . . . . . . . . . . . . . . . . . .

1. Nature of Problem . . . . . . . . . . . .

2. Herbicides for Peanuts . . . . . . . . . . .

. . . . . . . . . . . . . . . .

VII . Tobacco

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