soil nitrogen - naldc

NITROGEN is essential to plants because it plays a jundamental part in the jormation oj the proteins that are the stuff of living protoplasm. At the same time it is an expensive fertilizer element and one very easily lost from the soil Where does it come from and how does it get into the soil in the first placel Under what conditions does the supply in the soil become depleted! What are the forms of nitrogen useful to plants, and what changes do they undergo'^ Besides considering such questions as these, the authors make a rough inventory of the amount of nitrogen existing in various great groups of soils in the United States under natural conditions.

Soil Nitrogen By OSWALD SCI-JREINEK and B, E. BUOWN ^

NITROGEN is absolutely^ esseiitial to the main ten an ce of soil fertility. This elemeiiit is so necessary to the growth and reproduction of both plants and animals that all life would cease

to exist without it. Soil nitrogen, is important from both an agro- nomic and a commercial vie\Npoint. The higher cost of fertihzer idtrogen; the many combinations into wliich'nitrogen enters; the many transformations, both chemical and bacterial, that nitrogen, particularly organic nitrogen undergoes; the ease with which available inorganic nitrogen compounds may be lost from the soil through leachhig, erosion, crop removal, etc.; the relatively low content "^of total nitrogen in most soils, of which onty an insignificant, but nevertheless highly important, part is in a form available to plants; and the importance of this available nitrogen in tlie synthesis of complex organic substances, such as the essential proteins—these are some of the reasons why jutrogen is so important.^

GENERAL ROLE OF NITROGEN IN LIFE PROCESSES In combination with carbon, hydrogen, oxygen, and occasional^ other elements, nitrogen forms an essential constituent of all plant and animal tissues and plays a major role in the development and functions of protoplasm^ in plant and animal structures. Nitrogen is found especially in tissues that are concerned in growth and reproduction. There is evidence at hand to show that the rate of growth of plants is more dependent upon nitrogen than upon any other element. In

1 Oswald Schreiner is Principal Biochemist and B. E. Brown is Senior Biochemist, Division of Soil Fertility Investi.^iations, Bureau of Plant Industry.

2 Other phases of the nitroiïeu question are discussed in articles in this Yearbook as follows: Soil Ori>anic Matter and Soil Humus, p. 929; Fauna and Flora of the Soil, p. 940; Crop liotation, p, 406; Some llelation- ships of Soil to Plant and Animal Nutrition, p. 777; and three articles on fertilizers, pp. 4G9 to 545.

361

362 ^^ Yearbook, 1938

its combined foriii Jiitrogen is Tiniversally distributod m animals and plants in albuminoid or protein substances, such as the casein of milk or the ghiten of wheat. The vital importance of nitrogen may be further appreciated when it is considered that without nitrogen there can be no growth or reproduction on the part of plants or animals.

PRIMARY SOURCE OF SOIL NITROGEN

The inineral compounds of the soil, like calcium, magnesium, silicon, and iron compounds, originated from tl^e decomposition of original rock material, but insofar as nitrogen compounds in the soil are concerned, the nitrogen they contain was derived from the air. The air is primarily a mixture of nitrogen and oxygen gases of which, about 80 percent by volume consists of nitrogeu in a free or uncombined state. It lias been estimated that over every acre of land surface there are about 145,000 to 150,000 tons of this free nitrogen. Plauts, however, are unable to avail themselves directl}^ of this immense supply of raw material.^ Before they can do so the clement has to enter a state of combination with other elements. The chief difíiculty in this is that nitrogen is very inert and very stubbornly opposed to entering into combination with other elemeiits. Only through the intervention of powerful influences, such as lightning discharges or very powerful chemical reactions on the one hand, or through the milder but just as effective influence of soil bacteria on the other, can nitrogen be *'fixed'^ with other clemeuts. The importance of atmospheric iiitrogen as a raw material to supply agricultural and industrial iiitrogen needs cannot bo too highly stressed or appreciated.

THE NITROGEN CYCLE As the products of plants or the plants tliemselves are utilized by animals, still further utilization of combined nitrogen is eft'ected. Complex protein compounds are built up for the animal tissues, blood, etc., but some of the nitrogen in. the food consumed is voided as waste with a resultant break-down of the complex nitrogen compounds originally in the food. AVhen plants and animals die and their bodies decay, nitrogen is released from the complex combinations either as free nitrogen or as ammonia, a compound of nitrogen and hydrogen. Thus there is a continuous cycle going on of building, tearing down, and rebuilding. Animals liave no means of utilizing free nitrogen or even the simple nitrogen, compounds of an inorganic nature. Because plants have the ability to synthesize simple inorganic compounds into complex proteins, animals are dependent upon plants for their sustenance, either directly or indirectly through the consumption of the products of other animals whose existence in turn depended in some measure upon plant products.

In figure 1 is given a diagrainmatic presentation of such a nitrogen cycle as illustrated by Blair (56'),'^ who makes the foUowhig explana- tory statement:

Nitrogen is taken from tlie air artificially or tliroiigli the agency of microscopÎG organisms living in association with leguminous plants, or by organisms not associated witli plants. As plants, or plant residues, it is introduced into the soil as organic m'trogen. Thrt)ugh decomposition of the organic matter a part

3 Italic niinibers in jHironiliesos rtîfor io Liiorature (-ited, p. 1 ISl.

Soil Nitrogen 4^ 363

of this nitrogen niay be returned to the air as gaseous nitrogen or as ammonia, a part is converted into ammonia and nitrites, and finally into nitrates which are used by plants or lost in drainage waters, and a part remains in the soil for a long time as inert organic matter.

If the plants are used as food for animals a part of the nitrogen is recovered as manure and by-products of the slaughter house. When these are added to the soil, decomposition goes forward, as in the case of the plants or plant residues, and thus the cycle is completed.

AIR

'"'^^'^s^^DïïïïuirîssS^" FiGiJii]! 1.—The nitrogeTi Gyclo.

HOW NITROGEN GETS INTO THE SOIL As has been pointed out, the primary source of soil nitrogen is the free nitrogen of the air, which is fixed by two kinds of processes— natural and artificial. Lightning discharges unite nitrogen and oxygen to form oxides of nitrogen. These unite with moisture in the air to form either nitrous or nitric acid which is washed out of

364 ^ Yearbook, 1938

the atmosphere b^^ rain or snow and enters^ the soil. In addition a small amonnt of ammoniacal and organic nitrogen gets into the soil from the atmosphere where these forms of nitrogen occur as impurities from dust and gaseous contaminations. ^ Even at best, however, the aggregate amonnt of fixed nitrogen derived from the atmosphere in such a wa3' is rehxtively small. It has been estimated that a total of 5 to 7 ponnds of nitrogen per acre annually is added to the soil in this way. The amounts bronght down, fkictnate widely with seasonal conditions and proximity to factories and cities, being greater near factory towns and cities than in the open, country. Table 1 shows the ainounts of nitrogen brought down in rain as ammoniacal and nitrate nitrogen in different parts of the world {226, p. 299),

Table 1.—Amounts of nitrogen brought down in rain

Tjocaliori >'cars I

of ■ KaiTifall record ¡

K iiroííen added per acre per year

llarpeiidcn, Kuglaiul Garford, England _ riahult, Sweden .. Groningen, Netherlands ._ Blocmfontoin and Durhan, Hont h Africa. Ottawa, Canada Jtliaca, X. Y

X umber 2S

3 1

Ammonia- cal

nitrogen

Inch(S \ I'oiinds 2S. S i 20.9 i 32.5 27. G I

29.5 '

:. fil ti. 48 8. 32 4. o4 4.02 4. 42 7.10

Nitrate nitrogen

i.:r.î 1.93 i.'M) 1.40 1.39 2. H) .80

These results amjDly confirm the statejnent that the amount of nitrogen accndng to the soil from the atmosphei'e in rain and snow is comparatively small and would by no means take care of the nitrogen requirements of growing crops.

While plants such as corn, wheat, or cotton cannot directly ntilize the nncombincvl nitrogen of the air, there are vaiious micro-organisms inhabiting the soil that in their life function extract elemental nitrogen from the air and transform it into fixed-nitrogen compounds essential to plants. In addition to such bacteria, laiown as nons3anbiotic, there' are other kinds of bacteria that in association with the roots of leguminons plants—alfalfa, clover, vetch, peas, beans, etc.—fix tb.e free nitrogen of the air in combined forms which the plant utilizes in its growth processes. These phenomena account for the greatest supply of fixed nitrogen available to plants. The quantity of atmospheric nitrogen fixed by the nonsymbiotic organisms, though, varying widely with seasonal conditions and the character of the soil, averages about 25 pounds per acre annnally, while the amount of nitrogen fixed by legume bacteria will average about 80 pounds per acre annually, although higher quantities have been reported. According to Lipman (220) j the total annual addition of nitrogen to cultivated soils from the growth of legumes and associated nitrogen-fixing organisms in the United States amounts to 1,750,000 tons of this element.

Another source of addition of nitrogen to the soil is barnyard manure resulting from the feeding of animals on the farm. In recent years,

Soil Nitrogen ^ 365

owing to the ever-increasing use of tractor power, this supply of nitrogen has diniinished. The nitrogen content of manure varies widely with the kind and age of animal, the feed consumed, and other factors. In general the average content per ton of manure from animals for all conditions is taken as approximately 10 pounds. The value of manure does not rest entirely on its nitrogen content, as it also contains valuable organic matter as well as small percentages of phosphorus and potassium.

The so-called artificial iixation of atmospheric nitrogen refers to the efforts of man to accomplish what hghtning and soil bacteria do, namely, to bring the free nitrogen of the atmosphere into combination with other elements. Scientific and engineering advances have brought about a tremendous change in the last 25 to 30 years in the relative importance of nitrogen sources dependent upon chemical

FIGURE 2.—Outline mai) showing average number of pounds of nitrogen per acre in various soil regions of the tJnitcd States to a depth of 40 inches. White areas represent soils the nitrogen content of which has not yet been estimated

in this manner.

production. In 1900 about two-thirds of the world's nitrogen supply was obtained from Chile and the rest from the manufacture of coke and gas. ^ Thirty-four years later, accordmg to Chemical Nitrogen, recently issued by the United States Tariff Commission (437), 74 K percent of the world's needed supply of nitrogen was being obtained from the air, with less than 7 percent coming from the nitrate deposits in Chile.

NITROGEN DISTRIBUTION IN SOILS OF THE UNITED STATES UNDER NATURAL CONDITIONS

The best information on the nitrogen content of soils in the united States is given by Marbut (288), who considered the analyses of 250 to 300 samples of soils carefully collected from selected areas under


natural conditions tlironghout tlie United States, principally oast oí the Rocky Mountains. These samples arc from six natural soil groups, and the nitrogen content to the depth of 40 inches has been calculated. This depth has been taken as representing approximately the depth to which plants would be expected to feed and draw upon the plant-food supply of the soil.

An outline map of these large soil groups of the United States showing the approximate extent and the avej-agc content of nitrogen per acre to a depth of 40 inches in the respective regions is given in figure 2.

Naturally each region contains soils that í)]'e high and soils that are low in niti'ogen, an(l such ñgures can only approximate an average content over the regions, but the difl'erences in these larger soil groups are nevertheless very interesting and furnish the best available pictuvc. of the amount of nitrogen in the soils of the United States. Table 2 gives a summation of these figures for the various regions outlhied.

Table 2.—Average nitrogen content in varions soil regions of the United States ^

Soil region

Brown Forosi.. _ . Ked and Yellow rrairie Chernozem and C'licrnozeiiiliko Í 'liestniit. ]3ro\vji .

Approxiinaíií area of region

! .\i)|)ro.\'imato iiil.rogGu in

iûrface G inches

\ \'cra;ic iiitroííen to depth of 40

inches

A verafio amount of

nitniêri i)cr acre f.o

depth of 40 inoiies

ACT i a 180,000,000 150,000,000 113,000,000 123,000,000 102, 000, 000

Ö2, 000, 000

PcrcrM Per cení 0.05-0.20 0. 05

. 05- . 15 . 03

.10- .25 . 12

. 15- . 30 , 12

. 1(>- . 20 .OS

.10- . 15 .00

Pounds (i, 700 4,000

16, 000 10, 000 10, 700 «. 000

L Computed from data in Marbui (^¿38).

The first region, is that ocxiupying the iiortheasterji section of the United States and comprising the States of Pemisylvania, Ohio, Indiana, Kentucky, West Virginia, Virginia, Maryland, Delaware, the southern half of New York, Michigan, and Wisconsin, a httle of New England, Tennessee, Arkansas, Îissouri, Illinois, Iowa, and Minnesota.

The second group encompasses the Red ami Yellow soils of the South Atlantic and Gulf States, extending well into Texas, and includes most of North Carolina, Tennessee, Arkansas, and Louisiana, all of Mississippi, Alabama, Georgia, Florida, and South CaroUna, and part of Texas—essentially the old Cotton Belt. The average nitrogen content of this region is about 0.03 percent.

The third region comprises the Prairie soils, covering about one- third of Minnesota, most of Iowa, the northwestern two-thirds of Missouri, the northern two-thirds of Illinois, the eastern third of Nebraska, the eastern half of Oklahoma, and about two-fifths of Texas. These soils contain on an average about 0.12 percent of nitrogen. These are the rich l^rairie soils, including the black Corn Belt soils.

Soil Nitrogen ^ 367

Tlic fourth region is a strip parallel to the Prairie soils and extend- mg north and south through the Dakotas, taking in the eastern two- thirds of North Dakota, the eastern third of South Dakota, the eastern half of Nebraska, a httle over half of the central part of Kansas and Oklahoma, and a strip down through the center of Texas, comprising about a fifth of the total area of Texas. These Chernozem and Chernozemlike soils have practically the same nitrogen content as the Prairie soils.

The fifth area lies west of the last two groups and has a somewhat lower rainfall than the fourth group and much lower than the Prairie soils of the third group. These soils are known as the Chestnut soils. Tlie States involved in this area are the eastern fifth of Montana, tlic southwestern two-fifths of North Dakota, the western three-fifths of South Dakota, the western half of Nebraska, the eastern quarter of Wyoming, the eastern fourth of Colorado, the w^estern eighth of Kaiisas, a small section in the Panhandle of Oklahoma, about one- eighth of northwestern Texas, and less than one-eighth of eastern New Mexico.

The sixth group of soils are the Brown soils, just west of the Chest- nut soil area, extending, like the latter, north and south. This group comprises a strip through the approximate center of Montana, dow^n through Wyoming, covering about three-quarters of the State; a narrow strip through eastern Colorado; approximately one-half of New Mexico; and about one-eighth of western Texas. This Brown group of grassland soils showed an average of 0.06 percent of nitrogen.

Information regarding nitrogen in the other soil groups of tlie cxtj-cmc northeastern section and the Pacific slopes is not yet aA^ail- able; further study is necessary. The significant generalization to be obtained from this stud}'^ of the nitrogen distribution in the natural soils of the United States is that this nitrogen supply is highest in the soils of the Prairie and Chernozem regions of the central band of the United States, where it amounts to 16,000 pounds per acre to a depth of 40 inches, and then declines eastward and westward as the o til er soil groups are approached.

RELATION OF TEMPERATURE TO THE AMOUNT OF NITROGEN IN SOILS

In tlie Uinted States the percentage of nitrogen in soils varies within wide limits from 0.01 to 1 percent or liigher, the mean annual temperature varies from 32^ to 72° F., and humidity factors also vary widely.

It is generally agreed that the distribution of nitrogen iji. soils is closely related to climatic conditions, elenny (183), working essentially with the central belt of soils referred to above as the Prairie and Chernozem grou])s, which have developed under semihumid and semiarid conditions, respectively, has examined statistically the nitrogcMi content of a very large number of surface-soil samples (6 to 7 inclies deep), as reported by Canadian and State experiment stations. A total of 348 soil samples were used in this study. He concluded that the total nitrogen content of the soil decreases in the United States from north to south and that this change is a negative exponential function, so that for every 18° F. fall in mean annual temperature the average nitrogen content of the soil increases two to three times.


This trend is well ilhistrated in iigiire 3 (183), wliich shows that froju the high nitrogen content of northern soils there is a decline southward through the scmihumid regions of Minnesota, the Dakotas, Iowa, Missouri, Arkansas, and Louisiana. Similarly figure 4 (183) shows tliis same decided trend from Canada southward to Texas in the semiarid region through the Dakotas, Nebraska, Kansas, and Texas.

These facts show why it is possible to build up the nitrogen content of northern soils by the addition of organic matter, as the low temperatures favor its preservation. Conversely, in the South it is rather diflicult to increase the nitrogen content permanently by green- manuring practices, because the high temperatures favor decomposition, and nitrogen in the soil does not accumulate to the same extent as in the more northern soils.

^ Under natural soil conditions, before the lands are plowed, an cquilib- riimi exists between the formation of oi'ganic matter by vegetation and its decomposition by micro-organisms, the bahmcc being deter- mined primarily by climatic conditions; so that under natural conditions the nitrogen content of a soil is fairly constant. But cultivation, disturbs this natural equilibrium and results in a decrease of the nitrogen content, because less organic matter is retu.rned to the soil and its decomposition is hastened by farming operations. In tlie wheat-growing regions this loss is approximately 20 to 40 percent of the original nitrogen in the natural soil, after a period of from 20 to 40 years. Figure 5, taken from a bulletin on soil fertility from the Missouri Agricultural Experiment Station (185), shows this general trend in the loss of nitrogen with 3'ears of cultivation uiuler common farming practices in the Mi(]dle West.

40° 50° 60° MEAN ANNUAL TEMPERATURE TF.)

32" 40" 50° MEAN ANNUAL TEMPERATURE n^J

FiGUKE 3.—Decline of introgcii content J^^IGIIKE 4.—i:>ec]inc of nitrogen content in soils with riiic in mean annual in soils with rise in mean annual temperature in the semihumid temperature in the semiarid

region. region.

Soil Nitrogen ^ 369

<-ioo

30 40 YEARS OF CULTIVATION

FiGUKE 5.—Decline of nitrogen content in soils with length of cultivation periods under average farming practices in the Middle West.

The loss is most rapid in the first 20 years, when it amounts to approximately 25 percent of the original quantity under natural conditions; a 10-percent further loss occurs during the second 20 years, and a 7-percent loss during the third 20 years, indicating that the nitrogen level does not decline indefinitely and that the end result will be a new equilibrium at a decidedly lower level than the original natural nitrogen content. That these losses can be controlled and soil fertility maintained at a higher level bjr appropriate rotations, manuring, and fertilizing is shown in other articles in the Yearbook.

As a result of growing w^heat on the same land for 12 years Snyder (372) in Minnesota showed that the nitrogen had been reduced 2,039 pounds, or about 26 percent of that originally found in the soil at the beginning of the test. The crops grown, however, accounted for less than 450 pounds of this nitrogen, showing that nearly 1,600 pounds had been lost, mainly through decay of the soil organic matter under this type of continuous farming. In Illinois a plot on which corn was grow^n for 16 years contained 4,000 pounds and adjoining pasture land 4,914 pounds of nitrogen in the surface soil, according to Hopkins (169), The Pennsylvania^ long-term experiments (4^6, p. 202) furnish a further example in that the untreated plots decreased in nitrogen from 0.124 percent in 1899 to 0.111 percent in 1921.

DISTRIBUTION OF NITROGEN IN SOILS ACCORDING TO DEPTH

As a general rule, the content of nitrogen in soils is greatest in the plow level and decreases with depth, which is also true of soil organic

r)t)i8:^''


matter. Analytical figures showing the distribution of nitrogen according to depth as given by Hopkins (from Bear (25, p. 55)) are presented in table 3.

Table 3.—Quantity oj nitrogen in an acre oj soil ^

Soil type 3 |{)-G5á inches

i

Deep peats - Black clay loams Brown silt loams Brown loams \ )eep gray silt loams Brown sandy loams Yellow-gray silt loams,. Gray silt loams Drab silt loams Yellow fine sandy loams Yellow silt loams Light-çray silt loams Sands '

34, 8S0 7, 2:i0 f), 035 4, 720 3, 020 3, 070 2.890 2, 880 2. 800 2, 170 2,020 1.890 ],440

G2/3-20 inches

i ; 20-40 inches

04,980 7,470 5, 920 f), cm 2. 2Ö0 3, 920 2,710 3, 210 3.160 2, 610 2, 050 1,920 2, 070

Pounds 97, 730

3, 2!0 3, Ö70 4, !-)() 2, 280 4, 160 3,240 3; 240 3, 400 2, 730 2,410 2,100 3, JOO

0-40 inches

Pound'^ 197,590 17,910 14, .')20 15, 530 8, 150 11.150 s; 840 9,330 9, 360 7,510 C, 490 5,910 Ü. Ü10

1 O^/i inches of lopsoil is represented to weigh 2,000,000 pounds. 2Types found in considerable areas in the North Central States.

NITROGEN AN IMPORTANT FACTOR IN MAINTAINING SOIL FERTILITY

The presence in the soil of an adequate supply of available nitrogen, is one of the most important factors relathig to the maintenance or improvement of soil fertility. The lack of sufficient amounts of available nitrogen in soils, particularly those that have been cropped for many years, has long been a limiting factor in crop production. A deficiency of available nitrogen results in plants of poor color and appearance, poor quality, and low production. A sufficient supply of available nitrogen, on the other hand, is largely instrumental in getting plants off to a quick start and has a subsequent tendency to encourage stem and leaf development. Such plants will make a more rapid, thrifty growth and possess a normal deep-green color and generally healthy appearance. It has been shown also that plants supplied with sufficient nitrogen are much better able to utilize otlier nutrient materials such as phosphorus and potassium compounds. The curtailed leaf surface resulting from a lack of available nitrogen in the soil is usually reflected in a lowering of yield, since yield is ordinarily proportional to leaf development. No amount of available phosphorus and potassium will overcome a deficiency of available nitrogen; it is generally recognized that one nutrient element cannot be substituted for another. An oversupply of available nitrogen in the soil, on the other hand, tends to cause late maturity and poor seed development and to make plants more susceptible to disease organisms on account of the development of more succulent tissue, and is decidedly uneconomical because of the unnecessary use of soil nitrogen above actual plant requirements.

HOW SOIL-NITROGEN LOSSES OCCUR From one cause or another, losses of nitrogen from soils are constantly going on. The main causes are leaching, removal by crops and ani-

Soil Nitrogen ^ 371

mais, soil erosion, decfw procossos, nuá, muler certain soil conditions, denitrification.

Leaching concerns largely the available forms of nitrogen, such as nitrates, and to some degree ammonium compounds. Nitrates are very soluble and move freely with up and down movements of soil moisture. Heavy rainfall tends to wash nitrates into lower soil depths away from the plant action. Ammonium c(mipounds move less freely in the soil than nitrates, being more liable to fixation by certain of the soil components. Leaching of soluble nitrogen compounds is more pronounced in light sands and sandy loams than i]i heavier soils, especially if the subsoil is pervious to any extent. The best cultural practice that tends to offset such losses is a growing crop to utilize the available nitrogen compounds in growth processes and convert them to forms that when turned back into the soil furnish a source of organic nitrogen less subject to leaching than the original nitrates.

Crops remove large quantities of nitrogen from the soil. If the wheat, corn, or other crop grown is largely sold off the farm, just so much nitrogen goes with them. The same applies in some measure to animals when meat and milk products are sold, although this loss is offset to some extent by the manure produced. Table 4 {/¡-54; pp. 18/^-185) WÛ11 show how much nitrogen certahi crops take out of the soil.

Table 4.—Nitrogen content oj different harvested crops

(TOJ) Aero yiolfl

Alfalfa, hay. liarloy:

( irain .. Straw.. -.

Xitro- iiCll

ciontent per

crof) a(;rc

Pounds

Crop Acre yield

Total..

Cabbapo. Clover, red (hay)..._ Clover, alsike Oiay)

Com: (irain-- fitalkv'i (SI. over) . Cobs

Total Corn, silage — .

CottoTi: Lint Seeds. Stalks and leaves..

'10 bushels 1 Ion... ._

Onions, bulbs T*otatoes, tubers.

1') tons. 2 tons.. 2 tons..

60 bushels.. 1.7.') tons 900 pounds.

35 12

lOo 84 82

f)() 33

3

500 bushels.. 200 busliels .

500 pounds... l,OOOT)ounds. 2,000 pounds.

S2

1.0 3S. 5 28. T)

Total

Total ! f)8. 0 : i ■I2'1 ;

! (îrain Straw..

Tolal . .

1

30 bushels l.fitons

80. 5 Cowpeas, liay . 2 tons.

50 bu.sliels ... 1.25 Ions

10.0

Oats: (îrain .. Straw

Total

35. 0 J5.0

50. 0

51.5

Wheat:

Nitro- gen

C(.>ru.ent per

crop aero

Pounds I GO ¡ 45

Rye: (iraiTi . 25 bushels 2C.5 Straw .. L25 tons 12.0

TOVAI .. . . . 38. 5

Sufiar beets: -lioots . . . . 15 Ions 78 Leaves (green) . f).2 tons ■10

Total 118 Sweet potatoes, 1 ubers... 200 bushels.--. 35 Timothy (hay).. ... 2 tons 40

Tobacco: Leaves.. ...... 1,500 pounds.. •11 Stalks 1,250 jiounds.. 2f>


This shows conclusively that the crops make a heavy demand on soil nitrogen, making it essential that the farmer meet such losses in order to maintain the productivity of his soil at a high level.

Soil-erosion factors concerned in soil losses are discussed elsewhere in this Yearbook, but any actual loss of soil caused by erosion, whether from water or wind, means a serious loss of nitrogen, difficult to replace. Prevention of soil erosion will automatically conserve organic matter with its essential nitrogen.

Another factor involved in nitrogen Josses is associated with the death and decay of plants and animals. During the process, losses of nitrogen, either in a free state or as ammonia, may ensue under certain conditions. The same is true of manures if improperly kept or utilized, when serious losses of nitrogen may take place by escape into the atmosphere. Poor soi.1 conditions, as in submerged land or swampy areas, favorable to denitrifjâng or anaerobic organisms, may cause losses whereby nitrogen is freed from its combinations and returned to the atmosphere. Losses of this character are avoidable if the soil drainage adequately aerates the soil.

Any attempt to set upa balance sheet for nitrogen is likely to be in error by a considerable margin. At the same time such appraisals are helpful hi bringing more forcibly to our attention an approximation of nitrogen income and outgo.

Lipman (220) has proposed certain data to indicate the status of soil nitrogen with reference to how much nitrogen is lost annually and how much is added to the soil to offset the losses. The balance sheet has been estimated as follows :

Tons

Losses (harvested crops, grazing, erosion, leaching) 22,899,046 Additions (fertilizers, manures and bedding, rainfall, irrigation waters,

seeds, nitrogen fixed) 16, 253, 862

Net annual loss 6, 645, 184

It is very evident that, even allowing for possible errors in these preliminary estimates, the amount of nitrogen lost from the soil is very great. This emphasizes in a striking manner the fact that the soils of the United States are suffering a steady decline in fertility. If such an enormous annual loss of nitrogen from the soil actually occurs, then the necessity of doing everything possible to conserve the nitrogen reserves of the soil is self-evident. Some of the steps to counteract such a loss would include a wider and more effective use of nitrogen-supplying leguminous crops, preservation and better utilization of farm manures, the prevention of soil erosion, the growing of cover crops to absorb available nitrogen instead of havhig it leached from the soil, and a cheapening of fertilizer nitrogen to encourage its greater use in fertilizer practices.

By growhig a cover crop during the winter season, particularly under southern soil conditions, leaching will be decreased considerabl}^ Greater care in maintaining the reaction of the soil at a point that would promote a heavier growth of leguminous or cover crops would help furnish more nitrogen. It would appear, however, that an important offset to any nitrogen deficit will have to come to a large extent from applications of nitrogen fertilizer materials, cither as such or in mixed fertilizers.

Soil Nitrogen ^ 373

NITROGEN FORMS UTILIZED BY PLANTS

The fairly general belief that nitrogen must be in nitrate form to be of service to plants is by no means true. This has arisen from the fact that chemical examination shows that comparatively little ammonium or nitrite compounds are present in the soil—that ]iitrates predominate. Moreover it is thought that ammonium compounds pass over to nitrates in relatively short order owing to bacterial action, the speed of w^hich depends largely upon moisture supply, soil temperature, and soil reaction. The nitrites in turn arc quicldy converted to nitrates, the ultimate oxidation product of soil nitrogen. However, there is fairly well established evidence to show that both ammonia and organic compounds are directly utilized by plants.

With reference to the utilization of nitrites by crop plants, studies indicate that nitrites are injurious to plants although small amounts of nitrite nitrogen appear to be utilized.

Ammonia is constantly being formed in soils as the result of the action of ammonifying bacteria on soil organic matter, but the quantity present is generally not great, normally only a few parts per million of soil. It is known that rice grown on flooded lowland soils responds more markedly to applied ammonium salts than to nitrates, owing probably to the submergence, which affects the nitrates more adversly thnn ammonium salts. Stewart, Thomas, and Homer (387) drew the conclusion that pineapple plants wêre able to utilize all required nitrogen in the form of ammonium compounds, although under field conditions both nitrate and ammoniacal nitrogen compounds wêro assimilated by this plant.

Allison (Ö), working with corn in culture solutions containing various ammonium salts in comparison with nitrates, showed that under the imposed conditions there was nothing to indicate any superiority of nitrate nitrogen over ammoniacal forms. He further points out tliat it is possible that more ammoniacal nitrogen is utilized by higher plants than is generally supposed.

Hutchinson and Miller (Î77) have given a good review of the wôrk done before 1911 in respect to organic forms of nitrogen assimulated by plants, and their own studies show that many organic compounds are utilized directly by higher plants. Schreiner and Skinner (350) tested a wide range of organic nitrogen compounds for plants, some of which proved beneficial, others harmful. The principal organic compounds utilizable by plants appear to be certain of the amino acids and other intermediate products resulting from the breaking dow^n of protein nitrogen into relatively simple water-soluble forms of organic nitrogen. Very little is known as to the amount of organic nitrogen actually assimilated by plants, and it may be that quick conversion of the organic nitrogen to ammoniacal and to nitrate fornis is a determining factor in the apparent response of plants to organic nitrogen compounds when applied to soils.

NITROGENOUS ORGANIC CONSTITUENTS OF SOILS

By far the greatest tie-uj) of nitrogen in the soil is in organic combinations with the soil organic matter. In this form the nitrogen is comparatively insoluble and therefore unavailable to higher plants. If


this wcro not so the nitrogeii supi)ly of soils would be depleted very rapidly. Soil organisms utilize this organic nitrogen and in so doing transform tiie complex organic nitrogen compounds into more simple compounds available to higher plants. Generally speaking, the break- down of any organic constituent added to soils in organic materials is in the direction of complex to simpler forms.^ At the same time it is known that soil organisms produce complex nitrogen compounds from the simple inorganic compounds, thus furnishing another interesting cycle of nitrogen transformation in the soil itself.

In a fertile soil the organic nuitter, and consequently the nitrogen, is continually changing from season to season. The very life of the soil and all its fertility-promoting factors depend on this change. AMien organic matter has ceased to change, has become chemi(;ally and bacteriologically inert, the result is iníFertility. It is the cliang- hig character of the orgardc matter that makes for soil fertility, not the mere presence or store of organic matter in the soil.

In order to understand soil fertility as influenced by organic manures, green manures, and good farming methods study should be made not so nuicli of the nitrogen content—except as this is a key to these dynamic factors—as of the organic chemical changes themselves. In this field of research much remains to be done, but a study of soil nitrogen compounds ^ has been in progress for some years. A dia- grammatic presentation of decomposition is shown in figure 6.

Some of these nitrogenous organic soil compounds are the units obtained when one of the complex proteins, nucleoproteins, etc., is resolved into simpler compounds by chemical means. Other organic soil compounds are derived by further decomposition; by deamidiza- tion, oxidation, and reduction from amino acids, carbohydrates, and other organic compounds, with liberation of ammonia, carbon dioxide, sulphuretted hydrogen, and other final as well as intermediate soil products. The diagram will make some of these exceedingly complex and involved chemical ])rocesses as clear as is possible in a nontechnical numner.

The diagram illustrates that a complex micleoprotein can be split into protein and nucleic acid, both of which have been found in soils. The protehi can be further split into a number of smaller units, known as cleavage products of protein or primary degradation products. These iniits out of which the complex structure of the protein is built comprise such nitrogen compounds as histidine, arginine, lysine, and others.^ Those mentioned have all been found in soils, and their beneficial action on i)lants has been demonstrated.

This process of taking the complex nitrogen compounds apart is accomplished b}^ fairly simple chemical means, involving chiefly the process knowTi as hydrolysis; but to effect further decomposition of these units means more deep-seated changes and, as it were, actual breaking up of the imits themselves. The diagram shows how by oxidation or reduction many of the soil compounds can arise during this process of change, and how finally the end products, ammonia, carbon dioxide, sulphuretted hydrogen, etc., are reached. In soils,

*Sonie of the iiitrogoiious organic constituents isolated from soils are creatinine, creatine, histidiue, arfiiuiue, clioline, adeniiie, ííuanine, xanthine, hypoxanthine. nicoline, carboxylic acid, iiiicleie acid, allan- toiii, cyariuric acid, írímethylaíiiiae, cylosine, and chitin (349),

NUCLEOPROTEIM

Carbon dioxide

CARBOHYDRATES (Sugars)

UNCLASSIFIED PRODUCTS

ALIPHATIC ACIDS

Saccharic Qcid

PURINE COMPOUNDS

Xanthine Hypoxanthine

Guanine Adenine

PYRIMIDINE COMPOUNDS

Cytosine

Corbon dioxide Carbon

ALIPHATIC COMPOUNDS

Isocrotonic acid Paraffinic acid Lignoceric acid Succinic acid Acrylic acid Oxalic acid Dihydroxysteoric ocid Monohydroxystearic acid Mannite Glycen'des Hentriacontane

Carbon dioxide Carbon

BENZENE COMPOUNDS

. I Vanillic acid Vanillin Benzoic acid Oxytoluic acid Salicylic aldehyde

NITROGEN SULPHUR NITROGEN RING COMPOUNDS COMPOUNDS COMPOUNDS

i ^ i . Çyanurîc acid Trithiobenzaldehyde Methylamine Creotinine Histidine Picoline carboxylic

acid

Arginine Lysine Choline

SOIL COMPOUNDS

Hydrogen sulphide Thiosulphate

Sulphate Sulphur

Ammonia Nitrites Nitrotes Nitrogen

SOIL END

PRODUCTS

FiGUKii 6.—])iagrani showing the dcconipositioii of a complex nitrogenous plant or animal product in the soil, giving rise to many intermediate substances as soil constituents and finalh' amjuonia, nitrites, nitrates, and elementary nitrogen.

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as is well knowTi, the ammonia is again changed to nitrite and nitrate, and thus, according to most authorities, the cycle is completed and the nitrogen is ready to start again on its mission to X)romote plant growth.

The decomposition of the nucleic acid, which was one of the com- ponent parts of the nucleoprotein used in the illustration, can be similarly traced. Like the protein part, nucleic acid yields smaller chemical uidts. These units of the structure of nucleic acid comprise such compounds as hypoxanthine, xanthine, guanine, adenkie, cystine, pentose sugars, phosphoric acid, and others. All those mentioned have been found in soils. Like the cleavage products of the protein these further decompose in the soil, and the nitrogen appears first as ammonia, then nitrite and nitrate, as with protein.

That nucleic acid and others of the protein-degradation products can serve directly as plant food in buüdmg up plant tissue has been sho\\Ti by Schreiner and Skinner (SoO). The action of some of these compounds in promoting growth resembles in efl'ect the growth-promoting inihiencos of vitamins in animal nutrition or hormones in plant physiology.

Such soil reseaxch is fundamental. It throws light upon the nitrogen changes that take place in soils and prepares the way for a study of the decreases in nitrogen in our agricultural lands, including those changes that take place as the result of human influence on the natural equilibrium established in a soil under its environment of climate, drainage, native vegetation, etc. The tide has turned from methods leading to decrease in soil fertility to an intelligent use of those methods that upbuild the Nation^s resources in productive land.

soil nitrogen - naldc

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