Nitrogen and Soil Fertility

Transcription

1 NITROGEN AND SOIL FERTILITY amounts involved are much smaller. The amount of boron in the 7-ton potato crop (if determined) would have been about 0.2 pound. In the case of boron (and to a lesser degree the other micronutrients) if the soil contains excessive amounts, toxic quantities are absorbed by plants and growth is restricted. The need for a reasonable balance in amounts of available nutrients in the soil has been postulated. When the available supply of a given nutrient becomes depleted, this supply is a limiting factor in plant growth. Then the addition of increments of this nutrient to the soil as a fertilizer will result in increased yields of dry matter. EXCESSIVE INCREMENTS of some nutrients may cause decreases in yields. Eilhardt Mitscherlich, a German agricultural scientist, was the first to suggest an equation to relate nutrient supply and growth. This equation, although it has many recognized limitations, has been widely used in the solution of practical problems of crop production. It has been the basis used by economists in predicting fertilizer needs and economic usage. If more than a single nutrient, two for instance, are in limiting supply, the addition of either alone may produce but small increases in growth as compared with a combination of both. This effect is termed a nutrient interaction. A field experiment with oats conducted at the Iowa Agricultural Experiment Station illustrates such a nutrient interaction. The results from the treatments, expressed as bushels per acre, are listed thus: No fertilizer, 15; nitrogen only, 20; phosphorus only, 28; nitrogen and phosphorus combined, 54. The application of nitrogen and phosphorus singly resulted in net increases of 5 and 13 bushels of oats, respectively. On the other hand, a combination of the same quantities of nitrogen and phosphorus produced an increase of 39 bushels. This example illustrates the importance of maintaining a reasonable balance of nutrients. Nitrogen and Soil Fertility Franklin E. Allison 85 Crop yields in regions where rainfall is adequate are determined more by soil nitrogen than by any other mineral element supplied by the soil. Improved agricultural methods and better crop varieties are demanding more and more of this element. Soils alone seldom can meet the increased demand because they were never well supplied with nitrogen or because they have lost much of their original supply during 50 to 100 years of cultivation. Nitrogen is of special importance because plants need it in rather large amounts, it is fairly expensive to supply, and it is easily lost from the soil. A major factor in successful farming is the farmer's ability to manage nitrogen efficiently. The functions of nitrogen in plant and animal life are many. Essentially all life processes depend directly on it. Nitrogen occurs chiefly as protein and nucleoproteins with smaller and widely varying amounts of amines, amino acids, amino sugars, polypeptides, and many miscellaneous compounds. The more active nitrogenous compounds occur largely in the protoplasm and nuclei of the cells of plants and animals. Among them are the enzymes that speed up biological processes; they are proteins. An abundant supply of the essential nitrogen compounds is required in each plant cell for a good rate of reproduction, growth, and respiration. Even the green leaf pigment chlorophyll, which enables plants to use the

2 86 energy of sunlight to form sugars, starches, and fats from carbon dioxide and water, is a nitrogenous compound. Closely associated with the nitrogenous constituents are the many nonnitrogenous substances, which serve chiefly as sources of energy for the various cell activities. Some nonprotein nitrogen compounds do not appear to be especially active biologically but probably serve chiefly as a part of the structure of the organism, just as cellulose and lignin do. One of them is chitin, a complex organic substance related to the carbohydrates. It occurs in bacteria, fungi, lichens, and worms and in the shells of lobsters, crabs, shrimp, and insects. Nitrogen occurs chiefly in the young, tender parts of plant tissues, such as tips of shoots, buds, and the opening leaves. This nitrogen, present chiefly as protein, is constantly moving and undergoing chemical changes. As new cells form, much of the protein may move from the older cells to the newer ones, especially when the total nitrogen supply of the plant is too low. Then the plant makes the maximum use of a minimum supply. The transfer of nitrogen from cell to cell may proceed to such an extent that only the growing tips still are functioning properly. The older cells may turn yellow, and many of them, even whole leaves, die and drop off. This yellowing and dropping of the leaves farthest from the growing shoots is the main symptom of nitrogen deficiency. Growth then is of a nonsucculent, high-carbohydrate, dwarfed type. The proper functioning of nitrogen in plant nutrition requires that the other essential elements, particularly phosphorus, potassium, calcium, and magnesium, be present in adequate supply. If the supply of one or more of them is inadequate, the addition of much nitrogen to most common crops may produce limited growth, and this may be very abnormal. Such plants often are unusually susceptible to diseases, and they mature late. But if the YEARBOOK OF AGRICULTURE 1957 nutrient balance and total supply have been adequate from the seedling stage, plants throughout show the stocky growth and dark-green foliage that is a mark of health and vigor. Plants tend to take up most of the available supply of nitrogen during the early stages of growth. The young plants gorge on nitrogen and hold it for later use. This excess of nitrogen may meet the needs of the plant for several days, but it is not adequate indefinitely for a rapidly growing crop: There has to be a continuous formation and release of available nitrogen from soil organic matter or it must be supplied from outside sources to insure a steady rate of growth and an adequate supply later for the synthesis of storage protein for producing seed. For a crop like grass that is grown for pasturage, it is even more important that adequate nitrogen be available throughout the growing season. Because pasture plants arc not allowed to reach maturity, the need for nitrogen does not diminish as the season advances. As maturity approaches in a grain crop, most of the protein moves from the vegetative parts of the plants into the seeds. A 30-bushel wheat crop, for example, commonly contains about 50 pounds of nitrogen in the grain and less than 20 pounds in the rest of the plant. This storage protein remains inactive as long as the seed is inactive but changes rapidly when germination begins. THE PRIMARY SOURCE of soil nitrogen is the air. Harry A. Curtis, of the Tennessee Valley Authority, calculated that there are about 34,500 tons of nitrogen over every acre of the land area. That is about four-fifths of the atmosphere. This inexhaustible supply remains constant, because nitrogen is being returned to the atmosphere at about the same rate as it is being removed. Higher green plants cannot utilize gaseous nitrogen directly. It must first be combined with other elements. The

3 NITROGEN AND SOIL FERTILITY process of producing such combinations is called nitrogen fixation. Nitrogen is an inert element and resists combining with other elements. Such combinations are brought about in several ways, chiefly by electrical discharges in the atmosphere, by various chemical reactions in industrial processes, and by several species of micro-organisms living in or on the soil, in plant tissues, and in fresh and salt waters. Nitrogen fixed by lightning combines with the oxygen of the air to form oxides of nitrogen. These oxides are washed out of the air by rain or snow and reach the soil in the forms of nitrous and nitric acids. The amount of nitrogen that enters the soil in these forms is usually not more than 2 pounds an acre a year. Rains commonly wash about 2 to 6 pounds of ammonia and organic nitrogen from the air. The total amount of nitrogen brought down by rains annually varies with the rainfall, frequency of electrical storms, and nearness to industrial areas where ammonia is being released. An average figure for cropped areas in the humid-temperate region is usually considered to be about 5 pounds of combined nitrogen an acre a year. As I stated, two-thirds or more of this is not newly fixed nitrogen but is combined nitrogen, chiefly ammonia that escaped from the soil or was released as a result of the burning of coal and other materials. A small percentage consists of micro-organisms and other forms of organic matter carried into the air by the wind. Large amounts of nitrogen are fixed in industrial nitrogen-fixing plants as ammonia and calcium cyanamide. The synthetic ammonia process is the more economical. In this process, nitrogen and hydrogen gases are made to combine under pressure in the presence of a catalyst. Such fixed ammonia now constitutes the main commercial source of nitrogen used in agriculture. It is commonly applied in the form of ammonia and ammonium salts, and as urea and nitrates produced from ammonia. As a result of the rapid expan- 87 sion of the industry, chemical nitrogen was available in 1956 in adequate amounts and at prices somewhat below those a few years earlier. The capacity of 50-odd anhydrous ammonia factories in the United States in 1956 was about 4.1 million tons of ammonia, compared to 1.8 million tons in Nature's method of fixing nitrogen still constitutes the chief source of nitrogen for farm crops. This nitrogen is fixed by microscopic plants that exist wherever plant life can exist. Some of them live in nodules on the roots of plants, chiefly legumes. Others lead an independent existence. These microscopic forms fix atmospheric nitrogen at ordinary temperatures and pressures. Legumes may fix 200 pounds or more of nitrogen an acre each year if effective strains of the proper root nodule bacteria are present in the soil or are added to the seed as commercial inoculants. These bacteria penetrate the root hairs, live in the root nodules formed, and in cooperation with the higher plant take nitrogen from the air for the use of both the bacteria and the crop. An average fixation value is usually 50 to 100 pounds, depending on the kind of legume. When available soil nitrogen is abundant, legumes are likely to use it in preference to atmospheric nitrogen. The amount of nitrogen fixed in nitrogen-deficient soils parallels closely the amount of carbohydrate photosynthesized by the plant and its dry weight. Farmers in the past depended largely on legumes as a source of nitrogen to supplement animal manures and soil nitrogen. They have turned more and more in later years to synthetic ammonia to meet crop needs. It is largely a matter of economics, which is in turn influenced by the type of farming and the use to be made of the legume. For example, in livestock farming legumes are especially valuable as feed and as suppliers of nitrogen. But in grain farming, where their feed value is not realized, they may be a more expensive source of nitrogen than commercial fertilizer nitrogen.

4 88 YEARBOOK OF AGRICULTURE 1957 Several nonleguminous plants have root nodules that arc produced by bacteria or fungi and can use atmospheric nitrogen. Most of these plants, such as the alder and various species of Casuarina, Elaeagnus^ and Cycas, are trees or shrubs and of little value in agriculture. Nitrogen-fixing trees, both leguminous (black locust, Acacia, and Mimosa) and nonleguminous, are of considerable importance in the growth of some forests. Bacteria are the chief free-living micro-organisms that fix nitrogen. A few fungi and yeasts also can do so. A few genera of blue-green algae, often observed as a green scum on ponds, also can use atmospheric nitrogen and are of economic importance where paddy rice is grown. We do not know exactly how much nitrogen is fixed by nonsymbiotic (freeliving) soil organisms, such as azotobacter and clostridia. J. G. Lipman and A. B. Conybeare, of the New Jersey Agricultural Experiment Station, estimated it to be an average of 6 pounds a cultivated acre a year in the United States. Others have given higher values. An accurate value for such fixation cannot be given because the errors involved in sampling and analyzing soils are larger than the values to be measured. This nonsymbiotic fixation of nitrogen is important, but this source of nitrogen does not meet the needs of large crops. There has been much interest in recent years, especially in the Soviet Union and Germany, in the possibility that nitrogen-fixing bacteria may live on the outside of plant roots and supply the crop with nitrogen. It is reported that as early as 1942 the Soviet Union inoculated 5 million acres of crops with a commercial preparation of azotobacter. Such preparations have been valueless in most countries. MISCELLANEOUS SOURCES of nitrogen not obtained directly from the air, including a few manufactured products and various byproducts and wastes, are a large part of the supply of nitrogen for crops on many farms. This nitrogen originally came from the air, but after fixation it has taken on various forms. Nitrate of soda from Chile, which used to be the world's leading source of fertilizer nitrogen, has been pushed into the background by the abundant supplies of domestic synthetic ammonia. Chilean nitrate remains an important fertilizer material, and costs largely determine whether it or synthetic nitrogen is used. Ammonium sulfate, a byproduct of the coke industry, is also one of the important sources of fertilizer nitrogen. Coal thus is a secondary source of nitrogen. All ammonium sulfate sold in 1957, however, was not produced from coal. Some of it (and also some sodium nitrate) was produced in nitrogen-fixation plants from synthetic ammonia. Animal manures are one of the chief and best sources of nitrogen for crops. Slaughterhouse byproducts and press cake, such as cottonseed meal, are also excellent sources but have practically disappeared from fertilizers. They are of more value when used as feeds. Miscellaneous industrial and canning wastes find their way by various routes to the farm. The amount of nitrogen they supply is not large, and its availability to crops varies widely. Low availability may be due to the inert nature of the product itself or the high proportion of carbohydrate to nitrogen in the added material. Leather scraps and some urea-resin compounds are examples of inert materials. Canning wastes are typical of those that commonly contain such a high proportion of carbon to nitrogen that on decomposition little nitrogen is released to the growing crop. Sewage sludge and garbage wastes are becoming more important as sources of crop nitrogen, yet only small percentages of these city wastes reach the farms. In the raw state they are decidedly objectionable for health and sanitary reasons, high in moisture, and unsuitable for general crop use. Sewage sludges that are activated

5 NITROGEN AND SOIL FERTILITY and dried commonly contain 5 to 6 percent nitrogen and are satisfactory sources of nitrogen. The digested product usually contains less than half as much nitrogen, and its availability is considerably lower. Myron S. Anderson, in Department of Agriculture Circular 972, discusses the various types of sludges and presents data on the availability of the nitrogen. He states that heat-treated sludges are usually safe for use from a sanitary standpoint, but unheated digested sludges should be used with caution if vegetable crops are to be grown. The cost of converting raw sewage and garbage into forms suitable for use as fertilizers and their comparatively low content of nitrogen make them uneconomical to produce if the sole aim is to produce fertilizer. Nevertheless they will undoubtedly be produced to a greater extent in the future because cities must dispose of their wastes regardless of costs. Other sources of nitrogen are the various crop residues. Green manures are essentially in the same status, although if they are legumes most of the nitrogen is obtained by them directly from the air. THE CHEMICAL NATURE of soil nitrogen is not well understood. All but a small part is present in organic forms. Our knowledge of the organic part is inexact because widely varying types of nitrogenous compounds gain entrance to the soil and soon start to decompose. The destructive processes are affected by almost every factor that affects life itself. Furthermore, accompanying the biological degradation process arc numerous synthetic processes. The micro-organisms that decompose the various compounds use a portion of the nitrogen to form new cells, which in turn die and become a new source of soil organic matter. The cycle never ends. Protein is believed to be the chief form of nitrogen in soils although direct quantitative determination has not been made. Indirect procedures, 89 involving treatment with acids, followed by identification of the breakdown products, chiefly amino acids, have usually been used in such studies. J. M. Bremner, of the Rothamsted Experimental Station in England, concluded that up to 50 percent of the nitrogen in somic soils is present as protein. Because micro-organisms decompose protein readily, it is not present as free protein but is probably in the form of ligno-protein and clayprotein complexes. In fact, there is some evidence that the amino acids present after chemical treatment of soils may have been there before such treatment as amino acids combined with (and protected by) clays, rather than as constituents of proteins. Chitin, already mentioned as a constituent of plants and animals, is also a common soil constituent. It is somewhat resistant to biological attack, but some micro-organisms decompose it. Heterocyclic nitrogen compounds, such as purincs and pyrimidines, occur in soils to some extent, but the amount is believed to be small. Lignin and several other organic materials combine with ammonia to form compounds that are resistant even to treatment with acids and alkalies. Considerable nitrogen may be present in such forms, but we have no proof of that. Organic-inorganic complexes are important parts of soils. Laboratory studies have shown, for example, that protein to the extent of 8 percent of the weight of some clays can combine with them. Any protein held in such a com^plex is attacked much more slowly by micro-organisms than if no clay is present. Several other compounds, including humic acids and amino acids, are so held. Some nucleic acids, found in the nuclei of bacteria and in the cells of higher plants, are present in soils. The amount of soil nitrogen present in that form may be less than 10 percent. Crop residues, also present in varying stages of decomposition, contain chiefly protein and its degradation

6 90 products. During decomposition, such compounds as histidine, argininc, creatinine, choline, cyanuric acid, and lysine are formed. The amounts of each present at any one time are small and are likely to be transitory. Some of them have been thought to have effects on plant growth far greater than might be expected from the concentrations present, but seldom have such effects been demonstrated. Inorganic forms of nitrogen, chiefly ammonia and nitrates, are always present in soils in quantities varying from traces to perhaps 5 percent of the total. The amount present in cropped soils is small. Many soils can hold ammonia nitrogen so tenaciously that it is released only slowly to micro-organisms and to higher plants. The minerals in soils that are responsible for this fixation and holding of ammonium ions are vermiculite, montmorillonite, biotite, and weathered illite, which usually contains the other three minerals. The quantities of such fixed ammonium normally present in cultivated surface soils is believed to be small. The ammonium-fixing power of subsoils is usually higher than that of surface soils and is increased by drying and heating. Low recoveries of nitrogen in the crop, following additions of ammonia to subsoils, might be expected on the basis of laboratory studies on soils high in the minerals I mentioned, but we have no field data to prove or disprove this. THE NITROGEN CONTENT of virgin soils in various parts of the United States, according to Oswald Schreiner and B. E. Brown, of the Department of Agriculture, varied from 0.01 to i percent or higher in the surface layer when first put under cultivation. The nitrogen level tended to vary with the climate and native vegetation, which are so important in soil formation. They put the approximate nitrogen content of the surface 6 inches of virgin soils in the various regions as follows: Brown Forest soils of the YEARBOOK OF AGRICULTURE 1957 Northeastern States and Red and Yellow soils of the Southeast, 0.05 to 0.20 percent; Prairie soils of the Central States, o.io to 0.25 percent; Chernozem soils of the eastern Dakotas, Nebraska, and Kansas and farther south, 0.15 to 0.30 percent; Chestnut soils in the western parts of those States, o.io to 0.20 percent; Brown soils still farther west, o.io to 0.15 percent. The western desert soils are very low in nitrogen. The variation in nitrogen content with depth is pronounced in the soils of the different regions. Soils in the East generally are shallow. Soils in the Central States are deeper. Under virgin conditions, there was an average of about 4 thousand pounds of nitrogen an acre in the upper 40-inch layer in the Red and Yellow soils of the Southeast and 16 thousand pounds in the Chernozem soils of the Central States. The figures emphasize that many of our soils were not high in nitrogen when first brought under cultivation, but most of them contained more nitrogen then than they do now. The statement that virgin soils are not necessarily fertile soils may seem strange to some readers, but that is true of many of our soils. The main factors that determine content of soil nitrogen, aside from the effect of farming operations, are temperature, rainfall, soil texture, the type of minerals present, and vegetation. Temperature and rainfall largely determine the plant cover under virgin conditions. The nitrogen content of soils in the United States usually tends to increase from south to north. Hans Jenny, at the Missouri Agricultural Experiment Station, found that the average content of nitrogen and organic matter in the soils of the central part of the country increases two to three times for each fall of 18 F. in annual temperature. This temperature effect corresponds closely to the known effect of temperature on the rate of both chemical and microbiological activities as observed in the laboratory.

7 NITROGEN AND SOIL FERTILITY This effect of temperature does not always hold for tropical soils, some of which are as high in nitrogen as are soils of the Temperate Zone. In tropical soils, other factors may mask the effect of temperature poor aeration in regions of high rainfall; absence of wetting and drying, with the accompanying increased oxidation; and absence of killing frosts. Dr. Jenny also observed that in the grassland soils of the Central States the nitrogen content increased with rainfall. This finding would probably not hold for many other regions, especially wehere the rainfall is very high. Generalizations about the effect of climatic factors on soil nitrogen obviously need to be limited to definite regions. The type of vegetative cover on a soil affects the nature of the soil that forms and its nitrogen content. In the humid places where forests predominate, the Podzol soil that forms is a shallow soil, and most of the organic matter and nitrogen is concentrated in the upper foot or less. The higher rainfall causes much leaching and the removal of most of the bases originally present. In contrast, Chernozem soils, formed under conditions of somewhat limited rainfall, retain their bases and are deep soils, largely because of the depth of penetration of the grass roots. Soil texture influences the amount of nitrogen that accumulates in soil. A clay or clay loam commonly contains two or three times as much nitrogen as a very sandy soil in the same type of climate. A part of the difference is due to the combining of organic nitrogen constituents with some of the clay minerals. Poorer aeration and less leaching also favor the retention of nitrogen in soils of finer textures. THE NITROGEN CONTENT of cultivated soils after 50 years or more of cropping is usually much lower than when first plowed, unless the soil initially had little nitrogen. The nitrogen content in virgin soils, kept under constant conditions for centuries, is stabilized. Cultivation introduces a new set of factors, and soil nitrogen immediately starts to respond to the new conditions and seeks a new level at which income and outgo of nitrogen are in balance. Cultivation accelerates microbiological activities and the rate of release of ammonia from soil organic matter. A considerable part of this nitrogen may be lost through leaching and erosion while the soils arc without adequate vegetative cover, but a larger proportion is removed in the crop. Besides, less vegetative material usually is returned to the soil than under virgin conditions. Unless adequate nitrogen and the other essential elements are added as animal manures, legumes, or as commercial fertilizers, the productivity of the soil gradually declines. Even if they are added and crop production is maintained, the nitrogen level seldom stays at the virgin level. The higher the soil was originally in nitrogen, and the warmer the climate, the greater the decline in soil nitrogen is apt to be. Studies of the decrease in soil nitrogen when soils are first put under cultivation show that the rate of decline is usually high the first few years and becomes less in later years. Reductions of 25 to 50 percent of the original nitrogen in the Central States have commonly occurred during the 75 or more years of cultivation. Usually about half of the loss occurred in the first 20 years and one-fourth in the next 20 years. It seems that the nitrogen content now has about reached stability in many localities, at least where farmmanagement practices are good. In dry-farming regions, where wheat is the main crop, the surface soils commonly have lost about a third of their nitrogen since At the southern end of the Wheat Belt and below it, the losses have usually been near onehalf. Losses from eastern and southeastern soils probably have been of about the same magnitude in terms of percentage of the original content. In all sections of the United States 91

8 92 arc soils that have not shown nearly as marked losses in nitrogen as the figures would indicate. Examples are the soils that have been in sod or trees for much of the time and have not been stirred very often. The examples emphasize that farming practices determine the extent and rate of change in soil nitrogen. In fact, the trend toward lower soil nitrogen levels often can be reversed slowly. THE RELEASE of soil nitrogen to crops occurs as a result of the activities of many kinds of micro-organisms. Two broad groups are primarily responsible for making it available to higher plants the ammonifiers (ammoniaproducers) and the nitrifiers (nitriteand nitrate-producers). The ammonifiers comprise most of the bacteria and fungi that live in soil. They decompose soil organic matter primarily in order to obtain a supply of energy for growth. In the process they liberate ammonia, some of which they use themselves. The rest is set free as a byproduct. The nitrifiers then oxidize the ammonia to nitrites and then to nitrates. Some chemical oxidation of nitrites to nitrates may occur also in acid soils. Ammonia and nitrates are the main sources of nitrogen for plants and hence do not accumulate in the presence of an actively growing crop. Little ammonification and nitrification take place in the Temperate Zone during the winter. The rate of activity of micro-organisms above 45 F. increases twofold or threefold for each rise of 18 if moisture is adequate and the soil is not highly acid. Nitrate formation reaches a maximum in the late summer and decreases in early fall. The total amount of nitrates in the soil may be highest in late summer because of the high temperature and also because of less leaching and the slowing down of crop demands as maturity approaches. Any nitrate that accumulates in late summer is largely leached out during the winter and early spring in the humid region unless a cover crop is YEARBOOK OF AGRICULTURE 1957 grown. This yearly soil nitrate pattern under cultivated crops does not hold under sod crops; rarely is there more than a trace of available nitrogen in such soils at any time. There is less decomposition of soil organic matter under a sod, and any released nitrogen is quickly assimilated by the crop. The mass of undecomposed carbonaceous roots present also serves to keep nitrates low. Grasses can use much more nitrogen than they usually get. Good soil aeration also favors the release of nitrogen from soil organic matter. The degree of aeration is determined largely by soil texture, drainage, and cultivation. Aeration in cultivated soils is usually adequate for a satisfactory rate of formation of ammonia if it is adequate for healthy plant growth. An increase in the rate of oxidation of native organic matter, as evidenced by increased production of carbon dioxide, is sometimes observed after green manures and other fresh plant materials are added. Presumably nitrogen is also released, but it may be assimilated soon by the microflora. This increased destruction of native organic matter, which has been demonstrated by tracer techniques, is due to the increase in microbial growth with the accompanying increase in the production of cell enzymes. They attack the added plant material and native organic matter simultaneously. The magnitude of the efí*ect on the latter usually varies from a negligible increase to a i co-percent or 200-percent increase in carbon release from soils during the first week. A rapid decrease in the rate of release follows. Within a few days, a negative efí'cct may be observed. This phenomenon is of more scientific than practical interest; the overall effect during a growing season is small. Any such effect is much more than compensated for by the new soil organic matter formed from the added plant materials. The net amount of nitrogen released from an organic material depends on the nitrogen content of the material,

9 NITROGEN AND SOIL FERTILITY the completeness with which it is destroyed, and the amount of nitrogen used by the decay organisms. If the material is high in available energy and contains less than 1.5 percent of nitrogen, most of the nitrogen will be used by the micro-organisms. If the nitrogen content is above 1.5 percent, most of the extra nitrogen will be released for plant use. The percentage of soil nitrogen released annually to a cultivated crop varies widely and is affected by many factors. G. E. Smith, of the Missouri Agricultural Experiment Station, estimated the percentages released yearly to corn in that State to be 1.25 to 2.5 percent of the total from a clay or clay loam, 1.5 to 3 percent from a silt loam, and 4 to 6 percent from a sand or sandy loam. It is fortunate that humus is rather resistant to biological attack because it is usually more valuable for maintaining soil tilth and permeability to air and water than it is as a source of nitrogen. Great interest has been shown in rapid methods for estimating the amount of nitrogen that a soil can furnish a crop. No method has been proposed that is entirely satisfactory. The most used method, and probably the best, is to determine nitrate formation in a few grams of soil kept in the laboratory at constant temperature for about 2 weeks. The amount of nitrate formed is then compared with that formed in similar soils where crop yields have been determined. LOSSES OF NITROGEN from soils may occur through crop removal, erosion, and leaching, and as gases. The comparative importance of the four channels varies with the soil, crop, and soilmanagement practices. Losses through crop removal are proportional to the size of the crop. Such losses can be lowered only by changing the farming system. Grain and hay crops, for example, may be fed to animals on the farm rather than be sold. Erosion control is especially impor- 93 tant because erosion tends to loosen and float away much organic matter. The nitrogen in such eroded material may be several times higher than in the soil left behind. Leaching removes large amounts of nitrates from soils but no more than traces of other forms of nitrogen. The losses occur most readily from the more sandy cultivated soils, especially in warmer climates and in places where the rainfall is enough for the surface water to penetrate to the ground water several times a year. Losses of gaseous nitrogen from soils occur chiefly as ammonia, free nitrogen gas, and nitrous oxide. Traces of certain substances, such as amines, hydrocyanic acid, and nicotine, may also escape from growing plants. Ammonia, applied as a fertilizer, often is rapidly lost from soils to the extent of 25 percent or more. Such volatilization is largely limited to alkaline soils, although they need not be much above ph 7. It is accelerated by drying, especially if the temperature is high and the ammonia is near the surface. It occurs most readily from sandy soils and also from decaying masses of nitrogenous organic materials at or near the soil surface even if the soil is slightly acid, since the ammonia formed may raise the ph locally. Failure to incorporate manure with soil may help explain the low recovery of its nitrogen in the crop. Losses of nitrogen from soils in the form of free nitrogen gas and oxides of nitrogen are chiefly the result of bacterial action. Such losses are of considerable economic importance. Denitrifying bacteria that produce these gases are widely distributed. Ordinarily they use atmospheric oxygen for growth, but if the supply is deficient they can obtain oxygen from nitrates. In doing so they release gaseous nitrogen. The losses are greatest if soil aeration is poor and if nitrate fertilizers are applied in the presence of masses of plant materials that are undergoing decomposition. The eflbcient use of the nitrogen

10 94 sources available to the farmer involves the prevention of unnecessary losses, the return of manures and crop residues, use of Nature's methods of fixation, and the addition of commercial sources of nitrogen to bring the yield up to a satisfactory level. Attention should be given more to the use of good farm-management practices than to just the maintenance or building up of soil nitrogen. The level at which soil nitrogen can be held varies for each soil and for each climatic zone and is affected by the cropping system. With good farm management, including adequate use of fertilizers, good crops can be produced on soils of any nitrogen level if the environment can be made suitable for the crop. This does not imply, though, that abundant soil nitrogen is not highly desirable and favorable to larger and more profitable crops. In the control of soil erosion and leaching, a greater use of sod crops, catch crops, and cover crops, especially on rolling or hilly land, is needed. Application of soluble forms of nitrogen as short a time as possible before needed by cultivated crops will greatly increase the proportion that is utilized. This statement applies chiefly to the more sandy soils in regions where much leaching occurs just before and during the growing season. All crop residues should be returned to the soil unless insect infestation, diseases, or the crop to be grown make this impractical. The residues supply some nitrogen, help to hold soluble soil nitrogen, and replenish the supply of active organic matter. Special attention needs to be given to the preservation and use of animal manures. Gaseous losses of ammonia can be minimized by not applying anhydrous ammonia or ammonium salts to alkaline soils; if they are used, one has to make certain that they arc well mixed with the soil. Highly nitrogenous organic matter added to all soils should be handled in the same way. Losses of nitrogen gas and oxides of nitrogen can be reduced by avoiding YEARBOOK OF AGRICULTURE 1957 the application of nitrates to soils that are poorly aerated and drained. Large applications of undecomposed plant materials also may favor such losses by reducing the oxygen in the soil. More legumes should be grown in many areas, especially where moisture is not limiting and there arc enough livestock to utilize them. The deeprooted perennial or biennial ones that fix the most nitrogen and require the least cultivation are most beneficial to the soil. Because the amount of nitrogen fixed on nitrogen-deficient soils is likely to parallel closely the total dry weight of the legume, it is important that growth conditions, apart from nitrogen, be made satisfactory. Nitrogen fixation by free-living soil micro-organisms can probably be increased by the addition of crop residues, such as cornstalks and straw, that are low in nitrogen. Commercial nitrogen should be used as a supplement to the other forms of nitrogen to the extent needed. Other elements should be supplied in such quantities as to keep the proper nutrient balance. Soil Phosphorus and Fertility Sterling R. Olsen and Maurice Fried Phosphorus is present in all living tissue. It is particularly concentrated in the younger parts of the plant and in the flowers and the seed. It is necessary for such life processes as photosynthesis, the synthesis and breakdown of carbohydrates, and the transfer of energy within the plant. It is a major part of the nucleus of the cell and is present in the cyto-