EFFECTS OF NITROGEN FERTILIZER APPLICATION IN AGRICULTURE
D. J. Lathwell, D. R. Bouldin and W. S. Reid Reprinted from
Relationship of Anriculture to Soil and Wafr P-1ll*,ton Proceedings of Cornell University Lonterence on Agricultural Waste M, nagement, held in Syracuse, N. Y.
January 19-21, 1970.
Conference sponsored by the New York State College of Agricultuf e. A Statutory Colleg e c the Nevv .ork State
University of Cornell University, Ithaca, N. Y.
Effects of Nitrogen Fertilizer Applications
D. J. LATHWELL, D. R.'BOULDIN, and W. S. REID
Professor of Soil Science, 'Associate Professor of Soil Science, and Assistant Professor of
Soil Science, Respectively., Cornell University, Ithaca, New York
Effective use of fertilizer N is now an essential part ofmodern agriculture. The development of the fertilizer N industry :has been an essential component in increased crop production throughout the United States. Consumption of fertilizer N in the United Statesin '1945 was'595,313 t6ns, 2 ,685,572 tons in 1960, and by 1968 consumption -had reached 16,538,112 tofisj?'In- New t.'York. comparable figures are 21,353 tons in 1945, 45,522 tons, in1,960Yand. in, 1968 consumption of fertilizer N reached 67,699 tons. The trends in both the United States and in New York show a gradual increase in consumption from 1945 to 1960, and a more rapid increase in consumption from 1960 to 1968. Thompson (1969) has shown this graphically (Figure 1) for several Corn Belt States where N fertilizer rates on corn increased much more rapidly after 1960 than before 1960. For the period 1967 to 1969, however, little change occurred indicating that N consumption in the United States may well have reached a plateau, especially in those states where fertilizer usage has been well established.
Thompson (1969) also summarized the trends in yield of corn for the same Corn Belt States (Table 1) and his data show that while the annual increase in corn yields from 1930 to 1960 was about 0.8 bushels per acre per year, from 1960 to 1967 the annual increase was around 3.0 bushels per acre per year. Interestingly, trends in corn yields in New York as shown in Table 1 are very similar to those for the Corn Belt.
These observations demonstrate that corn yields in the United States and in New York have increased as the use of N fertilizer has increased. While N fertilizer is only part of the technology responsible for the increased corn yields, they could not have been obtained without sufficient inputs of fertilizer N.
Based on Figure 1, N consumption increased at a rate of about 10 pounds per acre' per year from 1960 to 1967. At the same time corn yields increased at the rate of about 3 bushels per acre per year. (Table 1). Thus, something over three pounds of N added resulted in a one bushel increase in yield. Assuming that only the grain was removed from the field, about one pound of the three pounds of N applied was actually removed from the land in the crop. Thus, some two-thirds of the N applied cannot be accounted for by crop removal. This then points up the problem of efficient use of fertilizer N in agriculture. The data of Thompson (1969) indicate an average application to corn in the Corn Belt of from 80 to 120 pounds of N per acre presently. These figures are not too different from the rates applied to corn in New York and are substantially lower than those applied to many vegetable crops in the state.
In the remainder of this paper we will examine the use of fertilizer N on corn and
Agronomy Dept. Paper No. 884.
TREND IN YIELD OF CORN IF FIVE U.S. CORN BELT
STATES AND IN NEW YORK (THOMPSON, 1969)
State Yield Annual Increase Yield Annual Increase Yield
1930 1930-1960 1960 1960-1967 1967
Illinois 42.9 0.87 68.9 4.10 97.6
Indiana 41.1 0.77 64.3 3.07 85.8
Iowa 44.2 0.74 66.5 3.70 92.4
Missouri 25.0 0.81 49.4 2.95 70.1
Ohio 40.0 0.82 66.6 2.15 81.7
New York 30.0 0.87 56.0 3.00 77.0
KILOGRAMS OF NITROGEN
140- PER HECTARE ON CORN
1945 190 15 960 1965 1970 1975
Figure 1. Trend in use of N on corn in several midwestern states. (Thompson 1969)
evaluate rate, method of application, and time of application on yield and efficiency of use of applied N.
YIELD RESPONSE AND NITROGEN UPTAKE BY CORN
Response to applied N on corn at several sites in New York is given in Figure 2. Twenty-one location years of data were available and the data were divided into the highest seven (I), the middle seven (11), and the lowest seven (111) yields and response curves for each set of data are plotted. As is typical of most data obtained from such experiments, response to applied N increased as yields increased ; that is response to N was greater at Group I sites than at Group III sites. Likewise, these results show the typical diminishing response with each additional increment of applied N. Usually the point of greatest economic return to applied N is at some point below thle maximum yield obtained.
50 5 8 5
Fort!II zer N added, lb/A
Figure 2. Yield of corn grain plotted against fertilizer N added for 21 location-years of data in New York.
The curve labeled I is the average of the 7 highest yieldings experiments, the curve labeled II is the average of the 7 intermediate yieldings experiments, and the curve labeled III is the average of the 7 lowest yielding experiments.
The N con tent of the above ground portion of the plants as shown in Figure 3 parallels the yield curves. As expected, the N content of the plants was higher at higher yields. The figures shown on the graph show the percent of the applied N that was recovered in the above ground portion of the plants. The recovery of applied N is defined here as the increase in N content of the above ground dry matter expressed as a percent of the fertilizer N applied. Two points are of considerable significance; first, as the rate of applied N increased, the percentage recovery decreased and this was true at each yield level. Secondly, at each rate of application of N the percentage recovery increased as yields increased. For example, percentage recovery of 50 pounds of applied N was higher at Group I sites than at Group III sites. Thus, in these experiments the percentage recovery ranged from a high of 83 percent at the lowest rate of applied N at the highest yield (Group 1), to a low of 10 percent at the highest rate of applied N at the lowest yield (Group III).
These data are typical of most response as a function of rate experiments. They illustrate vividly the diminishing response to each additional increment of input and the rapidly declining efficiency of utilization of fertilizer N as the rate of application increases. These data also illustrate the importance of higher yields in promoting the efficiency of use of fertilizer N. Thus, manipulation of factors which increase yield potential should increase the efficiency of applied fertilizer N and thereby enhance economic returns from fertilizer application and reduce possible N loss to other environments.
TIME AND METHOD OF APPLICATION OF NITROGEN
Before about 1945 most fertilizer, including N, was applied to corn at planting in a band. With higher rates of N, however, it is difficult, if not impossible, to apply all of the fertilizer at planting. Other possibilities exist including broadcasting most of the N
I- 8100 10%/
Zo 57%1 20
15 50 85 155
Fertilizer N added, lb/A
Figure 3. N content of the above ground dry matter of corn plotted against fertilizer N added for 21 location-years of data in New York.
Curves I, 11, and III correspond to the definitions given for Figure 2.
The numbers adjacent to the curves are the increases in N content resulting from each increment of N expressed as per cent of the added fertilizer N.
in the spring either before plowing or else disking in after plowing, but before the planting operation. Applying N in a band as a sidedress application after corn emerges, but before it enters its period of maximum growth and N demand has the potential to enhance efficiency and minimize losses. In recent years efficient labor use and reduced prices for off season purchase of N have resulted in much interest in fall application of N for the corn crop grown in the subsequent season. In most instances, fall applied N has been plowed under or injected into the soil. We will consider fall application, spring application, and summer sidedress as well as plowdown, disk in, and band application in our evaluation of the effectiveness of fertilizer N for corn.
COMPARISON OF FALL APPLICATION AND SUMMER SIDEDRESS APPLICATION OF NITROGEN ON CORN
In a series of experiments made in the Southeastern United States (Pearson et al, 1961) and on a Honeoye silt loam in New York, (Lathwell et al, 1966) several N sources were applied at a single rate in late fall for the corn crop planted the subsequent year. A response curve to N was evaluated by using ammonium nitrate as the N source applied as an application split between banding at planting (20 lb/A) and the remainder as a summer sidedress. As shown in Table 2, in New York the relative effectiveness of fall applied N on corn yields was quite low and varied widely among sources. The relative effectiveness of NH4NO3 was unusually low, but in other experiments has not been inferior to other sources. The values given in Table 3 show that percent recovery of N by corn on the Honeoye silt loam for sidedress N were quite high and significantly higher than those found for the fall applied N.
As shown in Table 4, the data from experiments in the Southeast were comparable to that obtained in New York although the relative effectiveness of the fall applied N was slightly higher (49 percent compared to 37 percent). Likewise, the variability among sources was not nearly as great on the average, although within locations there were differences among sources in some years.
RELATIVE EFFECTIVENESS OF FALL APPLIED
N COMPARED TO SIDEDRESS APPLIED N ON CORN YIELDS ON HONEOYE SILT LOAM IN NEW YORK
(AVERAGE OF 3 YEARS OF DATA) (Lathwell et al 1966)
of Fall Applied N
as Per Cent of
N Source Sidedress Applied N
PER CENT RECOVERY OF FALL AND SIDEDRESS APPLIED N
BY CORN ON HONEOYE SILT LOAM IN NEW YORK (AVERAGE OF 3 YEARS OF DATA) (Lathwell et al 1966)
Rate of N Per Cent N Recovery
Fall Sidedress NH4 NO3 NH3 Urea Cyanamid
0 40 82
0 80 55
80 0 5 16 14 25
Wide variability among locations with regard to relative effectiveness of fall applied N was observed. Pearson et al, (1961) found that differences in rainfall did not account for this variability. They, therefore, examined variations in soil texture and as shown in Table 4, the effectiveness of fall applied N was not related to differences in soil texture.
In an experiment on Romulus silt loam in New York, several rates of fall plowdown N were compared to the same rates of N applied as summer sidedress. As shown in Figure 4, yields from 60 to 120 pounds of N sidedressed were equal to those obtained with 120 and 240 pounds of N plowed down in the fall respectively.
Similar experiments (14 in total) comparing fall applied N and summer sidedressed N were also made in Nebraska (Olson et al, 1964). The results obtained in Nebraska (Figure 5) are similar to those from New York where 40 pounds of N sidedressed was equal to 80 pounds plowed down in the fall and 80 pounds sidedressed was equal to 160 pounds of N plowed down in the fall. In these experiments at the rates used, yields from fall plown N were always lower than from summer sidedress. Olson et al (1964) found no difference between nitrate and ammonium forms of N in their experiments.
Nitrogen recovery in the Nebraska experiments is given in Figure 5 and is similar to that found in New York. The percentage recovery of applied N was higher at the lower rates of application for the summer sidedressed N, but little effect of rate was
RELATIVE EFFECTIVENESS OF FALL APPLIED N COMPARED TO
SPRING APPLIED N ON CORN YIELDS IN SOUTHEASTERN
(AVERAGE OF 4 YEARS OF DATA)
(PEARSON ET AL 1961 )
of Fall Applied N
as Per Cent of
N Source Spring Applied NH4 NO3
(NH4)2 SO4 59
Greenville Sandy Loam 69
Greenville Fine Sandy Loam 61
Cecil Sandy Loam 48
Decatur Clay Loam 46
Houston Clay 45
Ruston Sandy Loam 45
Tifton Sandy Loam 13
8C 0 Fall Plowdown
o Summer Sidedress
6 II i
65 45 5 135 255
Fertilizer N added, lb/A
Figure 4. Yield of corn grain plotted against fertilizer N added for an experiment in New York on Romulus silt loam.
2e 586 31%
7"=! e Fall Plowdown
. Summer Sidedress
Fertilizer N added, lb/A
Figure 5. Yields of corn grain plotted against fertilizer N added by two methods for 14 experiments in Nebraska. Numbers adjacent to the curves represent the increase in N content of the above ground dry matter resulting from each increment of fertilizer expressed as per cent of the increment. (Olson et al 1964)
found for fall plow down N. At all rates, the recovery of N was lower for fall plow down than for summer sidedressed N.
These data reported for several parts of the United States are typical of other published data available. Welch et al (1966) have shown in their experiments on wheat in Illinois that fall applied N was only about 67 percent as efficient as spring applied N. Stevenson and Baldwin (1969) likewise found in Ontario that corn yields were inferior with fall applied N compared to summer sidedressed N. In their experiments no rate of fall applied N used gave yields as high as the optimum rate applied sidedress. In addition, they found no differences among sources of N used. Thus, the available evidence indicates that fall applied N is inferior to summer sidedressed N both from the standpoint of yield response of corn and from percentage recovery of applied fertilizer N. Also, different sources of N are not consistently different from one another regardless of time of application.
COMPARISONS OF EARLY SPRING APPLIED NITROGEN AND SUMMER SIDEDRESS APPLICATION OF NITROGEN ON CORN
In an experiment on Honeoye silt loam in New York, (Lathwell et al, 1966) several sources of N were broadcast at one rate and plowed under in early spring and compared to summer sidedressed N. As shown in Table 5, there was some decrease in relative effectiveness of the early spring plowdown treatment, but not nearly so marked as the decrease for fall application. Some variation in source was found, with urea in this instance appearing inferior to the other sources. Percent recovery (Table 6) of spring applied N was not greatly different from that of summer sidedressed N except for urea at the 80 pound rate. Recovery from the 80 pound rate was significantly lower than that for the 40 pound sidedress rate. In addition, there was no difference in yield between the 40 and 80 pound sidedress rate of N.
RELATIVE EFFECTIVENESS OF EARLY SPRING APPLIED N COMPARED TO SIDEDRESS APPLIED N ON CORN ON HONEOYE SILT LOAM IN NEW YORK (1 YEAR DATA)
(Lathwell et al 1966)
Relative Effectiveness of
Early Spring Applied N
as Per Cent of
N Source Sidedress Applied N
PER CENT RECOVERY OF EARLY SPRING AND SIDEDRESS APPLIED IN
BY CORN ON HONEOYE SILT LOAM IN NEW YORK (1 YEAR DATA)
(Lathwell et al 1966)
Rate of N Per Cent N Recovery
Spring Sidedress NH4 NO3 NH3 Urea Cyanamid
0 40 82
0 80 42
80 0 34 54 10 44
In another experiment on Lima silt loam (Figure 6) at low rates of N, summer sidedress N was superior to spring plowdown. At the 120 pound rate, little difference was found between times of application while at the highest rate, the yields from the plowdown N was slightly higher than from the summer sidedressed N. When soil samples were taken at sidedress time, however, only about 30 percent of the spring applied N could be recovered in the upper 16 inches of soil regardless of rate of application of N.
In the same experiments described earlier, Olson et al (1964) found that summer sidedressed N was superior to spring applied N at all rates tested (Figure 7). These results showed little difference between fall and early spring applied N. Likewise, percentage recovery of spring applied N was only slightly higher than that for fall applied N and substantially lower than the recovery of summer sidedressed N.
In contrast to the results reported here, Stevenson and Baldwin (1969) on fine textured soils in Ontario found no difference in effectiveness of spring applied and summer sidedressed N on corn. Thus, it appears that early spring applications of N may be as effective as summer sidedress application under certain circumstances and definitely inferior in other situations. There appear to be no particularly reliable criteria to predict which condition may prevail.
RESIDUAL EFFECTS OF APPLIED NITROGEN
Even under the most favorable conditions of application, from 75 to 80 percent of the applied N is recovered in the crop to which it is applied and more likely only 50
65 Spring Plowdown
o Summer Sidedress
15 45 75 135 255
Fertilizer N added, Ib/A
Figure 6. Yield of corn grain plotted against fertilizer N added by two methods for 2 years to Lima silt loam in New York.
100 % 34%
* Spring Applied o Summer Sidedress
) 40 80 160
Fertilizer N added, lb/A
Figure 7. Yield of corn grain plotted against fertilizer N added by two methods for 14 experiments in Nebraska. Numbers adjacent to the curves represent the increase in N content of the above ground dry matter resulting from each increment of fertilizer expressed as per cent of
the increment. (Olson et al 1964)
to 75 percent will be recovered. Under less favorable times and methods of' application, recovery may fall as low as 10 percent and routinely is in the range of 215 to 35 percent. Under many circumstances, then, well over half the applied N is not utilized by the crop to which it was applied. As shown in Table 7, on several soils in New York, any spring applied N that had not been utilized by the corn crop to which it was applied had disappeared from the upper 16 inches of soil by the following spring. Thus, any residual effect from the unutilized N is improbable.
In a laboratory and greenhouse experiment, samples of a Williamson silt loam on which 4 consecutive crops of corn receiving differential N fertilization had been grown were collected. As shown in Table 8, there was no difference in the inorganic N content of the soil as taken from the field or after being incubated for 60 days.
INORGANIC NITROGEN IN THE UPPER 16 INCHES OF SOIL IN THE SPRING
FOLLOWING DIFFERENTIAL NITROGEN FERTILIZATION OF CORN
Inorganic N in
Surface 16" of
N Removed in Soil, in Spring
Year of Quantity of Corn Grain + Following Year Location Application N Applied Stover of Application
Langford Silt Loam 1966 135 136 22
Langford Silt Loam 1966 15 99 10
Dunkirk Silt Loam 1965 135 168 8
Dunkirk Silt Loam 1965 15 81 12
Lima Fine Sandy Loam 1966 135 185 32
Lima Fine Sandy Loam 1966 15 137 36
Romulus Silt Loam 1967 255 198 52
Romulus Silt Loam 1967 25 101 37
Howard Gravelly Loam 1967 135 166 44
Howard Gravelly Loam 1967 15 129 48
RESIDUAL NITROGEN AS MEASURED BY LABORATORY AND GREENHOUSE EXPERIMENTS IN SAMPLES OF THE SURFACE HORIZON OF A
WILLIAMSON SILT LOAM FROM SARATOGA COUNTY AFTER 4 YEARS
OF DIFFERENTIAL NITROGEN FERTILIZATION UNDER CORN
Total Nitrogen Greenhouse Experiment
Fertilizer Nitrogen Grain + Stover
Applied 1965 + '66 1965 + '66 + '67 Yield of Oats
+ '67 + '68 + '68 Inorganic N in Soil in Greenhouse
lb/A Original Incubation gm/Pot
700 646 13 41 1.86
60 539 12 46 1.92
INitrogen additions equivalent to 100# of N per 2 X 106 pounds of soil resulted in yields of 4.02 grams per pot with both sets of soil samples.
Likewise, yields of oats grown on these samples in the greenhouse were not different indicating that no differences in residual N as a consequence of differential fertilization were present. These data show that in this soil, at least, the unutilized fertilizer N was not incorporated into a readily mineralizable organic form which could become available to a subsequent crop.
In a field experiment to evaluate the residual effects of N on subsequent crops, oats were planted on a Troy silt loam following three years of differential N fertilization of corn. In this field experiment, no difference in oat yields was measured and the N content of the crop was not different (Table 9). Thus, the residual effect of the applied N was negligible on this soil and supports the results found by measuring the inorganic soil N content and that released by incubation. These experiments indicate that under the conditions found in New York, residual effects of applied N are unlikely.
Under certain conditions some residual effects of applied N may be found. As shown in Table 10, Pearson et al (1961) found a winter oat forage crop responded substantially to N applied the previous spring to corn. Even in the second year, corn showed some response to residual N, especially at the highest rate. They found, however, that fall applied N fertilizers increased the small grain forage yield following the corn crop only slightly. In addition, carry-over to the second corn crop, planted some 18 months after fertilizer application, was negligible.
RESIDUAL NITROGEN IN TROY SILT LOAM IN WASHINGTON COUNTY AS MEASURED BY AN OAT CROP FOLLOWING 3 YEARS OF DIFFERENTIAL FERTILIZATION OF CORN
Oat Crop 1968
Removed in Inorganic N
Fertilizer Nitrogen Grain + Stover in Soil BeApplied, 1965 + '66 1965 + '66 fore Plant- Yield' N Content of'1 + '67 + '67 ing Oats Dry Master Oats
525 446 25 8700 52
45 274 18 8600 58
IAddition of 20# N/Acre to a portion of either set of plots increased yield to 9600# of dry matter containing 64 pounds of N.
RESPONSE OF SUCCESSIVE CROPS TO A SINGLE APPLICATION OF N AT
VARYING RATES MADE IN SPRING BEFORE FIRST CORN CROP PLANTED [PEARSON ET AL 1961]
Increase in Yields over Check
Rate of N Applied to First Corn Crop
in Spring Corn Bu/A Oat Forage lb/A Corn Bu/A
50 29.4 191 3
100 42.7 600 7
200 47.7 1600 19
Studies of residual effects of fertilizer N were undertaken in Nebraska and reported by Olson et al (1964). Their results are shown in Figure 8, and reveal increasing residual effects from increasing rates of applied N. Even more striking, however, was the greater residual effect from sidedressed N compared to earlier times of application. There was little yield response to N additions above 40 pounds per acre on these irrigated soils in the year of application due to high initial levels of soil N. There was considerable residual effect from the higher rates in the second year, especially where N was sidedressed. Utilization efficiencies ranged from 20 to 50 percent over the two years with the sidedressed N being most efficient at all rates of application. Olson et al (1964) point out that any residual N in the year of treatment would have equal opportunity for stable combination in the soil by the year after treatment regardless of application time so that the differences in residual effectiveness of applied N reflect losses that had occurred.
160 LBS. N
40 2ND YEAR RESPONSE
I ST YEAR RESPONSE
F- FALL ........
80 LOS. N
40 OS. N F...
Figure 8. Mean two year response of corn to one application of fertilizer N on two irrigated soils in Nebraska. (Olson et all 1964)
White and Pesek (1959) in Iowa soils showed that significant quantities of residual N may be found after the growing of a corn corp. In their experiments the residual N was found chiefly as nitrate and was located in the 6 to 21 inch layer. Little residual ammonium N was found and the effects of residual N on nitrifiable forms of N was negligible.
FATE OF UNUTILIZED NITROGEN
The data presented reveal that from 20 to 90 percent of the N applied to corn was not utilized by the crop to which it was applied. Variable amounts of this N may be
recovered by subsequent crops. Both the data of White and Pesek ( 1959) and that of Olson et al (1964) show that while residual effects of N are significant in some situations, still some 50 percent or more of the applied N was not recovered in the crops grown and thus, remained unaccounted for.
A possible fate of the applied N is immobilization into the organic fraction of the soil. An evaluation of the data indicates, however, that this is not a likely fate for any significant portion of the unutilized N. White and Pesek (1959) were unable to show residual effects to other than the nitrate form. Likewise, in the data reported for the Williamson silt loam from New York (Table 8), incubation tests showed no difference in inorganic N mineralized as a result of previous N application. Bouldin and Lathwell (1968), on a long term rotation experiment on Honeoye silt loam, were unable to show any increase in the organic N content of this soil as a result of inorganic fertilizer N application. They concluded that the N was lost from the soil system and therefore not immobilized. From these results it seems unlikely that substantial net immobilization of inorganic fertilizer N occurs.
Regardless of the form of inorganic N applied to soils, the evidence indicates that it is converted rapidly to nitrate under most conditions (Lathwell and Peech, 1964). Nitrate being an anion does not react with the soil and, thus, is not retained by the soil. It, therefore, moves through the soil profile in the percolation water and can ultimately reach the ground water. Johnston et al (1965) in California found the N content of tile drainage effluent from fertilized irrigated soils to range from 1.8 to 62.4 ppmn and the quantity found was correlated with that applied to the soils. Most of the N lost in the drainage water was found to be nitrate in their studies. Their figures showed that as much as 70 percent of an application of 240 pounds of N per acre could be lost in the tile drain effluent.
Zwerman et al (1969) found that the tile drain effluent from a recently heavily fertilized Honeoye silt loam could contain as high as 140 ppmn N as nitrate following a heavy rain. Their figures also indicate that large quantities of N can be lost in the drainage water resulting from natural rain fall.
The classical lysimeter experiments of Bizzell and Lyon (1927) showed on cropped soils losses of N in the range of 6 to 8 pounds per acre per year in the percolate, while on bare soils they found losses ranging from 4,0 to 70 pounds per acre. These soils, however, had received relatively little N fertilizer compared to present day practices.
These data indicate large quantities of N can be lost from soils in the drainage water. It is also apparent that once N has moved into the profile below the zone of organic matter accumulation, it is quite stable and remains as nitrate indefinitely under many conditions. The data of Herron et al (1968) in deep boess-derived soils in Nebraska showed that large amounts of nitrate could be stored within the surface 6 feet. A large portion of the N was found in the zone between 1 foot and 6 feet and apparently remained there as nitrate. Obviously this N would be available to any subsequent crop which rooted to these depths. White and Pesek (1959) also found that the N in Iowa soils below the plow layer remained as nitrate.
As long as the N remains as nitrate and escapes from the rooting environment of plants, it is a potential pollutant of the environment. It may escape to the groundwater where it may accumulate and pose a potential hazard to the health of humans if they use the groundwater as a source of drinking water. On the other hand, nitrate may escape to streams and other aquatic environments and cause sufficient enrichment of the water to cause significant entrophication. This may reduce the usefulness of streams and lakes sufficiently to be a serious environmental problem.
The third possible fate of unutilized inorganic N in agricultural soils is denitrification. In this process nitrate is reduced to elemental nitrogen (NA) nitrogen dioxide (NO, ) or nitrous oxide (N2 0), of which the most significant is elemental nitrogen.
Denitrification is favored by anaerobic conditions where certain organisms are able to use nitrate or nitrite in place of elemental oxygen. Readily decomposable organic matter is essential for denitrification to occur also.
Allison (1955), in his study of N balance sheets, showed that in almost all experiments "unaccounted for losses" occurred. Allison concluded from his study, which included lysimeter data from Ithaca, New York, that these losses occurred as a consequence of denitrification. Viets (1960) reported that some 60 percent of applied N was not accounted for in crop removal or increased soil N at Brawley, California where no leaching losses occurred. Olson et al (1964) also presented indirect evidence to show that significant losses of N from Nebraska soils occurred as a result of denitrification.
In evaluating our data from New York, we have concluded that some significant portion of the applied fertilizer N is lost by denitrification. Our soils are high in organic matter and a readily decomposable carbon supply is available. These soils are also fairly impermeable so that water movement through them is rather slow. Under these conditions, we believe that both aerobic and anaerobic zones can coexist in the same soil. This may be particularly true after heavy rains in late spring and early summer. In soild normally considered to be aerobic, significant losses of applied N by denitrification can thus occur in local zones where the environment is anaerobic. As pointed out by Black (1968), adjacent anaerobic and aerobic microenvironments have the potential for continuous loss of N from the soil by denitrification.
It seems evident from these considerations that significant loss of N from soils occurs by denitrification although actual quantities lost are not known with any precision. Quantitative data on the magnitude of field losses of N by denitrification would contribute greatly to our understanding of the fate of applied N in soils. Meanwhile, only curde estimates can be made of the magnitude of the losses. We estimate that losses by denitrification are at least as great as those by leaching in most experiments and may be far greater than leaching losses in others. Nitrogen that escapes from the soil as N2 merely mixes with the N2 already present in the atmosphere and is so insignificant in quantity compared to the N2 content of the atmosphere as to be undetectable. Unutilized fertilizer N that is lost from the soil system by denitrification then does not contribute to environmental pollution.
IMPLICATION FOR FERTILIZER NITROGEN MANAGEMENT
Fertilizer N is necessary for efficient crop production throughout the world. Vast increases in consumption of fertilizer N in the past decade are a reflection of this need. The evidence summarized in this discussion shows that even under the most favorable conditions some fertilizer N will not be utilized by the growing crop and, therefore, may be lost to the environment.
We believe a careful economic analysis of application rates must be made since at high rates little yield response to the final increments is obtained and the efficiency of use is very low. Thus, rate of application must be adjusted to maximize return and minimize loss to the environment.
From the data presented, proper timing of application appears to hold the most promise of influencing the efficiency of applied fertilizer N. the longer fertilizer N remains in the soil, the greater is the probability of loss by one mechanism or another. Less N, therefore, will be required to produce the same yield the closer it can be applied to the time of maximum demand by the crop. For corn summer sidedress applications fit this requirement. The number of experiments made under the wide variety of conditions examined here lead us to conclude that to get maximum efficiency and to minimize environmental pollution most, if not at all, fertilizer N applied to corn should be summer sidedressed.
For all crops to maximize efficiency of use and to minimize potential loss, fertilizer N should be applied as close as possible to the period of greatest crop demand.
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nitrate nitrogen in fertilized deep loess-derived soils. Argon. J. 60: 477-482
5. Johnston, W. R., Ittihadiek, F., Daum, R. M., and Pillsbury, A. F., 1965. Nitrogen and Phosphorus in Tile Drain Effluent. Soil Sci. Soc. Amer. Proc. 29: 287-289
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Cornell University, Ithaca, N. Y.
8. Olson, R. A., Dreier, A. F., Thompson, C., Frank, K., and Grabouski, P. H., 1964.
Using Fertilizer Nitrogen Effectively on Grain Crops. Nebra. Agr. Exp. Sta. Bul.
S. B. 479
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and White, A. W., 1961. Residual Effects of Fall- and Spring-Applied Nitrogen Fertilizers on Crop Yields in the Southeastern United States. U. S. Dept. Agr.
Tech. Bul. 1254
10. Stevenson, C. K., and Baldwin, C. S., 1969. Effect of time and method of
nitrogen application and source of nitrogen on the yield and nitrogen content of
corn (Zea mays L.). Agron. J. 61: 381-384
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