Agriculture and Environment, 4 (1978) 57--64 57 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

A RATIONAL APPROACH FOR OPTIMIZING APPLICATION RATES OF FERTILIZER NITROGEN TO REDUCE POTENTIAL NITRATE POLLUTION OF NATURAL WATERS

BIJAY SINGH, C.R. BISWAS and G.S. SEKHON

Department of Soils, Punjab Agricultural University, Ludhiana 141004 (India)

ABSTRACT

Singh, B., Biswas, C.R. and Sekhon, G.S., 1978. A rational approach for optimizing application rates of fertilizer nitrogen to reduce potential nitrate pollution of natural waters. Agric. Environm., 4: 57--64.

Yield, N uptake and residual NO3--N data for wheat and maize, raised in a long-term experiment, have been discussed to determine optimum rates of fertilizer N application at which yields are least affected, while unused nitrogen, which is a potential pollutant, is reduced to a permissible level. Computation of fertilizer N rates corresponding to the points of greatest economic return and permissible N loss has been described. Optimum fertilizer N rate is the smaller of the two fertilizer N rates. By assuming a loss of 60 kg N/ha or less, as environmentally permissible, it was observed that a small yield increment of maize would have to be sacrificed for the protection of the environment; whereas, for wheat, nitrogen can be applied safely upto the point of greatest economic return. Coin- cidence of the rainy season with the growth period of maize in the study area seems to have reduced the fertilizer N rate, corresponding to the point of permissible N loss, lower than that for greatest economic return.

INTRODUCTION

Fertilizer nitrogen (N) is subject to loss from the soil--root zone via leaching of nitrate ions. This results in low recovery and efficiency of the applied nitrogen. With increasing rates of application, fertilizer efficiency decreases progressively, while leaving an increasing amount of unused nitro- gen as a potential pollution hazard. Since the point of greatest economic return from this nutrient is usually somewhere below the point of maximum yield, it should be possible to adjust fertilizer nitrogen rates for maximum return and minimum loss to the environment (Parr, 1973).

In view of the increased usage of fertilizer nitrogen for continuous profi- table crop production, and the potential contr ibut ion of this practice to eutrophicat ion of surfacewater resources, and to nitrate enrichment of groundwater with deep percolation and resultant health hazards, an ap- proach to formulate more rational N-fertilizer recommendations has been described. Data from irrigated, heavily fertilized crops of wheat and maize,

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raised in a long term fertility experiment, have been used to explain the use- fulness of the approach. Optimization of fertilizer N rates is calculated to ensure maximum crop yields in consonance with minimum risk of nitrate pollution of natural waters.

MATERIALS AND METHODS

Four treatments were selected in a long-term experiment progressing at Punjab Agricultural University farm, Ludhiana with maize-wheat~owpeas rotation. The experiment was started, in 1971, with maize in a Fatehpur sand (Typic Ustipsamment). The treatments included in the study were: no NPK, 50% NPK, 100% NPK and 150% NPK, where 100% NPK for wheat, maize and cowpeas were: 150 kg N, 32.7 kg P and 31.1 kg K per hectare; 150 kg N, 32.7 kg P and 62.2 kg K per hectare; and 20 kg N, 17.5 kg P and 16.6 kg K per hectare, respectively. Soil characteristics of the experimen- tal site are discussed elsewhere (Singh and Sekhon 1976b). The average rainfall during the periods July to September, October to March and April to June is about 47, 12 and 7 cm, respectively.

Soil samples were taken from the above four treatments at 15 cm inter- vals to a depth of 225 cm just after harvesting of the wheat and maize on 14 April and 16 October 1974, respectively. These samples were processed and analysed for nitrate content, as described by Singh and Sekhon (1976a).

Yields of wheat and maize were recorded at harvest, and plant samples were analysed for total nitrogen content. The uptake of nitrogen by dif- ferent crops was computed from these parameters.

RESULTS AND DISCUSSION

Grain yield and N-uptake data for wheat and maize are presented in Fig. 1 as a function of the rate of fertilizer nitrogen. Phosphorus and potassium were applied, along with nitrogen, in a fixed proportion. Singh and Sekhon (1976b) have observed that the amount of NOs--N left unutilized in the profile is much less with a balanced application of NPK than otherwise. A close fit of yield and N-uptake data to quadratic equations is indicated by high values of the coefficient of determination (Table I).

Efficiency of the fertilizer nitrogen can be measured in terms of yield or N uptake. In both cases it may be defined as the rate of increase of yield or of N uptake with the increase in the amount of the fertilizer. Since yield (Ym or Yw) and N uptake (Urn or Uw) for both maize and wheat are defined by quadratic equations (Table I), fertilizer N efficiency, as measured by slope functions of these curves, varies continuously with increasing rates of applied nitrogen.

If X represents the rate of application of nitrogen, the efficiency of the fertilizer to increase yield and N uptake is expressed by dY/dX and dU/dX, respectively. In the present investigation, both these parameters decrease

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I I I 0 75 150 225

Fert i l izer N ( k g / h a )

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Fig. 1. Quadratic response functions for yield and nitrogen uptake o f wheat and maize as a function of fertilizer nitrogen rates, yield; . . . . . . N-uptake; o, • wheat; 4 • maize.

linearly with increase in X, as shown in Fig. 2. Equations for these slope functions are given in Table I. The rates of decrease of dU/dX and dY/dX is given by slopes of these lines.

The rate of fertilizer N application to obtain maximum yield is defined by a point where d¥/dX = 0. The rate of increase in yield with increase in N application is extremely small, near the point of maximum yield, so that application of fertilizer is not economical around this point. Point of greatest economic return, which is always somewhere below the point of maximum yield, can be defined by X where dY/dX = dC/dX. Where C represents the cost of fertilizer. From the yield point of view, application of fertilizer N is economical upto this point.

If the amount of nitrogen lost via denitrification and volatilization is ap- proximately known for a given type of soil, together with an estimate of N-uptake by the crop it can give an idea of unutilized fertilizer N, a substan- tial port ion of which is lost to natural waters. Nitrogen uptake data have thus been used to supplement yield data in formulating opt imum rates of fertilizer N application, at which crop yields are least affected while poten- tially hazardous nitrogen is kept to a minimum.

As discussed for yield data, when dU/dX approaches zero, further incre- ments of the applied fertilizer remain unutilized in the soil profile, or are lost via denitrification, volatilization and leaching. Lines in Fig. 2 indicate that dU/dX decreases continuously, so that the rate of absorption of applied nitrogen differs at each point on the N-uptake curve (Fig. 1). Thus, there exists only one point on the N-uptake curve, at which a given fraction of the applied nitrogen is absorbed by the crop. If, supposing P kg N/ha or less from the applied nitrogen remains unutilized, and is not considered poten- tially hazardous, a rate of fertilizer N(X) can be computed at or below which

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61

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Fertilizer N ( k g / h a )

Fig. 2. Slope functions of yield and N-uptake curves of wheat and maize (shown in Fig. 1) as a function of fertilizer nitrogen rates, dY/dX; . . . . . dU/dX; o, • wheat; 4, • maize.

N uptake is X -- P or more. For example, the X-value for wheat in the present investigation is obtained by solving: X -- P = 19.89 + 0.67 X - 0 .00053 X 2 . Application of fertilizer N upto this rate would thus create no pollution problem. This may be termed the point of permissible N loss.

The difference between fertilizer N rate at the point of permissible N loss and the point of maximum N uptake (dU/dX = 0) is directly proportional to the rate of decrease of dU/dX with increasing X. This parameter is denoted by d 2 U / d ~ and is given in Table I. Closeness of the point of permis- sible N loss to the point of maximum N uptake is determined by the curvature of the N-uptake curve. Radius of curvature of a quadratic curve is inversely proport ional to d2U/dX 2 and at the point where dU/dX = 0, radius of curvature is given by (d2U/dX ~) -x.

A simultaneous consideration of the points of maximum economic return and of permissible N loss suggests that the opt imum rate of fertilizer N application to minimize potential nitrate pollution of natural waters must be the smaller of the rates corresponding to the two points. A critical look at (1) yield and N uptake data given in Fig. 1, (2) slope lines shown in Fig. 2 and (3) amount (Table III) and profile distribution (Fig. 3) of NO3--N under wheat and maize verifies these concepts of op t imum rate of fertilizer N application.

Soft nitrate profiles under four fertilizer treatments at periods immediate- ly fol lowing the harvest of wheat and maize are exhibited in Fig. 3. Similar nitrate distribution patterns for maize plots with increasing fertilizer N levels have been reported by Linville and Smith (1971) and MacGregor et al. (1974), in soils differing widely in texture. Also, in soils which are light

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TABLE II

Nitrate nitrogen in 180 cm soil profile after harvest of crops which received different fertilizers

Crop Rate of fertilizer N (kg N/ha)

0 75 150 225

(kg NO3--N/ha after crop harvest) Wheat 14.96 13.08 15.82 16.38 Maize 15.45 27.51 37.70 108.91

textured and poor in organic matter, content estimation of nitrate nitrogen seems to be a reasonably good measure of the amount of fertilizer nitrogen left unutilized. The work of Olsen et al. (1970) also indirectly supports this statement. An estimation of NO3--N content , to a depth of 180 cm, of different t reatment profiles was made from a measurement of the area under different curves in Fig. 3. A bulk density of 1.50 g/cm 3 was assumed throughout the profiles. These data are given in Table II.

If 60 kg N/ha (P) of the applied nitrogen can remain unutilized without creating any pollution threat, op t imum rates of fertilizer N corresponding to this point of permissible N loss work ou t to be 186 and 111 kg N/ha for wheat and maize, respectively. These estimations are supported by the data

Soil nitrate content (rag NO3/kg )

0 5 10 15 0 5 10 15 0 5 10 1~ 20 0 5 10 15 20 ~3 30 35 40 I ~1 i I I I I I

~ 13 i , , ~ "", , . %

A

225"-

Fig. 3. P r o f i l e d J s t ~ b u t i o n o f n i t r a t e in f o u r t r e a t m e n t ~ o f t h e l o n g - t e r m e x p e r i m e n t ~¢f~r h a r v e s t o f w h e a t and I ma ize . A - - n o N P K ; B - - 50% N P K ; C - - i 0 0 % IN?K ; D - - 1 5 0 % N P K ; o - - w h e a t ; ~ . . . . . ma i ze .

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in Table II. In maize plots, about 35 kg of nitrate-nitrogen per hectare is found in the profile at the point of permissible N loss. The amount of NO3--N in the profile increases rapidly with increasing fertilizer dose. In the case of wheat, the rate of permissible N loss is about 1.6 times higher than that for maize, so that the amount of residual nitrate nitrogen is very small upto a N-application rate of 225 kg/ha. In contrast to maize, residual NO3--N in the wheat plots increases very slowly with increasing fertilizer N dose.

Solid lines, shown in Fig. 2, indicate that maximum yields are obtained at 260 and 195 kg N/ha for maize and wheat, respectively. Taking into con- sideration the radius of curvature near the point of maximum yield and the data reported by Parr (1973), it can safely be assumed that the points of greatest economic return would be obtained at fertilizer rates which are about 85% and 75% of the rates at the point of maximum yield for wheat and maize, respectively. It would mean that maximum economic returns for wheat and maize will be obtained at about 166 and 195 kg N/ha, respec- tively.

It can easily be inferred from the magnitudes of fertilizer N rates cor- responding to the points of maximum economic return and permissible N loss, that the op t imum rate of fertilizer N application for wheat is the rate at which maximum economic returns are obtained, the latter being the smaller amongst the two. Application of further doses of N does no harm, as far the environment is concerned, but no farmer will apply such uneconomical high rates. In the case of maize, the opt imum fertilizer N rate is determined by a point of permissible N loss. No doubt application of nitrogen beyond the op- t imum rate will increase the yield economically, but the amount of residual NO3--N remaining in the profile becomes a potential pollution threat. A small yield increment has to be sacrificed to protect the environment.

Perhaps the major factor governing the shape of the yield and N uptake curves for maize is the coincidence of the rainy season with the growth period of the crop in the study area. Heavy rain decreases the susceptibility of nitrates for absorption by plant roots, because it carries them to deeper soil layers in the initial stages of plant growth, from whence roots cannot absorb nitrates, even at advanced growth stages.

From the foregoing, it seems that the opt imum rate of fertilizer N applica- tion is affected by all those factors which determine yield and N-uptake curves. Important factors include soil, environment, and farm-management practices. Since the probability of various types of climate are generally known, it seems that the approach described in this paper can be used to optimize fertilizer N rates, applicable over wide areas.

ACKNOWLEDGEMENTS

The first author is grateful to the Indian Council of Agricultural Research, New Delhi, for the award of a Senior Research Fellowship during the course of this investigation.

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REFERENCES

Linville, K.W. and Smith, G.E., 1971. Nitrate content of soil cores from corn plots after repeated nitrogen fertilization. Soil Sci., 112: 249--255.

MacGregor, J.M., Blake, G.R. and Evans, S.D., 1974. Mineral nitrogen movement into subsoils followingcontinued annual fertilizationfor corn. Soil Sci. Soc. Am. Proc., 38: 110--112.

Olsen, R.J., Hensler, R.F., Attoe, O.J., Witzel, S.A. and Peterson, L.A., 1970. Fertilizer nitrogen and crop rotation in relation to movement of nitrate nitrogen through soil profiles. Soil Sci. Soc. Am. Proc., 34: 448--452.

Parr, J.F., 1973. Chemical and biochemical considerations for maximizing the efficiency of fertilizer nitrogen. J. Environ. Qual., 2: 75-84.

Singh, B. and Sekhon, G.S., 1976a. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. I. Proper coordination of nitrogen splitting with water management. Plant Soil, 44: 193--200.

Singh, B. and Sekhon, G.S., 1976b. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. II. Balanced fertilization. Plant Soil, 44: 391--395.