N. D. University of Agriculture and Technology, Kumarganj, Faizabad

Long-Term E�ects of Fertilizers on the Soil Fertility and Productivity of aRice±Wheat System

A. Kumar and D. S. Yadav

Authors' address: Drs A. Kumar (corresponding author) and D. S. Yadav, Department of Agronomy, N. D. Universityof Agriculture and Technology, Kumarganj, Faizabad, U. P. 224 229, India

With 5 ®gures and 2 tables

Received January 7, 2000; accepted May 31, 2000

AbstractRice±wheat cropping system to which graded levels of NPKfertilizers had been applied for 20 years were compared for

yield trends, and changes in response function, soil organic-C and available N, P, K and S status. This study of system

in which only chemical fertilizers had been used over a long

period enabled long-term yield declines of rice and wheat atdi�erent levels and combinations of NPK fertilizers to be

evaluated. The highest rate of yield decline in both rice and

wheat was found when 120 kg haÿ1 N was applied alone.The lowest rate of decline was observed when all three nutri-

ents (N, P and K) were applied, at 40, 35 and 33 kg haÿ1 forN, P and K, respectively, followed by 120, 35 and33 kg haÿ1 (currently recommended levels). The yield

response of rice and wheat to N fertilizer declined over the20 years, with a higher rate of decline in wheat. In contrast,

the response to applied P and K increased with time in both

crops, with a higher response rate in wheat. With continu-ous application of N and P fertilizers, there was a marginal

change in available N and K in the soil over time, but an

approximately 3-fold increase in available P and anapproximately 2-fold increase in available S were obtained

by regular dressing of P fertilizer (SSP: 7% P, 12% S) over

20 years. The results revealed that balanced, high doses ofNPK fertilizers are required to maintain soil fertility and

raise grain yields.

Key words: cropping system Ð fertilizer Ð rice±wheat Ð soil fertility

Introduction

In India, the rice±wheat rotation is the dominantcropping system across the Indo-Gangetic ¯oodplain and in the Himalayan foothills. Approxi-mately 33% of India's rice is grown in rotationsinvolving wheat, while about 42% of wheat isgrown in rotation with rice. Nearly 65% of thetotal amount of fertilizer used in the country(13.88m tons) is applied to rice and wheat crops.

Fertilizer consumption in India is grossly imbal-anced, tilted towards nitrogen (N) followed byphosphorus (P). This has implications for the yieldresponse to fertilizers as it decreases crop qualityand adversely a�ects overall soil fertility and pro-ductivity.There seems to have been a decline in the produc-

tivity of the rice±wheat rotation in the Indo-Gange-tic alluvial plains, despite the application ofoptimum nitrogen/phosphorus/potassium (NPK)fertilizer inputs (Nambiar and Abrol 1989). De®-ciencies of secondary nutrients and micronutrientsare also a�ecting the performance of this region innewer areas (Hegde and Dwivedi 1992).Data from a long-term fertility experiment were

analysed here to examine the impact of chemicalfertilizers on productivity and soil health withregard to the sustainability of the system.

Materials andMethodsA long-term manurial experiment was started in 1977±78 at

Faizabad, India (26�430 N, 82�80 E) under the Cropping Sys-tem Research Network Programme. The climate of the

experimental site is subhumid subtropical with hot summers

and fairly cool winters. The soil of the experimental site is

alluvial, having developed from the alluvium deposited by

rivers. The soil belongs to the order Inceptisol with silty

loam texture (52.9% silt, 22.3% clay). Potassium-bearing

minerals such as feldspar, micas and micaceous clays (illite)

predominate in the soil. The feldspar and micas are present

in the coarse fraction (sand and silt) and the illite in the clay

fraction of alluvial soils of Uttar Pradesh (Singh et al.

1991). Initially, the surface soil (0±20 cm) had a pHof 7.7,

an organic carbon content of 4.5 g kgÿ1 soil, and available

N, P, K and sulphur (S) contents of 65, 5, 57 and

11.4mg kgÿ1 soil, respectively.Eighteen fertility combinations comprising three levels of

N (40, 80 and 120 kg haÿ1), three levels of P (0, 17.5 and

35 kg haÿ1) and two levels of K (0 and 33 kg haÿ1) were

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implemented in a 3� 3� 2 factorial partially confoundeddesign. The 18 combinations were allocated to three rowswith one control (N0P0K0) in each row. Thus a total 21treatments were tested with four replications. However, inthe present study, data from only seven treatments, i.e. 0-0-0, 40-35-33, 80-35-33, 120-35-33, 120-0-0, 120-35-0 and 120-0-33 kg haÿ1 N-P-K, respectively, were evaluated. The N, Pand K were supplied through urea (46% N), single superphosphate (7% P) and muriate of potash (49% K), respec-tively. Single super phosphate also provided 12% S in addi-tion to P. In rice, zinc was applied as a foliar spray at therate of 5 kg ZnSO4� 2.5 kg Ca(OH)2 ha

ÿ1 in 1000 l ofwater, as and when de®ciency symptoms appeared.

Each year, rice was transplanted in July and harvested inOctober. Wheat was sown in November and harvested inApril. The crops were grown under irrigated conditionsfollowing the recommended practices. The crops wereharvested from ground level manually by sickle and above-ground biomass was removed from the plots of each treat-ment. After the wheat harvest of April 1998, soil sampleswere drawn from the 0±20 cm soil layer in each plot andanalysed for organic-C (Walkley & Black method), avail-able N (alkaline KMnO4 method), 0.5M NaHCO3 (pH8.5)-extractable P, 1N NH4OAC-extractable K and 0.15%CaCl2-extractable S following Jackson (1973).

Grain yield data for rice and wheat from 1977±78 to1997±98 were subjected to time trend analysis. To reducethe e�ect of variation in climate, 3-year moving averageswere used for responses and yield trends ®tted to regression.

The response to applied N over time was estimated bycalculating the increase in grain yield in the 120-35-33 treat-ment compared to the 40-35-33 and 80-35-33 treatments foreach season as:

N response �kg grain kgÿ 1 N�

� Yn120 ÿ Yn4080

andYn120 ÿ Yn80

40

Similarly, the response to applied P and K was calculatedfrom the increase in grain yield in the 120-35-33 treatmentcompared to the 120-0-33 treatment for P, and the 120-35-0

treatment for K, as:

P and=or K response �kg grain kgÿ 1 P or K�

� Yp35 ÿ Yp035

and=orYk33 ÿ Yk0

33

Results and Discussion

Trends in yield

The 20-year yield data for the seven selected treat-ments, depicted in Figs 1 and 2, showed that yieldsof both rice and wheat started to decline after 8years of cropping, even in the treatment receivingrecommended doses of N-P-K (120-35-33 kg haÿ1).However, yields fell steeply when fertilizer P wasomitted from the treatments. In plots treated with120 kgNhaÿ1 alone, wheat yields decreased by197 kg haÿ1 yearÿ1 and rice yields by 105 kg haÿ1

yearÿ1 (Table 1). In contrast, when the cropsreceived 120 kg N along with 35 kgP haÿ1, the rateof decline was reduced to 113 and 45 kg haÿ1 yearÿ1

for wheat and rice, respectively. Application of33 kg K with 120 kgNhaÿ1 did not decrease theyield reduction. However, the reduction in yieldwas further decreased to 102 and 38 kg haÿ1 yearÿ1

for wheat and rice crops, respectively, by the appli-cation of 120-35-33 kg haÿ1 N-P-K, and the highestyields for both crops were consistently obtained inthis treatment. This decline is probably associatedwith a de®ciency or imbalance in one or more soilnutrients.

Response to applied nutrients

The incremental yield response of the wheat crop tofertilizer N declined with time for the 120 vs. 80 and120 vs. 40 kgNhaÿ1 comparisons (Fig. 3), but the

Table 1: Rate of decline (kg haÿ1 yearÿ1) in grain yields of rice and wheat in a continuous rice±wheat system from1977 to 1996 under di�erent NPK fertilizer treatments

Fertilizer treatment (kg haÿ1) Yield decline rate over 20 years (kg haÿ1 yearÿ1)

N P K Rice Wheat

0 0 0 36 2640 35 33 30 6080 35 33 37 93

120 35 33 38 102120 0 0 105 197120 35 0 45 113120 0 33 85 196CD 5% 18 21

48 Long-Term E�ects of Fertilizers in a Rice±Wheat System

yield response of the rice crop did not declinesmoothly over the 20-year cropping period. Despitethis trend, the incremental e�ciencies for fertilizerN were relatively high (15±25 kg grain kgÿ1 appliedN) for both rice and wheat crops (Peng et al. 1996).

The decline in the response to applied N may havebeen a result of changes in soil properties caused byrepeatedly ¯ooding and drying of rice±wheat ®elds.These changes reduce the nitrogen-supplying capa-city of soils by inhibiting the release of native soil N

6

4

5

3

2

0

1

Gra

in y

ield

(t h

a–1)

1 2

Y = 1.28 – 0.038 t

R2 = 0.81

(a) 0-0-0

Rice

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 3.46 – 0.032 t

R2 = 0.53

(b) 40-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 4.36 – 0.039 t

R2 = 0.52

(c) 80-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 5.05 – 0.040 t

R2 = 0.58

(d) 120-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Years

6

4

5

3

2

0

1

1 2

Y = 4.55 – 0.110 t

R2 = 0.89

(e) 120-0-0

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 4.99 – 0.048 t

R2 = 0.69

(f) 120-35-0

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 4.50 – 0.089 t

R2 = 0.79

(g) 120-0-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Fig. 1: E�ect of di�erent fertility treatments (a, 0-0-0; b, 40-35-33; c, 80-35-33; d, 120-35-33; e, 120-0-0; f, 120-35-0; g,120-0-33 kg haÿ1 N-P-K, respectively) on rice yield (solid line) and the linear trend (dotted line) in a continuous rice±wheat system

49Kumar and Yadav

(Olk et al. 1996). This suggests that it is necessary toincrease the doses of fertilizer N to sustain yieldlevels.The incremental responses of rice and wheat to

fertilizer P at 35 kgP haÿ1 increased over time, but

the rate of increase was higher in wheat (Fig. 4).The average incremental response of rice at 35 kgPhaÿ1 increased from 22 kg grain kgÿ1 P during1977±78 to 49 kg grain kgÿ1 P during 1996±97,while the response of the wheat crop increased from

6

4

5

3

2

0

1

Gra

in y

ield

(t h

a–1)

1 2

Y = 1.04 – 0.027 t

R2 = 0.68

(a) 0-0-0

Wheat

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 3.08 – 0.063 t

R2 = 0.94

(b) 40-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 4.35 – 0.098 t

R2 = 0.91

(c) 80-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 5.24 – 0.107 t

R2 = 0.94

(d) 120-35-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Years

6

4

5

3

2

0

1

1 2

Y = 4.87 – 0.207 t

R2 = 0.93

(e) 120-0-0

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 5.22 – 0.119 t

R2 = 0.94

(f) 120-35-0

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

6

4

5

3

2

0

1

1 2

Y = 4.97 – 0.206 t

R2 = 0.94

(g) 120-0-33

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Fig. 2: E�ect of di�erent fertility treatments (a, 0-0-0; b, 40-35-33; c, 80-35-33; d, 120-35-33; e, 120-0-0; f, 120-35-0; g,120-0-33 kg haÿ1 N-P-K, respectively) on wheat yield (solid line) and the linear trend (dotted line) in a continuousrice±wheat system

50 Long-Term E�ects of Fertilizers in a Rice±Wheat System

20 to 74 kg grain kgÿ1 P during the same period.Phosphorus availability is increased under the sub-merged conditions needed for rice cultivation, andhence the wheat crop has a greater response toapplied P than rice (Hegde and Dwivedi 1992). Asreviewed by Tandon (1987), a greater response ofwheat to P compared to rainy season (`kharif')crops has been observed in many cropping systems

in which wheat is rotated with rice, cotton, ground-nut or maize.The rice and wheat crops did not respond greatly

to applied K during the initial 10 years (Fig. 5).Thereafter, the responses to K increased in bothrice and wheat. In the 20th year the incrementalresponse of rice and wheat to K fertilizer reached6.4 and 8.3 kg grain kgÿ1 K, respectively. It is

30

20

25

15

10

0

5

Incr

emen

tal g

rain

yie

ld r

espo

nse

to 1

20 v

s. 4

0 kg

N h

a–1

(kg

grai

n kg

–1 N

)

0 105

Y = 18.85 + 0.19 t – 0.01 t2

R2 = 0.33

Rice

15 20

30

20

25

15

10

0

5

0 105

Y = 21.73 + 0.83 t – 0.06 t2

R2 = 0.96

Wheat

15 20

30

20

25

15

10

0

5

Incr

emen

tal g

rain

yie

ld r

espo

nse

to 1

20 v

s. 8

0 kg

N h

a–1

(kg

grai

n kg

–1 N

)

0 105

Y = 12.77 + 1.66 t – 0.08 t2

R2 = 0.55

Rice

15 20

30

20

25

15

10

0

5

0 105

Y = 19.73 – 1.13 t + 0.28 t2 – 0.01 t3

R2 = 0.85

Wheat

15 20

Years

Fig. 3: Incremental grain yield response of rice and wheat to N application at 120 vs. 40 kg haÿ1 and 120 vs. 80kg haÿ1 over 20 years in a continuous rice±wheat system

Fig. 4: Incremental grain yield response of rice and wheat to P application at 35 kg haÿ1 over 20 years in a continuousrice±wheat system

51Kumar and Yadav

expected that the magnitude of the K responsewill increase over time. In alluvial soils, release ofnon-exchangeable K from illitic clay minerals isresponsible for the lack of response of crops to Kapplication (S.C. Modgal, unpubl. data ).

Changes in soil fertility

After 20 years of continuous rice±wheat cropping,soil organic-C increased by 35, 27 and 20% in plotsreceiving 120-35-33, 120-35-0 and 80-35-33 kg haÿ1

N-P-K, respectively, relative to the initial value of4.5 g Ckgÿ1 soil in 1977. In contrast, organic-Cdeclined in the unfertilized treatment by 62% com-pared with the initial level and by 49% in the treat-ment receiving 120 kgNhaÿ1 alone (Table 2). ThusP fertilizer had a bene®cial impact on organic soil Cas it had a favourable e�ect on microbial activity.The quantity of organic-C in the soil also dependson the annual turn-over of root residues (Subrama-niam and Kumaraswamy 1989).

Available N was also higher in those treatmentsin which organic-C was higher. A marginal decreaseof 12±17% in the available N was observedin the treatments receiving the optimal dose of120 kgNhaÿ1 to each crop regularly over 20 years.In treatments receiving suboptimal doses of N(40 and 80 kg Nhaÿ1), available N declined by 20±48%, with the smallest values in the unfertilizedtreatment. The decline in available N (mineralizedN) seemed to be associated with immobilization offertilizer N (Subba Rao and Ghosh 1981) and itsleaching from the ploughed layer (0±20 cm) underthe submerged conditions of rice farming (Nambiarand Abrol 1989). The results of fertilizer trials con-tinued over more than 100 years have revealed thatmineral fertilizer application signi®cantly increasesthe organic-C and organic-N of Morrow plot soils(Odell et al. 1984).The available P content of the soil increased

about 3-fold in plots receiving 35 kg P haÿ1 cropÿ1

12

8

10

6

4

0

2

Incr

emen

tal g

rain

yie

ld r

espo

nse

to K

(kg

grai

n kg

–1 K

)

0 105

Y = 5.44 – 1.65 t + 0.22 t2 – 0.007 t3

R2 = 0.88

Rice

15 20

12

8

10

6

4

0

2

0 105

Y = 2.47 – 0.22 t + 0.03 t2

R2 = 0.90

Wheat

15 20Years

Fig. 5: Incremental grain yield response of rice and wheat to K application at 33 kg haÿ1 over 20 years in a continuousrice±wheat system

Table 2: E�ects of di�erent fertilizer NPK treatments on soil organic-C and available N, P, K and S status over20 years of a continuous rice±wheat cropping system

Fertilizer treatment (kg haÿ1)Organic-C Available N Available P Available K Available S

N P K (g kgÿ1 soil) (mg kgÿ1 soil) (mg kgÿ1 soil) (mg kgÿ1 soil) (mg kgÿ1 soil)

0 0 0 1.7 34 2.6 50 7.240 35 33 4.8 44 15.7 62 25.480 35 33 5.4 52 16.8 63 23.0

120 35 33 5.8 56 15.8 61 23.7120 0 0 2.3 55 2.1 45 6.4120 35 0 5.7 57 16.4 42 24.4120 0 33 2.6 54 1.9 64 5.9CD at 5% 0.6 4.6 2.3 3.6 3.9Initial values in 1977 4.5 65 5.0 57 11.4

52 Long-Term E�ects of Fertilizers in a Rice±Wheat System

over 20 years (Table 2). Omission of phosphorusfertilizer reduced available P (Olsen's P) to 2.6 and2.1mgkgÿ1 soil (lower critical limit: 5mg Pkgÿ1

soil) in unfertilized treatments and treatment with120 kgNhaÿ1 alone, respectively. A signi®cantbuild-up of available P over a period of 14 cycles ofa maize±wheat system has been reported by Singhand Brar (1986). Continuous cropping of rice±wheat without K fertilizer for 20 years depleted soilavailable K only by 12±25%, while regular applica-tion of K fertilizer gave a marginal increase of7±13% in available K. Alluvial soils contain a largepool of non-exchangeable K (1078±3069mgkgÿ1)which serves as a source of K for the replenishmentof the exchangeable K when the latter is exhaustedby crops to a certain threshold value (Sharma andMishra 1987). Soils in Pakistan rich in mica werealso found to release high amount of K from themica after cultivation of various crops (Mengel andRahmatullah 1994).

Single super phosphate (P fertilizer) is also anindirect source of sulphur (12% S), and the avail-able S increased appreciably in the treatments thatreceived P fertilizer. The level of available Sincreased about 2-fold in 20 years from its initiallevel (11.4mg kgÿ1 soil) in the treatments receiv-ing P fertilizer (SSP) regularly. Omission of Pfertilizer depleted the available sulphur from 11.4to 6.4 and 7.2mgS kgÿ1 soil (lower critical limit:10mgS kgÿ1 soil) in the plot with N alone (120-0-0)and the unfertilized plot (0-0-0), respectively. Soilanalysis data revealed that continuous inadequateand imbalanced use of fertilizers leads to over-exploitation of the natural fertility and eventuallyloss of productivity.

The study concludes that balanced, higher dosesof N, P and K, with a preference for sulphur-con-taining P fertilizers, are required to maintain soilfertility status in the long term and to increase thegrain yields of the rice±wheat system.

Zusammenfassung

Langzeitwirkungen von DuÈ nger auf die Bodenfrucht-barkeit und die ProduktivitaÈ t von Reis-Weizen-Anbausystemen

Die Ergebnisse unterschiedlicher Konzentrationen von

NPK-DuÈ ngern in einem 20 Jahre waÈ hrenden Reis-Weizen-

anbausystem hinsichtlich der Ertragstrends, AÈ nderungen in

der Reaktionsfunktion, bodenorganischem C und verfuÈ gba-

ren N, P, K, S wurden untersucht. Die Anwendung auf

Dauer von nur einem chemischen DuÈ nger weist Ertrags-

ruckgaÈ nge von Reis und Weizen in unterschiedlichem

Ausmaû im Zusammenhang mit NPK-DuÈ ngern auf. Die

hoÈ chste Ertragsabnahme von Reis und Weizen wurde

gefunden, wenn mit 120 kgNhaÿ1 allein geduÈ ngt wurde.

Die geringste Abnahme wurde beobachtet, wenn alle drei

NaÈ hrsto�e (N, P, K) angewendet in Mengen von 40-35-

33 kg haÿ1 wurden, gefolgt von 120-35-33 kg N-P-Khaÿ1

(z. Zt. empfohlene Mengen). Die Ertragsreaktionen von

Reis undWeizen gegenuÈ ber N-DuÈ nger nahm uÈ ber die zwan-

zig Jahre ab, wobei der RuÈ ckgang in Weizen staÈ rker war.

Die Reaktion auf P und K verstaÈ rkte sich im Laufe der

Jahre in den BestaÈ nden beider Arten, wobei die Reaktion

bei Weizen staÈ rker war. Bei einer anhaltenden Anwendung

von N- und P-DuÈ ngern war nur eine marginale AÈ nderung

im verfuÈ gbaren N und K im Boden im Laufe der Jahre fest-

zustellen; es wurde aber eine dreifache Zunahme im verfuÈ g-

baren P und etwa eine zweifache Zunahme im verfuÈ gbaren

S bei einer regulaÈ ren Anwendung von P-DuÈ nger (SSP: 7%

P, 12% S) im Verlauf von zwanzig Jahren beobachtet. Die

Ergebnisse weisen darauf hin, dass Ausgeglichenheit und

hoÈ here Anwendungsmengen von NPK-DuÈ ngern benoÈ tigt

werden, um die Bodenfruchtbarkeit zu erhalten und die

KornertraÈ ge zu steigern.

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Jackson, M. L., 1973: Soil Chemical Analysis. Prentice Hallof India, Ltd, New Delhi.

Mengel, K., and X. Rahmatullah, 1994: Exploitation ofpotassium by various crop species from primary mineralsin soils rich in micas. Biol. Fert. Soils 17, 75Ð79.

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Olk, D. C., K. G. Cassman, E. W. Randall, P. Kinchesh, L.J. Sangar, and J. M. Anderson, 1996: Changes in chemicalproperties of soil organic matter with intensi®ed rice crop-ping in tropical lowland soils. Eur. J. Soil Sci. 47,293Ð303.

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54 Long-Term E�ects of Fertilizers in a Rice±Wheat System