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Published in Agron J 99:1436-1447 (2007)
DOI: 10.2134/agronj2006.0283
© 2007 American Society of Agronomy
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Site-Specific Analysis & Management

Performance of Site-Specific Nutrient Management for Irrigated, Transplanted Rice in Northwest India

Harmandeep S. Khuranaa,*, Steven B. Phillipsb, Bijay-Singhc, Achim Dobermannd, Ajmer S. Sidhuc, Yadvinder-Singhc and Shaobing Pengd

a Virginia Tech, Dep. of Crop & Soil Environ. Sci., Blacksburg, VA 24061
b Int. Plant. Nutr. Inst., Southeast Region, Melfa, VA 23410
c Dep. of Soils, Punjab Agric. Univ., Ludhiana 141004, India
d Int. Rice Res. Inst. (IRRI), DAPO Box 7777, Manila 1271, the Philippines

* Corresponding author (hsk{at}vt.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
 REFERENCES
 
Like in other parts of Asia, irrigated, transplanted rice (Oryza sativa L.) yield increases in Punjab, India, have slowed down in recent years. Further yield increases are likely to occur in smaller increments through fine-tuning of crop management mainly by accounting for the large spatial and temporal variation in soil characteristics. On-farm experiments were conducted from 2002 to 2004 at 56 sites in six key irrigated rice-wheat (Triticum aestivum L.) domains of Punjab to evaluate an approach for site-specific nutrient management (SSNM). Field-specific N–P–K applications were calculated by accounting for the indigenous nutrient supply, yield targets, and nutrient demand as a function of the interactions between N, P, and K. The performance of SSNM was tested for two rice crops. Compared with the current farmers' fertilizer practice (FFP), average grain yield increased from 5.1 to 6.0 Mg ha–1, while plant N, P, and K accumulations increased by 13 to 15%. The gross return above fertilizer cost (GRF) was about 14% greater with SSNM than with FFP. Improved timing and/or splitting of fertilizer N increased N recovery efficiency from 0.20 kg kg–1 in FFP plots to 0.30 kg kg–1 in SSNM plots. The agronomic N use efficiency was 83% greater with SSNM than with FFP. The year-wise effect on all parameters was, however, nonsignificant. As defined in our study, SSNM has potential for improving yields and nutrient efficiency in irrigated, transplanted rice.

Abbreviations: AEN, agronomic efficiency of applied fertilizer nitrogen • DAT, days after transplanting • FFP, farmers' fertilizer practice • FK, Faridkot • FP, Firozpur • GD, Gurdaspur • GRF, gross return above fertilizer cost • HO, Hoshiarpur • IKS, indigenous potassium supply • INS, indigenous nitrogen supply • IPS, indigenous phosphorus supply • LU, Ludhiana • PFPN, partial factor productivity of applied nitrogen • PT, Patiala • QUEFTS, Quantitative Evaluation of the Fertility of Tropical Soils (model) • REN, recovery efficiency of applied nitrogen • SSNM, site-specific nutrient management

Performance of Site-Specific Nutrient Management for Irrigated, Transplanted Rice in Northwest India

Harmandeep S. Khuranaa,*, Steven B. Phillipsb, Bijay-Singhc, Achim Dobermannd, Ajmer S. Sidhuc, Yadvinder-Singhc and Shaobing Pengd

a Virginia Tech, Dep. of Crop & Soil Environ. Sci., Blacksburg, VA 24061
b Int. Plant. Nutr. Inst., Southeast Region, Melfa, VA 23410
c Dep. of Soils, Punjab Agric. Univ., Ludhiana 141004, India
d Int. Rice Res. Inst. (IRRI), DAPO Box 7777, Manila 1271, the Philippines

* Corresponding author (hsk{at}vt.edu)

Received for publication October 13, 2006.
Like in other parts of Asia, irrigated, transplanted rice (Oryza sativa L.) yield increases in Punjab, India, have slowed down in recent years. Further yield increases are likely to occur in smaller increments through fine-tuning of crop management mainly by accounting for the large spatial and temporal variation in soil characteristics. On-farm experiments were conducted from 2002 to 2004 at 56 sites in six key irrigated rice-wheat (Triticum aestivum L.) domains of Punjab to evaluate an approach for site-specific nutrient management (SSNM). Field-specific N–P–K applications were calculated by accounting for the indigenous nutrient supply, yield targets, and nutrient demand as a function of the interactions between N, P, and K. The performance of SSNM was tested for two rice crops. Compared with the current farmers' fertilizer practice (FFP), average grain yield increased from 5.1 to 6.0 Mg ha–1, while plant N, P, and K accumulations increased by 13 to 15%. The gross return above fertilizer cost (GRF) was about 14% greater with SSNM than with FFP. Improved timing and/or splitting of fertilizer N increased N recovery efficiency from 0.20 kg kg–1 in FFP plots to 0.30 kg kg–1 in SSNM plots. The agronomic N use efficiency was 83% greater with SSNM than with FFP. The year-wise effect on all parameters was, however, nonsignificant. As defined in our study, SSNM has potential for improving yields and nutrient efficiency in irrigated, transplanted rice.

Abbreviations: AEN, agronomic efficiency of applied fertilizer nitrogen • DAT, days after transplanting • FFP, farmers' fertilizer practice • FK, Faridkot • FP, Firozpur • GD, Gurdaspur • GRF, gross return above fertilizer cost • HO, Hoshiarpur • IKS, indigenous potassium supply • INS, indigenous nitrogen supply • IPS, indigenous phosphorus supply • LU, Ludhiana • PFPN, partial factor productivity of applied nitrogen • PT, Patiala • QUEFTS, Quantitative Evaluation of the Fertility of Tropical Soils (model) • REN, recovery efficiency of applied nitrogen • SSNM, site-specific nutrient management


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
 REFERENCES
 
PUNJAB PROVINCE IN NORTHWEST INDIA accounts for 10% of the national rice production (Anonymous, 2005). About 61% of the total arable land in Punjab is used for growing irrigated, transplanted rice, mainly in the rice-wheat cropping systems that were adopted in the 1960s (Bijay-Singh et al., 2003a). During the green revolution phase (1960–1986), rice grain yields in Punjab increased at an average annual rate of 1.3% (Fig. 1 ). However, from 1987 to 2003, the average rice grain yield growth rate was only about 0.1% per year. More recent analysis of yield trends in several long-term experiments in Asia and more specifically, in Punjab (Duxbury, 2001; Ladha et al., 2003; Pathak et al., 2003a; Yadvinder-Singh and Bijay-Singh, 2003) also suggests that rice yields are either stagnating or declining. This has been mainly attributed to improper nutrient management approaches (FAO, 1994a, 1994b; Cassman et al., 1997; Timsina and Connor, 2001; Cakmak, 2002) that resulted in decreased nutrient supply capacity of soil and use efficiency of the applied fertilizer. For example, Dobermann and Cassman (2002) observed nonsignificant increases in fertilizer N efficiency in rice grown in different Asian countries during the past 30 yr. The average plant recovery efficiency of fertilizer N in rice is still only about 30% (Dobermann, 2000). Application of other macronutrients, such as K, has lagged behind leading to imbalanced plant nutrition and negative K input–output balances in many parts of Punjab and Asia (Bijay-Singh et al., 2003b; Dobermann and Witt, 2004). Environmental pollution by nutrient leaching or runoff from rice fields has become another concern across Asia (Bijay-Singh and Yadvinder-Singh, 2003; Dobermann et al., 2002).


Figure 1
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  		Fig. 1. Trend of rice productivity (or grain yield, GY) in Punjab, India, from 1960 to 2003. Annual rates of productivity changes were estimated from a piece-wise linear regression, which for the period 1960 to 1986 was y = 0.1307x – 202.6 (R2 = 0.958) and for the period 1987 to 2004 was y = 0.0104x – 50.44 (R2 = 0.687). (Data source: Annual statistical abstracts of Punjab, Economic and Statistical Organization, Government of Punjab, India.)

 
To overcome these challenges and meet the expected food demand in the next 30 yr, rough estimates for India (Dobermann and Witt, 2004) suggest the need to increase average farm productivity of the system, which is currently at 45 to 60% of the attainable yield potential, to 70 to 80% of the attainable potential. For Punjab, this represents average rice yields of about 7.9 Mg ha–1. Although strategic research is ongoing to develop new germplasm with increased yield potential, improved N-fixation characteristics, or host plant resistance to pests (Peng et al., 1999; Ladha and Reddy, 2000; Sheehy et al., 2000; Dobermann and Cassman, 2002), it is still uncertain whether any of these efforts will have measurable impact on increasing rice yields in the near future. At issue then is whether the existing yield gaps can be further exploited through improved nutrient and crop management, and whether better nutrient management might be able to restore some momentum to the yield growth of rice in the coming decades.

Recent research conducted in different continents (Angus et al., 1990; Wopereis et al., 1999; Wang et al., 2001; Dobermann et al., 2002) has demonstrated limitations of the current approach of fixed-rate, fixed-time fertilizer recommendations being made for large areas. This approach does not take into account the existence of large variability in soil nutrient supply and crop response to nutrients among, or even within, farms. For example, Cassman et al. (1996a, 1996b) observed that indigenous nitrogen supply (INS, defined as plant N accumulation in grain and straw at physiological maturity in a 0-N plot) was variable among fields and seasons across Asia, and was not related to soil organic matter content—an index commonly used to determine available N in Punjab soils. Similarly, research on P and K demonstrated that the existing soil test methods had limited applicability to lowland rice (De Datta et al., 1989; Dobermann et al., 1996a, 1996b, 1996c). Thus, it was hypothesized (Peng et al., 1996a, 1996b) that yields, profits, plant nutrient accumulations, and N use efficiencies could be significantly increased using plant-based, more knowledge-intensive, and site-specific strategies of nutrient management.

The original concept of SSNM to manage among-farm nutrient variability was developed in Asia in 1996 (Dobermann et al., 1996d; Buresh et al., 2005). They defined SSNM as a dynamic, field-specific management of nutrients in a particular crop or cropping system to optimize the supply and demand of nutrients according to their differences in cycling through soil–plant systems. A distinct feature of this SSNM approach is that it adds important regional and real-time components to the otherwise used approaches of SSNM in large-scale farming, which mainly focus on managing spatial variability of nutrients within large production fields using highly advanced tools (Pierce and Nowak, 1999; Robert, 2002). Thus, this approach adapts itself to small farms (commonly found in Punjab) without employing very expensive tools, but using the more reliable crop-based estimates of indigenous nutrient supply compared with the soil tests. The approach has shown the potential to improve productivity and profitability in intensive (double- or triple-) rice cropping systems of Asia (Dobermann et al., 2004) and Africa (Wopereis et al., 1999). However, more research is needed for irrigated, transplanted rice grown under the rice-wheat cropping systems. Therefore, the main objective of the present study was to evaluate this SSNM approach from agronomic and economic aspects for irrigated, transplanted rice under the rice-wheat cropping system in Punjab. Specific objectives of the study were to (i) quantify the variation in soil nutrient supply in irrigated, transplanted rice fields across Punjab, (ii) compare the agronomic performance of SSNM with the current practice of fertilizer use (blanket use) by Punjab farmers, and (iii) assess the costs and benefits of SSNM for irrigated, transplanted rice in Punjab.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
 REFERENCES
 
Site Characteristics
Punjab province is located between 29°30' and 32°32' N latitudes and 73°55' and 76°50' E longitudes in the northwestern part of India. It has a subtropical type of climate and hyperthermic temperature regime. Rice-wheat is the dominant cropping system of the region wherein rice is grown in the summer months (mid-June to October) followed by wheat in the winter months (November to mid-April) and a small fallow period from mid-April to mid-June. We conducted on-farm experiments with irrigated, transplanted rice on 56 farmers' fields in six rice-wheat production domains (sites) across the three major agro-climatic zones of Punjab (Table 1 ). The sites at which on-farm experiments were conducted were Gurdaspur (GD), Hoshiarpur (HO), Ludhiana (LU), Patiala (PT), Faridkot (FK), and Firozpur (FP). Each site represents a spatial domain in which on-farm experiments were conducted in 8 to 11 farms located within a radius of typically 9 to 27 km around a research station of Punjab Agricultural University (PAU), India. The domains varied in size, but were typically in the 100- to 250-km2 range. Farms were typically clustered into several villages and selected to represent different socioeconomic conditions, farm sizes, and the most common soil types and farm management practices in the region. Average farm sizes ranged from 0.8 to 6 ha. At each farm, a single farmer's field (0.1 to 0.4 ha) served as the principal experimental unit.


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  		Table 1. Experimental sites in Punjab, India.

 
On-Farm Experiments
On-farm experiments were conducted from 2002 to 2004 using irrigated, transplanted rice on 56 rice-wheat farms of Punjab to test the performance of SSNM approach from agronomic and economic aspects. All experiments followed a standardized experimental protocol with minor location-specific modifications to account for differences in field sizes, climatic seasons, and crop establishment techniques. Treatments are listed below.

Farmers' Fertilizer Practice (2002–2004)
All crop and fertilizer management was done by the farmer on a single field (0.1–0.4 ha) with no interference by the researcher. Ranges of farmers' fertilizer rates were 120 to 175 and 0 to 12 kg ha–1 N and P, respectively. Most farmers applied all fertilizer N within 6 wk of transplanting rice and at fixed schedules (0, 21, and 42 days after transplanting, DAT). All fertilizer P was drilled at the time of transplanting rice. No farmer applied fertilizer K in any of the 56 fields used for the experiment. This might be due to the fact that large quantities of available potassium (4.13–22.8 mg L–1 K with an average of 12.8 mg L–1 K) are present in irrigation waters of Punjab.1 The FFP treatment was sampled at each farm after the harvest of rice crop for each of the 3 yr (2002, 2003, and 2004) to estimate grain and straw yields and plant nutrient (N, P, and K) accumulations. In the first year, the purpose of this treatment was mainly to formulate yield goal for SSNM. In the second and third years, this treatment was used for comparison with SSNM for yield, plant nutrient accumulation, fertilizer use, N use efficiency, total fertilizer cost, and GRF.

Nitrogen Omission Plot (2002–2004)
Only P (15 kg P ha–1 [Yadvinder-Singh et al., 2000]) and K (30–50 kg K ha–1 [Wang et al., 2001]) were applied to 5 by 5 m plots embedded in the farmers' fields to ensure that macronutrients other than N did not limit plant N uptake from indigenous sources. The plot area was kept small to minimize the monetary loss to farmers, as grain yields are likely to decline drastically when no fertilizer N is applied. The range in fertilizer K was kept to accommodate the variation in available K present in irrigation waters across different sites. This treatment was sampled at each site after the harvest of the rice crop for each of the 3 yr (2002, 2003, and 2004) to estimate INS defined as plant N accumulation in grain and straw at physiological maturity in a 0-N plot. This treatment was used to estimate (i) N use efficiencies using the difference method and (ii) INS used as an input parameter for SSNM.

Phosphorus Omission Plot (2002–2004)
Only N (150 kg N ha–1) and K (30–50 kg K ha–1 [Wang et al., 2001]) were applied to a strip plot (40–90 m2) embedded in the farmers' fields to ensure that macronutrients other than P did not limit plant P uptake from indigenous sources. This treatment was sampled at each site after the harvest of the rice crop for each of the 3 yr (2002, 2003, and 2004) to estimate indigenous phosphorus supply (IPS) defined as plant P accumulation in grain and straw at physiological maturity in a 0-P plot.

Potassium Omission Plot (2002–2004)
Only N (150 kg N ha–1) and P (15 kg P ha–1 [Yadvinder-Singh et al., 2000]) were applied to a strip plot (40–90 m2) embedded in the farmers' fields to ensure that macronutrients other than K did not limit plant K uptake from indigenous sources. This treatment was sampled at each site after the harvest of the rice crop for each of the 3 yr (2002, 2003, and 2004) to estimate indigenous potassium supply (IKS) defined as plant K accumulation in grain and straw at physiological maturity in a 0-K plot.

There exists little within-field variability in Punjab probably due to small farm sizes (0.8–6 ha); thus, we used only two sets of these three different types of omission plots in a single field. However, omission plots for each year were moved to a location different from that used in the previous year, though all these locations were within the same field. This was done to represent normal conditions of indigenous nutrient supplies under typical crop management as well as to avoid nutrient depletion and residual effects. Depending on the initial soil test data, 50 kg ha–1 ZnSO4 was also applied before transplanting rice to prevent any potential Zn deficiency at 19 farms across all sites except GD.

Site-Specific Nutrient Management Plot (2003–2004)
Nutrient applications were prescribed to a larger plot (300–1000 m2) located within the farmer's field on a field- and crop-specific basis following the SSNM approach described below. Ranges of fertilizer rates in this treatment plots were 110 to 155, 10 to 23, and 12 to 50 kg ha–1 N, P, and K, respectively, across different farms and sites.

Agronomic Field Management and Measurements
Flooding and land preparation for rice started in late May or early June at all the sites. From 8 to 20 June, 5- to 6-wk-old rice seedlings (cv. PR-116 at 44 farms and Pusa-44 at 12 farms) were transplanted in the different treatment plots with row x plant spacing of 20 cm x 15 cm. Both the varieties of rice used in the study were modern semidwarf types with similar yield potential and harvest index (Yadvinder-Singh et al., 2000). Apart from the water received from rainfall, supplemental irrigation was provided using both well and canal water. Plots were kept flooded for 3 wk after transplanting; thereafter, rice was irrigated at 2-d intervals. Although soil did not remain flooded for more than 8 to 15 h after irrigation, anaerobic conditions prevailed for more than 75% of the rice growth period. Rice was harvested between the first and third weeks of October at all the 56 farms. Farmers did all the water management as well as weed and pest control following the commonly recommended methods. However, where problems were suspected or observed, measures to either control them in advance (prophylactic) or correct them, were implemented under the guidance of university researchers. At harvest, rice plants were cut close to the ground surface and after harvesting, the straw was removed from the plots.

Initial soil samples for the determination of general soil properties in the 0- to 15-cm depth were collected from each field in the last week of April 2002. Thereafter, soil samples were collected again at 20 to 30 d after planting from the nutrient omission plots and analyzed for organic carbon and total N in 0-N plots, Olsen-P in 0-P plots and 1 M NH4OAc extractable K in 0-K plots using standard procedures (see footnotes to Table 3).


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  		Table 3. Status and variability of soil fertility in 0- to 15-cm depth in 56 irrigated, transplanted rice fields of Punjab, India.

 
Plant sampling procedures followed a standard procedure at all the experimental sites (Witt et al., 1999; Dobermann and Fairhurst, 2000). Two sampling areas (6 by 6 m in FFP and SSNM; 4 by 4 m in 0-N, 0-P, and 0-K plots) were randomly selected in each treatment for replicated plant sampling. A 12-hill plant sample was collected at physiological maturity to determine yield components and nutrient concentrations in plant tissue. Grain yields were obtained from a central 5-m2 harvest area in each sampling plot at harvestable maturity and are reported at standard moisture content of 0.14 kg kg–1 H2O fresh wt. Grain and straw subsamples from the 12-hill sample were oven-dried to constant weight at 70°C. Straw yields were estimated from the oven-dry grain yield of the 5-m2 harvest area and the grain–straw ratio of the 12-hill sample. Nitrogen concentrations in grain and straw were measured by micro-Kjeldahl digestion, distillation, and titration (Bremner and Mulvaney, 1982). Tissue-P was measured by the molybdenum-blue colorimetric method and tissue-K by flame emission spectrophotometer after wet digestion (Walinga et al., 1995). Other measurements included chlorophyll or SPAD (Soil Plant Analysis Development) meter (SPAD 502, Minolta, Ramsey, NJ) readings of the uppermost fully expanded leaf in the SSNM and FFP treatments. Beginning at 14 DAT, 15 leaf readings per plot were averaged, and measurements continued in 7- to 10-d intervals until about 10 d after flowering.

Site-Specific Nutrient Management Approach
The SSNM approach used in this study focused on managing spatial variation in indigenous N, P, and K supplies among individual fields. Soil and plant nutrient analysis has indicated that nutrients other than N generally did not limit rice growth in Punjab (Yadvinder-Singh et al., 2003). Therefore, SSNM mainly involved prediction of field-specific optimal fertilizer rates and development and implementation of a site-specific N management scheme that accounted for real-time variation in crop N demand at major growth stages of rice.

A modification of the QUEFTS (Quantitative Evaluation of the Fertility of Tropical Soils) model (Janssen et al., 1990; Smaling and Janssen, 1993; Witt et al., 1999) was used to work out field-specific fertilizer recommendations for each farm at the beginning of growing season of rice crop. Information needed to estimate the total amount of N, P, and K to be applied included (i) climatic yield potential; (ii) yield goal; (iii) definition of the relationship between grain yield and nutrient accumulation; (iv) recovery efficiencies of fertilizer N, P, and K; (v) field-specific estimates of the indigenous N, P, and K supply; and (vi) potential constraints to fertilizer use.

On the basis of the previous crop simulation analysis (Pathak et al., 2003b), the climatic yield potential for irrigated, transplanted rice in Punjab varied from 10.25 to 11.1 Mg ha–1 for different sites. Yield goals were constrained to a range of 70 to 80% of the climatic yield potential because beyond that level, internal nutrient efficiencies in the plant decline (Witt et al., 1999). Moreover, practical experience indicates that yields of about 80% of the climatic yield potential appear to represent a ceiling for what can be achieved by most farmers under field conditions (Cassman and Harwood, 1995). In fields with low indigenous supply of one or more nutrients (e.g., at GD, HO, FK, and FP sites), the yield goals were, however, lowered from the 70 to 80% range to first slowly build up soil fertility and raise the yield goal over time. This is because even very high mineral fertilizer rates cannot fully substitute for lower attainable yields in these fields because of low inherent soil fertility (Dobermann et al., 2004). Actual yield goals for irrigated, transplanted rice in this study ranged from 51 to 78% of the climatic yield potential. The generic empirical models proposed for rice (Witt et al., 1999) were used to model the relationship between grain yield and plant accumulation of N, P, and K. Average first crop recovery fractions of 0.4, 0.2, and 0.5 kg kg–1 were assumed for fertilizer N, P, and K, respectively (Wang et al., 2001). The potential supply of N, P, and K from soil and other indigenous sources was estimated as plant nutrient accumulation in the nutrient omission plots. Indigenous macronutrient (N, P, and K) supply values measured in 2002 and 2003 were used as model inputs along with target yields for different farms. A linear optimization procedure was used to find the best combination of N, P, and K fertilizer rates to achieve the yield goal under the constraint of optimizing the internal N, P, and K efficiencies in the plant. The model was constrained to arrive at a solution close to the situation of most balanced nutrition, that is, where the ratio between accumulation and potential supply of each macronutrient was close to 0.95 (Janssen et al., 1990). Upper limits of 180 and 35 kg ha–1 N and P, respectively, were set to avoid predicting unrealistically high yields and fertilizer rates on soils with low fertility, assuming that application of N, P, and K alone cannot completely substitute for low inherent soil fertility. Excessive N rates would also increase the risk of pest damage or lodging. Lower limits of 10 and 30 kg ha–1 P and K, respectively, were set as the minimum amount to be applied to replenish net removal of these nutrients from field and minimize risk of any macronutrient deficiency. However, where the total amount of available K (from both irrigation well water and exchangeable soil K) exceeded the total K accumulated by rice, K fertilization was omitted to avoid any leaching losses.

Field-specific, a priori N recommendations, calculated using QUEFTS, assume average climatic conditions and no or minimal stresses. Under field conditions, the actual climate varies, and some stresses such as water, pests, and mineral excess or deficiency cannot fully be excluded. Our goal was, therefore, to gradually develop a N management strategy that allows adjusting N rates according to the climatic differences among sites and seasonally within each field and for each rice crop. For this, we used chlorophyll or SPAD meter and developed N management strategies in irrigated, transplanted rice for the selected sites in Punjab (Khurana et al., 2005) that are outlined in Table 2 .


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  		Table 2. Site-specific N management strategies for irrigated, transplanted rice followed during 2003 and 2004 across different sites in Punjab, India.

 
These N management strategies accounted for variation in INS during early growth and variation in late-season N demand depending on the actual growth conditions. Late N was only applied in cases with good crop stand to support the extra yield potential by adding more N for grain filling (Perez et al., 1996). Fertilizer sources used were urea granules (46% N), diammonium phosphate (DAP; 18% N and 20% P), and muriate of potash (KCl, 50% K). Because DAP also contained N, the fertilizer application rates of urea were adjusted so as not to exceed the total amount of N applied. All P and K fertilizer was incorporated into the soil before transplanting rice (100% basal).

Calculations and Statistical Analysis
Nitrogen use efficiencies were estimated using the differences between N-fertilized treatments and the 0-N plots, as described by Cassman et al. (1998). Terms used are agronomic efficiency of applied N (AEN; kg grain yield increase kg–1 N applied), apparent recovery efficiency of applied nitrogen (REN, kg N taken up kg–1 N applied), and partial factor productivity of applied nitrogen (PFPN, kg grain kg–1 N applied). Economic calculations were made using U.S. dollars as standard currency:

Formula 1[1]

Formula 2[2]
where TFC = total fertilizer cost ($ ha–1); PN = price of N fertilizer ($0.32 kg–1 N); FN = amount of N applied (kg N ha–1); PP = price of P fertilizer ($1.81 kg–1 P); FP = amount of P applied (kg P ha–1); PK = price of K fertilizer ($0.50 kg K–1); FK = amount of K applied (kg K ha–1); GRF = GRF ($ ha–1); PR = price of rice ($0.12 kg–1 paddy); and YR = rice yield (kg ha–1).

The prices used were minimum procurement prices for the Government of India for rice grains and average retail prices for different fertilizers in Punjab for the year 2004. National policies affecting rice prices and input prices also affect the profitability of a new technology, and both factors are in principle important for technology adoption. However, it is better to avoid a discussion of idiosyncratic national pricing policies, especially since the effect of different national policies on profitability is not large. As the data on land rental costs were not easily available and because of the difficulties in imputing costs to family labor, it was not possible to calculate the absolute level of profit with and without SSNM. This is not a major drawback, since the absolute level of profits is less important than the change in profits due to adoption of the technology. The incremental profitability of SSNM ({Delta}GRF, $ ha–1) was therefore measured as the difference in GRFs due to different grain yields for SSNM and FFP minus the change in total fertilizer costs due to different fertilizer usage in the two treatments:

Formula 3[3]

PROC GLM of SAS (SAS Institute, 1988) was used to perform ANOVA on the differences between SSNM and FFP ({Delta} = SSNM – FFP) measured at each farm for two rice crops grown during 2003 and 2004 using the following model: Site df = 5; Farm within site df = 50; Crop year df = 1; Site x crop year df = 5; Residual df = 51.

A fixed-effects model was used to analyze the on-farm data because the sampling locations were not selected truly randomly. For variables with missing observations, the denominator mean square was adjusted using the Satterthwaite approximation (Satterthwaite, 1946). All effects, except site, were tested against the residual. Site effect was tested against farm within site as error term. Since the crop year effect was nonsignificant for all parameters in our study, we tabulated the average data for 3 yr (2002–2004) for grain yield and plant nutrient (N, P, and K) accumulation in nutrient omission plots and for 2 yr (2003–2004) for differences between SSNM and FFP treatments for grain yield, plant nutrient (N, P, and K) accumulation, fertilizer use, N use efficiency, TFC, and GRF. The level of significance used was 5% (P = 0.05).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
CONCLUSIONS AND SCOPE
 REFERENCES
 
Spatial Variation in Indigenous Nutrient Supply
Initial soil analysis indicated large variation in soil fertility characteristics among the six rice-wheat production domains as well as among farms within each domain (Table 3 ). For example, organic carbon values—an index used to determine available N in Punjab soils—ranged from low (<4 g kg–1) to high (>7.5 g kg–1) with an average within-domain coefficient of variation (CV) of 19%. Similarly, extractable K and Olsen-P values also ranged from low (<120 and <12.5) to high (>280 and >22.5 kg ha–1, respectively) among the experimental sites. Potassium extracted by 1 M ammonium acetate ranged from 86.2 to 343 kg ha–1, but one-fourth of all fields had <175 kg K ha–1, a critical level often used for rice soils with little K fixation (Dobermann and Fairhurst, 2000). Interestingly however, plant-based indicators of the IKS indicated greater available K reserves than suggested by extraction with1 M NH4OAc (see below).

Two- to four-fold ranges of grain yields and plant nutrient accumulations in nutrient omission plots were found among fields within all domains, with CVs mostly in the 10 to 18% range (Table 4 ). Average rice grain yields in nutrient omission plots increased in the order 0-N (3.82) < 0-K (5.41) = 0-P (5.45 Mg ha–1). This confirmed the observations of Yadvinder-Singh et al. (2003) that N deficiency was an ubiquitous feature of irrigated rice-wheat environments in tropical and subtropical areas of India, whereas at the present soil fertility levels, P and K supplies are seemingly less limiting factors for rice production. Plant N accumulation in the 0-N plot, also known as INS, ranged from 19.8 to 86.6 kg ha–1, IPS ranged from 7.8 to 25.1 kg ha–1, and IKS ranged from 48.4 to 124 kg ha–1. This field-to-field variability includes spatial variability due to rotating omission plots, climatic and crop management factors, and the errors associated with plant sampling and chemical analysis. However, no consistent difference was observed among the 3 yr of experimentation for indigenous N, P, and K supplies. Coefficients of variation within a domain were generally largest for INS (12–27%). In fact, Dawe and Moya (1999) suggested that large variability in INS was probably one of the major reasons for the large temporal fluctuations in optimal fertilizer N-rates observed. Depletion of the median IKS to levels below present average yields of irrigated rice has not yet occurred widely, but, considering the widespread negative K input-output balances in irrigated rice-wheat systems (Bijay-Singh et al., 2003b), this appears to be only a matter of time.


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  		Table 4. Variability of grain yield and plant nutrient accumulation in nutrient omission plots in 56 irrigated, transplanted rice fields of Punjab, India. Descriptive statistics are based on three rice crops sampled in each field from 2002 to 2004.

 
Grain Yield and Nutrient Accumulation
Compared with the FFP, SSNM significantly increased grain yield and plant N, P, and K accumulations at all sites in the two rice crops grown in 2003 and 2004 (Table 5 ). However, year-wise no significant differences were observed. The average yield difference between SSNM and FFP was 0.9 Mg ha–1 (17%). Maximum increase in rice grain yield was observed at FP (35%) and FK (24%) sites having a majority of poor-fertility soils, followed by GD (16%) and HO (16%) sites where soils are generally deficient in extractable K. The sites with good soil fertility, such as LU and PT, showed minimum yet significant increases in grain yields of 9 to 11%. In five farms at LU, rice yields exceeded 7.7 Mg ha–1, with a maximum of 8.2 Mg ha–1. At 21 of the total 56 farms studied, grain yield increases were ≥1 Mg ha–1 with SSNM compared with FFP, showing the potential of the SSNM approach used.


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  		Table 5. Effect of site-specific nutrient management (SSNM) on paddy grain yield and plant nutrient accumulation at six sites in Punjab, India, during 2003 and 2004.

 
There were significant increases in plant N, P, and K accumulations in SSNM compared with FFP treatments (Table 5). On average, plant N accumulation increased by 11 kg ha–1 (14%), P accumulation by 1.9 kg ha–1 (13%), and K accumulation by 12 kg ha–1 (15%). Among sites, maximum increase in plant N and P accumulations was observed at FP (23 and 27%, respectively) and FK (23 and 16%, respectively) sites having minimum organic carbon and Olsen-P values, while maximum increase in plant K accumulation was observed at HO (41%) and GD (29%) where soils are generally deficient in extractable K.

Fertilizer Use
Average fertilizer N use in rice in the FFP at all sites in Punjab (148 kg N ha–1) (Table 6 ) was relatively higher than the fertilizer use in other regions in Asia (Dobermann et al., 2002) but lower than that used in China (Wang et al., 2001). However, most farmers had no means of adjusting their N fertilizer rates according to the actual soil N status. Correlation between N rate and INS in rice was –0.21, confirming similar observations in China (–0.34, Wang et al., 2001) and other parts of Asia (Olk et al., 1999). The poor correlation was precisely the reason why despite higher N use under FFP, grain yield and N accumulation were low as compared with that under SSNM (Table 5). In rice, P application was generally skipped in FFP at four sites (GD, HO, LU, and PT). This might be due to the medium content of P in soils at these sites as well as increased availability of P under flooded conditions. Like N, P rates were not significantly correlated with IPS (r = 0.01). Potassium use in FFP was highly restricted in Punjab probably because of substantial contribution of K (4.1–22.8 mg K L–1 with an average of 12.8 mg K L–1) from irrigation water. This might also be the reason why rice grown in some soils testing low in available K (particularly at GD and HO sites) did not give any response to fertilizer K application.


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  		Table 6. Effect of site-specific nutrient management (SSNM) on fertilizer use in irrigated, transplanted rice fields at six sites in Punjab, India, during 2003 and 2004.

 
On average, 12 kg ha–1 (8%) less fertilizer N was used with SSNM compared with FFP (Table 6). In contrast, there were significant increases in P and K use with SSNM compared with FFP. These increases could be attributable to the fact that in SSNM lower limits of 10 and 30 kg ha–1 P and K, respectively, were applied to replenish net removal of these nutrients from the field and to minimize risk of any macronutrient deficiency. However, farmers applied limited amounts of P and no K at any of the sites used in this study. Crop year effects were all nonsignificant for N–P–K fertilizer applications.

Nitrogen Use Efficiency
Significant increases in N use efficiency were achieved through the field-specific N management practiced in the SSNM treatment (Table 7 ). In general, compared with the FFP, less fertilizer N was applied (Table 6) and AEN, REN, and PFPN were significantly increased with SSNM. On average, AEN was increased by 7.3 kg kg–1 (83%), REN by 0.10 kg kg–1 (50%), and PFPN by 9.5 kg kg–1 (27%). Differences in the impact of SSNM on N use efficiency between 2003 and 2004 were not significant. At the FP and FK sites, AEN and REN were more than doubled because of generally poor fertility soils, clearly bringing out the effect of good N management under SSNM. In contrast, average PFPN increased more at HO (42%) and GD (39%) probably because of greater organic carbon and lesser N fertilizer use (Table 6) than at the other sites.


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  		Table 7. Effect of site-specific nutrient management (SSNM) on fertilizer N use efficiency in irrigated, transplanted rice fields at six sites in Punjab, India, during 2003 and 2004.

 
Compared with FFP, N applications under SSNM were more uniform among farms, spread more evenly through the growing season, and avoided heavy single applications at early growth stages (Fig. 2 ). At GD, many farmers applied N in two large doses of about 35 to 75 kg N ha–1 each during the first 24 DAT but used lower rates (25–50 kg N ha–1) thereafter (Fig. 2a). Likewise, many farmers used N fertilizer to stimulate tillering and therefore, applied large doses of N from about 25 to 35 DAT. As a result of this, midseason drainage was difficult to control and caused large N losses in the FFP. In the SSNM treatment, however, preplant N application was much smaller and more uniform (35–45 kg N ha–1) than in the FFP treatment. The top dressed N applications at 21 to 35 and 35 to 49 DAT were also similar (35–45 kg N ha–1). But in contrast to the FFP where farmers generally stopped applying N at about 45 DAT, N application in the SSNM treatment extended up to 57 DAT. The importance of sufficient late-season N supply for achieving higher rice yields has been highlighted in other studies (Perez et al., 1996; Peng et al., 1996b; Peng and Cassman, 1998; Bijay-Singh et al., 2002). Similar results were obtained at FP site (Fig. 2b) where N fertilizer was generally applied in three splits under the FFP but required four splits as evidenced under SSNM mainly because of the coarse-textured soils here. This might be a reason why FFP was leading to heavy doses of N at a particular time and thereby caused greater N losses without contributing much toward increased grain yields.


Figure 2
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  		Fig. 2. Fertilizer N applications in irrigated, transplanted rice in farmers' fertilizer practice (FFP) and site-specific nutrient management (SSNM) plots at (a) Gurdaspur (GD) and (b) Firozpur (FP) sites in Punjab, India, during 2004.

 
Our results indicate that current N management practices in Punjab might be inconsistent with the physiological requirement of the rice crop, thereby, leading to large N losses. Nitrogen supply appears to be excessive during early vegetative growth but deficient during grain filling. During early growth, chlorophyll or SPAD meter readings in the FFP plots were mostly larger than those in SSNM plots, but the reverse was true during reproductive growth stages (Fig. 3a ). This was true for GD, HO, LU, and PT sites. Other studies (Zheng et al., 1997; Wang et al., 2001) also showed similar results. However, at the FP site, N application in SSNM was greater even during the early phase (Fig. 3b) probably because rice grown in generally coarse-textured soils here required an additional dose of 10 to 20 kg N ha–1 during 7 to 14 DAT as was given under SSNM but was usually skipped by local farmers.


Figure 3
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  		Fig. 3. Difference between chlorophyll meter readings ({Delta}SPAD) in site-specific nutrient management (SSNM) and farmers' fertilizer practice (FFP) during early vegetative and late reproductive growth periods of irrigated, transplanted rice at (a) Ludhiana (LU) and (b) Firozpur (FP) sites in Punjab, India, during 2004.

 
Data gathered from this study indicates that when grown under optimal crop management, rice is capable of utilizing fertilizer-N very efficiently. The gains in N use efficiency were all achieved with top-dressed applications of prilled urea and no major changes in other cropping practices, that is, without using expensive slow release fertilizers or labor-intensive deep-placement techniques. Spreading N applications more evenly throughout the growing season was probably the major factor for the increases in N use efficiency and also reduces the risk for environmental pollution associated with gaseous N losses or losses through runoff or leaching after a heavy fertilizer application. A more balanced NPK nutrition practiced in the SSNM may have contributed to increases in AEN and REN through more vigorous plant growth and greater resistance to diseases.

Profitability of Site-Specific Nutrient Management
Site-specific nutrient management led to an increase in the average fertilizer cost by $27.3 ha–1 crop–1 (52%) and an increase in GRF by $79.3 ha–1 crop–1 (14%) compared with FFP (Table 8 ). No consistent difference was, however, observed between the 2 yr of experimentation for both the average fertilizer cost and the GRF. Increase in the average fertilizer cost under SSNM was mainly attributed to increases in P and K fertilizer use. Phosphorus and K are important inputs from balanced crop nutrition point of view but generally not applied in the FFP in Punjab. Among sites, the largest increase in average fertilizer cost was observed at PT (84%) followed by LU (79%) mainly because of the increased use of N, P, and K fertilizers under SSNM at these sites. Maximum increase in GRF was observed at FP (36%) site having majority of poor-fertility soils followed by FK (23%), GD (13%), HO (13%), PT (6%), and LU (5%) sites. Attributing meaning to the calculation of GRF implicitly assumes that the only difference in crop management between SSNM and FFP is different quantities of nutrients and different timing of a certain constant number of applications so that all other management practices and quantities of input use are held constant.


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  		Table 8. Effect of site-specific nutrient management (SSNM) on total fertilizer cost (TFC) and gross return above fertilizer cost (GRF) in irrigated, transplanted rice fields at six sites in Punjab, India, during 2003 and 2004.

 
At issue is to what extent slight differences in crop management occurred and how they would affect the profitability of SSNM. First, a cost is associated with obtaining field-specific estimates of INS, IPS, and IKS. The combined size of these plots for an individual field was <0.025 ha, and yield losses would mainly occur in 0-N plots. Assuming that a broader use of the SSNM approach will probably not be based on placing such an omission plot into each field, the cost per hectare appears to be negligible. Second, the real-time N management approach used for SSNM is associated with an extra cost. Using a chlorophyll meter or a simple leaf color chart (Tao et al., 1990; Bijay-Singh et al., 2002) to gather information about crop N status requires about 0.5 h per field. However, if this can be done by paid crop consultants or local extension aides, the cost per hectare becomes small—probably well below $5 ha–1 per crop cycle.

Another issue is that of labor for applying N fertilizer because SSNM might be associated with an extra top-dressing of N as was the case in our study at 8 farms at FP and 3 farms at FK sites (Fig. 2b). Assuming that it takes one person about 3 h to apply N on 1 ha, the additional cost compared with FFP is <$2 ha–1. However, again as our study showed, this additional cost was not applicable at 80% of the selected farms. Thus, although the SSNM was associated with an additional cost, these expenses were far below the increase in GRF. Further, the environmental benefits that could be generated by SSNM in long-term and which have not been accounted for in these calculations might help in further increasing the profitability of this SSNM approach, for example, farmers may get environmental credits under the Kyoto Protocol.


   CONCLUSIONS AND SCOPE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
REFERENCES
 
In most irrigated rice-wheat areas of India, further yield increases are likely to occur in small, incremental steps that involve gradual buildup of soil fertility and fine-tuning of crop management. Future strategies for nutrient management in these systems must, therefore, become more site-specific and dynamic to manage spatially and temporally variable resources based on a quantitative understanding of the congruence between nutrient supply and crop demand. The SSNM concept has demonstrated promising agronomic and economic potential. It can be used for managing plant nutrients at any scale, that is, ranging from a general recommendation for homogenous management of a larger domain to true management of between-field variability. Assessment of pest profiles in FFP and SSNM plots suggests that SSNM may also reduce pest incidence, particularly diseases that are often associated with excessive N use or unbalanced plant nutrition (Sta. Cruz et al., 2001).

Field-specific management of macronutrients increased nutrient accumulation, yields, and N use efficiency at majority of the 56 irrigated rice fields in Punjab, India. Grain yield increases averaged 0.9 Mg ha–1 across sites. On a global scale, such yield increases would be sufficient for matching about 7 to 12 yr of annual growth in rice demand in India. Yield increases were achieved with a decrease in average N rate, but were associated with increased N accumulation mainly due to improved N management. Compared with the current farmers' practices, N losses from fertilizer were typically reduced by 50 to 80% and profitability increased by 14% of the total average net return. Site-specific nutrient management requires little in the way of credit for financing or complex coordination among farmers. Significant performance differences among sites suggest further scope for improvement by alleviating other crop management constraints to nutrient use efficiency.

The major challenges for SSNM will be two-fold: (i) to retain the success of the approach and (ii) to build on what has been already achieved using this approach while reducing the complexity of the technology as it is disseminated to farmers. For example, despite an overall 50 to 80% increase, N use efficiencies obtained with SSNM remained below an AEN of >20 kg kg–1 and REN of > 0.5 kg kg–1 typically achieved in irrigated rice with good crop management (Peng et al., 1996a; Peng and Cassman, 1998). Thus, there is a need to further refine location-specific N management strategies and test them against other forms of N management. The nature of the SSNM approach will need to be tailored to specific circumstances in different situations (IRRI, 2006). In some areas, SSNM may be field- or farm-specific, but in many areas, it is likely to be just domain- and/or season-specific. A simplified future SSNM approach should combine decisions that are made on a field-specific basis as well as decisions that are valid for somewhat larger recommendation domains with similar socio–economic and biophysical conditions. Estimates that allow placing a field into one of several broad categories of indigenous nutrient supply are probably sufficient for most SSNM applications and are easier to follow.


   ACKNOWLEDGMENTS
 
We are grateful to the farmers in Punjab for their patience and excellent cooperation in conducting the on-farm experiments for this study.


   NOTES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
 REFERENCES
 
1 Rice consumes an average of 1000 mm of water from rainfall (300–450 mm) and supplemental irrigation water sources (well and canal waters) (500–700mm). Back


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 NOTES
 RESULTS AND DISCUSSION
 CONCLUSIONS AND SCOPE
 REFERENCES
 





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