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Agronomy Journal 94:821-829 (2002)
© 2002 American Society of Agronomy

NITROGEN MANAGEMENT

Chlorophyll Meter– and Leaf Color Chart–Based Nitrogen Management for Rice and Wheat in Northwestern India

Bijay Singha, Yadvinder Singha, Jagdish K. Ladha*,b, Kevin F. Bronson{dagger},c, Vethaiya Balasubramanianb, Jagdeep Singha and Charan S. Khinda

a Dep. of Soils, Punjab Agric. Univ., Ludhiana 141004, Punjab, India
b IRRI, DAPO Box 7777, Metro Manila, Philippines
c Soil and Water Sci. Div., IRRI, DAPO Box 7777, Metro Manila, Philippines

* Corresponding author (j.k.ladha{at}cgiar.org)

Received for publication August 7, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Overapplication of N in cereal crops leads to low N recovery efficiency and risk of NO3 pollution of ground water. The chlorophyll meter, also known as SPAD meter, is a simple, portable diagnostic tool for identifying crop N status. We used it to test need-based N management approaches for rice (Oryza sativa L.) and wheat (Triticum aestivum L.) on a loamy sand in northwestern India. Applying 30 kg N ha-1 each time the SPAD value fell below the critical value of 37.5 resulted in application of 90 kg N ha-1, which produced rice yields equivalent to those with 120 kg N ha-1 applied in three splits. Using a SPAD value of 35 was inadequate for the two rice cultivars because it resulted in application of only 60 kg N ha-1 and, thus, low yields. With high inherent soil fertility resulting in rice yield of >3 Mg ha-1 in zero-N plots, applying N basally or a week after rice transplanting did not further increase yield. Limited experimentation with leaf color chart (LCC) indicated that N management based on LCC shade 4 helped avoid overapplication of N to rice. Wheat responded to N application at maximum tillering (MT) when SPAD value fell below 44. Wheat yield increased by 20% when 30 kg N ha-1 was applied at SPAD value of 42 at MT. Results show that plant need–based N management through chlorophyll meter reduces N requirement of rice from 12.5 to 25%, with no loss in yield.

Abbreviations: AE, agronomic efficiency • CRI, crown root initiation • DAT, days after transplanting • LCC, leaf color chart • MT, maximum tillering • RE, recovery efficiency • SPAD, soil plant analysis development


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN THE QUEST of achieving high yields of rice (Oryza sativa L.) and wheat (Triticum aestivum L.), farmers in many parts of the world tend to apply N in excess of the requirements. This is particularly true in sequentially grown rice and wheat in the Indo-Gangetic plain in the northwestern India, and it leads to further lowering of N fertilizer recovery efficiency (RE), which is already not more than 50% (Katyal et al., 1985, 1986; Bijay-Singh et al., 2001). Due to the predominantly alkaline reaction of the soil, urea [(NH2)2CO] topdressed in both rice and wheat is preferentially lost via NH3 volatilization. In highly permeable soil profiles with alternating aerobic and anaerobic soil conditions under rice, applied N is readily converted to NO3, which is prone to loss via leaching, nitrification–denitrification, or both (Aulakh and Bijay-Singh, 1997; Bijay-Singh et al., 2001). High levels of NO3–N in the region's ground water have been reported recently (Bijay-Singh et al., 1995). Furthermore, large field-to-field variability of soil N supply restricts efficient use of N fertilizer when broad-based blanket recommendations for fertilizer N are used (Adhikari et al., 1999).

When N application is not synchronized with crop demand, N losses from the soil–plant system are large, leading to low N fertilizer use efficiency. Peng and Cassman (1998) demonstrated that RE of topdressed urea during panicle initiation stage could be as high as 78%. Hence, plant need–based application of N is crucial for achieving high yield and N use efficiency. Soil tests for N fertilizer recommendations in flooded rice soils have not been successful (Stalin et al., 1996; Adhikari et al., 1999). The chlorophyll meter (SPAD-502, Minolta, Ramsey, NJ), also known as SPAD (soil plant analysis development) meter, can quickly and reliably assess the N status of a crop based on leaf area. It has been successfully used for rice (Balasubramanian et al., 1999; Hussain et al., 2000), corn (Zea mays L.) (Peterson et al., 1993), and wheat (Follett et al., 1992). Two approaches have been used to apply fertilizer N in rice using chlorophyll meter: (i) when sufficiency index (defined as SPAD value of the plot in question divided by that of a well-fertilized reference plot or strip) falls below 0.90 (Hussain et al., 2000) and (ii) when SPAD value is less than the set critical reading. The sufficiency index approach of Hussain et al. (2000) may be disadvantageous because it requires a well-fertilized area.

In the Philippine dry season, application of 30 kg N ha-1 to rice cultivar IR72 when SPAD value was below the critical value of 35 resulted in higher agronomic efficiency (AE) compared with recommended splits. However, the critical value had to be reduced to 32 during the wet season due to continuous cloud cover for most of the growing season (Balasubramanian et al., 1999). In another study carried out in South India (IRRI-CREMNET, 1998), a value of 37 was found to be critical for obtaining high yields and N use efficiency of short-statured improved indica cultivars. These studies indicate the need for determining chlorophyll meter threshold values of different rice-growing environments. No attempt has been made to establish critical SPAD values for rice in northwestern India.

One-third of the 120 kg N ha-1 for rice has been recommended to be applied basally immediately before soil puddling and rice transplanting. Many farmers also apply a dose of N about 1 wk after transplanting in lieu of basal application. As rice seedlings take about 7 d to recover from transplanting shock (Meelu and Gupta, 1980), it is very likely that most N applied around 7 d after transplanting (DAT) is not used by plants and is lost. As chlorophyll meter–guided N management in rice starts at 2 wk after transplanting, the usefulness of applying a dose of N basally or at about 7 DAT needs to be examined.

The high cost of the chlorophyll meter keeps it out of reach of many Asian farmers. The leaf color chart (LCC) is an inexpensive alternative to the chlorophyll meter (Furuya, 1987; IRRI, 1996). Like the chlorophyll meter, the critical color shade on the LCC needs to be determined to guide N applications.

Wheat grown in northwestern India has a recommended dose of 120 kg N ha-1 applied in two equal splits—basal N at land preparation and N topdressing at crown root initiation (CRI). As N applications to dry-season wheat are linked to irrigation events, farmers often apply a dose of N with the maximum-tillering (MT) irrigation. However, we lack suitable criteria to determine whether a N application at MT is needed. The chlorophyll meter can help in establishing the need for N application at MT, which will largely depend on soil N supply, date of planting, and seasonal temperature.

The field experiments described in this study were carried out during 1996 through 2000 to refine the fixed critical-reading approaches for N management using a chlorophyll meter. We also evaluated need-based N management strategies for rice using LCC. For wheat, the chlorophyll meter was tested as an indicator of the need for N application at MT stage. The objectives were to (i) compare two SPAD values of 35 and 37.5 as threshold values in rice in northwestern India, (ii) determine the need for basal N application in SPAD-based N management for rice, (iii) compare the efficiency of two critical LCC values for guiding fertilizer applications in rice, and (iv) establish the role of the chlorophyll meter in guiding N application at MT stage of wheat.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Site
Field experiments were conducted with rice from 1997 to 2000 and with wheat from 1996 to 1999 on a Typic Ustipsamment (Fatehpur loamy sand) at the experimental farm of the Punjab Agricultural University, Ludhiana (30°56' N, 75°52' E), located in the Indo-Gangetic alluvial plains in the state of Punjab, northwestern India. The area receives on average 800 mm yr-1 rainfall, about 80% of which occurs from June to September. Mean maximum and minimum temperatures are 35 and 18°C during rice cropping (June to October) and 22.6 and 6.7°C during wheat (November to April) season. Soils are well drained. Table 1 shows the physical and chemical properties of soil samples (0–15 cm) from different field sites.


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  		Table 1. Soil (0–15 cm) properties of rice and wheat experimental sites, Ludhiana, India (1996–2000).

 
Experimental Design and Treatments
Rice
A total of four experiments were conducted. Experiments I, II, and III were primarily designed to use SPAD and Experiment IV to use LCC. In addition, Exp. II also included treatments involving LCC. Experiment I (1997 and 1998) was laid out in a split-plot design with rice cultivars PR106 and PR111 in the main plots in three replicates. Six fertilizer N (as urea) management treatments in subplots are described in Table 2. In the recommended splits, N was applied at transplanting, midtillering (21 DAT), and panicle initiation (42 DAT). Chlorophyll meter readings were taken weekly starting from 2 wk after transplanting, and 30 kg N ha-1 was applied whenever SPAD reading was below the critical value of 35 or 37.5. While threshold SPAD value of 35 was picked up from the studies carried out by Peng et al. (1996), a preliminary experiment (not reported) indicated that a threshold SPAD value of 37.5 offered savings in N without reducing yield.


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  		Table 2. Treatments used in Rice Experiment I during 1997 and 1998, Ludhiana, India.

 
In Experiments II and III conducted during 1999 and 2000, the amount of N applied in response to SPAD values below the critical limit of 37.5 varied during the high-demand period of rapid plant growth between 29 and 49 DAT. Before 29 DAT and after 49 DAT, 30 kg N ha-1 was applied based on the chlorophyll meter. Forty-five and 40 kg N ha-1 were applied during rapid crop-growth stage of rice in 1999 (Exp. II) and 2000 (Exp. III), respectively. Another objective of these experiments was to examine how application of a dose of 20 kg N ha-1 at 7 DAT rather than a basal application influences the SPAD-based N management in rice. Experiments were laid out in a completely randomized block design with three replicates and treatments as described in Table 3. The two treatments with LCC included in Exp. II consisted of applying N when rice leaf color was less than shade 4 (LCC < 4) or 5.


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  		Table 3. Treatments used in rice experiments during 1999 and 2000, Ludhiana, India.

 
Experiment IV was conducted in 2000 using a completely randomized block design with three replicates to evaluate need-based N management strategies using LCC. The five N treatments included in this experiment are described in Table 3. Measurements with LCC were made every week starting at 15 DAT.

Wheat
Experiment V on wheat was conducted during three seasons (1996–1999) and was laid out in a split-plot design with three replicates. The N levels of 0 (except in 1996–1997), 60, 80, 100, and 120 kg N ha-1, applied in two equal splits (basally at planting and at CRI), constituted the main plots. The two subplots consisted of applying 0 or 30 kg N ha-1 at MT stage after taking chlorophyll meter readings.

Crop Management
After removing crop residues, the land was plowed, puddled, and leveled for rice transplanting on 23 June 1997 and 12 June 1998 (Exp. I), 22 June 1999 (Exp. II), 26 June 2000 (Exp. III), and 10 June 2000 (Exp. IV). Two 4- to 5-wk-old rice seedlings were transplanted at 20- by 15-cm spacing in 20- to 33-m2 plots in different experiments and years. A dose of 26 kg P ha-1 {as monocalcium phosphate [Ca(H2PO4)2]} and 25 kg K ha-1 (as KCl) was incorporated into the soil before last puddling. During the rice season, along with rainfall, 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 the soil did not remain flooded for more than 8 to 10 h after irrigation, anaerobic conditions prevailed for >75% of the rice growth period. Different rice varieties grown (Tables 2 and 3) were modern semidwarf types with similar yield potential and harvest index. Hand-weeding was done, and pest control followed standard practices.

Wheat cultivar PBW343 was sown in rows 20 cm apart in 16.8- to 24-m2 plots on 28 Nov. 1996, 18 Nov. 1997, and 5 Nov. 1998. Before seeding, the land was plowed twice to about 20-cm depth and leveled. After seeding with a hand-drawn seed-cum-fertilizer drill, a plank was dragged over the plots to cover the seed. All P [26 kg P ha-1 as Ca(H2PO4)2] and K (25 kg K ha-1 as KCl) were drilled below the seed at sowing. The basal dose of N per treatment was mixed in the soil just before sowing. In wheat, three to four irrigations were given at CRI, MT, and flowering stages using both well and canal water. While CRI stage coincided with time of first irrigation 3 wk after sowing wheat, MT stage varied in the 3 yr, depending primarily on the date of sowing and the climate. Maximum tillering occurred on 30 Jan. 1997, 12 Jan. 1998, and 19 Jan. 1999 (64, 56, and 76 d after sowing, respectively). Weeds, pests, and diseases were controlled as required.

Crops were harvested by hand at ground level at maturity on 10 Oct. 1997 and 15 Oct. 1998 (Exp. I); 30 Sept. 1999 (Exp. II); 20 Oct. 2000 (Exp. III); 4 Oct. 2000 (Exp. IV); and 8 Apr. 1997, 11 Apr. 1998, and 9 Apr. 1999 (Exp. V). Grain and straw yields were determined from an area (12.6–15.4 m2 for rice and 8–13.2 m2 for wheat) located at the center of each plot. Grains were separated from straw using a plot thresher, dried in a batch grain dryer, and weighed. Grain moisture was determined immediately after weighing, and subsamples were dried in an oven at 65°C for 48 h. Grain weights for rice and wheat were expressed at 140 and 120 g kg-1 water content, respectively. Straw weights were expressed on oven-dry basis.

Plant Sampling and Analysis
Grain and straw subsamples were dried at 70°C and finely ground to pass through a 0.5-mm sieve. Nitrogen content in grain and straw was determined by digesting the samples in sulfuric acid (H2SO4), followed by analysis for total N by a micro-Kjeldahl method (Yoshida et al., 1976). The N in grain plus that in straw was taken as the measure of total N uptake.

Chlorophyll Meter and Leaf Color Chart Measurements
Chlorophyll meter readings were taken weekly with a Minolta SPAD-502 chlorophyll meter, starting 14 DAT. Twenty hills of rice were chosen at random in each plot. From each hill, three readings were taken from the uppermost fully expanded leaf. SPAD readings were taken up to 50% flowering stage. In wheat, SPAD readings were taken at the MT stage only. Ten plants per plot were read, consisting of three readings per leaf.

The LCC developed by International Rice Research Institute (IRRI, 1996) consisted of six green strips showing increasing greenness with increasing number. As with the chlorophyll meter, the chart was used to take weekly readings starting 15 DAT. Twenty disease-free rice plants were randomly selected in the plot, and the color of the youngest fully expanded leaf of the selected plant was compared by placing its middle part on top of the color strip in the chart. Like chlorophyll meter, LCC readings were taken up to 50% flowering stage. If 12 or more leaves read below a critical value (of LCC 4 or 5), a dose of 30 kg N ha-1 was applied.

Data Analysis
Analysis of variance was performed on yield parameters to determine effects of cultivars, N management treatments, and their interaction using IRRISTAT version 1992 (IRRI, Manila, Philippines). Duncan's Multiple Range Test was used at 0.05 level of probability to test differences between treatment means. Simple linear regression analysis was performed to study the response of wheat to N application at MT.

The N use efficiency measures, RE (Dilz, 1988) and AE (Novoa and Loomis, 1981), were calculated as follows:


where TNU is the total N uptake in grain and straw.

Percent grain yield response of wheat to application of 30 kg N ha-1 at MT stage was calculated at different levels of basal + CRI N applications as:


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Threshold SPAD Values for Rice
A significant positive response of rice grain yield and total N uptake to N application relative to the zero-N control was observed in all treatments of Exp. I for both cultivars in 1997 and 1998 (Table 4). The relatively high yield of >4 Mg ha-1 in control plots in 1997 may have been related to the relatively high soil NH4–N levels at transplanting (Table 1). In 1997, the effects of cultivar and N management were significant for grain yield and N uptake. In 1998, these effects along with cultivar x N management interaction were also significant for RE and AE. When SPAD value of 35 was used as a critical value for applying 30 kg N ha-1 to rice, irrespective of basal application of 30 kg N ha-1 (T3 and T4), a total of 30 or 60 kg N ha-1 was applied to both cultivars (except PR 111 in 1997) in both years. Accordingly, grain yield and N uptake by rice in these treatments were significantly less than those observed in recommended fixed-time application of 120 kg N ha-1. The only exception was cultivar PR111 in 1997, to which a total of 90 kg N ha-1 was applied and which had a grain yield not different from that obtained with 120 kg N ha-1. Thus, a critical SPAD value of 35 was not found to be suitable for guiding N application in the two cultivars widely grown in northwestern India. These results are in contrast to the findings of Peng et al. (1996) in the Philippines that N management based on a critical SPAD value of 35 could produce yields in IR72 similar to those obtained by applying 120 kg N ha-1. This may be due to difference in rice cultivars and growing conditions between the two studies.


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  		Table 4. Rice grain yield, N uptake, total fertilizer N applied, and recovery and agronomic efficiencies (RE and AE, respectively) of two rice cultivars grown during 1997 and 1998 using different need-based fertilizer N management criteria at Ludhiana, India (Exp. I).

 
Using the criteria of applying 30 kg N ha-1 each time the SPAD value falls below 37.5 (T5 and T6) always resulted in rice grain yield equivalent to that obtained with 120 kg N ha-1 in three fixed-time splits (Table 4). The only exception was rice cultivar PR106 in 1998 when chlorophyll meter–based N management was used along with a basal N application of 30 kg N ha-1. In treatments receiving all N doses starting from 14 DAT at a critical SPAD value of 37.5 (T6), rice grain yields equivalent to those produced by applying 120 kg N ha-1 (T2) were obtained with 90 kg N ha-1. This was true for both varieties and in both years. As expected, AE was greater when less N fertilizer was used, but this was achieved with the use of the chlorophyll meter without sacrificing yield.

The threshold SPAD value of 35 for semidwarf indica varieties in transplanted rice systems during the dry season in the Philippines has to be reduced to 32 during the wet season when solar radiation is relatively low (Balasubramanian et al., 1999). In South India, a critical SPAD value of 37 was found to be appropriate for rice grown in the summer monsoon season (IRRI-CREMNET, 1998). It has also been suggested that different threshold SPAD values may have to be used for different varietal groups (Balasubramanian et al., 2000; Thiyagarajan et al., 2000).

Basal Nitrogen Application with Chlorophyll Meter–Guided Nitrogen Management
In Exp. I (1997), when need-based N management using 37.5 as the critical SPAD value was followed along with a basal application of 30 kg N ha-1 (T5), grain yields of the two cultivars were similar to those of treatments with no basal application (T6) (Table 4). But in 1998, in the SPAD 37.5 treatment without basal N (T6), cultivar PR106 yielded significantly higher than SPAD 37.5 with basal N (T5). In cultivar PR111, it was necessary to apply 120 kg N ha-1 to produce a grain yield that was not significantly different from that obtained by recommended three fixed-time split applications of 120 kg N ha-1 (Table 4). These results suggest that (i) a basal dose of 30 kg N ha-1 was not efficiently used by the crop and is possibly prone to losses or immobilization and (ii) N applied starting at 14 DAT based on crop need determined by the chlorophyll meter was used more efficiently. Rice seedlings need about 7 d to recover from transplanting shock (Meelu and Gupta, 1980); thus, N uptake within 2 wk of transplanting should be very small.

After establishing the superiority of N management based on a critical SPAD value of 37.5 with no basal N application, Exp. II and III were conducted in 1999 and 2000 with two modifications in the chlorophyll meter–based N management treatments. First, instead of 30 kg N ha-1, 40 kg N ha-1 (45 in 1999) was applied during the rapid growth stage of rice between 28 and 49 DAT when SPAD value was below 37.5. Second, a dose of 20 kg N ha-1 was applied at 7 DAT, followed by chlorophyll meter–guided N applications after 14 DAT. Table 5 shows results from the two experiments. Grain yield of rice and N RE were similar for the two chlorophyll meter–based N management treatments (T3 and T4) and the recommended dose of 120 kg N ha-1 (T2) in both years. However, total fertilizer N applied in the treatment with no basal N (T4) was 20 and 15 kg N ha-1 less than that in the treatment with N application at 7 DAT (T3) or the recommended splits (T2), respectively. The AE in the chlorophyll meter–guided N management treatment without basal N was significantly higher than in the other two treatments in 1999 (Exp. II).


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  		Table 5. Rice grain yield, N uptake, total fertilizer N applied, and recovery and agronomic efficiencies (RE and AE, respectively) of two rice cultivars grown during 1999 and 2000 using different need-based fertilizer N management criteria at Ludhiana, India (Exp. II and III).

 
Season as well as variety influenced the AE. In 1997 and 1998 (Exp. I), responses to fertilizer N were lower than in 1999 and 2000 (Exp. II–IV), resulting in much lower AE values (Tables 4 and 5). However, in all 4 yr, irrespective of the extent of response to applied N, chlorophyll meter–based N management had significant N savings while maintaining grain yield. As in 1997 and 1998, rice yields in zero-N plots were >3 Mg ha-1 in both 1999 and 2000 (Table 5), indicating that basal N application had no added effect. Results of this study support the hypothesis of Balasubramanian et al. (1999) that soils producing a grain yield of >=3 Mg ha-1 without any fertilizer application do not need basal N application. Because this finding has an important bearing on the overall N management of rice, further studies should be planned to determine the minimum yield level in zero-N plots above which no basal N is required.

Schedule of Fertilizer Nitrogen Application in Rice
Application of basal N is recommended to farmers throughout northwestern India though its need has not been calibrated with soil N supply. Color of leaves as read by chlorophyll meter during early stages of rice growth should reflect the status of soil N supply. For example, in Exp. I in 1997, SPAD readings of rice leaves up to about 50 DAT in recommended splits (T2) were as high as 4.9 units over those in the chlorophyll meter–based N management treatment with no basal N application (T6) (Fig. 1) . But, thereafter, the trend was reversed, suggesting that savings in fertilizer N can be accomplished without sacrificing yield using the chlorophyll meter–based N management strategy. This avoids excessive N supply during early crop-growth stages and instead provides adequate N during later stages when the plant requires it most. The chlorophyll meter showed that plants required N at or beyond 50 DAT in all experiments in this study (Tables 2 and 3). Because the number of chlorophyll meter–based N applications did not exceed three, this suggests no additional labor cost over the treatment with recommended splits.



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  		Fig. 1. Chlorophyll meter readings (SPAD values) as affected by N management in rice (1997) following recommended three fixed-split applications of 120 kg N ha-1 and application of 30 kg N ha-1 each time SPAD value fell below 37.5 (a total application of 90 kg N ha-1) at Ludhiana, India (Exp. I).

 
Leaf Color Chart for Nitrogen Management in Rice
Table 6 presents results of LCC experiments conducted to evaluate the performance of LCC in guiding need-based N application in rice. Two shades of greenness—LCC 4 and LCC 5—were used as threshold values. In Exp. II (1999), when a basal dose of 20 kg N ha-1 was applied at 7 DAT before starting LCC-guided N applications (T5 and T6), a total of 110 kg N ha-1 was applied following both LCC 4 and LCC 5 to produce a grain yield similar to that obtained by applying 120 kg N ha-1 in fixed-time splits. In Exp. IV (2000), application of 30 kg N ha-1 at LCC < 4 (T2) resulted in a total N application of 90 kg N ha-1 and grain yield that was statistically similar to that obtained with 120 kg N ha-1 in recommended splits (T1). Similar to Exp. II and III with the chlorophyll meter (1999 and 2000), applying 20 kg N ha-1 at 7 DAT (T3) did not increase rice yield. In 2000, total N applications with LCC 5 as threshold value (T4 and T5) were 120 and 140 kg N ha-1, depending on whether basal N dose was applied or not. These N levels did not show any savings in N fertilizer. It appears that LCC 5 represents a shade greener than the color corresponding to a chlorophyll meter reading of 37.5.


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  		Table 6. Rice grain yield, N uptake, and total fertilizer N applied in 1999 and 2000 following different criteria for need-based fertilizer N management using leaf color chart (LCC) at Ludhiana, India (Exp. IV).

 
Chlorophyll Meter–Guided Nitrogen Management of Wheat at Maximum-Tillering Stage
Wheat yield increased with basal plus CRI N rates of 0 to 120 kg N ha-1 (Exp. V) (Table 7). The only exception was the 1997–1998 season when a significant increase was observed only up to 100 kg N ha-1. However, wheat yield further responded to N application at MT. The yield increased up to 0.80 Mg ha-1 with an additional N application of 30 kg ha-1.


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  		Table 7. Wheat yield as influenced by application of different N levels at basal and crown root initiation (CRI) stages and 30 kg N ha-1 at maximum tillering (MT) stage at Ludhiana, India (1996–1999) (Exp. V).

 
Grain yield from plots that did not receive N at MT correlated positively with SPAD values at MT (r2 = 0.75) (Fig. 2) . When percent grain yield response to N application at MT was regressed against SPAD readings at MT, the relationship was negative, with r2 = 0.84 (Fig. 3) . This relationship clearly shows that at a SPAD <=42, applying N at MT can increase yield by >=20%; at SPAD >=44, no yield response is likely to be expected. Below this critical limit, the extent of response increased with decreasing SPAD value at MT. A critical SPAD value of 42 for MT in wheat corresponds with the result of Follett et al. (1992) in dryland winter wheat in Colorado. The validity of this critical reading for wheat at MT in northwestern India needs to be further examined under diverse agroecological conditions.



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  		Fig. 2. Relationship between grain yield of wheat with different preplant + crown root initiation (CRI) N levels and SPAD values recorded at maximum-tillering (MT) stage for three seasons (1996–1999) at Ludhiana, India (Exp. V).

 


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  		Fig. 3. Relationship between percent grain yield response of wheat to 30 kg N ha-1 applied at maximum tillering (MT) and SPAD values recorded at MT before N application for three seasons (1996–1999) at Ludhiana, India (Exp. V).

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The optimum use of N results from matching supply with crop demand. We use the chlorophyll meter, a reliable and nondestructive tool, to determine the right time for N topdressing of rice and wheat. The five cultivar–season combinations evaluated over 4 yr showed that chlorophyll meter–based applications of N produced rice yields similar to those of existing fertilizer recommendations. However, chlorophyll meter–based N management saved 12.5 to 25% of the existing fertilizer N recommendation. The SPAD value of 37.5 was found to be critical for northwestern India, unlike the value of 35 recommended for the Philippines. This presumably reflects difference in rice cultivars tested under northwestern Indian and Philippine conditions. In this study, data also showed the need for more N application at tillering and up to flowering stage instead of basal application at the beginning of crop growth. These results suggest that current recommendations of timing of fertilizer N application should be revised. Data presented in this study suggest that applying 90 to 105 kg N ha-1 in three splits at 14, 35, and 50 DAT should lead to better AE.

The cost of the chlorophyll meter restricts its widespread use by farmers. The LCC, a simple tool used to measure leaf color intensity, was also tested. Results of N applications to rice based on LCC shade 4 were reasonably consistent with those using the chlorophyll meter.

Results presented in this study provide strong evidence that current fertilizer N recommendations are inadequate for maintaining current yields of wheat and lead to application of excess N fertilizer to rice. The chlorophyll meter–based N management in rice suggests that N can be saved with no yield loss by appropriately revising the blanket fertilizer recommendations. The LCC is a simple and easy tool that can help farmers avoid overapplication of N to rice.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
{dagger} Current address, Texas A&M Univ., Texas Agric. Exp. Stn., Route 3, Box 219, Lubbock, TX 79401. Back


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




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M. Olivier, J.-P. Goffart, and J.-F. Ledent
Threshold Value for Chlorophyll Meter as Decision Tool for Nitrogen Management of Potato
Agron. J., April 11, 2006; 98(3): 496 - 506.
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S. O. PB. Samonte, L. T. Wilson, J. C. Medley, S. R. M. Pinson, A. M. McClung, and J. S. Lales
Nitrogen Utilization Efficiency: Relationships with Grain Yield, Grain Protein, and Yield-Related Traits in Rice
Agron. J., January 5, 2006; 98(1): 168 - 176.
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M. M. Alam, J. K. Ladha, S. R. Khan, Foyjunnessa, Harun-ur-Rashid, A. H. Khan, and R. J. Buresh
Leaf Color Chart for Managing Nitrogen Fertilizer in Lowland Rice in Bangladesh
Agron. J., May 13, 2005; 97(3): 949 - 959.
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W. R. Raun, J. B. Solie, M. L. Stone, D. L. Zavodny, K. L. Martin, and K. W. Freeman
AUTOMATED CALIBRATION STAMP TECHNOLOGY FOR IMPROVED IN-SEASON NITROGEN FERTILIZATION
Agron. J., January 1, 2005; 97(1): 338 - 342.
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A. K. Shukla, J. K. Ladha, V. K. Singh, B. S. Dwivedi, V. Balasubramanian, R. K. Gupta, S. K. Sharma, Y. Singh, H. Pathak, P. S. Pandey, et al.
Calibrating the Leaf Color Chart for Nitrogen Management in Different Genotypes of Rice and Wheat in a Systems Perspective
Agron. J., November 1, 2004; 96(6): 1606 - 1621.
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L. G. Bundy and T. W. Andraski
Diagnostic Tests for Site-Specific Nitrogen Recommendations for Winter Wheat
Agron. J., May 1, 2004; 96(3): 608 - 614.
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T. T. Chua, K. F. Bronson, J. D. Booker, J. W. Keeling, A. R. Mosier, J. P. Bordovsky, R. J. Lascano, C. J. Green, and E. Segarra
In-Season Nitrogen Status Sensing in Irrigated Cotton: I. Yields and Nitrogen-15 Recovery
Soil Sci. Soc. Am. J., September 1, 2003; 67(5): 1428 - 1438.
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W.-H. Yang, S. Peng, J. Huang, A. L. Sanico, R. J. Buresh, and C. Witt
Using Leaf Color Charts to Estimate Leaf Nitrogen Status of Rice
Agron. J., January 1, 2003; 95(1): 212 - 217.
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