Optimizing chlorophyll meter (SPAD) reading to allow efficient nitrogen use in rice and wheat under rice-wheat cropping system in eastern India

Figures & data

Table 1. Monthly weather parameters during the study period at Kharagpur, India.

	Tmax (ºC)		Tmin (ºC)		Rainfall (mm)		RH (%)		Sunshine hours
Month	2010–11	2011–12	2010–11	2011–12	2010–11	2011–12	2010–11	2011–12	2010–11	2011–12
July	33.2	32.5	26.7	25.8	173.8	339.7	86.4	84.9	9	7
August	33.6	32.1	27.1	25.8	148.3	309.4	83.1	89.1	8	6
September	32.6	32.2	26.2	25.2	217.3	252.6	87.2	88.6	7	7
October	31.9	31.6	24.3	22.8	91.0	74.8	85.0	76.5	7	8
November	31.8	29.2	22.0	18.1	1.5	17.8	74.8	69.7	9	8
December	25.9	26.2	13.8	13.9	11.7	6.2	72.0	66.7	9	8
January	25.5	25.4	13.5	14.2	12.7	31.8	64.9	75.3	8	9
February	29.4	30.3	16.9	15.6	23.8	42.0	61.6	55.8	8	9
March	34.3	35.4	21.3	22.3	52.3	0.0	60.3	50.8	8	10

Tmax = Maximum temperature; Tmim = Minimum temperature; RH = Relative humidity

Table

T1 = S34N15	T4 = S36N15	T7 = S38N15	T10 = FTNM (120–21.8–50 kg N-P-K ha^−1)
T2 = S34N25	T5 = S36N25	T8 = S38N25	T11 = FFP (60–13.1–25 kg N- P-K ha^−1)
T3 = S34N35	T6 = S36N35	T9 = S38N35	T12 = Control (No application of fertilizer)

Table

T1 = S38N15	T4 = S40N15	T7 = S42N15	T10 = FTNM (120–21.8–50 kg N-P-K ha^−1)
T2 = S38N25	T5 = S40N25	T8 = S42N25	T11 = FFP (80–26.2–37.5 kg N- P-K ha^−1)
T3 = S38N35	T6 = S40N35	T9 = S42N35	T12 = Control (No application of fertilizer)

Table 2. SPAD-based timing of N application in rice.

	Total N applied (kg ha^−1)						Time of N application (DAT)
	2010			2011			2010			2011
Treatments	R1	R2	R3	R1	R2	R3	R1	R2	R3	R1	R2	R3
S34 N15	60	60	45	60	60	45	0, 30, 50	0, 40, 50	0, 30	0, 40, 50	0, 30, 40	0, 40
S34 N25	80	55	55	55	55	55	0, 30, 60	0, 50	0, 30	0, 40	0, 40	0, 40
S34 N35	65	100	65	65	65	65	0, 60	0, 30, 50	0, 50	0, 40	0, 30	0, 40
S36 N15	75	60	75	60	60	60	0, 30, 50, 60	0, 50, 60	0, 30, 50, 60	0, 30, 40	0, 30, 40	0, 30, 40
S36 N25	80	55	105	80	80	80	0, 40, 60	0, 50	0, 30, 50, 60	0, 30, 40	0, 30, 50	0, 30, 40
S36 N35	65	100	100	65	100	100	0, 50	0, 30, 50	0, 30, 60	0, 30	0, 30, 50	0, 30, 50
S38 N15	90	90	75	90	75	75	0, 30, 40, 50, 60	0, 30, 40, 50, 60	0, 30, 50, 60	0, 30, 40, 50, 60	0, 30, 40, 50	0, 30, 40, 50
S38 N25	105	105	80	105	80	105	0, 30, 50, 60	0, 30, 50, 60	0, 40, 50	0, 30, 40, 60	0, 30, 40	0, 30, 40, 60
S38 N35	100	135	100	100	100	100	0, 30, 60	0, 30, 50, 60	0, 40, 60	0, 30, 40	0, 30, 40	0, 30, 40
FTNM	120	120	120	120	120	120	0, 20, 40, 60	0, 20, 40, 60	0, 20, 40, 60	0, 20, 40, 60	0, 20, 40, 60	0, 20, 40, 60
FFP	60	60	60	60	60	60	0, 40	0, 40	0, 40	0, 40	0, 40	0, 40
Control	0	0	0	0	0	0	–	–	–	–	–	–

S = SPAD threshold value for rice; N = nitrogen topdressing (kg N ha⁻¹); FTNM = fixed-time nitrogen management; FFP = farmers’ fertilizer practice; DAT = days after transplanting (basal dose of 30 kg N ha⁻¹ applied to all plots except control plots); R₁ = Replication 1; R₂ = Replication 2; R₃ = Replication 3.

Table 3. SPAD-based timing of N application in wheat.

	Total N applied (kg ha^−1)						Time of N application (DAS)
	2010–2011			2011–2012			2010–2011			2011–2012
Treatments	R1	R2	R3	R1	R2	R3	R1	R2	R3	R1	R2	R3
S38 N15	90	90	75	75	60	75	0, 35, 65	0, 35, 65	0, 35	0, 45	0	0, 65
S38 N25	85	110	110	85	60	60	0, 35	0, 35, 65	0, 35, 65	0, 65	0	0
S38 N35	95	130	95	95	95	95	0, 35	0, 35, 65	0, 35	0, 65	0, 65	0, 45
S40 N15	90	75	90	105	75	90	0, 35, 65	0, 35	0, 25, 35	0, 25, 45, 65	0, 45	0, 25, 45
S40 N25	85	110	85	110	85	110	0, 35	0, 35, 65	0, 35	0, 45, 65	0, 65	0, 25, 45
S40 N35	95	130	130	130	95	130	0, 35	0, 35, 65	0, 35, 65	0, 25, 45	0, 45	0, 25, 45
S42 N15	90	90	90	105	105	105	0, 35, 65	0, 35, 65	0, 25, 35	0, 25, 45, 65	0, 25, 45, 65	0, 25, 45, 65
S42N25	110	135	85	110	110	110	0, 35, 65	0, 35, 65	0, 35	0, 45, 65	0, 45, 65	0, 25, 45
S42 N35	130	95	130	95	165	95	0, 35, 65	0, 35	0, 35, 65	0, 45	0, 25, 45, 65	0, 45
FTNM	120	120	120	120	120	120	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45
FFP	80	80	80	80	80	80	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45	0, 25, 45
Control	0	0	0	0	0	0	–	–	–	–	–	–

S = SPAD threshold value for wheat; N = nitrogen topdressing (kg N ha⁻¹); FTNM = fixed-time nitrogen management; FFP = farmers’ fertilizer practice; DAS = Days after sowing (basal dose of 60 kg N ha⁻¹ applied to all plots except control and FFP plots; FFP received 40 kg N ha⁻¹ as basal); R₁ = Replication 1; R₂ = Replication 2; R₃ = Replication 3.

Figure 1. Rice grain and straw yield under different N management practices during 2010 and 2011 (vertical lines indicate the standard error).

Figure 2. Wheat grain and straw yield under different N management practices during 2010–11 and 2011–12 (vertical lines indicate the standard error).

Figure 3. Relationship between grain yield and SPAD value at (a) active tillering, (b) panicle initiation and (c) flowering stages of rice; and (d) active tillering, (e) earhead initiation and (f) flowering stages of wheat (over 2 years' data).

Figure 4. SPAD meter readings at different growth stages of rice under different N management practices.

Figure 5. Leaf N content at different growth stages of rice under different N management practices (vertical lines indicate the standard error).

Figure 6. Soil available N status of 0–20 cm depth at different growth stages of rice under different N management practices (vertical lines indicate the standard error).

Figure 7. SPAD meter readings at different growth stages of wheat under different N management practices.

Figure 8. Leaf N content at different growth stages of wheat under different N management practices (vertical lines indicate the standard error).

Figure 9. Soil available N status of 0–20 cm depth at different growth stages of wheat under different N management practices (vertical lines indicate the standard error).

Table 4. Correlations among SPAD values, leaf N content, soil available N status and grain yield of rice (Over 2 years' data).

Characters	Leaf N at AT	Leaf N at PI	Leaf N at FL	SPAD at AT	SPAD at PI	SPAD at FL	Soil N at AT	Soil N at PI	Soil N at FL	Yield
Leaf N at AT	1.00
Leaf N at PI	0.55**	1.00
Leaf N at FL	0.46**	0.60**	1.00
SPAD at AT	0.62**	0.53**	0.67**	1.00
SPAD at PI	0.53**	0.90**	0.68**	0.61**	1.00
SPAD at FL	0.51**	0.69**	0.78**	0.77**	0.82**	1.00
Soil N at AT	0.35**	0.28*	0.34**	0.53**	0.27*	0.29*	1.00
Soil N at PI	0.37**	0.55**	0.52**	0.66**	0.51**	0.53**	0.59**	1.00
Soil N at FL	0.50**	0.50**	0.70**	0.80**	0.50**	0.73**	0.50**	0.81**	1.00
Yield	0.46**	0.52**	0.58**	0.49**	0.66**	0.70**	0.10	0.14	0.34**	1.00

Table values of correlation coefficient at 70 error df at 5% and 1% P level are 0.229 and 0.298 respectively; AT = Active tillering; PI = Panicle initiation; FL = Flowering; * indicates significance at p < 0.05; ** indicates significance at p < 0.01.

Table 5. Relationships between leaf N content and grain yield, leaf N content and SPAD value and leaf N content and soil available N at different growth stages of rice and desired leaf N, SPAD value and soil available N for maximum grain yield.

Growth stages	(i) Relationship (leaf N and grain yield)	R^2 value	Desired Leaf N content†
Active tillering	Y = −1771X^2 + 10466X – 10329 (1.88 ≤ X ≤ 3.24, n = 72)	0.29**	2.95 ± 0.03
Panicle initiation	Y = −1545X^2 + 7735X – 4443 (1.28 ≤ X ≤ 2.70, n = 72)	0.34**	2.50 ± 0.03
Flowering	Y = −873X^2 + 4295X – 48.74 (1.04 ≤ X ≤ 2.90, n = 72)	0.42**	2.46 ± 0.05
Maturity	Y = −34576X^2 + 45994X – 10010 (0.42 ≤ X ≤ 0.80, n = 72)	0.41**	0.67 ± 0.01
Growth stages	(ii) Relationship (leaf N and SPAD value)	R^2 value	Desired SPAD value
Active tillering	Y = 5.706X + 24.67 (1.88 ≤ X ≤ 3.24, n = 72)	0.39**	41.5 ± 0.28
Panicle initiation	Y = 8.217X + 16.89 (1.28 ≤ X ≤ 2.70, n = 72)	0.81**	37.5 ± 0.28
Flowering	Y = 5.073X + 25.07 (1.04 ≤ X ≤ 2.90, n = 72)	0.61**	37.5 ± 0.33
Growth stages	(iii) Relationship (leaf N and Soil avail. N)	R^2 value	Desired soil availability N (kg ha^−1)
Active tillering	Y = 0.0017X + 2.226 (226 ≤ X ≤ 512, n = 72)	0.12**	426 ± 6.2
Panicle initiation	Y = 0.0035X + 1.175 (181 ≤ X ≤ 388, n = 72)	0.31**	378 ± 5.0
Flowering	Y = 0.0067X + 0.288 (146 ≤ X ≤ 331, n = 72)	0.49**	324 ± 5.3

(i) Y = grain yield; X = leaf N content; (ii) Y = SPAD value; X = leaf N content; (iii) Y = leaf N content; X = soil available N status; ** indicates significance at p < 0.01; †values are as mean±SE.

Table 6. Correlation among SPAD values, leaf N content, soil available N status and grain yield of wheat (over 2 years' data).

Charaters	Leaf N at AT	Leaf N at EI	Leaf N at FL	SPAD at AT	SPAD at EI	SPAD at FL	Soil N at AT	Soil N at EI	Soil N at FL	Yield
Leaf N at AT	1.00
Leaf N at EI	0.76**	1.00
Leaf N at FL	0.77**	0.72**	1.00
SPAD at AT	0.63**	0.54**	0.39**	1.00
SPAD at EI	0.60**	0.53**	0.34**	0.71**	1.00
SPAD at FL	0.57**	0.44**	0.66**	0.42**	0.48**	1.00
Soil N at AT	0.68**	0.63**	0.47**	0.78**	0.70**	0.47**	1.00
Soil N at EI	0.64**	0.64**	0.78**	0.42**	0.41**	0.57**	0.46**	1.00
Soil N at FL	0.77**	0.74**	0.76**	0.59**	0.59**	0.77**	0.64**	0.71**	1.00
Yield	0.72**	0.64**	0.69**	0.53**	0.59**	0.69**	0.67**	0.71**	0.78**	1.00

Table values of correlation coefficient at 70 error df at 5% level and 1% level are 0.229 and 0.298, respectively; AT = Active tillering; EI = Earhead initiation; FL = Flowering; ** indicates significance at p < 0.01.

Table 7. Relationships between leaf N content and grain yield, leaf N content and SPAD value and leaf N content and soil available N at different growth stages of wheat and desired leaf N, SPAD value and soil available N for maximum grain yield.

Growth stages	(i) Relationship (leaf N and grain yield)	R^2 value	Desired Leaf N content†
Active tillering	Y = −525.3X^2 + 5082X – 8204 (2.70 ≤ X ≤ 5.00, n = 72)	0.59**	4.84 ± 0.06
Earhead initiation	Y = −606.3X^2 + 4558X – 4441 (1.76 ≤ X ≤ 4.47, n = 72)	0.58**	3.76 ± 0.06
Flowering	Y = −684.1X^2 + 4365X – 2739 (1.31 ≤ X ≤ 3.24, n = 72)	0.53**	3.19 ± 0.04
Maturity	Y = −5730X^2 + 10697X – 759.8 (0.34 ≤ X ≤ 1.23, n = 72)	0.62**	0.93 ± 0.02
Growth stages	(ii) Relationship (leaf N and SPAD value)	R^2 value	Desired SPAD value
Active tillering	Y = 4.213X + 23.88 (2.70 ≤ X ≤ 5.00, n = 72)	0.40**	44.3 ± 0.38
Earhead initiation	Y = 2.787X + 30.43 (1.76 ≤ X ≤ 4.47, n = 72)	0.28**	40.9 ± 0.33
Flowering	Y = 5.293X + 24.90 (1.31 ≤ X ≤ 3.24, n = 72)	0.43**	41.8 ± 0.35
Growth stages	(iii) Relationship (leaf N and Soil avail. N)	R^2 value	Desired soil availability N (kg ha^−1)
Active tillering	Y = 0.0065X + 2.0061 (213 ≤ X ≤ 435, n = 72)	0.2**	436 ± 6.0
Earhead initiation	Y = 0.0080X + 0.9018 (171 ≤ X ≤ 382, n = 72)	0.42**	357 ± 5.1
Flowering	Y = 0.0088X + 0.5480 (98 ≤ X ≤ 256, n = 72)	0.57**	300 ± 3.8

(i) Y = Grain yield; X = leaf N content; (ii) Y = SPAD value; X = leaf N content; (iii) Y = leaf N content; X = soil available N status; ** indicates significance at p < 0.01; †values are as mean±SE.

Table 8. Agronomic N use efficiency (AE_N), nitrogen recovery efficiency (RE_N) and partial factor productivity of applied N (PFP_N) of rice grown under different N management practices (average data over 2 years).

Treatment	N rate (kg ha^−1)	AEN (kg kg^−1)	REN (kg kg^−1)	PFPN (kg kg^−1)
S† 34 N15	55.0	21.8*	0.53*	89.3*
S 34 N25	59.2	22.8*	0.64*	87.0*
S 34 N35	70.9	22.3*	0.67*	75.0
S 36 N15	65.0	22.3*	0.59*	78.1
S 36 N25	80.0	21.4*	0.60*	67.3
S 36 N35	88.3	20.3*	0.64*	63.5
S 38 N15	82.5	18.0*	0.52*	62.3
S 38 N25	96.7	18.1*	0.55*	56.3
S 38 N35	105.9	17.2*	0.55*	52.0
FTNM	120.0	13.5	0.45	43.6
FFP	60.0	11.1	0.39	71.3
Control	0.0	–	–	–
SE		2.0	0.04	3.7
LSD (0.05)		5.8	0.13	10.8

†S = SPAD threshold value; N = nitrogen top dressing in kg N ha⁻¹ in each application; FTNM = fixed-time nitrogen management; FFP = farmers’ fertilizer practice; SE = standard error of mean; LSD = least significant difference; * = values statistically differed over FFP.

Table 9. Agronomic N use efficiency (AE_N), nitrogen recovery efficiency (RE_N) and partial factor productivity of applied N (PFP_N) of wheat grown under different N management practices (average data over 2 years).

Treatment	N rate (kg ha^−1)	AEN (kg kg^−1)	REN (kg kg^−1)	PFPN (kg kg^−1)
S† 38 N15	77.5	19.5	0.53	46.5
S 38 N25	85.0	20.3	0.70	46.5
S 38 N35	100.9	17.7	0.69	38.5*
S 40 N15	87.5	21.4	0.59	45.1
S 40 N25	97.5	25.2	0.80	46.6
S 40 N35	118.3	18.3	0.70	36.1*
S 42 N15	97.5	21.1	0.60	42.2
S 42 N25	110.0	19.4	0.71	38.3*
S 42 N35	118.3	16.8	0.74	34.9*
FTNM	120.0	15.9	0.70	32.9*
FFP	80.0	20.2	0.69	45.8
Control	0.0	–	–	–
SE		1.7	0.06	2.2
LSD (0.05)		5.1	0.17	6.3

†S = SPAD threshold value; N = nitrogen top dressing in kg N ha⁻¹ in each application; FTNM = fixed-time nitrogen management; FFP = farmers’ fertilizer practice; SE = standard error of mean; LSD = least significant difference; * = values statistically differed over FFP.