Table 2.

Effect of soil plant analysis development (SPAD) chlorophyll meter-based fertilizer N management on fertilizer N use efficiency in rice in Asia.

Region	Threshold SPAD Value	Agronomic Efficiency †, kg grain kg N−1		Recovery Efficiency †, kg N kg N−1		Partial Factor Productivity †, kg grain kg N−1		Reference
		Farmer’s Fertilizer Practice ‡	SPAD	Farmer’s Fertilizer Practice ‡	SPAD	Farmer’s Fertilizer Practice ‡	SPAD
Philippines, Nueva Ecija ⁋	35	17.7a §	15.7a	-	-	53.9a	52.2a	Balasubramanian et al. [91]
South India, Old Cauvery Delta	35	18.6a	41.2b	-	-	58.1a	117.2b
South India, New Cauvery Delta #	35	8.8a	51.0b	-	-	51.6a	118.4b
Vietnam, Cai Lay District	32	9.8a	17.8b	-	-	33.0a	57.5b
East India, Nadia, West Bengal	37	24.3a	42.4b	0.43a	0.55b	56.6a	77.3b	Maiti et al. [92] ‡
South India, Aduthurai, Old Cauvery Delta	35	13.9a	16.0b	0.39a	0.46b	32.8a	38.0b	Nagarajan et al. [93]
North western India, Punjab	36–37.5	8.8a	16.1b	0.20a	0.30b	34.7a	44.2b	Khurana et al. [102]
North western India, Ludhiana, Punjab	37.5	20.0a	23.7b	0.44a	0.51b	-	-	Bijay-Singh et al. [90] ‡
South India, Thanjavur, New Cauvery Delta	35	13.6a	15.0b	0.45a	0.46a	27.9a	31.0b	Nagarajan et al. [93]
South India, Hyderabad, Telangana	37	-	-	-	-	42.8	54.4	Suresh et al. [103]
North western India, Ludhiana, Punjab (dry direct seeded rice)	37	14.1a	22.1b	0.29a	0.35b	40.0a	70.0b	Ali et al. [97]

^† Agronomic efficiency of applied N = (grain yield in the N fertilized plot—grain yield in the zero N plot) ÷ total amount of fertilizer N applied; Recovery efficiency of applied N = (total N uptake in grain and straw in the N fertilized plot—total N uptake in grain and straw in the zero N plot) ÷ total amount of fertilizer N applied; Partial factor productivity of applied N = grain yield in the N fertilized plot ÷ total amount of fertilizer N applied. ^‡ Farmers’ fertilizer practice: All fertilizer management was done by the farmer without any interference by the researcher. However, in some studies conducted on research farms and not in actual farmers’ fields, farmer’s fertilizer practice denotes fixed-schedule N application, for example, in Maiti et al. [92] @100 kg N ha⁻¹, in Bijay-Singh et al. [90] @120 kg N ha⁻¹ and in Ali et al. [97] @150 kg N ha⁻¹. ^§ For each N use efficiency index (agronomic efficiency, recovery efficiency or partial factor productivity) and site, values with different letters are significantly different by Duncan’s Multiple Range Test at 0.05 probability level (Balasubramanian et al. [91], Maiti et al. [92], Bijay-Singh et al. [90] and Ali et al. [97]), Least Significance Difference at 0.05 probability level (Nagarajan et al. [93]) and p-value < 0.001 (Khurana et al. [102]). ^⁋ Data averaged for 12 farms. ^# Data averaged for 20 farms.