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Table 4 Factor levels used in central composite design (CCD)

From: Improvement of growth performance of Amorpha fruticosa under contrasting regime of water and fertilizer in coal-contaminated spoils using response surface methodology

Independent variables

Codes

Symbols

Coded and actual values

- 1.682

−1

0

+ 1

+ 1.682

Soil-water (% FC)

x1

W

40

48.1

60

71.9

80

Nitrogen (N) rate (mg kg−1)

x2

N

0

24.32

60

95.68

120

Phosphorus (P) rate (mg kg−1)

x3

P

0

36.49

90

143.51

180