The soil line, in reference to remote sensing analyses, is a plot line of digital number values that typically fall 45° along this line. These values represent red light wavelengths against very-near infrared light (NIR) wavelengths. Generally, soil values fall close to the plot line and vegetation fall away from it. Thus, the vegetation plotting will be inclined to reflect NIR.
Reference:
(Last Edit 12 Feb, Kellee)