An improved method for reconstructing a soil profile from analyses of a small number of samples

SUMMARY Most soil properties vary down a soil profile. In nearly every case a graph of the value of the property against depth (the ‘true’ curve) is smooth. However, a soil sample is usually taken from the whole of a horizon so that its analysis is the mean value for the horizon, and a graph of analyses against horizon depth is a ‘stepped’ profile. It is possible to produce a smooth profile by attributing each analysis to the mid‐depth of its horizon and fitting a polynomial or spline curve through these values, but the former may produce erratic results, and both of them produce curves in which the true maxima and minima may be considerably damped. This paper explores the possibility of deriving a better reconstruction of the true profile from the same number of analyses by: taking the samples for analysis from narrow layers at depths that appear to correspond to maxima, minima, or points of inflexion on the true curve, and fitting spline curves through their values; fitting a modified ‘equal‐area’ spline curve to conventional whole‐horizon samples. The latter is considerably superior and, in most cases, achieves a surprisingly close reconstruction of the true profile from which the samples were taken.