There is a lack of quantified precision in the term “upper part”, but that imprecision is quite likely by design. The 12th Keys to Soil Taxonomy use the term “upper part” 22 times. A few references are to the “upper part” of an organic layer or a permafrost layer, but the vast majority of references are to the diagnostic pedogenic features of argillic or kandic horizons (horizons of clay illuviation). The term “lower part” is used 16 times and typically in reference to the base of the mollic epipedon, or the particle size control section.
The use of upper or lower implies that within the distinguishable feature to which it refers, such as an argillic horizon or mollic epipedon, there are clearly discernable subhorizons, or there exists an observable gradient of development. When used in reference to a pedogenic processes, the terms upper and lower also mark the transition in the dominant soil forming processes. In the Alfisols, the use of ‘upper part’ is to identify that portion of the horizon that transitions from the overlying eluvial horizon or layers. The abruptness or clarity around the transition is the indicator of the degree of development and also an indicator of a long term stability and equilibrium of the climate and soil landscape. This is the concept of ‘Pale’ (old or excessive development) where the soil forming factors have been acting relatively constant for a long period such that an equilibrium is approached. In the case ‘Pale’ subgroup in Alfisols, the majority of clay subject to translocation has moved to a depth in the soil to be in equilibrium with precipitation. The transition from clearly eluvial to clearly illuvial process is very short and near the top of the argillic horizon. Clay translocation in Pale subgroups is in equilibrium with the potential afforded by the climate. Often, the accumulation of translocated clay can be observed before the increase required for an argillic horizon is met. Also, the requirement for argillic horizon is often met above the criteria for ‘Pale’. The transition from eluvial layers to maximal development in the illuvial layer is intended to be short in the Pale Great Group of Alfisols and similar soils with argillic horizons. The use of the term ‘upper part’ permits this relatively short transition to meet the concept of ‘Pale’ without assigning an arbitrary depth requirement below the top of the argillic, nor require an abrupt boundary.
Motherlode series fails the requirement of ‘Pale’ because the clay increase appears stepped within the argillic and possibly even a result of a lithologic discontinuity arising from differences in parent material emplaced by an aggrading colluvial process even if acting concurrently with pedogenic development. The clay increase required by Pale does not occur in the ‘upper part’ of the argillic.
