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The first of the passive soil formers is parent material, the residual or transported overburden of disintegrated rock making up the bulk of the soil. Certain of the original rock forming minerals have been thoroughly changed chemically into new compounds and reduced to colloidal size. Although many people think that the type of parent material alone determines the kind of soil that is present, this is an inaccurate concept. For example, the same granite forms the bedrock in the Piedmont region in both Maryland and Georgia, but because of climatic differences the soils of these two states are somewhat different. On the other hand, soils of the same major groups may be found to overlie two different types of overburden or bedrock.

An exception to the general rule that soil type is independent of parent material origin is found in young soils that have not had enough time to develop, and in some limestone areas where the influence of the rock is especially strong. Parent material may locally exert a strong control over the soil texture.

Another passive soil former is landform, or ground-surface configuration. Where slope is steep, surface erosion by runoff is more rapid and water penetration is less than on gentle slopes. As a result, the soil will be thinner on steeper slopes. Flat upland areas accumulate a thick soil that has a thick layer of dense clay (clay pan) and is excessively leached. There the products of weathering tend to remain in place. Flat bottom lands likewise have thick soils, but they are poorly drained and dark coloured. Here, constant saturation retards decay of vegetation and allows organic matter to accumulate. Gentle slopes where drainage is good but erosion is slow are considered the norm for soil formation. Slow, continuous erosion is a normal geologic soil process whereby the removal balances the formation, of new soil from the parent material. Only when this erosion is greatly accelerated does it become harmful to the soil.

Another influence of landform is the slope aspect or direction of exposure of the surface to the slanting rays of the sun. In middle latitudes it is common to find that south-facing slopes, exposed to the warming and drying effects of sunlight, have different conditions of vegetation and soils from north-facing slopes, which retain cold and moisture longer.

A third passive factor in soil formation is time. A soil is said to become mature when it has been acted upon by all soil-forming processes for a sufficient long time to have developed a profile that changes only imperceptibly with further passage of time. Soils that are evolving from recently deposited river alluvium or glacial till, for example, are considered young. In young soils the characteristic horizons are absent or poorly developed. No age in terms of years can be given to all mature soils because the rate at which a mature soil is developed depends on many other factors. Some soils of humid regions in sandy localities may require 100 to 200 years to develop, whereas more commonly, several thousand years may be needed to produce a mature soil. Some soils of tropical and equatorial regions are thought to be as old as one to six million years, or of Pliocene geologic age. As with development of landforms, age of soils is purely relative. Another way of defining a mature soil might be to say that it is in equilibrium with the many processes and forces acting upon it.

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