Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 483

developing per unit area. As a guide for the following discussion, the NRCS typically
recognizes soil loss rates of 3 to 5 tons/acre (6.7-11.2 Mg/ha) as tolerable (discussed
in more detail later in this chapter). A soil loss or development rate of 11.2 Mg/ha
equates to a depth of soil (assuming a bulk density of 1.3 Mg/m 3 ) of 0.85 mm/year.
For the subsequent discussions, development rates or soil erosion rates expressed on
a mass-per-unit basis have been converted to depth assuming a soil material bulk
density of 1.3 Mg/m 3 .
Soil renewal rates vary widely between different studies and, as might be
expected, depend heavily on the component of the soil profile being addressed and
methods used. Alexander (1988), based on data from 18 watersheds, concluded that
rates of soil formation ranged from 0.002 to 0.09 mm/year, while Wakatuski and
Rasyidin (1992) estimated soil formation on a worldwide basis of 0.056 mm/year.
Using present morphological descriptions to determine soil thickness for four well-
drained loess derived Mollisols (Tama, Marshall, Otley, Galva) with cambic hori-
zons or minimal argillic horizons in Iowa on stable geomorphic surfaces dating from
14,000 YBP, the average rate of soil formation is about 0.11 mm/year, with a range
of 0.11 to 0.13 mm/year. Reported soil formation rates for these three studies varied
by approximately two orders of magnitude, illustrating the challenge associated with
relating soil loss to soil sustainability and the implication of methods used in esti-
mating formation rates.
The surface soil horizon seems the most rapidly/easily modified soil profile com-
ponent but also is the most sensitive to degradation through mismanagement and/or
soil erosion. Other profile components change considerably slower. For a 100- and
a 50-year period in the Central and North Central United States, surface horizon
formation rates of 3 mm/year were observed by Hallberg et al. (1978) and Simonson
(1959). Ruhe et al. (1975) observed an average A horizon formation rate of approxi-
mately 6.3 mm/year for soil developed on the Missouri River floodplain, but for a
relatively short time of less than 100 years. When additional subsurface horizons
are considered, the formation rate decreases. Crocker (1960) concluded that organic
carbon increased rapidly in the early stages of all sequences he studied. In one
sequence, he reported a rapid buildup of organic carbon in the first 60 years and an
apparent decreasing accumulation rate after that time. Parsons (1962) reported that
the formation of a 25.4-cm A + A2 (now E) horizon took 1000 years, which equates
to 0.254 mm/year. Using data from Parson et al. (1962) for three B horizons with an
estimated formation time of 2500 years, the formation rate for the total soil (A, E,
and B) was 0.31 mm/year. Correcting for a soil organic matter loss from the surface
horizon might be relatively quickly fixed, but that does not necessarily equate to
soil formation rates in other horizons or correcting other interrelated properties and
processes impacted by soil loss.
Soil formation contributes to sustaining the soil resource but is counterbalanced
by soil loss through erosion processes. Recently, global cropland erosion rates sum-
marized by Montgomery (2007) averaged 0.64 mm/year, more than an order of mag-
nitude greater than the global soil formation rates given by Wakutuski and Rasyidin
(1992) or Alexander (1988). In contrast, if the T value of 0.85 mm/year recognized
for many US soils by the NRCS appropriately reflects sustainable soil loss rates glob-
ally (a very big assumption), soil degradation through erosion might not be as serious