Calcic and Petrocalcic Horizons - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 114

septal structures, calcified filaments, coated grains, spherulites, calcified root cells,
and calcispheres that indicated biogenic origins, mainly induced by plant root-
related microbial activity (Shankar and Achyuthan 2007 ). Durand et al. ( 2007 )
found Pseudomycelium in laminations and rhizoconcentrations in the lower part of
the petrocalcic horizon in a desert soil in India.
In the Negev desert of Israel, calcic horizons occur only in soils that receive less
than 50 mm/yr of precipitation (Amit et al. 2010 ) (Table 12.3 ). The calcic horizon
was attributed to rare rainstorms that move the carbonates down the soil profile. A
number of studies suggest that petrocalcic horizons generally result from multiple
episodes of aeolian sedimentation and soil formation (Wells et al. 1987 ; Chadwick
and Davis 1990 ; Gustavson and Holliday 1999 ; Kleber 2000 ; Amiotti et al. 2001 ;
Brock and Buck 2009 ). Although Harper ( 1957 ) and Marion ( 1989 ) related the
depth to the carbonate layer to mean annual precipitation, our analysis suggests that
other factors may play a role, such as relief and age of parent material.
Several studies report that petrocalcic horizons are deeper in stable upland
positions than in depressions (Abtahi 1980 ; Shankar and Achyuthan 2007 )
(Table 12.3 ). In TX Udolls with calcic horizons occur on landforms with
microhighs, and noncalcareous Aqualfs occur on those with microlows (Sobecki
and Wilding 1982 ). Groundwater close to the surface enables upward movement of
CaCO 3 to form calcic (Shankar and Achyuthan 2007 ) and petrocalcic (Sobecki and
Wilding 1983 ; Dhir et al. 2004 ).
Several case studies in the USA, especially those in TX, suggest that weathering
in situ is more important than aeolian inputs as a source of CaCO 3 (Sobecki and
Wilding 1983 ; Rabenhorst et al. 1984 , 1986a , b ; West et al. 1988a , b , c ; Boettinger
and Southard 1991 ). However, in southwestern USA, aeolian inputs appear to be
particularly important, especially in the case of petrocalcic horizons (Machette
1985 ; Chadwick and Davis 1990 ; van der Hoven and Quade 2002 ). Data from
strontium isotopes suggest that 94-98 % of the CaCO 3 in petrocalcic horizons of
soils of New Mexico (NM) could be attributed to aeolian inputs (Capo and
Chadwick 1999 ). Reheis ( 2006 ) established an extensive dust-monitoring network
in southwestern USA, recording dust deposition rates of 2-20 g/m 2 /yr. CaCO 3
accumulation rates vary from 0.22 to 0.51 g/cm 2 /kyr in southwestern USA
(Machette 1985 ).
The time factor is particularly important with regard to development of calcic
and petrocalcic horizons (Table 12.3 ). The work of Gile et al. ( 1966 ) and Machette
( 1985 ) suggests that soils with calcic horizons (stage I through III carbonates) have
formed during the Holocene but that soils with petrocalcic horizons (stage IV
through VI) began forming in the mid- to late Pleistocene.
These findings are borne out by our analysis of the SSURGO dataset. Soils with
calcic horizon had a mean carbonate stage of 1.8, which would of “latest Pleisto-
cene” age (Gile et al. 1966 ) and those with a petrocalcic horizon had a mean
carbonate state of 5, which would assign them to a mid- to late-Pleistocene age
class.