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Published online 1 May 2008
Published in Soil Sci Soc Am J 72:838-847 (2008)
DOI: 10.2136/sssaj2007.0159
© 2008 Soil Science Society of America
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SOIL MINERALOGY

Tectonic Inclusions in Serpentinite Landscapes Contribute Plant Nutrient Calcium

Donald G. McGahan*, Randal J. Southard and Victor P. Claassen

Univ. of California, Davis, CA 95616

* Corresponding author (soilman{at}ucdavis.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Serpentinite-derived soils give rise to botanically distinct systems primarily because of inadequate parent material Ca content. We hypothesized that Ca content varies widely in what have been mapped as soils derived from serpentinite. An exchangeable Ca/Mg ratio <0.7 is often used to relate the imbalance of these nutrient elements in serpentinite-derived soils. We sampled six parent materials and soils from the Coast Ranges of California in Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) modal location map unit polygons. Parent material total CaO content varied from 1.0 to 230 mg kg–1, and CaO/MgO varied from <0.1 to 4. A combination of x-ray diffraction (XRD), polarized light microscopy (PLM), and electron microscopy was used to identify the Ca-bearing accessory minerals diopside, grossularite, andradite, and tremolite. Accessory mineral content was often too low to be detected by XRD or minerals were too finely disseminated and difficult to detect in thin section by PLM. Electron microscopy, in concert with XRD and PLM, was needed to fully characterize the mineral assemblage. Two sites, Napa and Tehama, contained no serpentine minerals, were not serpentinites, and were tectonic inclusions in the serpentinite landscape. Napa rocks contained almost no Ca-bearing minerals and would be identified as a serpentinite if relying on elemental analysis CaO/MgO ratio alone. Tectonic inclusions and Ca-bearing accessory minerals affect Ca distribution and presumably its availability for plants. Careful mineralogical analysis may be required to identify Ca-bearing accessory minerals.

Abbreviations: BSE, backscatter electron microscopy • CL-VR, interstratified chlorite–vermiculite • EDX, energy dispersive x-ray spectroscopy • PLM, polarized light microscopy • XRD, x-ray diffraction


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Serpentinized ultramafic lithologies give rise to botanically distinct ecosystems (Walker, 1948). Debates have abounded regarding edaphic influences for this effect, but the Ca/Mg ratio has been used to express the high Mg and low Ca condition that exists in these soils. Early researchers attributed the poor growth of vegetation on serpentine soils to Mg toxicity (Loew and May, 1901; Gordon and Lipman, 1926). Vlamis and Jenny (1948) demonstrated that Ca deficiency, rather than Mg toxicity, was the primary cause of poor plant growth. Walker et al. (1955) demonstrated that non-native species yields were reduced when the exchangeable Ca was 20% or less of the cation exchange capacity, and had little or no growth or yield below 10% exchangeable Ca. Native plant species, however, were better able to extract Ca; yield was only reduced 24% in the 5 to 3% exchangeable Ca range, whereas non-native plant species had a 90% yield reduction.

Soils dominated by Mg silicates, with a potential Ca deficiency and its resulting effects on plant growth in agricultural or native settings, are recognized in Soil Taxonomy at the family mineralogical class level as "magnesic" (Soil Survey Staff, 1999). Identification of Mg silicates requires XRD and PLM. Still, total or extractable Ca/Mg has been a sometimes useful indicator of serpentinite-derived soils. An NH4OAc-extractable Ca/Mg of 0.7 or greater is generally desired for agricultural crop production (Alexander et al., 1985; Brooks, 1987).

Serpentinite is a product of the low temperature and pressure metamorphism and metasomatism of ultramafic rocks (e.g., peridotite). Metasomatism is a process of indefinite replacement, loss, or addition of elements as a result of percolating solutions (Merrill, 1906). As a result, during metamorphism or metasomatism, the anhydrous peridotite minerals become more hydrous and Ca content decreases relative to the original rocks, resulting in a relative enrichment of Mg (Page, 1966, 1967; Coleman and Keith, 1971; Alexander et al., 2007). Olivine is the dominant mineral in peridotite, the idealized serpentinite precursor, and is readily altered to serpentine during metasomatism. Serpentinous pseudomorphs after the peridotite chain (pyroxene and amphibole) and layer (talc) silicates are commonly identified in thin section studies of serpentinized rocks. These pseudomorphs of chain and layer silicates in serpentinites are called bastites (Merrill, 1906; Wicks and Whittaker, 1977; O'Hanley, 1996). Wicks and Whittaker (1977) asserted that bastites should be considered a textural rather than a mineralogical term because once serpentinization is complete and they become serpentine, it is often impossible to distinguish a pyroxene bastite from an amphibole bastite. If distinguishable, however, bastites after clinopyroxene or calcic-clinoamphibole may indicate that, before metasomatism, the peridotite contained more Ca than if the bastites identified in the serpentinite were after orthopyroxene or orthoamphibole. The pyroxene and amphibole in a peridotite are less susceptible to serpentinization than olivine, and it is feasible that not all the Ca in the clinopyroxene or clinoamphibole was completely removed from the lithologic unit during metasomatism to serpentinite. Together with metamorphic secondary minerals such as garnets that may contain Ca, it is reasonable to assume that the Ca content of soils on serpentinitic landscape is variable and, therefore, influences plant nutrient supplying status as a result of serpentinite weathering.

Less mafic lithic inclusions in serpentinite landscapes can influence the total and extractable Ca/Mg contents of the bulk parent material and the soil. Rabenhorst and Foss (1981) attempted to predict mafic or ultramafic parent lithology while mapping soils of the eastern Piedmont of Maryland. Based on their study of 39 samples, if the exchangeable Ca/Mg ratio ranged from 0.0 to 0.1, the probability that the soil was formed from serpentinite rather than a mafic lithology was 98%. An exchangeable Ca/Mg ratio of 0.2 to 0.3 yielded a probability of 53%, whereas a 0.6 to 0.7 exchangeable Ca/Mg ratio reduced the probability to 21%. Clearly at exchangeable Ca/Mg ratios >0.3, the likelihood that the soil is derived solely from ultramafic parent material is low.

A basic question is "Do some lithologies underlying serpentinitic landscapes contain Ca-bearing minerals that can significantly alter extractable Ca/Mg ratios in the soils weathered from the serpentinites?" We studied the variation in parent material (nominally serpentinite) mineralogy beneath magnesic pedons from the Coast Ranges of California. This focus on bulk mineralogy and Ca-bearing accessory mineralogy of the parent material is necessary as a requisite to follow-up pedologic studies at these sites. We expected to find a variation in accessory minerals, specifically Ca-bearing minerals, in these serpentinite parent materials and that these accessory minerals would have a profound influence on the total and extractable Ca/Mg ratios in the soils weathered from the serpentinites.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Field
Sampling locations were from six California soil survey areas within Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) map units (Fig. 1 ). These are "soils formed in material weathered from serpentinite and rocks of similar mineralogy" (Soil Survey Staff, 2005). We sampled pedons near the location of the modal pedon for the Henneke series for each soil survey. To minimize the impacts of colluviation, we sampled pedons on summits above the modal pedon location and within the polygon containing the modal location. Pits were excavated by hand tools. Soils were described and sampled by horizon using conventional procedures, and rocks samples were collected within the Cr or R layers (Soil Survey Division Staff, 1993).


Figure 1
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  		Fig. 1. Pedon sampling locations from Henneke soil series (clayey-skeletal, magnesic, thermic Lithic Argixerolls) map units within six California soil survey areas.

 
Woody vegetation species composition and percentage of cover at the sites is presented in Table 1 .


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  		Table 1. Woody vegetation coverage at sites on serpentinite-derived landscapes.

 
Laboratory
Three rocks from each parent material were ground with an agate mortar and pestle to pass a 0.106-mm sieve. The three rock powders for each parent material were mixed, ground for 10 min with an agate mortar and pestle (Jackson, 2005), mounted on porous ceramic tiles, washed with MgCl2 or KCl salt solutions, rinsed with deionized water to remove excess salts, and then reoriented by smoothing with a glass slide held at an angle to the sample (Whittig and Allardice, 1986). X-ray analyses were made with a Diano XRD 8000 diffractometer (Diano Corp., Woburn, MA) fitted with a nickel filter and curved graphite monochromator to produce Cu K{alpha} radiation. After the initial diffraction analysis, the MgCl2-treated samples were treated with glycerol and reanalyzed. The KCl samples were reanalyzed after 350 and 550°C heat treatments (Whittig and Allardice, 1986). Major oxide content of the rock powders was determined by inductively coupled plasma emission spectrometry following a LiBO2 fusion and dilute HNO3 digestion (Sawhney and Stilwell, 1994).

Three rock samples from each R or Cr layer were impregnated with Petropoxy-154 resin (Palouse Petro Products, Palouse, WA). Thin sections of these samples were prepared and examined with a polarizing light microscope (Drees and Ransom, 1994; Stoops, 2003). Selected thin sections were polished and analyzed using backscatter electron microscopy (BSE) and energy dispersive x-ray spectroscopy (EDX) on a Cameca SX-100 electron probe microanalyzer (Cameca, Paris).

Selected soil properties are presented in Table 2 . Soils were air dried, sieved to pass through a 2-mm sieve, and the fine earth fraction was analyzed for particle size distribution by the pipette method as described by Gee and Bauder (1986). Fine-earth-fraction extractable cations were measured by displacement with pH 7.0, 1 mol L–1 NH4OAc; concentrations of Na, Ca, Mg, and K in the leachate were measured with flame atomic absorption or emission spectrometry. Cation exchange capacity (CEC-7) was measured by saturating samples with 1 mol L–1 NH4OAc, at pH 7, washing with 95% ethanol, extracting with 2 mol L–1 KCl to remove adsorbed NH4 (Soil Survey Staff, 2004), and determining the NH4 concentration in the leachate conductimetrically (Carlson, 1978). The pH of the fine earth fraction was determined at 1:1 soil/water on a mass basis (Soil Survey Staff, 2004).


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  		Table 2. Selected chemical properties, particle size distribution, and colors of soils from serpentinite-derived landscapes.{dagger}

 

   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Non-serpentinites
Napa and Tehama County parent materials were not dominated by serpentine minerals and, therefore, not serpentinites. The Tehama and Napa County parent materials contained clinopyroxene but were quite different in the composition and proportions of the remainder of the parent material mineral suite.

Tehama County
The Tehama County parent material contained garnet (0.299- and 0.266-nm peaks), clinopyroxene (0.32-nm peak), interstratified chlorite–vermiculite (CL-VR) (1.4-nm peak), and pumpellyite (0.29-nm peak), but no serpentine. The CL-VR was characterized by a 1.4-nm Mg-saturated peak that collapsed with K saturation and heating treatments, leaving a plateau of peaks between 1.4 and 1.0 nm (Fig. 2A ).


Figure 2
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  		Fig. 2. Mineralogical properties of Tehama County parent material: (A) x-ray diffractograms of interstratified chlorite–vermiculite, grossularite, diopside, and pumpellyite; (B) photomicrograph under crossed polarizers—the garnets (GN) are isotropic dark purplish black tightly packed small interlocking euhedral crystals and the ilmenite (IL) is brown; (C) backscatter electron (BSE) micrograph of diopside (Di), titanite (Tt), ilmenite (IL), and garnet (grossularite, GN); and (D) BSE micrograph of lighter gray pumpellyite (PU) intergrown into darker gray interstratified chlorite–vermiculite (CL-VR) upper left and center, with smaller masses of euhedral grossularite garnet crystals (GN) lower center but predominantly tightly packed and interlocked into larger masses (lower right).

 
Garnets have the idealized formula of X3Y2(SiO4)3 and are further divided into the pyralspite and ugrandite groups where Y is Al in pyralspite, X is Ca in the ugrandite group, and X is not Ca in the pyralspite group (Table 3 ). Energy-dispersive x-ray spectroscopy of the Tehama County garnet corroborated XRD results indicating that the garnet was grossularite of the ugrandite group (Fig. 3A ).


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  		Table 3. Chemical composition of garnets (reproduced after Hurlbut and Klein, 1977).

 

Figure 3
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  		Fig. 3. Energy dispersive x-ray spectra (EDX) of Tehama County parent material minerals: (A) grossularite demonstrating some inclusion of Ti; (B) diopside; (C) pumpellyite; (D) titanite; (E) ilmenite; and (F) chlorite–vermiculite.

 
In thin sections with plane-polarized light at lower magnifications, the small euhedral interlocking grains of grossularite were nearly indistinguishable and formed large cinnamon-brown, somewhat granular masses. The masses were isotropic under crossed polarizers (Fig. 2B).

Pyroxene is a common inclusion in ultramafic serpentinite protoliths and is more resistant to alteration than is olivine, the most common mineral in the ultramafic protolith (Goldich, 1938; Huang, 1989). The idealized formula for pyroxene is XYSi2O6. Clinopyroxene is distinguished optically from orthopyroxene by inclined, rather than parallel, extinction and by its higher interference colors as viewed in cross-polarized light. For orthopyroxene, X is Mg or Fe, but for clinopyroxene X is Ca, Na, or Li and therefore a potential Ca source on weathering (Table 4 ).


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  		Table 4. Chemical composition of pyroxenes and amphiboles (reproduced after Hurlbut and Klein, 1977).

 
The clinopyroxene diopside was identified in thin sections by inclined extinction. The clinopyroxene was determined by EDX to be a member of the diopside–hedenbergite series with considerably greater Ca than Fe and therefore may be referred to as diopside (Fig. 3B).

Pumpellyite, a sorosilicate [idealized formula Ca2MgAl2(SiO4)(Si2O7)(OH)2·(H2O)] could be easily missed in the x-ray diffractogram and was not identified in thin section, but EDX confirmed its presence (Fig. 3C). The BSE micrograph clearly showed that it was intergrown with the CL-VR and chemistry was confirmed by EDX (Fig. 3F).

Among the six parent materials, the Ti and Al contents were highest in the Tehama County parent material, while Si content was the lowest (Table 5 ). The Ti was a trace element in grossularite (Fig. 3A) but a major element in ilmenite and titanite (Fig. 3D and 3E). Titanite (sphene) was finely disseminated and associated with the diopside (Fig. 2C).


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  		Table 5. Content of selected elements in "serpentinite" parent material rocks beneath the six pedons.

 
Ugrandite garnets are characteristic of rodingites (Coleman, 1979, 1980). Rodingites are a massive, dense, buff-to-pink rock, typically rich in grossular garnet and calcic pyroxene, and enveloped in serpentinite. They are the result of localized metasomatism of non-peridotites in association with the metasomatic alteration of peridotite to serpentinite. These rodingites can be tectonic inclusions within the serpentinite mass (endogenous) or can occur at metasomatic contacts with the country rock (exogenous). Tectonic inclusions within serpentinite bodies could be from mafic dikes in the peridotite or country rock inclusions during emplacement of the more plastic serpentinite. The replacement or invasion by calcisilicate minerals, such as Ca-garnets, into the protomineralogy of the tectonic inclusions is coupled with the Ca lost from the peridotite during metasomatism. Chlorite and calcic clinopyroxenes are commonly associated with ugrandite garnets in rodingites (Coleman, 1979, 1980). This mineral suite fits well with the observed mineralogy of the Tehama County parent material, which was dominated by grossularite and also contained calcic clinopyroxene. The CL-VR in the parent materials is probably the result of interlayer stripping from a precursor chlorite in the hard rock underlying the Cr layer we analyzed.

Rodingites are not serpentinites but probably occur frequently enough in serpentinitic landscapes to supply significant amounts of Ca to the soil solution on weathering.

Napa County
Based on XRD, the Napa County parent material was dominantly vermiculite (1.4-nm peak with Mg treatment collapsed to 1.0 nm with K treatment and heat) and subdominant plagioclase feldspar (0.630-, 0.374-, 0.365-, 0.318-, and 0.293-nm peaks) (Fig. 4A ). Energy-dispersive x-ray spectroscopy confirmed the plagioclase, and elemental ratios suggested that the plagioclase was toward the albite end of the plagioclase solid-state solution (Fig. 4C).


Figure 4
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  		Fig. 4. Mineralogical properties of Napa County parent material: (A) x-ray diffractograms of vermiculite and albite plagioclase; (B) backscatter electron micrograph of Napa County parent material (Di = diopside, P = plagioclase feldspar, VR = vermiculite); (C) energy dispersive x-ray spectra (EDX) of Napa County parent material plagioclase; (D) EDX of Napa County parent material diopside; and (E) EDX of Napa County parent material vermiculite.

 
The mineralogy, based on XRD peak intensity, suggested a greater amount of plagioclase than would be expected from the elemental analysis (Table 5). This is probably due to attenuated vermiculite peaks due to interstratification by chlorite and its presumed precursor mica in amounts too small to be detected by XRD (Moore and Reynolds, 1997).

A pyroxene XRD peak was not evident, but a pyroxene with inclined extinction was clearly identified by PLM. The clinopyroxene, like the clinopyroxene in the Tehama County parent material, contained more Ca than Fe and had a small amount of Al (Fig. 4D). The vermiculite contained more Al and less O (Fig. 4E) than the Tehama County CL-VR (Fig. 3F). The dominant source of Ca was probably the clinopyroxene (diopside) identified by PLM and electron microscopy (Fig. 4C). This mineral was not identified by XRD and therefore was apparently only a minor component of the parent material (Fig. 4A). Low abundance of diopside, together with high vermiculite content, was probably responsible for a CaO/MgO < 0.1 (Table 5). Napa County parent material contained no detectable serpentine, so was not a serpentinite. It did not have Ca-garnets and was not a rodingite. It was an example of a mafic tectonic inclusion in the serpentinitic terrain. Weathering of this parent material resulted in soil Ca enrichment (Table 2), probably as a result of rapid weathering of the plagioclase feldspar and biocycling of the Ca as the diopside weathered (McGahan, 2007).

Serpentinites
Shasta County
Shasta County parent material was dominated by serpentine (XRD peaks at 0.724, 0.455, and 0.362 nm) and talc (0.93-, 0.466-, and 0.31-nm peaks) with a subdominant component of chlorite (persistent 1.425-nm peak and 0.475-, 0.71-, 0.355-, and 0.284-nm peaks) (Fig. 5A ). Not identified by XRD but clearly distinguishable in thin sections by PLM and BSE were masses of radiating columnar or needlelike crystals of calcic clinoamphibole, probably tremolite, as confirmed by EDX (Fig. 5B).


Figure 5
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  		Fig. 5. Mineralogical properties of Shasta County parent material: (A) x-ray diffractograms of serpentine, talc, and chlorite; and (B) energy dispersive x-ray spectra of calcic clinoamphibole (tremolite).

 
Tremolite contains Ca, and yet the CaO content of the parent material was very low (Table 5); therefore, the tremolite was probably not an abundant accessory component of the rock.

Glenn County
Glenn County parent material was dominated by serpentine (0.731-, 0.457-, 0.363-, and 0.250-nm peaks) with a trace of magnetite (0.253-nm peak) (Fig. 6A ). Enstatite was not identified by XRD but was clearly identified by PLM by its parallel extinction. Energy-dispersive x-ray spectroscopy confirmed that it contained no Ca (Fig. 6B).


Figure 6
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  		Fig. 6. Mineralogical properties of Glenn County parent material: (A) x-ray diffractograms of serpentine and magnetite; and (B) energy dispersive x-ray spectra of enstatite.

 
The CaO content of the Glenn County parent material was similar to the CaO content of the Shasta County parent material (Table 5), and no Ca-containing minerals were identified in the Glenn County parent materials.

Kings County
Kings County parent material was dominated by serpentine (0.724, 0.455, 0.363, and 0.250 nm) (Fig. 7A ). There were also traces of andradite garnet (0.302, 0.271, and 0.246 nm) and magnetite (0.297- and 0.253-nm XRD peaks) detected by XRD (Fig. 7A). The andradite XRD peaks were very weak and could easily be missed. Polarized light microscopy clearly showed garnet interspersed with magnetite among the serpentine (Fig. 7C). The Fe content of andradite increases backscattered electron fluorescence and therefore the andradite may easily be mistaken for magnetite in BSE images (Fig. 7D). Andradite is easily distinguished from magnetite by PLM, EDX analysis, or by adjusting the contrast of the BSE image.


Figure 7
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  		Fig. 7. Mineralogical properties of Kings County parent material: (A) x-ray diffractograms of serpentine, andradite, and magnetite; (B) backscatter electron (BSE) micrograph of talc (TA) among serpentine (SY) of varying shades of gray—bright white spots are either magnetite or andradite garnet; (C) photomicrograph shown in plane-polarized light of a mass of talc (TA), dark magnetite (MG), and individual dodecahedral crystals of andradite garnet (GN) in a serpentine (SY) matrix; and (D) BSE micrograph with bright white magnetite and andradite garnet among serpentine.

 
The serpentine EDX was representative of the serpentine in all of the parent materials (Fig. 8B ). Talc [idealized formula Mg3Si4O10(OH)2] was not identified by XRD but could be seen in thin section by PLM (Fig. 7C) and in BSE images (Fig. 7B). The talc chemistry was confirmed by EDX (Fig. 8A). Magnetite chemistry was also confirmed by EDX (Fig. 8D). The garnet was confirmed as andradite by EDX and contained more Fe than grossularite (Fig. 8C and 3A, Table 3). Unlike the grossularite in the Tehama County parent material, the andradite garnet in Kings County occurred as individual crystals or in small clusters in the dominantly serpentine Kings County parent material (Fig. 7C).


Figure 8
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  		Fig. 8. Energy dispersive x-ray spectra of Kings County parent material minerals: (A) talc; (B) serpentine; (C) andradite garnet; and (D) magnetite.

 
Talc does not contribute to soil Ca (Fig. 8A) nor does serpentine (Fig. 8B), but the andradite could contribute to the soil solution Ca on weathering (Fig. 8C). The total CaO content was far less (3.2 mg kg–1) in the Kings County parent material than in the Tehama County parent material (229 mg kg–1), and the CaO/MgO ratio was markedly lower (<0.1) than for the Tehama County parent material (4.0) (Table 5).

San Benito County
San Benito County parent material was dominated by serpentine (0.73-, 0.45-, 0.36-, and 0.27-nm peaks) (Fig. 9A ). No accessory pyroxenes or amphiboles were identified. Polarized light microscopy clearly identified opaque inclusions (Fig. 9B). The opaque minerals seen in PLM were resolved by BSE (Fig. 9C) and EDX to be magnetite (Fig. 10A ), chromite spinel (Fig. 10B), and Cr-rich andradite garnet (Fig. 10C). The andradite contained some Ca and probably contributed to the parent material CaO content of 14 mg kg–1 (Table 5, Fig. 10C).


Figure 9
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  		Fig. 9. Mineralogical properties of San Benito County parent material: (A) x-ray diffractogram of serpentine; (B) photomicrograph under plane-polarized light of serpentine (SY) with opaque inclusions (15-mm-wide field of view); and (C) backscatter electron micrograph increased contrast and lowered brightness eliminates outlines of serpentine (SY) crystals that dominate the parent material but distinguishes between chromite spinel (SP) at center surrounded by chrome-rich andradite garnet (GN), while lightest gray is magnetite (MT).

 

Figure 10
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  		Fig. 10. Energy dispersive x-ray spectra of San Benito County parent material: (A) chromite spinel; (B) chrome-rich andradite; and (C) magnetite.

 
Implications for Calcium Supply and Plant Growth
We asked a most basic question: How do you know if a soil is derived from serpentinite? The simple answer is to characterize the parent material and ensure that it is dominated by serpentine minerals. If a parent material is dominated by serpentine minerals and is therefore a serpentinite, could it still contain Ca-bearing minerals? Do accessory minerals influence the Ca/Mg ratio?

Elemental analysis clearly demonstrated that Ca content was variable on nominally serpentinite landscapes. The parent material of two out of six pedons contained no serpentine minerals. One parent material, from Tehama County, had abundant Ca-bearing minerals, and the other, from Napa County, contained very few Ca-bearing minerals.

It is clear that reliance solely on CaO/MgO from parent material analysis would have incorrectly identified the Napa County parent material as serpentinite. It was not a serpentinite and despite the low parent material total CaO/MgO (<0.01), the soils derived from it had relatively high extractable Ca/Mg (Table 2).

The Tehama County parent material total CaO/MgO (4.0) (Table 5) readily indicated a non-serpentinite parent material. The soil extractable Ca/Mg was more favorable to vegetation and increased with proximity to the surface (Table 2).

Further complicating interpretation of the total CaO/MgO is the fact that serpentinite parent materials can contain Ca-bearing minerals. We could not identify Ca-bearing trace accessory minerals in many of the parent materials using XRD. Kings, Shasta, and San Benito County parent materials were examples of serpentinites that had no obvious accessory minerals, other than magnetite, as detected by XRD. They did have trace accessory minerals detected by PLM or BSE and EDX. Soil extractable Ca/Mg ratios at these sites increased with proximity to the surface (Table 2). Kings County soil extractable Ca/Mg approached 0.6 in the A horizon, a ratio that suggests a low probability of serpentinite (Rabenhorst and Foss, 1981), whereas this ratio in the A horizons of the San Benito and Glenn County pedons was 0.2. The variability of the soil extractable Ca/Mg in pedons clearly derived from serpentinite underscores the influence of Ca-bearing accessory minerals on the variation of Ca as a potential plant nutrient. Our results suggest that total or soil extractable Ca/Mg measured on surface horizon grab samples is an unreliable predictor of the presence of serpentine in nominal serpentinitic landscapes.

Weathering of these parent materials, like the non-serpentinite tectonic inclusions such as rodingites (Tehama County) and mafics (Napa County), could result in downslope Ca enrichment. Their impact by soil solution or sediment transport could be further varied based on downslope dynamics, e.g., water-gathering vs. water-spreading landscape positions.

With other supporting analyses such as PLM and electron microscopy, we identified the minerals contributing Ca to the soils: clinopyroxene (diopside), ugrandite garnets (grossularite and andradite), and calcic clinoamphibole (tremolite). Ugrandite garnets were a common Ca source in serpentinites and rodingites. Identification of these Ca-bearing accessory minerals by a screening process could be especially beneficial to researchers investigating "serpentine soil" to avoid anomalous results arising from tectonic inclusions on serpentinitic landscapes or calcic accessory minerals in serpentinites. Tectonic inclusions in the landscape and, to a lesser extent, Ca-bearing accessory minerals have the potential of acting as "landscape fertilizers" of Ca. Landscape managers or revegetation efforts may also benefit from identification of site sources of Ca, as such identification may help to adjust practices of managing amendment application rates.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Received for publication May 1, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome