Geoderma, 41 (1988) 337-351 337 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Morphology, Fertil ity and Classification of Productive Soils on Serpentinized Peridotite in California (U.S.A.)

E.B. ALEXANDER

USDA Forest Service, Juneau, AK 99802 (U.S.A.)

(Received December 30, 1986; accepted after revision July 3, 1987)

ABSTRACT

Alexander, E.B., 1988. Morphology, fertility and classification of productive soils on serpentinized peridotite in California (U.S.A.). Geoderma, 41: 337-351.

Most soils in humid climates decrease in fertility as weathering releases plant nutrients from their parent materials and the nutrients are leached away. Young soils on peridotite, however, have low productivity which increases as weathering and leaching remove Mg, Ni, and Co. This is illustrated by comparing a Xerochrept and a more weathered and leached Haplohumult from the Klamath Mountains. All exchangeable Ca/Mg ratios are < 0.4 in the Ochrept and > 1.0 in the Humult. The ratio of total Ni and Co to total Fe, Mn, Cu, and Zn are all lower in the Humult. The Dunning timber site index (height of conifers at 300 years) increases 24% from shallow to deep Ochrepts, attributed to increases in available water capacity (AWC). It increases another 27% from deep Ochrepts to deep Humults with relatively small increases in AWC. Therefore, increased productivity from the Ochrepts to the Humults appears to be due more to increased fertility than to increased AWC. The timber site index increases another 10% from the Humult to a Humox (tentative classification) in the Sierra Nevada, but it is uncertain whether this increase is due to increase in AWC or in fertility. None of these soils on peridotite are completely accommodated in the Soil Taxonomy. The Ochrepts fit well to the subgroup, the Humult to the suborder, and the Humox fits only to the order level.

INTRODUCTION

Soils on pe r ido t i t e s a n d s e rpen t in i t e s genera l ly have poo r vege ta t ive cover a n d low p roduc t i v i t y due to low fer t i l i ty , which is the e l e m e n t a l ava i lab i l i ty- a spec t o f soil p roduc t iv i ty . Ecologis t s c o m m o n l y l u m p soils on per ido t i t e wi th those on se rpen t in i t e ( P r o c t o r a n d Woodell , 1975), because the p l a n t cover is genera l ly ind i s t ingu i shab le f r o m one to the o ther . W h i t t a k e r (1960) found the vege ta t ion on these u l t r a m a f i c rocks in t he h u m i d K l a m a t h M o u n t a i n s of Cal- i forn ia a n d Oregon so d i s t inc t ive t h a t he doub t ed t h a t the soils on t h e m would ever s u p p o r t t he s a m e vege t a t i on as t he soils on m o r e silicic ( less maf i c ) rocks

0016-7061/88/$03.50 © 1988 Elsevier Science Publishers B.V.

338

in the area. This impression is justified for serpentinite in the Klamath Moun- tains because the mountains are so steep and the serpentinite terrains are so unstable that there are no old land surfaces. In peridotite terrains of the Kla- math Mountains, however, there are a few small remnants of old surfaces on which soils have developed to stages supporting vegetation much like that of soils on more silicic rocks in the area. These old soils on peridotite, which is usually at least partially serpentinized, appear to be as productive for timber as any of those on more silicic rocks.

The increase in productivity with greater weathering and leaching of soils on serpentinized peridote is unusual, because most soils in humid climates decrease in productivity as the plant nutrients released in weathering are leached away (Kronberg and Nesbitt, 1981 ). Considerable knowledge of this phenomenon was gained in a reconnaissance soil inventory of western Siski- you County which included about 200 km 2 of ultramafic rock terrain in the Preston Peak (Snoke, 1977) and Seiad ( Lieberman and Rice, 1986) ophiol- ites. Xerochrepts predominate in the peridotite terrain - Lithic Rup- tic-Xerorthentic Xerochrepts on bedrock and Typic Xerochrepts in colluvium, both with open stands of relatively slow-growing trees. The oldest soils over serpentinized peridotite are Xeric "Orthoxic" Haplohumults with dense stands of more rapidly growing trees. Entisols (Lithic Xerorthents) were found only on olivine-rich peridotite (that is, dunite) and are not extensive in the peri- dotite terrain.

The evidence gained during a soil inventory is the basis for this more inten- sive investigation designed (1) to check the apparent increase in tree growth with greater soil development on serpentinized peridotite, and (2) to learn what soil properties are related to increased soil development and might be responsible for increased tree growth. A soil on serpentinized peridotite in a similar climate of the Sierra Nevada is more weathered and more productive than any soils on peridotite in western Siskiyou County; it was therefore in- cluded in this investigation in order to extend the developmental sequence.

MATERIALS AND METHODS

Tree ages ( assuming 10 years to breast height) and heights were determined in old-growth stands at several sites on serpentinized peridotite while soils were being mapped in western Siskiyou County. Soils were sampled in pits at those sites and horizon samples from two of them (Fig. 1) were sent to the National Soil Survey Laboratory (NSSL) in Lincoln, Nebraska for standard analyses (Soil Conservation Service, 1972). Also, horizon samples were col- lected from a pit at the type location of the Forbes series.

Sand grains from selected horizons were fractionat.ed by sieving, following citrate-dithionite treatment, and then were separated into heavy and light fractions with bromoform ( SG-- 2.89 ). Grains were immersed in oils of differ-

339

Fig. 1. A map of California showing the locations of the Ochrept (I) and the Humult (U) in the Klamath Mountains and the Humox (O) in the Sierra Nevada.

ent refractive indices for microscopic identification. Counts were made with various numbers of grains from about 50 to 300 per separate. Because counts are not readily translated into mass (kg/kg), minerals were placed in abun- dance classes and are reported in the order of abundance. The abundance classes are 0-10%, 10-30%, 30-60%, and 60-100%. In reporting their relative abun- dances, minerals are separated by > > >, > >, >, or a comma, depending on whether their counts are different by 3, 2, or 1 abundance classes or are in the same class.

Acid extracts were obtained by adding 3 ml of concentrated HNOs to 300 mg of fine earth and digesting it overnight at 150 ° C in a pressurized teflon bomb. Although the digestion leaves a solid residue, extraction of the chemical ele- ments determined was nearly complete as indicated by comparisons to total elemental analyses. All elements were determined on a Spectrospan III B emis- sion spectrophotometer in the NSSL.

RESULTS AND DISCUSSION

The first and second soils described in Table I and their vegetative cover are typical of the more extensive younger soils (Ochrepts, pedon $76CA093-6 on a convex mountain sideslope) and the older soils (Humults, pedon $78CA093- 1 on a midmountain bench 180 m above the East Fork of Indian Creek) on serpentinized peridotite in western Siskiyou County. The Ochrepts range from shallow to very deep, due to accumulations of colluvium rather than to differ- ences in weathering and leaching. Diminished contents of rock fragments, in- creased clay, abundant nodules (spherical glaebules) visible in the very fine

340

TABLE I

Pedon descriptions and environmental characteristics

Hor.* ~ Depth/ Color, Field Structure Consistence, Bound. dry/moist grade dry,moist/wet (cm)

Coatings

Pedon $76CA093-6, Ochrept, 41 °55 N, 123°09 W, SW 75% slope, 1000 m altitude

Oi 3-0 Loose conifer needles

A 0-2 as

BA 2-15 gs

Bw 15-27 gb

Bt 27-45 ab

R 15-45 +

5YR 4/5 gsl 2 vf gr so,vfr 5YR 3/3 ss,po

2.5YR 4/6 gl 2 f,m sbk so,fr 2.5YR 4/5 s,sp

1 np f 2.5YR 3/6

2.5YR 4/6 vgl 2 m sbk sh,fr 2.5YR 4/6 s,p

l n p f 2.5YR 3/6

5YR 4/6 egl I f sbk so,fr 5YR 4/6 s,p

2 n p f 2.5YR 4/6

hard, slightly fractured peridotite below 15 to 45 cm depths

Vegetative cover, open conifer forest: Pinus jeffreyi, Calocedrus decurrens, Pseudotsuga menziesii, Cupressus bakeri mathewsii; Arbutus menziesii; Arctostaphylos viscida, Quercus vaccinifolia, Garrya buxifolia, Ceanothus cuneatus; Festuca idahoensis, Agropyron spicatum; Cordylanthus viscidus, Monardella odoratissima, Eriophyllum lanatum lanceolatum, Galium bolanderi

Pedon $78CA093-1, Humult, 41 °53 N, 123°24 W, N 14% slope, 790 m altitude

Oi 3-0 Loose/weakly matted conifer needles, mosses over 15% of surface

A1 0-6 5YR 4/4 vgl 2 vf sbk sh,fr cw 5YR 3/4 ss,sp

A2 6-27 5YR 4/5 gcl 2 vf sbk sh,fr cs 5YR 3/4 ss,sp

Btl 27-52 5YR 4/6 gc 2 vf sbk h,fi gs 2.5YR 3/6 s,p

Bt2 52-105 2.5YR 4/6 gc I vf, f sbk h,fi diffuse 2.5YR 3/6 s,p

Bt3 105-210 + 2.5YR 4/6 gc massive h,fi 2.5YR 3/6 s,p

2 n p f 2.5YR-10R 3/6

2 n pf,tpo 2.5YR- 10R 3/6

3 n tpo 2.5YR- 10R 3/6

TABLE I (continued)

341

Hor. .1 Depth/ Color, Field Structure Consistence, Bound. dry/moist grade dry,moist/wet (cm)

Coatings

Pedon $78CA093-1, Humult. 41 °53 N, 123°24 W, N 14% slope, 790 m altitude

C 7.5YR 5/8 sicl massive sh,fr 7.5YR 4/7 ss,sp

C horizon described in a ditch 30 m from sampled pedon

Vegetative cover, dense conifer forest: Pseudotsuga menziesii, Pinus lambertiana; Quercus keUoggii, Chrysolepis chrysophylla, Lithocarpus denisiflorus, Arbutus menziesii, Cornus nuttaUii, Quercus chrysolepis; Symphorocarpus acutus, Rosa sp; Festuca cali[ornica; Chimaphila umbellata occidentalis, Pyrola picta, Vancouveria sp, Pteridiurn aquiUnum pubescens

Pedon $82CA061-1, Hurnox. 39°07N, 120°46 W, E 12% slope, 1220 m altitude

Oi 4-0 loose/weal~ly matted conifer needles

A 0-23 2.5YR 3/5 gl 1 vf, f,m gr s,fr gs 2.5YR 3/4 ss,sp

AB 23-51 2.5YR 3/6 gcl 2 vf, f sbk sh,fr cs 2.5YR 3/5 ss,sp

Bo 51-94 2.5YR 4/6 cl massive sh,fi ci 2.5YR 3/6 ss,sp

BC 94-135 7.5YR 5/8 cl massive h,fi cw 5YR 4/6 s,sp

C 135-150+ 10YR 6/8 l massive sh,fi 7.5YR 4/6 s,sp

3n pf

1 mk fractures

Vegetative cover, dense conifer forest: Pinus ponderosa, Abies concolor, Pinus larnbertiana, Calocedrus decurrens, Pseudotsuga menziesii; Ceanothus prostratus, Ceanothus integerrimus; Festuca idahoensis, Stipa stillrnanii; Trientalis latifolia, Asteraceae, Polygala cornuta, Iris sp, Pteridium aquilinum pubescens

*~The soil horizon designations are those currently used in the U.S.A. (Guthrie and Witty, 1982).

pebble (2 -5 ram) and very coarse sand fract ions, and th i cke r sola in the Hu- mul t s indicate t h a t t hey are much more wea the red t h a n the Ochrepts . T h e B- hor izons in bo th the Ochrep t and the H u m u l t r ep re sen ted in Tab le I have red colors o f high c h r o m a and low values charac te r i s t i c of soils developed f rom se rpen t in ized per idot i te . Wea the r i ng and leaching p r o m o t e lower p H values

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TABLE II

Dunning site indices (height in 300 years) for mixed conifers of soils on serpentinized peridotite in western Siskiyou County

Soil n Mean Std. Grouping* height dev. by t-test (m)

Xerochrept: shallow 7 30.2 5.2 A mod. deep 3 33.8 4.6 AB deep 7 37.2 4.9 B

Haplohumult: deep 7 48.2 6.3 C

"1P=0.05.

and the accumulation of clay in Bt-horizons, features which are more pro- nounced in the Humult. Clay coatings described in the field were not evident in thin-sections from the Ochrept or from the Humult. Any birefringence due to the preferential orientation of clay minerals is apparently masked by sec- ondary iron compounds which are common in both soils.

Tree cover and timber site indices increase slightly as the depths of younger soils increase, but the usual range within the group of Ochrepts is generally somewhat less than the differences between the tree densities (or basal areas) and the timber site indices from Ochrepts to Humults. The Humults have mixed conifer-black oak (Quercus keUoggii) cover in drier areas toward the east and Douglas-fir (Pseudotsuga menziesii) -sugar-pine (Pinus lambertiana) -tanoak ( Lithocarpus densiflora) cover in wetter areas toward the west but similar tim- ber site indices in contrast to lower indices on the Ochrepts. Whittaker (1960) and Waring (1969) give more detailed descriptions of the vegetative cover in adjacent areas, and Kruckeberg (1984) has summarized the plant distribution on ultramafic rocks throughout California.

Jeffrey pine (Pinusjeffreyi) trees were measured on the Ochrepts and Doug- las-fir and sugar-pine trees on the Humults (Table II). Their growth rates are compared by their Dunning site indices (Dunning, 1942 ) for old-growth trees, 300 years old. No basal areas nor volumes were determined but apparently they increase as the site indices increase for trees on serpentinized peridotite. The mean indices are 30.2 m (99 ft.) for seven trees on shallow (25-50 cm deep) Ochrepts, 33.8 m (111 ft.) for three trees on moderately deep (50-100 cm deep) Ochrepts, 37.2 m (122 ft. ) for seven trees on deep Ochrepts, and 48.2 m (158 ft.) for seven trees on deep Humults. Site (index) differences due to altitude, slope aspect, soil depth, and precipitation (100 to 250 cm/year, very little summer rainfall) are relatively small compared to those between the lesser developed Ochrepts and the more developed Humults.

343

Thin-sections of the Humult in the Klamath Mountains (pedon $78CA093- 1 ) showed strained quartz grains and fine fragments of phyllite which were not evident in macroscopic observations. This extraneous material may have been mixed with ultramafic detritus and deposited over the serpentinized per- idotite by a Pleistocene earthflow or by solifluction from the Galice Formation which occurs upslope from the Humult. There does not appear to be sufficient phyllit~ in the Humult to appreciably affect its development and fertility.

A more weathered soil has been mapped on serpentinized peridotite in the Sierra Nevada (Humox, Fig. 1 ). In the Forbes series, it has been sampled at the type location (pedon $82CA061-1 on a convex hillslope on a mountain plateau), where the mean annual precipitation is about 125 cm with negligible summer rainfall. Four ponderosa pine trees 106 to 134 years old sampled there have a mean Dunning site index of 53.0 + 4.9 m, and a site index of 21.1 _+ 2.3 m in 50 years for second-growth trees (Dunning and Reineke, 1933).

Laboratory analyses (Table III) indicate that both the Humult and the Hu- mox have more free iron ( citrate-dithionite extractable Fe) and greater vari- able charge (A CEC) than the Ochrepts. Leaching is expressed by lower base saturation and higher Ca/Mg ratios in the Humult and the Humox. Soils of the Forbes series (pedon $82CA061-1 ) are the only ones in California which have been found to have higher pH values in a neutral salt solution (CaC12, Table III) than in distilled water, indicating net positive charges in their Bo- and C-horizons. Two independent samples from the C horizon of the Humox at the Forbes type location were found to have mean pH values of 6.3 in dis- tilled water and 7.0 in 1 M KC1 (NSSL).

Recognizable rock fragments are sparse and nodules are abundant in the coarser sand fractions of the Humult and the Humox (Table IV). The citrate- dithionite t reatment bleaches the nodules and reduces their densities such that many appear in the light separates. A black crust is evident on many of the bleached nodules which remain in the heavy separates. Olivine and serpentine are abundant in the finer sand fractions of the Ochrept, but serpentine is sparse and olivine is absent in the sola of the older soils. Olivine in the C-horizons of the Humult and the Humox is strongly limonitized or iddingsitized, such that few grains of it are identifiable. X-ray analyses of clay from the C-horizon of the Humox indicated chlorite, talc, and goethite (NSSL). Chlorite, smectite, and goethite are the principal ingredients of bowlingite and iddingsite, miner- alogically variable alteration products of olivine; iddingsite being the result of alteration or weathering of olivine in more oxidizing environments (Deer et al., 1982 ). Delvigne et al. (1979) provide a more complete discussion of olivine weathering. The talc may be produced by the weathering of orthopyroxenes, as observed by Nahon and Colin (1982). Grains which appear to be sericitized ( SR, Table IV) may be pyroxenes and amphiboles altered to microcrystalline talc or kerolite, as defined by Brindley et al. (1977), with completely altered grains appearing in the light separates. Parent materials of the Humult and

344

+~'~

~q

v

O ~ v

v

° ~

~b ¢:b

TABLE IV

The characterization of sand fractions *~

345

Hor. Size Light fraction *~

wt. composition (%)

Heavy

wt. composition (%)

Total weight (%)

before after treat.

Pedon $78CA093-6, bedrock density 3.10 Mg/m J

A v.co 0.7 RF co. 1.3 R F > > > M C med. 1.1 S P > > > T R fine 2.8 SP > > > TR

AB v.co. 0.4 RF co. 0.6 R F > > > M C reed. 0.9 SP > > > TR fine 2.0 SP > > > TR

Bw v.co. 0.4 RF > > > MC co. 0.8 RF > > > MC med. 0.8 SP > > > TR fine 2.3 SP > > TR

Bt v.co. 0.7 R F > > > MC co. 1.2 R F > > > M C reed. 0.7 SP > > > TR fine 1.3 SP > > TR

0.6 RF 3.2 1.3 2.1 R F > > > B M 8.8 3.4 6.5 F F > > H B > B M , P C 7.1 7.6

22.0 F F > > H B > P C , B M {27.0) 24.8

O.7 RF 2.O 1.1 2.0 R F > > > B M 4.0 2.6 5.4 F F > > B M > H B , P C , P O 5.4 6.3

25.6 F F > > >PC,HB,BM,PO {26.0) 27.6

1.0 RF 1.7 1.4 3.0 R F > > > B M 3.6 3.8 6.2 F F > > >BM,HB,PO,PC 5.2 7.0

24.9 F F > > H B > P C , P O , B M (27.0} 27.2

0.8 RF 1.7 1.5 2.6 R F > > > B M , F F 3.7 3.8 6.2 F F > > B M > P O , P C , H B 5.1 6.9

25.5 F F > > >BM,HB,PC,PO {25.9) 26.8

Pedon $78CA093-1, bedrock density 2.84 Mg/m 3

A1 v.co. 9.2 GN > > > RF co. 5.7 G N > > > R F med. 2.5 G N > > > R F fine 2.7 GN > QZ

A2 v.co. 3.7 G N > > > R F co. 4.5 GN > > > RF reed. 2.0 GN > > QZ fine 3.1 GN,QZ

v.co. 2.1 G N > > > R F co. 3.2 GN > > > RF med. 1.7 G N > > >QZ fine 4.2 GN > QZ

Bt l

v.co. 1.2 G N > > > R F co. 1.7 G N > > > R F reed. 1.2 GN > > > QZ fine 3.3 GN > QZ > > SP

Bt2

3.2 G N > > B M 21.6 12.4 2.4 G N > > B M 8.2 8.1 1.0 GN,BM 1.6 3.5 0.6 B M > G N > Z R > H B {3.3) 3.3

1.7 G N > > B M 12.3 5.4 2.1 G N > > B M 5.6 6.6 0.5 GN,BM 2.0 2.5 0.5 B M > G N > >ZR,HB (4.5) 3.6

0.6 G N > > > B M 11.4 2.7 1.6 GN,BM 5.8 4.8 1.0 B M > > G N 2.2 2.7 1.4 B M > G N > >ZR,HB,AC,AN {4.9) 5.6

0.6 GN 10.7 1.8 1.5 GN 5.9 3.2 0.8 B M > G N > >AC 2.2 2.0 1.0 B M > G N > >HB,ZR,AC,AN (4.4) 4.3

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TABLE IV (continued)

Hor. Size Light fraction .2

Heavy Total weight (%)

wt. composit ion wt. composit ion before after ( % ) ( % ) treat.

Bt3 v.co. 1.1 G N > > > R F 0.7 GN 15.4 1.8 co. 1.8 G N > > > R F 1.3 GN 7.1 3.1 med. 1.8 GN > > > QZ 1.2 B M > > GN > AC 2.4 3.0 fine 5.2 G N > > Q Z > S P 1.4 B M > G N > A C > Z R , A N , H B (4.7) 6.6

v.co. 0.0 - 0.3 B M > > R F > G N - 0.3 co. 0.2 S R > > > G N 1.1 B M > > > R F , G N - 1.3 med. 0.8 S R > > > S P 2.9 B M > > A C > A N , R F - 3.7 fine 8.8 S R > S P 4.5 B M > > A C > A N 13.3

Pedon $82CA061-1, bedrock density 2.7 Mg/m ~

Bo

BC

v.co. - GN - GN 5.7 3.1 co. - . . . . . . . 3.6 2.6 reed. - . . . . . . . 2.7 1.5 fine 3.7 G N > > Q Z , S R > S P 0.5 B M > > G N > P O , P C , A C , H B (7.5) 4.2

v.co. - GN - GN 1.7 0.2 co. - . . . . . . . 1.0 0.4 med. - . . . . . . . 1.0 0.5 fine 1.2 G N > S R , Q Z > S P 0.6 B M > > >GN,AC,HB,PO,PC (6.9) 1.8

v.co. - RF,GN - GN,RF 0.5 0.3 co. - . . . . . . 1.9 1.0 med. - . . . . . . 3.5 2.0 fine 2.9 S R > > > G N 1.9 S R > B M > > G N , P O , P C (11.3) 4.8

*1Symbols: AC, t remoli te or actinolite; AN, anthophyl i te ; BM, black or metallic grains; FF, oli- vine; GN, nodules or rounded glaebules; HB, common hornblende; MC, mica; PC, clinopyroxene; PO, orthopyroxene; QZ, quartz; RF, rock fragments; SP, serpent ine or chlorite; SR, limonitized, iddingsitized, or kerolitized grains, unidentifiable; TR, tremolite; ZR, zircon; and - - -, no density separations. *2Sand size fractions: v.co., 1-2 ram; co., 0.5-1 ram; med., 0.25-0.5 mm; and fine, 0.075-0.25 ram. Weights before t r ea tmen t are from the NSSL and weights after c i t ra te-di thioni te t r ea tmen t are from the authors laboratory. Since a 0.074-ram sieve is not used routinely in the NSSL, fine sand {0.075-0.25 m m ) was est imated from cumulative plots of particle-size dis t r ibut ions and placed

in parentheses.

t h e H u m o x h a v e l e s s o l i v i n e b u t m o r e s e r p e n t i n e t h a n t h e O c h r e p t , w h i c h i s

r e f l e c t e d i n l o w e r b e d r o c k d e n s i t i e s ( T a b l e I V ) . T h e r e f o r e , t h e d i m i n i s h e d

s e r p e n t i n e c o n t e n t s i n t h e H u m u l t a n d i n t h e H u m o x s e e m s c l e a r l y d u e t o

s t r o n g w e a t h e r i n g . S o m e s e r p e n t i n e p e r s i s t s i n d e n s e n o d u l e s , a s w a s s e e n i n

347

TABLE V

Elements (g/kg) extracted from the fine earth of Klamath Mountain soils with heated concen- trated nitric acid; mean total analyses of some rocks and soils are added for comparison

Hot. Na Mg A1 K Ca Cr Mn Fe Co Ni Cu Zn

Pedon $76CA093-6 A 0.58 68. 12 0.7 6.5 0.5 1.5 85 0.15 1.7 0.06 0.07 AB 0.54 90. 15 0.6 5.5 0.9 2.1 130 0.22 2.6 0.09 0.10 Bw 0.59 77. 15 0.6 5.7 1.0 2.1 130 0.22 2.6 0.10 0.09 Bt 0.74 91. 21 0.7 6.7 1.1 1.9 120 0.25 2.7 0.13 0.08

Pedon $78CA093-1 A1 0.04 3.9 140 2.4 4.4 2.9 5.1 130 0.15 1.0 0.14 0.15 A2 0.05 4.1 98 2.2 2.0 2.0 3.7 120 0.13 0.9 0.13 0.16 Btl 0.02 4.5 69 1.7 1.2 3.1 4.0 140 0.13 1.1 0.13 0.14 Bt2 0.03 3.5 79 1.7 0.4 2.5 1.0 140 0.12 1.1 0.14 0.21 Bt3 0.03 3.1 87 1.5 0.3 3.2 0.9 150 0.11 1.2 0.23 0.17

Soils of the U.S.A., Shacklette and Boerngen (1984): mean 12.0 9.0 72 15.0 24.0 0.1 0.6 26 0 . 0 1 <0.1 0.02 0.06

Earth's crust, Bowen (1979): mean 23.0 23.0 82 21.0 41.0 0.1 1.0 41 0.02 0.1 0.05 0.08

Peridotite (saxonite or harzburgite, 4 samples), Hotz (1964): mean 0.1 245.5 7 0.1 6.3 2.5 1.0 58 0.28 2.6

a thin-section from the A-horizon of the Humult, even though individual grains of serpentine have disappeared from the sand fractions of the A-horizon. About one-half of the nodules in the C-horizon of the Humox are strongly attracted by a magnet, with the magnetic portion increasing upward to nearly 100% in the A-horizon. Hotz (1964) found that similar nodules in the Klamath Moun- tains are predominantly maghemite with lesser amounts of goethite.

Acid extractions from the fine earth fractions of the Klamath Mountain soils (Table V) indicate that the more mobile elements such as Na, Mg, and Ca are greatly diminished in the Humult as compared to the Ochrept. The least mo- bile elements, such as A1 and Cr (Esson, 1983; Schellmann, 1964), are more concentrated in the Humult, although the higher A1 content may be due largely to phyllite. Shrewry and Peterson {1976) found that boiling concentrated ni- tric acid extracted all of the Ni from soils but left a residue of chromite; thus, the total Cr of soils on serpentinized peridotite may be a little higher than indicated in Table V. Very high Cr, Mn, Fe, Co, Ni, and Cu contents in the Humult, compared to the means from an extensive sampling of soils in the U.S.A. ( Shacklette and Boerngen, 1984), confirm the field assessment that it is developed essentially from ultramafic material even though it has been con-

348

taminated with phyllite from the Galice Formation. The higher K contents of the Humult may be due to hydrous micas inherited from the phyllite or to greater productivity and hence more biocycling of nutrient elements than in the Ochrept or to both. Appreciable biocycling is suggested by the greater con- tent of K in the A-horizon than in the B-horizon of the Humult. (Table V).

Soil-productivity relationships

Proctor and Woodell (1975) have reviewed the soil properties which might account for poor plant growth on soils from ultramafic rocks. They include low Ca, or low Ca/Mg ratios, low P, and Ni, Cr, and Co toxicities (Alexander et al., 1985). Exchangeable Ca/Mg ratios increase from the Ochrepts to the Hu- mults ( Table III) and total Ni and Co contents decrease ( Table V ). The soils on serpentinized peridotite have relatively large amounts of organic C { Table III) and therefore probably have relatively large amounts of N, although the amounts of plant-available N may be relatively low. The Ochrepts are more friable than the Humult or the Humox and there are no root-restrictive layers in the deeper Ochrepts; thus the physical properties of the Ochrepts, other than AWC which is related to soil depth, are no less favorable for root growth and plant production than in the older soils.

The timber site indices increase 24% from shallow to deep Ochrepts (Table II) due to the greater available water capacities (AWC) of the deeper soils. There are relatively slight increases in AWC from deep Ochrepts to deep Hu- mults, due mainly to lower contents of coarse fragment in the Humults, yet the timber site index increases another 27%. Therefore, the greater timber site indices of the Humults, compared to deep Ochrepts, appear to be due more to greater fertility than to greater AWC. The Dunning timber site index increases 10% from the Humult to the Humox.

The greater fertility of the Humult and the Humox may be due to the leach- ing of Mg, increasing the Ca/Mg ratio. Ca is a higher percentage of the ex- changeable cations in the A-horizons of the Humult and the Humox than in the A-horizon of the Ochrept (Table III). The higher Ca contents in the A- horizons of the Humult and the Humox, however, may be a result of greater productivity and thus greater biocycling rather than the cause of higher fertil- ity. Cr is more concentrated in the Humult than in the Ochrept, but the con- centrations of Ni and Co, which are more mobile than Cr, are lower in the Humult. The relatively high organic matter contents and moderately low pH values in the Humults favor trivalent Cr over the more toxic hexavalent Cr (Adriano, 1986), making Cr toxicities unlikely. Nevertheless, Cr may have a negative effect on fertility by reacting with P and other nutrient elements to reduce their availabilities. Ni or Co toxicities are more likely, but the higher total Ni and Co contents of the Ochrept are insufficient evidence to suggest that toxicities of either are the reason for lower fertility than in the Humults.

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Interactions between Ni and nutrient elements in row 4 of the periodic table may cause nutrient deficiencies (Adriano, 1986), but the ratios of total Ni and Co to total Fe, Mn, Cu, and Zn decrease from the Ochrept to the Humult ( Ta- ble V).

Soils developed over bedrock of most lithologies become less fertile as the plant nutrients are released by waathering and leached from their sola ( Kron- berg and Nesbitt, 1981 ). Soils on peridotites and serpentinites, however, are relatively infertile before they are leached because of low Ca and Mo contents and toxic quantities of some of the elements in the first transition of the peri- odical grouping (Proctor and Woodell, 1976; Alexander et al., 1985). They become more fertile as weathering depletes the source minerals and toxic ele- ments are leached from their sola. The most common problems seem related to the fact that peridotites and serpentinites have very high Mg contents and little Ca (Alexander et al., 1985). Initially, soils on serpentinized peridotite have very low Ca/Mg ratios. The Ca/Mg ratio is 0.35 in the A-horizon of the Ochrept (pedon S76CA093-6) and < 0.03 in its B-horizon (Table III). Weath- ering and leaching losses are much greater for Mg than for Ca, such that the Ca/Mg ratios are >/1.0 in all horizons of the Humult and the Humox (Table III). Ratios of Ca/Mg > 1.0 do not appear to be detrimental to plant growth (Alexander et al., 1985).

Soil classification

Soil Taxonomy (Soil Survey Staff, 1975) seems to be adequate for classi- fying the Ochrepts on serpentinized peridotite to the subgroup level (Lithic Ruptic-Xerorthentic and Typic Xerochrepts). The family, however, should indicate fertility lower than that of most Xerochrepts. A serpentinitic family would indicate low fertility, but the Ochrepts on peridotite do not have enough serpentine to qualify. Heavy minerals, particularly olivine and spinel group minerals such as chromite and magnetite, predominate in the sand fractions (pedon S76CA093-6, Table IV). An ultramafic family, as proposed by Alex- ander et al. (1985), would be quite suitable for the Xerochrepts. Presently they are in loamy-skeletal, oxidic, mesic families.

The Humults of the Klamath Mountains (pedon $78CA093-1 ) have colors red enough for Rhodoxeralfs if their BC-horizons are not included in their argillic horizons. There is, however, no comparable group in the Humult sub- order. The clay content in the Bt3-horizon of pedon $78CA093-1 is a little too low, compared to that in the Bt2-horizon, for a Palehumult. If the soil were a Palehumult, it would be in an Orthoxic subgroup. I have used this group mod- ifier and call Humults of the Klamath Mountains Xeric Orthoxic Haplohu- mults, although Rhodohumults might be a more appropriate great group. They are in a clayey, oxidic, mesic family.

The Humox profile of the Sierra Nevada is more weathered than the Hu-

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mults, as indicated by greater variable charge (Table III). Its Bo-horizon is massive and has too little clay, compared to overlying horizons, for an argillic horizon. Grain counts indicate 6% pyroxene plus amphibole in the heavy and 1% serpentine in the light separate of the fine sand fraction (Table IV); but this is less than 2% (weight) weatherable minerals, because the light separate weighs much more than the heavy separate of the Bo-horizon. The AB-horizon might have enough clay for an argillic horizon, except that with a 1.5 MPa water to clay ratio > 0.6, 2.5 times the 1.5 MPa water retention is substituted for the clay content ( Soil Survey Staff, 1975 ). There is practically no differ- ence between the 1.5 MPa water retention in the A-, AB-, and Bo-horizons ( Table III ). The very high 1.5 MPa water retention in the BC- and C-horizons may be due to amorphous ferrosilicates, much as it is due to aluminosilicates in soils in volcanic ash, but this hypothesis has not been tested. A sample from the C-horizon rose to pH 9.3 in M NaF in 2 min, indicating moderately active A1 or Fe, but only minute quantities of Al, Fe, and Si were extracted with acid oxalate (John Kimble, personal communication, 1986). The key in Soil Tax- onomy (Soil Survey Staff, 1975) indicates that the soil is an Orthox, but the Orthox description excludes soils with xeric moisture regimes. Therefore, I have placed it in the Humox suborder, even though it has only 12.3 kg of or- ganic carbon per m 2 to a depth of 1 m. Xeric Haplohumox (or Rhodohumox) seems to be the most suitable placement, although Soil Taxonomy would have to be ammended before this subgroup could be recognized.

ACKNOWLEDGEMENTS

W.D. Nettleton was most helpful in coordinating analyses in the National Soil Survey Laboratory, Lincoln, Nebraska. George Holmgren supervised the analyses of concentrated nitric acid extracts and John Kimble supervised the analyses of acid oxalate extracts. D. Ernstrom sampled the Forbes Series at its type location.

E. Horton, formerly with USDA Forest Service, identified some of the forbs from the Xerochrept (pedon $76CA093-6) and Mary Barkworth, Utah State University, identified the Stipa stillmanii.

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