total-iron ratios of soils in the Western Po Valley, Italy — A discussion

Geoderma, 35 (1985) 2 5 7 - - 2 5 9

257

Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in The N e t h e r l a n d s

Discussion ESTIMATING R E L A T I V E AGES FROM IRON-OXIDE/TOTAL-IRON RATIOS OF SOILS IN THE WESTERN PO VALLEY, ITALY -A DISCUSSION

E.B. A L E X A N D E R

USDA Forest Service, San Francisco, CA 94111 (U.S.A.)

Arduino et al. (1984) determined the iron in dithionite (Fed) and oxalate (Feo) extracts and the total (Fet) in the fine earth fraction of 19 samples from as many soils on different terrace levels. Their data show clearly that the (Fed--Feo)/Fet ratio is more closely related to terrace or soil age than the Feo/Fed ratio suggested by me (Alexander, 1974). However, I do not understand why Arduino et al. (1984) utilized a redness rating scale which does n o t differentiate between soils with different values and chroma in the 10YR hue. All soils with a 10YR hue were assigned redness ratings equal to zero, following the procedure of Torrent et al. (1980). A convenient formula for redness rating is R r = ( K - - H ) C V -1 ; where K is a constant, H is a number assigned to the hue following the procedure of Hurst (1977), C is the chroma, and V is the color value. H is assigned by adding zero to the number preceding R, 10 to the number preceding YR, or 20 to •

25

I"

2o

~1o

/

O0

I

I

I

2

I

I

I

3 Fed - Feo ,

I

4 %

s

I

i

5

Fig. 1. R e l a t i o n s h i p b e t w e e n r e d n e s s rating (Rr2s) a n d Fed m i n u s Feo. Rr2 s = 3.1 + 3.85 ( F e d - - F e o ) , n = 18, r 2 ffi 0.929.

258 the n u m b e r preceding Y in Munsell c o l o r n o t a t i o n . When K is 20, the R r scale (Rr2o) is equivalent to t h a t o f T o r r e n t et al. (1980). I believe t h a t a n u m b e r greater t h a n 20 for K w o u l d be m o r e a p p r o p r i a t e , because it w o u l d provide for the assignment o f d i f f e r e n t positive n u m b e r s to d i f f e r e n t soil samples with 1 0 Y R hues. Utilizing the data of A r d u i n o et al. (1984), I f o u n d t h a t 25 is a b e t t e r c h o i c e for K t h a n 20, 22.5, or 30. The c o r r e l a t i o n b e t w e e n Fe d or Fed--Feo and R r , w h e n K = 25, is r = 0 . 9 5 8 or 0.964, respectively. T h e redness rating is p r e d i c t e d b y Rr2s = 3.1 + 3.85 ( F e d - - F e o ) with r 2 :- 0.929, c o m p a r e d to R 2 = 0.925 f o r the e q u a t i o n o f A r d u i n o et al. ( 1 9 8 4 ) , utilizing d a t a f o r the same 18 samples (Fig. 1). Thus m y p r o p o s e d R r scale permits the p r e d i c t i o n o f R r f r o m F e d - - F e o as well as t h e scale utilized b y A r d u i n o et al. ( 1 9 8 4 ) even t h o u g h m y e q u a t i o n contains just o n e i n d e p e n d e n t variable (16 degrees o f f r e e d o m ) instead o f t h r e e (14 degrees o f f r e e d o m , A r d u i n o et al., 1984). Alt h o u g h the R r is a linear f u n c t i o n o f Fed--Feo for soils with low to intermediate iron c o n t e n t s , a logarithmic f u n c t i o n o f F e d - - F e o w o u l d u n d o u b t e d l y be m o r e a p p r o p r i a t e u p o n t h e inclusion o f soils with higher iron c o n t e n t s , due to c o l o r s a t u r a t i o n as r e c o g n i z e d by T o r r e n t et al. (1983).

REFERENCES Alexander, E.B., 1974. Extractable iron in relation to soil age on terraces along the Truckee River, Nevada. Soil Sci. Soc. Am. Proc., 38: 121--124. Arduino, E., Barberis, E., Carraro, F. and Forno, M.G., 1984. Estimating relative ages

from iron-oxide/total-iron ratios of soils in the western Po Valley, Italy. Geoderma, 33: 39--52. Hurst, V.J., 1977. Visual estimation of iron in saprolite. Geol. Soc. Am. Bull., 88: 174-176. Torrent, J., Schwertmann, U. and Schulze, D.G., 1980. Iron oxide mineralogy of some soils of two river terrace sequences in Spain. Geoderma, 23: 191--208. Torrent, J., Schwertmann, U., Fechter, H. and Alferez, F., 1983. Quantitative relationships between soil color and hematite content. Soil Sci., 136: 354--358.

ESTIMATING RELATIVE AGES FROM IRON-OXIDE/TOTAL-IRON R A T I O S O F SOILS IN T H E W E S T E R N P O V A L L E Y , I T A L Y -- A R E P L Y

E. ARDUINO "Istituto di Chimica Agraria, Via Giuria 15, 10126 Torino (Italy)

We t h a n k Mr. A l e x a n d e r f o r his interesting c o m m e n t s and we agree with him t h a t t h e redness rating values o b t a i n e d with K = 25 are b e t t e r t h a n t h o s e o b t a i n e d with K = 20. In fact it provides t h e possibility o f assigning p o s i t i v e