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total-iron ratios of soils in the Western Po Valley, Italy — A reply
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
259 numbers to soil samples with 10YR hues. That is consistent with the F e d _ o of soils th at we studied. In optimizing the fitness of the experimental values to an interpolation curve the best result is obtained with a cubic polynomial model even using K=25.
The equation Rr2s = 4.82 + 0.90 x + 1.14 x2--0.12 x 3 (r 2 = 0.937, F = 209, p = 0.001) allows to obtain the minor residual sum of squares (49.14) and SE of the estimate (1.75). We f u r t h e r m o r e agree in affirming t hat the different chromatic index, whatever the adopted scale, always defines a discontinuous function, while an instrumental chromatic index would optimize correlation between chemical and empirical parameters.
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
259 numbers to soil samples with 10YR hues. That is consistent with the F e d _ o of soils th at we studied. In optimizing the fitness of the experimental values to an interpolation curve the best result is obtained with a cubic polynomial model even using K=25.
The equation Rr2s = 4.82 + 0.90 x + 1.14 x2--0.12 x 3 (r 2 = 0.937, F = 209, p = 0.001) allows to obtain the minor residual sum of squares (49.14) and SE of the estimate (1.75). We f u r t h e r m o r e agree in affirming t hat the different chromatic index, whatever the adopted scale, always defines a discontinuous function, while an instrumental chromatic index would optimize correlation between chemical and empirical parameters.