FACTORS AFFECTING CLAY FORMATION by

I N T R O D U C T I O N T h e s t u d y o f c lay f o r m a t i o n i n soils m a y b e d i v i d e d i n t o t h r e e p a r t s : (1) a q u a n t i t a t i v e e v a l u a t i o n o f t h e a m o u n t o f c lay f o r m e d f r o m a g i v e n a m o u n t o f p a r e n t m a t e r i a l , (2) a q u a l i t a t i v e e v a l u a t i o n of t h e k i n d o f m i n e r a l s a n d o t h e r s u b s t a n c e s p r e s e n t i n t h e c l ay f r a c t i o n , a n d (3) a n e v a l u a t i o n o f t h e r e a c t i o n s a n d t h e a g e n t s i n v o l v e d i n t h e f o r m a t i o n o f clay. F u r t h e r m o r e , a n a d e q u a t e u n d e r s t a n d i n g of c lay f o r m a t i o n r e q u i r e s a s o l u t i o n t o a l l t h r e e p r o b l e m s i n t h e c o n t e x t of t h e f a c t o r s o f soil f o r m a t i o n : c l ima te , p a r e n t m a t e r i a l , o r g a n i s m s , t o p o g r a p h y , a n d t i m e ( J e n n y , 1941). T h e p r e s e n t p a p e r dea l s w i t h t h e f i r s t a s p e c t o f c l ay f o r m a t i o n , n a m e l y , a q u a n t i t a t i v e e v a l u a t i o n of c lay f o r m a t i o n as i t is a f fec ted b y t h e f a c t o r s o f soil f o r m a t i o n . H o w e v e r , b e i b r e p r o c e e d i n g f u r t h e r i t w o u l d be b e s t t o de f ine 110 ]~ACTORS AFFECTING CLAY FOI~I~IATION 111 the following terms which will be used in this paper : clay, clay formation, clay accumulation, and clay depletion. By "clay " is meant the whole inorganic fine fraction of a soil consisting of particles having a diameter of less than 2#, 5# or l#, depending on the analysis available. By " clay ibrmation " is meant the conversion of particles greater than clay size to those of clay size regardless of the path and mechanism of formation. By " clay accumulation " is meant the gain of clay by a soil material other than by " clay formation ", and by " clay depletion " is meant the loss of clay due to migration, cementation, or solution. I t is not possible to use the " clay content " of a soil as a measure of clay formation, because the clay content of a soil sample represents not only the clay formed but also the clay originally present in the parent material as well as the clay gained from an outside source by accumulation or the clay remaining after clay losses have occurred by depletion. I t is therefore clear that the clay content of a given amount of soil from different soils may represent different amounts of parent materials; consequently, the relative clay content in such soils is not a direct measure of the relative intensity of clay formation. To obtain such a measure it is necessary to determine clay formation for all soils per unit volume of parent material having identical areal extent and identical depth functions because the agents causing clay formation operate on an areal basis and with a definite relation to depth. ~r for obtaining such measurements were developed recently (Barshad, 1955) but at present few data are available to which these methods may be applied. In the absence of data suitable for calculating clay formation on a volume basis, the best alternative is to calculate clay formation on a constant weight of the nonclay fraction of the parent material since this is the fraction from which the clay is formed. The present paper is a report of such an application. The objective in making such calculations was to correlate the intensity of clay formation with the factors which operate in soil formation, namely, climate, parent material, topography, organisms, and time (Jenny, 1941). Since the amount of clay in any given soil sample represents the net effect of the original clay content of the parent material, clay formation, and either clay accumulation or depletion , it is obvious that it would be impossible to evaluate clay formation in an isolated soil sample. I t can be done only in the context of.a soil profile--a three-dimensional section of a soil body from the surface to the parent material. In practice such a section is subdivided into several thin layers (horizons) and a soil sample from each layer is collected and studied separately. The subdivision of the profile into layers and the decision as to what layer is the parent material presents many problems. These problems and their solutions were discussed in a previous publication (Barshad, 1955). Since the present paper is based on data for soils collected by various workers over a considerable length of time, the parent material for calculating clay formation is herein defined as the nonclay fi'action of the deepest soil horizon of each profile studied, or the underlying rock if such was present. 112 SlxrrH NA~IO~iL CON~EI~ESIC~ ON CLAYS A~D CLAY MINERALS The amount of clay formed is expressed in terms of grains of clay formed per 100 g of nonclay of parent material. M A T E R I A L U S E D The soils studied were chosen to represent a cross section of the Great Soil Groups as well as a number of soils within each group to illustrate the effect of variations in one or more of the soil-forming factors. The original data for the soils used in the present study are presented in the original papers as listed in the references and in Table 1. No a t tempt was made in the present paper to rename the Great Soil Group by which the authors described their soils, even though in the terminology of present day pedology some of the Great Soil Groups are known by different names. The method used to investigate the soils reported in the various papers included (1) field survey methods; (2) visual examination to describe the morphological features; (3) mechanical analysis to ascertain the textures; (4) total chemical analysis of the soils and of their separated clays ; and (5) mineralogical methods, for a few soils, to determine their content of resistant mineral. EXPERIMENTAL To evaluate clay formation in a soil from its parent material two methods were developed: one based on mineralogical analysis and another on total chemical analyses of the soil and of the clay in the soil, and the amount of clay in the soil. These methods are described in detail in a previous publication (Barshad, 1955). In the present paper the majori ty of the results obtained are based on the second method. For the present discussion it need be stated only that the basic assumption underlying both methods is tha t the differences in the mineralogical composition or the chemical composition of the non-clay fraction of any soil from that of the parent material resulted from the formation of clay and other changes which may occur during soil development such as leaching of lime or gain in water of hydration and organic matter. The results obtained in the present study are reported only in terms of number of grams of clay formed from 100 g of nonclay parent material. The following factors affecting clay formation were considered : (t) relative position of the soil from the surface ; (2) c l imate-the effect of temperature and rainfall; (3) effect of topography--mainly good drainage versus poor drainage as caused by the position of the soil in the landscape ; (4) effect of vegetat ion--mainly grass versus tree vegetation ; (5) effect of parent material ; and (6) effect of t ime or the age of the soil. To evaluate factors (1), (2), (3), (4) and (6), it is necessary to compare soils developed from similar or identical parent materials. To compare clay formation among the different soils studied, it was four~d more helpful and informative to report the results graphically, as shown in Figs. 1 to 14, rather than in tabular form. ~ACTORS AFFECTING CLAY ~OI~MATION 113