A . N A T U R E O F T H E S O I L C O L L O I D S
T h e colloidal fraction of soils is p a r t l y inorganic (clay) a n d p a r t l y organic ( h u m u s ) , t h e two forming in most soils a c l a y - h u m u s complex.
crops. Nevertheless t h e evidence is not sufficient to prove t h a t t h e element is essential for n o r m a l growth of plants. Crops w h i c h respond well to applications of sodium salts include celery, mangold, sugar beet, table beet, a n d t u r n i p . Sodium, like potassium, is present as a n exchangeable cation in n o r m a l soils a n d ranges in a m o u n t from 100 to 200 pounds per acre (6-inch surface l a y e r ) . It is present as sodium
carbonate in alkali soils.
Knowledge of t h e n a t u r e a n d properties of soil colloids is essential if one is to u n d e r s t a n d t h e p a r t t h e y p l a y i n p l a n t nutrition. Colloidal surfaces have t h e p r o p e r t y of adsorbing ions: this adsorption m a y be designated as positive (cations), negative ( a n i o n s ) , or chemical.
1. Clay
T h e clay fraction of soils includes all inorganic particles w i t h a diameter of < 2 μ; those particles < 1 μ a r e t e r m e d colloidal clay a n d consist m a i n l y of weathered minerals. T h e n a t u r e of these materials h a s given rise to a great deal of speculation. T h e earliest t h e o r y re-garded kaolinite as t h e essential m i n e r a l of clay, this being formed from orthoclase felspar according to the equation
K2O A l203- 6 S i 02 + 3 H20 = A l203- 2 S i 02- 2 H20 + 2KOH + 4 S i 02
Kaolinite does not possess all t h e properties i n h e r e n t in clay, a n d it is n o w k n o w n to be only one of a n u m b e r of clay minerals. Later, workers tried to characterize clays b y their solubility in hot m i n e r a l acids a n d placed t h e m in categories according to their S i 02/ A l203 ratios.
V a n Bemmelen, 1 8 8 8 , a n d later S t r e m m e , 1 9 1 1 , divided their colloidal fraction into two parts. One part, w h i c h w a s soluble in h y d r o -chloric acid, t h e y called allophaneton, a n d a second p a r t not soluble in hydrochloric acid but soluble in hot concentrated sulfuric acid c a m e to be called kaolinton. A t t e m p t s w e r e m a d e to classify clay materials on the basis of their kaolinton a n d allophaneton content. W i e g n e r , 1 9 3 6 , considered t h e colloidal exchange m a t e r i a l as being m a d e u p of three parts: ( a ) a kernel, (b) a layer of adsorbed anions external to t h e kernel b u t l y i n g i n contact w i t h it, (c) exchangeable cations attracted to the particle b y t h e adsorbed anions. T h e kernel w a s considered to be a h y d r o u s compound chiefly of a l u m i n a a n d silica of variable composi-tion a n d of u n k n o w n structural attributes. Mattson, 1 9 3 8 , regarded the colloidal complex as a crystalline kernel covered w i t h a n amorphous heterogeneous coating w h i c h lacks a definite composition a n d is not identical w i t h t h e nucleus ( 8 9 ) .
F o r m a n y years it h a d been suggested that clay materials w e r e com-posed of extremely small particles of a limited n u m b e r of crystalline minerals, b u t prior to about 1 9 2 0 t h e r e w e r e no adequate research tools to provide the positive evidence. I n 1 9 2 3 , H a d d i n g in Sweden, and in 1 9 2 4 R i n n e in G e r m a n y , working quite independently, published t h e first X - r a y diffraction analyses of clay minerals. Both these workers found crystalline m a t e r i a l in the finest fraction of a series of clays.
About 1 9 2 4 , Ross a n d his colleagues, in America, on the basis of work with t h e pétrographie microscope, also showed t h a t clay minerals w e r e
C . B O U L D
largely crystalline a n d w e r e limited in n u m b e r . T h e y confirmed these findings later b y m e a n s of X - r a y analysis ( 8 9 ) .
By the early 1930's w h a t has come to be k n o w n as the clay-mineral concept became firmly established. According to this concept, clays generally are composed of extremely small crystalline particles of one or m o r e m e m b e r s of a small group of minerals. T h e clay m i n e r a l s a r e essentially h y d r o u s a l u m i n u m silicates w i t h m a g n e s i u m or iron sub-stituting wholly or in p a r t for the a l u m i n u m i n some minerals, a n d w i t h alkalis or alkaline earths present as essential constituents in some of them. T h e classic investigation of P a u l i n g in 1930 provided t h e basic ideas w h i c h permitted the elaboration of t h e structure of the layer clay minerals ( 8 9 ) .
T w o structural units are involved in the atomic lattices of most of the clay minerals. One u n i t consists of two sheets of closely packed
ο b FIG. 4. Diagrammatic sketch showing (a) single octahedral unit and (b) the sheet structure of the octahedral units. Open circles, hydroxyls; filled circles, aluminums, magnesiums, etc. From Grim (89).
oxygens or hydroxyls in which a l u m i n u m , iron, or m a g n e s i u m atoms are embedded in octahedral coordination, so t h a t t h e y are equidistant from six oxygens or hydroxyls (Fig. 4 ) . W h e n a l u m i n u m is present, only two-thirds of t h e possible positions a r e filled to balance t h e structure, w h i c h is t h e gibbsite structure a n d has t h e formula A l2( O H )6. W h e n m a g n e s i u m is present, all t h e positions a r e filled to balance the structure, which is the brucite structure and has the formula M g3( O H )6. T h e second u n i t is built of silica tetrahedrons. I n each tetrahedron a silicon atom is equidistant from four oxygen, or hydroxyls if needed to balance the structure, a r r a n g e d in t h e form of a tetrahedron w i t h a silicon atom at the center. T h e silica tetrahedral groups are a r r a n g e d to form a hexagonal network, which is repeated indefinitely to form a sheet of composition S i406( O H )4 (Fig. 5 ) . T h e terahedrons are so a r r a n g e d t h a t their tips all point in the same direction, a n d their bases are all in the same plane. Combination of these two units, w i t h modi-fications a n d substitutions, give rise to t h e lattice structure of clay minerals ( 8 9 ) . Those clay minerals usually found in soils include kaolinite, halloysite, montmorillonite, illite, chlorite, a n d vermiculite.
T h e clay m i n e r a l s can be classified according to their lattice struc-ture, of w h i c h t h e r e a r e two basic types. Kaolinite has a 1 : 1 lattice a n d m a y be described as a single silica t e t r a h e d r a l sheet topped b y a slightly distorted gibbsite sheet, both being joined b y condensation and
FIG. 5. Diagrammatic sketch showing (a) single silica tetrahedron and (b) sheet structure of silica tetrahedrons arranged in a hexagonal network. Larger circles, oxygens; smaller circles, silicons. From Grim (89).
C-AXIS Ö Ο 6 X> Ö 0 4 0 . 2 ( 0 H ) 4 Si 6 0 b-AXlS
KAOLINITE (0H)eAI4Si40l0
FIG. 6. Schematic diagram of the crystal structure of kaolinite. Courtesy of J. W.
Grüner and Akademische Verlagsgesellschaft Geest and Portig K.-G. (publishers of Z. Krist.). From Toth (247).
splitting off of w a t e r (Fig. 6 ) . Lattice substitution for all practical purposes does not exist. Montmorillonite has a 2: 1 lattice a n d consists of a single gibbsite sheet between two sheets of silica tetrahedrons (Fig. 7 ) . Lattice substitutions include Al a n d Ρ for Si, a n d M g , Fe, N i , a n d Li for Al. Because of these lattice substitutions, t h e atomic charges w i t h i n the lattice are unbalanced. Compensation for the u n b a l a n c e
in-eludes substitution of O H for Ο in t h e octahedral layer, a n d adsorption of exchangeable cations. I n kaolinitic types, broken bonds a r o u n d t h e edges of the silica-alumina sheets are largely responsible for their cation exchange capacity ( 2 4 7 ) .
It should be pointed out t h a t clay minerals seldom occur in a p u r e state b u t are contaminated w i t h inorganic amorphous isoelectric pre-cipitates a n d organic matter. T h e inorganic contaminants m a k e little
bAXIS -MONTMORILLONITE ( 0 H )4A l4s ie02àn HZ °
FIG. 7. Schematic diagram of the crystal structure of montmorillonite. Courtesy of V. Hofmann et al. and Akademische Verlagsgesellschaft Geest and Portig K.-G.
(publishers of Z. Krist.). From Toth (247).
contribution to t h e cation exchange capacity of soils b u t are responsible for a considerable proportion of the anion exchange capacity.
2. Organic Matter
Soil organic m a t t e r consists of p l a n t residues together w i t h the products of decomposition, t h e excretions from soil microorganisms a n d microbial cells. T h a t p a r t w h i c h h a s u n d e r g o n e advanced change and lost its original structure is referred to as h u m u s . T h e t e r m h u m u s however has no precise chemical definition. T h e earlier soil chemists attempted to separate a n d fractionate h u m u s b y the use of solvents.
T o some of these fractions t h e y gave the n a m e s fulvic acid, hemato-melanic acid, a n d h u m i c acid (see Scheme I ) . Later, workers showed t h a t these fractions w e r e not chemical entities. W a k s m a n (262) a n d his co-workers applied a n e w technique in w h i c h t h e y fractionated organic m a t t e r into k n o w n biochemical constituents. By this m e a n s t h e y w e r e able to account for m o r e t h a n 9 0 % of t h e total m a t t e r in h u m u s .
I n the process of biological decomposition t h e chemical a n d physical n a t u r e of organic m a t t e r is changed, a n d it acquires certain colloidal properties w h i c h p l a y a n i m p o r t a n t p a r t i n p l a n t nutrition. Carbo-h y d r a t e s a r e decomposed w i t Carbo-h t Carbo-h e formation of C 02 a n d H20 a n d t h e synthesis b y the microorganisms of polyuronides, substances w h i c h h a v e a n i m p o r t a n t bearing on soil structure. D u r i n g t h e partial decomposition of lignins t h e cation exchange capacity increases, owing to t h e forma-tion of phenolic a n d carboxylic groups. Organic m a t t e r is dealt w i t h in somewhat m o r e detail b y Quastel (see Chapter 6 ) .
SCHEME I