2. CHARACTERISTICS OF GYPSIFEROUS SOILS

2.1 Introduction
2.2 Physical Properties
2.3 Chemical Properties

2.1 Introduction

Rozanov (1961) and Kurmangaliyev (1966b), who studied the gypsiferous soils of the Soviet Union, state that the physical and physico-chemical properties of gypsiferous soils are closely related to those of non-gypsiferous soils when the soils are developed under similar soil forming factors, and the gypsic layer is deep in the profile. However, where gypsum particles are present in the surface layer, their type, amount, and the degree of crystallization have a profound impact on the physical and physico-chemical properties of the soil as a medium for plant growth. Due to its solubility, gypsum is leached downward by rainfall or irrigation, and is moved upwards by capillary rise when the water-table is high. The gypsum leached from the upper soil layers tends to precipitate in lower horizons in the form of fine gypsum crystals, or as lumps consisting of soil particles cemented by gypsum, or as gypsum rosettes. In extreme cases it can form a hard horizontal crust. Where pedogenic gypsum accumulates in the subsurface horizons to the extent that the S-matrix becomes plugged, the growing gypsum crystals tend to interlock and indurate the horizon (Kulchitskii 1956), causing a serious impediment to root extension. The depth of the gypsum layer in the soil profile usually depends on the amount of water percolating through the soil. Summarizing, the physical properties of gypsiferous soils, as a medium for plant growth, depend on the surface gypsum content, the depth of the gypsic layer and its degree of induration.

2.2 Physical Properties

2.2.1 Particle-size distribution
2.2.2 Structure
2.2.3 Soil-water relationships

2.2.1 Particle-size distribution

Standard laboratory determination of the particle-size distribution (texture) of gypsiferous soils is a tedious and time-consuming process. Gypsum, which inhibits soil dispersion, is usually removed from soil samples before analysis by leaching the soil either with distilled water, or by extraction with a solution of ammonium oxalate (Coutinet 1965), or any concentrated chloride solution in which gypsum is more soluble than pure water. Because gypsum has marked effects on the physical properties of soils it is most desirable to determine the particle-size distribution without removing the gypsum fraction. Independently both Hesse (1974) and Matar and Douleimy (1978) developed tentative methods for the preparation of a stable suspension of gypsiferous soils without removing the gypsum fraction. Vieillefon (1979) gives a substantially improved method based on that of Hesse. Details of these procedures are discussed in Chapter 6.

At present no method for the determination of the particle-size distribution of gypsiferous soils is entirely satsifactory and the development of a reliable method without removing the gypsum fraction is needed.

The estimation of texture under field conditions is misleading because of the presence of gypsum crystals in various sand-sized fractions. The forms and degree of crystallization of gypsum particles influence the feel of the soil and as a result field estimates of texture are generally coarser than indicated by laboratory determinations.

A great range of textures has been observed in gypsiferous soils around the Mediterranean. Clay contents ranging between 2 and 50 percent have been recorded, depending upon the genesis of the soil (Mousli 1980; Van Alphen and de los Rios Romero 1971; Mardoud 1980; Dekkiche 1976; Barzanji 1973; Barzanji et al. 1975).

Experience of analysis of gypsiferous soils suggests the following relationships:

1. The sum of the various size-fractions of gypsiferous horizons may be less than 100 percent due to dehydration of CaSO₄. 2H₂O during oven drying at circa 105°C.

2. The distribution of gypsum within the various size-fractions depends on the total amount of gypsum (Vieillefon 1979). The difference between the weight of such fractions obtained after treatment at 50°C and 105°C shows the following:

i. the fraction less than 20 µm is greatest when the total content of gypsum is about 10 percent and decreases with the gypsum content up to about 25 percent. It is constant and small when gypsum is between 25 and 45 percent and increases again when gypsum is more than 45 percent

ii. the amount of the fractions greater than 20 µm increases as the total amount of gypsum increases. For most samples with a gypsum content between 10 and 25 percent, the relationship is not significant.

3. Gypsum is found in all size fractions, but it is mostly linked with the coarse and fine sand fractions (0.05 - 2 mm) followed by the silt fraction.

2.2.2 Structure

Soil particles in gypsiferous soils are weakly aggregated as the cohesive forces attracting single soil particles are very weak. Gypsum particles have no cation exchange capacity. Erosion of gypsiferous soils can be very serious because of poor aggregation. Boyadgiev (1974) noticed that a gypsum content over 15 percent tends to give an unstable structure. There is however a tendency to increase in stability as the gypsum content exceeds 25 percent. He concludes from the results obtained in the Euphrates basin of Syria, that soils containing 10 to 35 percent gypsum are permeable, have poor structural stability and small water retention capacity, and are considered as dangerous for the construction of irrigation canals. He also notes that caverns, which will cause the collapse of irrigation canals, can be formed through the removal of fine soil particles by mechanical means, when gypsum is in a crystallized form and its content ranges from 10 to 35 percent.

Smith and Robertson (1962), working in Iraq, found that 3 to 10 percent of gypsum does not interfere significantly with soil characteristics such as structure, consistency and water holding capacity; while in soils containing 10 to 25 percent of gypsum, the gypsum crystals tend to break the continuity of the soil mass. This is also the case for the gypsiferous soils of the USSR. They observed that soils with more than 25 percent of gypsum do not provide a good medium for plant growth and the soil material lacks plasticity, cohesion and aggregation and becomes completely unstable in water.

The different observations by the various authors on the structural properties of gypsiferous soils could be due to the variation in the degree of hardness and crystallization of gypsum particles in the soils. Castroviejo and Porta (1975) found in the Giguela area that the A horizon of typic Gypsiorthids is brown in colour with good structure and organic matter content. The gypsic horizon however is structureless and poor in organic matter. The Giguela area is more humid and has a higher precipitation than the Euphrates basin and other gypsum-affected areas in Tunisia, Algeria, Saudi Arabia, Iran and elsewhere.

Most gypsiferous soils are poorly aggregated and consequently the structure of the surface layer is dominantly massive or fine platy. The gypsic layer can be strongly aggregated with the formation of hard crusts sometimes impeding the downward movement of water and the extension of roots. In field and laboratory studies, Abrukova and Isayev (1983) established qualitative and quantitative differences in the nature of structural bonds and also in the deformational behaviour of grey-brown gypsiferous soils studied as a function of their gypsum content.

The tolerance of various crops and land suitability for crop production depend on the depth and structure of the gypsic layer as well as the gypsum content of soils. Fruit and forest trees are most affected by the type of gypsic layer and its hardness.

2.2.3 Soil-water relationships

Soil moisture retention

The energy of retention of soil moisture in gypsiferous soils is considered by many workers to be similar to that in non-gypsiferous soils and the available moisture to plants is considered to be that held between 0.3 and 15 bar. Van Alphen and de los Rios Romero (1971) discuss some of the published data on available moisture to plant growth. Minashina (1956) found that available moisture in the non-gypsic surface layer or the gypsic subsoil layer with 80 percent gypsum was in the range of 11 to 22 percent by volume. Similarly, Amami et al. (1967) found in the oasis of Gabès in Tunisia, where soils are coarse-textured with 10 percent of clay and about 20 percent of gypsum, that available moisture retained by these soils ranged from 10 to 12 percent. Van Alphen and de los Rios Romero (1971) mentioned that in the fine-textured soils of the Ebro Valley of Spain, with 40 percent clay and 1 to 9 percent gypsum, the available moisture was higher, ranging from 23 to 38 percent in the non-gypsic surface layers. These figures of available moisture are considered high even for non-gypsiferous soils.

Mousli (1980) mentioned that available moisture in the Balikh and Maskaneh basins of Syria ranged between 9 to 15 percent, depending upon the soil texture and gypsum content of soils. However, when gypsum occurs as crystals in the size of coarse to very coarse sand, available moisture becomes lower and could be as low as 5 percent by volume.

Recently, Heinze and Fielder (1984) found in a study on nine different gypsiferous soil profiles of various textures and degrees of maturity that gypsiferous soils have a large pore volume, especially the medium-size pores, and a water content comparable to calcareous or silicate soils of comparable texture and degree of maturity.

The results of soil moisture retention curves of certain gypsiferous soils conducted by Van Alphen and de los Rios Romero (1971) in the Euphrates Basin, found that an important part of soil water is retained in the low moisture tension part, between pF 1.5 and 2.7 (0.03 and 0.5 bar); and assumed that available moisture ranged between pF 2.0 and pF 4.2 (0.1 and 15 bar). The available moisture ranged between 13 and 22 percent by volume for the non-gypsic layer and 15 to 31 percent in the gypsic subsoil layer.

In a comparative study on the moisture characteristics of soils in the Hodna region (Algeria), Balikh basin (Syria), and Karak area (Jordan), Boyadgiev (1974) noted the following:

i. the retained and available water content in the soils is minimal when the gypsum content is about 25 percent

ii. when the gypsum content is between 25 and 35 percent, the retained water content depends upon the forms of gypsum and the soil texture

iii. an increase in retained water was observed when the gypsum content increased from 35 percent and when gypsum content fell below 10 percent

iv. the marked irregularity of the distribution of water content is typical of gypsiferous soils. In the soils with the highest amounts of gypsum (more than 35-45 percent) the soil moisture content is uniform only in the first 10-15 cm from the surface. Below this depth the Water is localized in channels 10 to 20 cm in size and all other parts are dry. This irregularity of distribution of water is due to crevasses which are characteristic of hypergypsic horizons and which conduct the water through the soil profile (Figure 2.1).

Regarding the availability of soil moisture to plants, it was found by Hernando et al. (1963) that soil water in the range of pF 2.3 to 4.2 (0.2 to 15 bar) is not equally available to plants grown in gypsiferous soils. Yields of corn grown in soils containing 8 percent gypsum were greatest when the soil moisture content was 50 percent of field capacity. Consequently, it was found that the addition of gypsum to the soil decreases the yield but less so when the soil moisture was at a high or low moisture tension. Plant species were found to respond very differently to the soil moisture level at which irrigation application will give the maximum yields (Doorenbos and Pruitt 1976).

The interaction between the soil moisture energy and the effect of gypsum on plant growth is an important aspect of the management of gypsiferous soils which is relevant to the determination of optimum irrigation scheduling.

Water movement in soils

The internal water movement in most gypsiferous soils is normally moderate to rapid, except where gypsum incrusted layers impede the downward movement of water. Mousli (1980) states that hydraulic conductivity of gypsiferous soils in the Meskaneh Plain, Syria, ranges between 0.12 and 1.3 metres per day (m/d); and 1.6 to 8.2 m/d for the surface layers in the Euphrates Basin. Water permeability in the Granada experimental farm, Syria, ranged between 0.15 and 0.42 m/d for medium gypsiferous soils and 0.23 and 0.47 m/d for shallow gypsiferous soils (Mardoud 1980). Van Alphen and de los Rios Romero (1971) found that the hydraulic conductivity of the Kirovabad Massif (USSR) varies between 0.2 and 1.0 m/d for the gypsic subsoil and between 1.2 and 1.9 m/d for the non-gypsic surface layer. Amami et al. (1967) found that the hydraulic conductivity in the gypsum-bearing surface soil in the oasis of Gabès ranged between 2.0 to 2.5 m/d; and in the gypsum incrusted subsoil layer between 1.6 and 2.5 m/d. Porta Casanellas (1975) found that the permeability of many gypsiferous soils in the Margenes del Rio Giguella of Spain ranged between 0.8 and 1.0 m/d.

Figure 2.1 Diagram showing soil moisture content of gypsiferous soils

The water movement in gypsiferous soils is sometimes excessive, as can be seen from the above examples, so waterlogging is not a significant problem except where incrusted gypsic layers are found in the soil profiles. When gypsiferous soils are irrigated, the gypsum is leached and translocated and in some cases a gypsum layer is formed which reduces the hydraulic conductivity. Keren et al. (1980) suggest that the reduction in the hydraulic conductivity is due to the plugging of soil pores by the precipitation of the leached gypsum.

2.3 Chemical Properties

2.3.1 Solubility relationships
2.3.2 Gypsum and calcium carbonate interaction
2.3.3 Cation exchange properties and exchangeable cations

2.3.1 Solubility relationships

The solubility of gypsum depends on particle size (Figure 2.2), and several other factors.

Figure 2.2 The effect of particle size on the solubility of gypsum in water

In strictly thermodynamic terms the solubility of a sparingly soluble binary weak electrolyte such gypsum, is given by:

K = fA⁺[A⁺] × fB^-[B^-]

where K is a constant, the activity product; fA⁺ and fB^- are the activity coefficients of the cation and anion respectively, and [A⁺] and [B⁺] are the cation and anion concentrations. For very dilute solutions, such as we are considering, the relationship:

f¥(1 - kÖc)

also holds, where c is concentration and k is a constant. Thus when c is very small f is approximately unity and:

K » [A⁺][B^-]

K is then still the activity product. Since, in dilute solutions of weak electrolytes, the measured value for the molar concentration of a simple dissociated ion is close to its activity, we can write for gypsum:

K_sp = [Ca²⁺][SO₄^2-]

where K_sp is the solubility product. Thus, the amount of gypsum that will dissolve is controlled by the molar concentrations of calcium and sulphate ions in solution. For a pure system containing only water and solid gypsum in excess, Glas et al. (1979) found that the calcium concentration varied only slightly between 14.9 and 15.8 millimoles dm^-3. The results of Bennett and Adams (1972) show that activity coefficients must be about 0.5.

However, soil solutions are rarely simple and other factors need to be considered. One such, is that other more soluble minerals may supply calcium or sulphate ions to the soil solution. These common ions will help satisfy the solubility product relationship and hence depress the solubility of the solid gypsum phase.

Equally important is the concept of ionic strength (S) which is defined as:

where z_i is the valency of the ion and m_i is its molar concentration. Ionic strength becomes important because in dilute solutions an increase in ( ) will cause a decrease in the activity coefficient of the sparingly soluble ions according to the relationship:

log f_i = -0.505z_i². S⁰⁵

where f_i is the activity coefficient.

Thus in a solution of increased ionic strength, where there are no ions common with the sparingly soluble phase, the concentration of the ions of the latter will increase to compensate for the decrease in f_i while K_sp remains constant. This means that, in practice, the amount of gypsum brought into solution will increase. These principles are demonstrated by the results of Bennett and Adams (1972) shown in Table 2.1.

Table 2.1 CALCIUM AND SULPHATE CONCENTRATION AND IONIC STRENGTH OF AQUEOUS SOLUTIONS SATURATED WITH CaSO₄. 2H₂O AT 25°C, WITHOUT AND WITH COMMON IONS

(Bennett and Adams 1972)

Solutions without common ions -			Solutions with common ions
Medium	Ca2+ = SO42	Ionic strength	Medium	Concentration	Ca2+	SO42-	Ionic strength
	mol m-3	M		mol m-3	mol m-3	mol m-3	M
KCl	18.3	0.113	K2SO4	14	12.1	25.9	0.090
KCl	26.2	0.345	K2SO4	83	10.1	93.2	0.290
NH4CI	18.5	0.115	Ca(NO3)2	13	23.3	10.5	0.081
KH4CI	26.6	0.315	Ca(NO3)2	79	90.6	9.0	0.279
KNO3	23.8	0.228	MgSO4	9.7	13.2	22.9	0.092
KNO3	27.0	0.373	MgSO4	58	11, 3	69.6	0.278
NH4NO3	17.4	0.108	nitrates1	27	20.7	16.8	0.104
NH4NO3	29.9	0.354	nitrates	163	40.4	21.1	0.324
KH2PO4	19.4	0.117	chlorides1	26	19.6	17.2	0.107
KH2PO4	33.7	0.372	chlorides	158	38.8	20.7	0.313
MgCl2	18.2	0.110	sulphates2	12	12.8	25.1	0.096
MgCl2	28.0	0.332	sulphates	74	10.8	84.0	0.284

¹ Nitrate and chloride solutions contained as cations: Ca, Mg, K and NH₄
² Sulphate solution contained as cations: Mg, K and NH₄

Thus the solubility of gypsum, its dissolution and transport in the soil profile, and to some extent its effect on the ionic nutrient composition of the soil solution, could be explained by its solubility-product relationships and ion-pair formation which in turn are dependent on the concentration and types of salts present in the soil.

2.3.2 Gypsum and calcium carbonate interaction

Most gypsiferous soils contain calcium carbonate in various amounts and forms. Gypsum and calcium carbonate in soils are not independent of each other except in soils with little profile development, such as Sierozems. It is also commonly observed that when the calcium carbonate content decreases, the gypsum content increases. Boyadgiev (1974) tried to explain this phenomenon on the basis of forms of calcium carbonate and gypsum and of the presence of soluble salts in the soils. He found the following in the Euphrates basin of Syria:

i. when the form of calcium carbonate and gypsum is soft and powdery, the relationship between the gypsum and calcium carbonate content was significant and expressed by the following equation:

Percentage gypsum = 100 - 1.281 CaCO₃ percent

ii. when the gypsum is of sand grade and the calcium carbonate is present as nodules and crusts or when soluble salts are present, the relationship between gypsum and calcium carbonate contents becomes insignificant. In Tunisia, Vieillefon (1976) found a negative relationship between gypsum and calcium carbonate contents for soils with surface gypsiferous layers or gypsiferous incrustations (Fig .2.3).

The following paragraphs discuss the chemistry of the CaCO₃. CaSO₄. 2H₂O system. Firstly, the solubility equilibria of pure calcium carbonate CaCO_3(s) in soils is dealt with. Secondly, the solubility equilibria of the CaCO₃. CaSO₄. 2H₂O present in soils is discussed. This helps to clarify the interaction between gypsum and calcium carbonate solid phases and their effect on the ionic composition of the soil solution, and consequently on the way their chemical properties affect the soil as a whole.

The CaCO_3(s). CO_2(g). H₂O₍₁₎ system

Let us consider the system of pure water originally free of carbonate in solution and free of the gaseous phase but saturated with CaCO₃ (solid phase). Under these conditions, the composition of the solution is governed by the solubility product of calcite as follows:

CaCO3(s) Ca2+ + CO32-	(1)
where log Ksp (the solubility product) is equal to -8.35
Because of the protonization of the CO32- the following reactions take place:
CO32- + H+ HCO3-	(2)
with log K° (the thermodynamic equilibrium constant) equal to 10.33 and
HCO3- + H+ H2CO3	(3)
with log K° equal -6.35

If the system CaCO_3(s). H₂O is exposed to the air, which contains CO₂ then another source of carbonate is introduced into the system, which is the result of the solubility of CO_2(g) in water which is governed by Henry's Law for gases at low pressure (Barzanji et al. 1975). The total concentration of dissolved CO₂ as H₂CO₃ could be found from the following equations:

CO2 + H2O H2CO3	(4)
with log K° equal to -1.46

As a result, a general expression describing the equilibrium composition of the solution with calcite, and in the presence of CO₂ of the air could be derived by adding equation (1) to (4) to get the general form (5)

CaCO3(s) + 2H+ Ca2+ + CO2(g)+ H2O	(5)
where log K° is equal to 9.79.

And by taking the logarithms of equation (5) and the arrangements of terms, that gives:

log Ca2+ = 9.79 - 2pH - log PCO2

(6)

where P_CO2 is the partial pressure of CO₂ in millibars and Ca²⁺ denotes the ion activity in solution at equilibrium.

The Ca²⁺ activity in solution could be determined by multiplying the measured concentration of Ca²⁺ in solution by the activity coefficient calculated either from the Debye-Hückel theory or the Davies equation (Bolt and Bruggenwert 1976).

From equations (5) or (6) it appears that the solubility of CaCO₃ decreases at higher pH values and high CO₂ partial pressure.

Under field conditions, the CO₂ levels in the soil air is far above the 0.03 percent by volume of CO₂ in the atmospheric air, due to biological activity of roots and micro-organisms.

A higher level of CO₂ in the soil air would lead, on the one hand, to a drop in soil pH and a consequent increase in the solubility of CaCO₃ but, on the other hand, this is counteracted by the higher partial pressure of CO₂. As a result, the final solubility of CaCO₃ will then be determined by the equilibrium conditions of equation (6).

The CaCO_3(s). CaSO₄. 2H₂O. CO₂ system:

In a system where both CaCO₃ and gypsum co-exist with the Ca²⁺ ion in common, the solubility of both minerals and the ionic composition of the equilibrium solution will be governed by the simultaneous solubility products of both reactions:

CaCO3 + 2H+ Ca2+(s) + H2O + CO2(g)	(7)
with log K° = 9.79
CaSO4. 2H2O Ca2+ + SO42-+ 2H2O	(8)
with log K° = 4.64.

Following the same thermodynamic derivations, it will be found as shown in Figure 2.4 that:

1. The pH of calcareous-gypsiferous soils in equilibrium with CO₂ is determined by the simultaneous solubility of CaCO₃ and gypsum.

2. The Ca²⁺ activity is much higher and the soil pH lower in the CaCO₃-gypsum-CO₂ system than in the CaCO₃-CO₂ system.

3. The effect of CO₂ partial pressure on the Ca²⁺ activity is much less in the CaCO₃-gypsum-CO₂ system as compared to its effect on the CaCO₃-CO₂ system.

4. The solubility of CaCO₃ should be much lower in the presence of gypsum due to the common ion effect.

Figure 2.4 Calcium, carbonate, hydrogen and sulphate ions in equilibrium with calcite with or without gypsum as a function of CO₂ level, in an aqueous system (Harmsen 1984)

From the soil genesis point of view, where soils contain a mixture of calcium carbonate and gypsum, several groups of soils could form under arid or semi-arid conditions. Typically, CaCO₃ accumulates in the middle part of the profile in the form of calcareous concretions or nodules, with a gypsic horizon at depth. Soil profiles of this type are classified according to Soil Taxonomy as Calcic Gypsiorthids. This horizon sequence could be explained by the fact that gypsum, which is more soluble than calcite, is leached first to form a gypsic layer, to be followed above by lime nodules or a calcareous crust.

The interaction between gypsum and calcium carbonate thus has importance in soil genesis and also influences the soil medium for plant growth.

2.3.3 Cation exchange properties and exchangeable cations

Gypsum particles have no negative charge and consequently it is expected that the total exchange capacity of gypsiferous soils decreases as the gypsum content of the soil increases. An example of the gypsiferous soils of Balikh basin of Syria is given in Table 2.2.

Table 2.2 RELATIONSHIP BETWEEN CATION EXCHANGE CAPACITY (CEC) OF SOILS AND GYPSUM CONTENT IN THE BALIKH VALLEY OF SYRIA

% gypsum	0	10	25	50	75
CEC (mEq/100 g)	24	21	18	11	5

The cation exchange capacity of soils depends, in general, on the organic matter content, soil texture and type of clay minerals. Most gypsiferous soils are poor in organic matter and consequently the contribution of organic colloids to cation exchange capacity is a small part of the total.

The percentage clay in gypsiferous soils depends on the kind of soil developed. In the Typic Gypsiorthids of Iraq (Barzanji 1974), Algeria (Dekkiche 1976), Syria (Ilaiwi 1983) and elsewhere, the clay content rarely exceeds 20 percent by weight and the cation exchange capacity ranges between 7 and 14 mEq/100 g of soil. However, in Calcic Gypsiorthids with a calcic horizon overlying the gypsic layer and smaller amounts of gypsum, the clay content ranges between 20 and 50 percent by weight (Barzanji 1974) and cation exchange capacity ranges between 14 and 18 mEq/100 g. Porta Casanellas (1975) found, however, that the organic matter content of the A horizon in many of the gypsiferous soils of Spain ranged between 0.5 and 3.5 percent, with a cation exchange capacity as high as 50 mEq/100 g (see Appendices).

The types of clay mineral in a soil influence its physico-chemical properties, including the cation exchange capacity, structure, shrinkage and swelling, water-holding capacity, stickiness, and tillage properties.

In gypsiferous soils, many workers (Dekkiche 1976; Barzanji et al. 1975; Altaie 1968; Yahia 1971; Shadfan et al. 1984; Lee et al. 1983), have found that attapulgite is the main mineral in the fine fractions except in soils formed in recent deposits. Attapulgite is found mainly in gypsic and petrogypsic horizons. In addition to attapulgite (palygorskite) other minerals are found, such as chlorite, montmorillonite, illite, vermiculite, sepiolite and kaolinite. Attapulgite represents 40 to 80 percent of the clay minerals of the gypsic horizons in various areas of Iraqi soils and Barzanji et al. (1975) found a highly significant linear relationship between gypsum contents of soils (X) and the attapulgite content of the clay fraction (Y)

Y = 0.89X+ 15.85

with r = 0.963.

Barzanji et al. (1975) found some evidence, using a scanning electron microscope for the neoformation of attapulgite and its preferential occurrence on gypsum grains. He considers that attapulgite is formed later than the gypsum. In a later study in Iraq, he also found that with increasing rainfall attapulgite decreases and the montmorillonite content of soils increases.

However, Boyadgiev (1974) mentions that, in Calcic Gypsiorthids of Jordan, no attapulgite is found in the gypsic horizons, but a limited amount is found in the calcic horizon of the soil. Lee et al. (1983) from a study of gypsiferous soils of the eastern region of Saudi Arabia, concludes that palygorskite originates from the underlying Mio-Pliocene limestone in the basin and was distributed by alluvial and eolian processes. Aba-Hysayn and Sayegh (1977) report that attapulgite and illite were found as the most abundant and common clay minerals in the soil and strata of Al-Hasa soils of Saudi Arabia, and the origin of these minerals and their relative abundance is believed to be associated with the source of the sediment since minimum alteration seems to have taken place after deposition. From these and other observations, the relative roles of gypsiferous or calcareous environments in attapulgite formation cannot be separately estimated.

The dominant exchangeable cation in gypsiferous soils is calcium, followed by magnesium, potassium and sometimes sodium. The impact of cation exchange capacity and exchangeable cations on soil nutrient availability to plants will be discussed in the following chapter.