1. GYPSIFEROUS SOILS IN THE WORLD

1.1 Introduction
1.2 Origin of Gypsum in Soils
1.3 Forms of Gypsum in Soils
1.4 Classification of Gypsiferous Soils
1.5 Distribution of Gypsiferous Soils in the World

1.1 Introduction

Gypsiferous soils are soils that contain sufficient quantities of gypsum (calcium sulphate) to interfere with plant growth. Soils with gypsum of pedogenic origin are found in regions with ustic, xeric and aridic moisture regimes (Nettleton et al. 1982). They are well represented in dry areas where sources for the calcium sulphate exist. They do not usually occur under wet climates. In most cases the gypsum is associated with other salts of calcium and salts of sodium and magnesium.

1.2 Origin of Gypsum in Soils

Gypsum formation has been described by various authors (Moret 1979). Kovda (1954) states that gypsum accumulation occurs in two ways: by the evaporation of mineralized groundwater and by the precipitation within the groundwater itself. Gomez-Miguel et al. (1984) describe an area of Spain where generation and accumulation of gypsum takes place by evaporation of a fluctuating water-table.

The origin of sulphate ions in the soil solution is in some circumstances due to the presence of sulphur-rich minerals such as pyrite in the parent material. By weathering and oxidation, the sulphur in these minerals is transformed into sulphuric acid which in calcareous soils reacts with CaCO₃ to form gypsum.

On irrigated land, leaching of saline soils containing sulphate and calcium in the soil solution leads in some circumstances to the precipitation and accumulation of gypsum in the subsurface horizon. The formation of gypsum may result from replacement of NaCl by CaSO₄ when the irrigation water contains a substantial amount of calcium and sulphate. But it could be also a result of a partial leaching of salts from the soil because NaCl is much more soluble than CaSO₄. It has been observed in the Euphrates Basin, that gypsum is recrystallized and redistributed in the soil profile after leaching of other, more soluble, salts.

Buringh (1960) states that gypsiferous soils in Iraq are mainly associated with a geological substratum containing gypsum and anhydrite interlayers or with Pleistocene terraces connected with such deposits. Barzanji (1973) points out that the basin of Iraq was partly filled with detrital sediments during the Lower Miocene. Inland seas were formed at this time in which, on evaporation, gypsum, and limestone were formed characterizing the Lower Fars Series of the Middle Miocene. He concludes that this geological formation is the origin of most gypsiferous soils in Iraq. Segalen and Brion (1981) state that gypsum originates from the material in the catchment areas that has been eroded and re-deposited.

Van Alphen and de los Rios Romero (1971) mention that solid or interbedded gypsum deposits are found in the Ebro Valley of Spain. These date from the Upper Miocene and Oligocene-Eocene transition periods. Other gypsum deposits are found in the Tajo Valley, south-west of Madrid which are probably of the lower Miocene age.

Mulders (1969) states that in the Balikh Basin, Syria, a large part of the region is covered by a sandy gypsum deposit with a thickness of 3-8 m, but his hypothesis that such deposits are the products of the weathering of lagoonal gypsum in situ is very debatable.

Most gypsiferous soils are located in arid areas, some in semi-arid ones. The presence of gypsum reflects the characteristic of wetness that was present in geological time, or it is actually related to the presence of a brackish water-table. River beds and lagoons are ideal locations for the formation of secondary gypsum deposits.

Gypsum can be deposited in other ways. It can be transported by water or wind and re-deposited in new localities forming individual gypsum dunes or becoming incorporated with the soil. Eolian deposits of gypsum are well known in the south-western United States, North Africa (Vieillefon 1976), Asia and south-east Australia. The neoformations of secondary gypsum in the soil are concentrated first in the cracks, voids and channels. Later gypsum becomes diffused throughout the soil matrix.

Concerning the origin and genesis of soils in which gypsum has accumulated, it can be concluded from Boyadgiev (1974) and other studies conducted on gypsiferous soils (Abdulgawad et al. 1977, Adams 1971, Alimaganbetov and Akhanov 1970, Basinski 1967, Carenas and Marfil 1979, Castroviejo and Porta 1975, Denagyer-De Smet 1966, Sekhara Reddy et al. 1976) that:

1. Gypsiferous soils usually develop in arid and semi-arid areas with less than 400 mm annual rainfall where abundant sources of gypsum exist.

2. Gypsum accumulation in soils is related to Quaternary deposits (glacis and terraces) of all ages. However, in deposits of Moulouyen (Villefranchian) and Pliocene age the gypsum accumulation occurs in the form of a very hard and thick (>1 m) crust, of yellowish colour.

3. Gypsiferous soils are widespread in areas affected by tectonic movement, exactly where the synclinal and anticlinal axes, faults and underground ridges have a perpendicular orientation to the natural movement of water.

4. Maximum accumulation of gypsum is observed on the fringe of terraces, detrital cones and slope deposits where bordered by hills.

5. A water-table at less than 5 m depth with mineralized waters, whose saline nature is either sulphato-chloride or chlorido-sulphate leads to gypsum accumulation in the soils.

6. Accumulation of gypsum and soluble salts may take place over a very short period (Bower et al, 1952). Gypsum and other soluble salts are precipitated simultaneously provided that they are in solution, that movement takes place over a short distance and that evaporation is rapid.

7. The form of the gypsum depends mainly upon the thermodynamic conditions of its precipitation and is not related to the age of the deposits. It is sometimes difficult to distinguish recent gypsum from old or residual forms.

8. The amount of sulphate and chloride in the underground seepage and in plants, is higher than sodium. No sodium bicarbonate occurs and consequently soil sodification is reduced. However, the proportion of sodium in the adsorption complex is increased as a result of exchange processes:

Ca²⁺ + Na₂SO₄ = 2Na⁺ + CaSO₄

9. In soils with a recent accumulation of gypsum, the salt-affected horizon overlies the gypsic horizon. In the case of old or residual gypsum, the accumulation of soluble salts occurs either in the gypsic horizon or at lower depths.

The mixed accumulation of salts in soils of arid and semi-arid regions can be the result of various processes, superimposed on each other during successive climatic periods.

Mixed accumulation arises in areas where the normal geochemical distribution of salts of variable solubility is disturbed by the geological, geomorphological or climatic factors.

The following relationships have been observed by the authors:

Gypsum is precipitated and accumulates when the electric conductivity is less than 60 mS cm^-1. The possibilities of concentrated solutions and the formation of deposits of gypsum will increase gradually with the drop in conductivity. The ionic product (Ca²⁺)(SO₄^2-) of the solution will not be attained if the electric conductivity is higher than 60 mS cm^-1.

As the gypsum content increases, the calcium carbonate content tends to decrease and vice-versa. However, the forms of CaCO₃ and gypsum, as well as the presence of soluble salts, influence this relation. These effects appear m the following ways:

i. when CaCO₃ and gypsum are in a powdery soft form, the correlation is high;

ii. when the CaCO₃ is in the form of nodules or crusty formations, and gypsum is found in the form of sand or individual crystals, the relationship is less marked;

iii. in the presence of soluble salts (EC > 16 mS cm^-1) the calcium carbonate content is small and it is not correlated with the gypsum content.

1.3 Forms of Gypsum in Soils

Gypsum is a soluble salt, hydrous calcium sulphate CaSO₄. 2H₂O. Its solubility is 2.6 g dm^-3 of pure water at 25°C and a pressure 1 atmosphere. Segalen and Brion (1981) note that the solubility of gypsum is influenced by the presence of other salts in the solution. The maximum solubility occurs at 35-50°C. The solubility is always higher than 2.2 g dm^-3. Salts that have common ions, calcium bicarbonate and sodium sulphate, decrease the solubility of gypsum while others increase it. More details are given in Chapter 2.

In arid and semi-arid areas gypsum is dissolved in the wet season and tends to be precipitated when the soil is dry.

Whatever the source of calcium and sulphate ions, the precipitation of gypsum in soils is a function of the nature and the composition of the soil solution, as well as the effects of alternating wet and dry seasons.

Gypsum can be transformed into anhydrite upon heating. Dehydration starts at 40°C and reaches a level corresponding to the semi-hydrate (bassanite) at 70-90°C. Above 100°C or even 200°C, the salt still contains about 0.01 mole of water per mole.

Gypsum is formed in two ways: by crystallization from an aqueous solution, or by the hydration of anhydrite. It crystallizes as monoclinic prisms. The mineral occurs in lamellar, prismatic, fibrous or massive forms. Alabaster is a fine grained, massive transparent variety of gypsum. The twinned formations have different shapes for example in the form of an arrowhead with platy cleavage faces. Selenite is a variety that has three directions of cleavage, one of which gives perfect mica-like plates. In other forms, the crystals are grouped together forming a flower-like shape called a desert rose. Gypsum has hardness 2 on the Moh's scale. Some of the important forms of gypsum in Spain have been described by Carenas and Marfil (1979).

Gypsum of pedogenic origin varies in shape, size, form and distribution. Several names are given to the different kinds.

Bureau and Roederer (1960), described the form of gypsum in Tunisian soils as follows:

Amas (accumulations):	local precipitations of fine crystalline gypsum grains
Pulverulant (powder):	a secondary deposit of very fine powdery non-coherent gypsum
Encroûtement (crusty):	a gypsum layer that is not hard and massive
Croûte (crust):	a relatively hard and massive gypsum layer
Polygonal:	is attributed to a form of crust that is exposed to the surface and the weather. Drying by evaporation leads to the formation of vertical intersecting cracks in the material giving a polygonal structure. The alternate dissolution and recrystallization of gypsum at the surface crust, gives a harder thin layer covering this crust and having its external shape.

Some authors have simplified this series of forms but others have recognized more varieties. Gibb et al. (1967) observed twelve forms of gypsum in the Balikh Basin in Syria. Houérou (1960) described the hydromorphic encrustations of intumescence layers in Tunisia, as well as the "pulverulent gypsum agglomerated into a rather hard sandstone, vibrating, and having a cavernous structure". Pouget (1968) considers that gypsum in the soil evolves in the following sequence: very fine crystals-spots-encrustations, and then crusts.

As seen under the electron microscope (Beutelspacher and Van der Marel 1966) gypsum crystals appear either as elongated spots ("Parallelepipedic crystals"), dark with clear, irregular edges (gypsum crystals of the Dead Sea), or dark spots in the shape of an hourglass with irregular boundaries (altered gypsiferous rock in Italy).

Taking into account their mode of formation, the gypsum forms observed in the field by Boyadgiev (1974, 1975) may be grouped as follows:

1. Mycelium, spots (amas), elongated accumulations of 1-2 mm in diameter, pellicular layers, moss-like and covered with white powder on the surface. These are neoformations of recent origin found in horizons of fine and medium texture resulting from rising water.

2. Separate crystals or crystals in small agglomerations: these are found in horizons of coarse texture immediately above the water-table or in the pores of fine-textured horizons; these too are neoformations.

3. Powdery compact gypsum (microcrystallized): this is found in horizons of medium to fine texture towards the upper limit of the capillary fringe if its origin is connected with the phreatic layer. It can be a recent or ancient neoformation. When microcrystallized gypsum rises to the surface of the soil or the borders of crevices, it gives birth to hard polygons (crust and encrustation).

4. Spongy gypsum (agglomerated crystals): these crystals are seen in well-drained horizons of coarse texture. These are residual neoformations. If the adjacent horizons are very rich in lime, the gypsum is powdery and fluffy with crystals or encrustation around the gravel.

5. Gypsiferous sand of which two kinds are known: (a) an aeolian accumulation at the surface of the soil which forms dunes of stabilized sand of various sizes; (b) a secondary gypsum accumulation through the profile of the soil: grains of sand are finer in the upper part and coarser underneath. These are recent or ancient neoformations.

6. Roses of the desert: small-winged (semi-arid climate) and large-winged (arid-climate).

7. Hard gypseous rock, well crystallized, of geological origin.

i. dark dots or speckles having a dimension of 0.01 to 0.04 µm in diameter

ii. spots or sand-glasses made up of dark dots scattered in the sand-glass and bordered by a kind of elongated crystal. The spots are about 2 µm long and 1.6 µm wide

iii. nodules with irregular, rounded outlines of 0.15 to 5 µm

iv. parallelepipedic crystals with clear outlines and well-marked 40° angles, generally without macles. The crystals are 1.5 µm long and 0.7 µm wide

v. canons having longer crystals, often with macles. The thinner crystals are 0.07 µm wide and 0.35 to 2 µm long, whereas the thicker ones are 0.7 µm wide and 2.5 µm long

vi. sponges constituted from masses of nodules 0.25 to 1.8 µm in diameter and attapulgite needles 0.02 to 2.0 µm long.

1.4 Classification of Gypsiferous Soils

1.4.1 The American classification
1.4.2 The FAO-Unesco legend
1.4.3 The French classification
1.4.4 The Russian classification
1.4.5 International Reference Base for soil classification
1.4.6 Other classifications

Gypsiferous soils were first recognized and introduced to soil science by W. Knop in 1871 (quoted by Dokuchaev vol. 4, 1896) under the name sulphate soils. In his classification, gypsum content was used to subdivide the soils at a low level. In recent decades interest in gypsiferous soils has increased so they are now recognized in almost all soil classifications at a high level. The place of gypsiferous soils in selected soil classifications is outlined below.

1.4.1 The American classification

In the first American system of soil classification gypsiferous soils are not separated from other soils. The soils of the dry areas are classified as Red Desert Soils equivalent to Argids, Calciorthids and Camborthids of the modern American system. The first system was elaborated from the classification of Marbut (1967) after some redefinition. It was revised several times subsequently.

Work on the new American soil classification system started in 1951. It went through a series of approximations and was published in 1975 under the title "Soil Taxonomy, A Basic System of Soil Classification for Making and Interpreting Soil Survey" (Soil Survey Staff 1975). This system has been further elaborated by the Staff of the Soil Conservation Service, US Department of Agriculture in collaboration with many pedologists from other countries.

To identify gypsiferous soils, Soil Taxonomy requires the presence of one of two different diagnostic horizons with high gypsum content: a gypsic horizon or a petrogypsic horizon.

A gypsic horizon is a subsurface diagnostic horizon that is enriched with secondary calcium sulphate. It is not cemented or is weakly cemented and occurs within 1 m of the surface. Its thickness is 15 cm or more, has at least 5 percent more gypsum than the C horizon or the underlying stratum, and in which the product of the thickness of the horizon in centimetres and the percentage of gypsum is equal to or more than 150.

A subsurface horizon that is 15 cm thick and has 10 percent of gypsum is considered as a gypsic horizon if the gypsum content of the underlying horizon is not more than 5 percent. A horizon 30 cm thick with 6 percent gypsum also qualifies as a gypsic horizon if the gypsum content of the underlying horizon is not more than 1 percent.

A petrogypsic horizon is a gypsic horizon strongly cemented with gypsum and is hard and massive. Dry fragments do not slake in water and roots cannot enter. The gypsum content is relatively high, usually exceeding 60 percent.

Gypsiferous soils are classified using these horizons within the order of Aridisols, suborder Orthids, at the great group level as Gypsiorthids. The presence of gypsum is not recognized in other categories of soils. Gypsiorthids are Aridisols that do not have an argillic or natric horizon. They include those Orthids that have a gypsic or a petrogypsic horizon whose upper boundary is within 1 m of the soil surface.

The Gypsiorthids are divided into the following sub-groups:

Typic Gypsiorthids

These have a gypsic horizon in which the product of the percentage of gypsum and the thickness in centimetres above a depth of 1.5 m is 3000 or more and do have a petrogypsic horizon within 1 m of the surface.

These have a calcic horizon above the gypsic horizon and do not have a gypsic horizon in which the product is 3000 or more.

These do not have a gypsic horizon in which the product of the percentage of gypsum and the thickness in centimetres above a depth of 1.5 m is 3000 or more, and do not have a calcic horizon.

These have a petrogypsic horizon within 1 m of the soil surface.

The definitions of Gypsiorthids need to be clarified to meet the following questions:

1. Are soils which have a calcic horizon above a gypsic horizon in which the product is 3000 or more Typic or Calcic Gypsiorthids?

2. Are soils otherwise like Cambic Gypsiorthids which have a low value of product (percent × thickness) Cambic Gypsiorthids or not?

3. How are Gypsiorthids classified which have a high salinity or stratified gypsic, salic and calcic horizons in various sequences?

4. How are soils with discontinuous gypsum accumulation in the form of pockets classified?

Figure 1.1 Distribution of gypsum in Typic and Calcic Gypsiorthids

1.4.2 The FAO-Unesco legend

The Food and Agriculture Organization of the United Nations (FAO) with the cooperation of the United Nations Education, Scientific and Cultural Organization (UNESCO) published a Soil Map of the World at a scale 1: 5 000 000 in 1975 (FAO 1975). The soils are classified into 26 units and 103 sub-units. The legend recognizes three textural classes (coarse, medium and fine) and three slope classes (gentle, 0-8%; rolling, 8-30%; and steep >30%). The map shows soil phases and miscellaneous land units. These are stony, lithic, petric, petrocalcic, petrogypsic, petroferric, phreatic, fragipan, duripan, saline, sodic and cerrado phases and dunes or shifting sands, rock debris, salt flats and glaciers and snow caps as miscellaneous land units. The phases were introduced as limiting factors related to surface or subsurface features which may constrain the use of the land.

On the map, gypsiferous soils are not considered at a high level of generalization. The presence of a gypsic horizon, as defined in US Soil Taxonomy, is used to define Gypsic Yermosols and Gypsic Xerosols at the second level. In the legend to the map a gypsic horizon is accepted in other sub-units for example in Luvic and Calcic Yermosols, Luvic and Calcic Xerosols, Luvic and Calcic Kastanozems, Luvic and Calcic Chernozems, Calcic Cambisols and Solonchaks.

A revised version of the Legend of the Soil Map of the World (FAO 1988) introduced two new major soil groupings, Calcisols and Gypsisols, while deleting the Yermosols and Xerosols. The Gypsisols have a gypsic or petrogypsic horizon; the Calcisols have a calcic or petrocalcic horizon or soft powdery lime, but not a gypsic horizon. The Soil Units of the Gypsisols are the following:

Petric Gypsisols	- having a petrogypsic horizon within 100 cm of the surface
Calcic Gypsisols	- having a calcic horizon
Luvic Gypsisols	- having an argillic horizon (clay accumulation)
Haplic Gypsisols	- other Gypsisols

In addition to the Gypsisols, the presence of a gypsic horizon or gypsiferous layer identifies the following soil units: Gypsic Regosols, Gypsic Vertisols, Gypsic Solonetz, Gypsic Solonchaks, Gypsic Kastanizems.

These and other soil units may be subdivided, at a third level, according to the presence and depth of gypsiferous layers and other criteria for scales more detailed than 1: 5 m, but these sub-units have not been defined yet. The systematic definition of such sub-units would increase the utility of the revised legend.

1.4.3 The French classification

The French classification, published in 1967 (C.P.C.S. 1967) is given below. It classifies soils into classes, subclasses, groups and subgroups. Gypsiferous soils are recognized within the system as follows:

Class	Sub-class	Group
Sols calcimagnésiques	Sols gypseux	Sols gypseux rendziniformes
		Sols bruns gypseux
Sols isohumiques	... pedoclimat frais...	Sierozems
Sols hydromorphes	Minéraux ou peu humifères	A redistribution du calcaire et du gypse

They are further divided at subgroup level into:

i. modal
ii. à nodules
iii. à encroûtement or encroûté (à croûte gypseuse).

The French classification needs to be improved as follows:

1. More precise definition of the groups and subgroups are required.

2. Quantification of the terms used (e.g. encroûté, à nodules, modal, rendziniforme, etc.) is needed.

3. By the introduction of integrated soil units.

1.4.4 The Russian classification

The Russian system of soil classification is one of the oldest. In 1896, Dokuchaev noted that the presence of gypsum in soils had been mentioned in several studies. K. Vesselovsky (1851, quoted by Dokuchaev vol. 2, 1896) distinguished clay solonetzoid soils on the European part of the map of Russia and showed outcrops of gypsum. Early Russian data showed the presence of gypsum in the meadow calcareous soils, solonetz soils and brown solonetz soils.

However, it can be considered that gypsiferous soil was introduced first as an independent soil unit by W. Knop in 1871 (quoted by Dokuchaev vol. 4, 1896). He distinguished three soil classes:

i. silicatic soils
ii. carbonatic soils
iii. sulphatic soils

The Russian classification has been modified and improved several times. Four types of classification can be recognized; Geographo-genetic, Historico-genetic, Geochemic and Organo-mineral.

As a guide for soil survey investigation "Classification and diagnostics of the soil in USSR" was published by the Soil Institute V.V. Dokuchaev in 1977. In this publication the presence of gypsum alone or together with calcium carbonate or soluble salt, is used to subdivide the soils at the genus (third) level of the following soil types:

Soil type	Soil genus
Meadow soils	Omerguelovanie (can contain gypsum)
	Zasolenie (can be gypsic)
	Silty (can be omerguelovanie)
Brown semi-arid soils	Gypsonosnie
Grey-brown arid soils	Obitichnie gypsonosnie
	Solonchako-gypsonosnie
	Takyrno-solonchaxovie-gypsonosnie, visokgypsonosnie (bozyngueny)
Arid sandy soils	gypsonovie
Meadow arid and semi-arid soils	sazovie
	sazovo-zasolenie
	sazovie solontzevatie
Irrigated grey brown arid soils	ostatachno-gypsonosnie
Irrigated meadow arid and semi-arid soils	sazovie
	sazovie zasolenie
Irrigated boggy arid and semi-arid soils	sazovie
Grey cinamonic soils	gypsonosnie (gajevie)
Boggy arid and semi-arid soils	sazovie
	sazovie zasolenie

The criteria for subdivision of the soil genus having a gypsiferous accumulation with more than 10 percent of gypsum and located 20 cm below the surface are as follows:

1. Subdivision according to the upper boundary of the gypsic horizon:

20- 60 cm	shallow
60-100 cm	moderately deep
100-200 cm	deep
>200 cm	very deep

2. Subdivision according to the content of gypsum:

10-20%	medium
20-40%	high
>40%	very high

3. Subdivision according to the thickness of the gypsic horizon:

<40 cm	slightly thick
40-100 cm	moderately thick
> 100 cm	thick

4. Subdivision according to the size of gypsiferous accumulation:

0.1 mm	microcrystalline
0.1-1.0 mm	micro-mesocrystalline
1.0- 10 mm	mesocrystalline
10-100 mm	macrocrystalline

5. Soils having a gypsic horizon within the uppermost 20 cm of soil with more than 40 percent of gypsum are considered as gypsic soils at the group level.

Our remarks on the Soviet classification are:

1. There are gaps between the gypsic soil, the genus of soils having a gypsiferous accumulation and the norms for subdivision of the soil at genus level. It would be better to represent all gypsiferous soils in one sequence of gypsum distribution and not to use this criterion at three different levels of generalization.

2. The definition and diagnosis of the units at genus level are not well specified, for example the presence of gypsum is mentioned for some sazic (sazovie) units and not for others.

3. The forms of the gypsum are not considered for defining the gypsiferous soils at a higher level of generalization.

1.4.5 International Reference Base for soil classification

At the meeting on the elaboration of an International Reference Base for Soil Classification, held in Sofia, Bulgaria, it was decided to separate 16 groups of soils at the higher levels of generalization (ISSS Bulletin No. 65). One of these groups was calcic/gypsic soils.

The first proposal for the classification of these soils was prepared and distributed for comments in 1984. On the basis of remarks, suggestions and recommendations of the specialists involved with the soils showing carbonatic or gypsic accumulation, a second proposal was prepared. In simplified form this proposal is presented below:

LEVEL 1	Unit	5	Calcic/Gypsic soils showing carbonatic and/or gypsic accumulation
LEVEL 2	Unit	5.1	Calcisols - soils with carbonatic accumulation
		5.2	Gypsisols-soils with gypsic accumulation (>25%)
		5.3	Calgypsols - soils with both carbonatic and gypsic accumulation
LEVEL 3	Unit	5.11	Petric Calcisols - soils having a calcareous crust layer
		5.12	Encrustic Calcisols - soils having an hypercalcic horizon
		5.13	Sazic Calcisols - soils having an hydromorphic accumulation of calcium carbonate in the form of a marly matrix
		5.14	Eluvic Calcisols - soils having a weathering profile
		5.15	Orthic Calcisols - soils having a nodular horizon
		5.16	Cambic Calcisols - soils having a diffuse distribution of calcium carbonate
	Unit	5.21	Petric Gypsisols - soils having a gypsiferous crust
		5.22	Arzic Gypsisols - soils having a hydromorphic accumulation of calcium sulphate in various forms (micro-macrocrystalline)
		5.23	Orthic Gypsisols - soils having a powdery-fibrous horizon
		5.24	Arentic Gypsisols - soils having an eolian accumulation of coarse-textured unconsolidated gypsum
	Unit	5.31	Encrustic Calygypsols - soils having a hypercalcic horizon (s) alternating with powdery-fibrous horizon (s)
		5.32	Orthic Calgypsols - soils having a nodular horizon (s) alternating with powdery-fibrous horizon (s)

LEVEL 4	Name	EC (mS cm-1)	Gypsum (%)
	Typic	<2	<3
	Halic	2-15	<3
	Gypsic	<2	3-25
	Halgypsic	2-15	3-25

LEVEL 5 (Phase level)

Surface mantle and non-pedogenetic phases: stony, gravelly, harrazy (stony, rocky desert), rocky, remly (windblown accumulation)

Pedogenetic phases: sombric, chromic, luvic, natric, oxic, vertic, andic, arenic, fluventic, areno-fluventic, liptic

Anthropogenic phases: Irrigo-cumulic, anthropic

1.4.6 Other classifications

Boyadgiev (1974) proposes changes in the French, the Russian and the American classification systems to accommodate gypsiferous soils (Appendix 2).

Other national soil classification systems cater for gypsiferous soils but they are not internationally known.

Soil classification is an aid to soil survey and mapping and for describing the relationship between soil conditions and plant growth. Development projects need estimates of the agricultural productivity and potential response to management of the soils. Existing soil classification systems do not provide adequate predictions; therefore several land classification systems have been developed, especially for irrigated agriculture (for example FAO 1985). The various classifications differ in their assessment of the suitability of gypsiferous soils for various land uses. The following examples of the limits used to rate gypsiferous soils in different countries illustrate the variations: if there are justifications for such variations they are not known.

Gypsiferous soils are well represented in the Euphrates river basin in Syria and Iraq where Gibb et al. (1967) distinguished three groups of gypsiferous soils: less than 10 percent gypsum suitable for all crops; 10-50 percent gypsum suitable for limited number of crops, more than 50 percent gypsum not suitable for irrigated agriculture.

In Algeria (Boyadgiev 1975) the following coefficients, based on the amount of gypsum, were applied to assess reduction of productivity.

Gypsum (%)	Coefficient
>50	0.3
25-50	0.6
15-25	0.8
5-15	0.9
<5	1.0

Sys and Riquier (1980) consider that the optimum amount of gypsum for most crop plants in the world is less than 5 percent. Plant growth is marginal when the amount of gypsum in the root zone is 5 to 25 percent and it is strictly limited when the gypsum content is higher than 25 percent.

In Iraq, Barzanji (1973) distinguished five classes of gypsiferous soils:

Gypsum (%)	Class name
<0.3	non-gypsiferous
0.3-10	slightly gypsiferous
10-15	moderately gypsiferous, root growth inhibited
25-50	highly gypsiferous, root growth is minimized, not suitable for irrigated agriculture

Another approach to the classification of gypsum formations is related to the geomorphological aspects of the soils. In this case gypsiferous soils are classified according to their distribution, with regard to topographic features such as piedmont, fans, terraces and lagoon deposits.

Some farmers give local names to gypsiferous soils such as Gatch in Kuwait, Terch in Tunisia, Gagi in the Soviet Union. These names reflect the ability of the soils to grow crops or their behaviour for tillage.

1.5 Distribution of Gypsiferous Soils in the World

The Soil Map of the World is the best summary source to estimate the distribution and extent of gypsiferous soils. Figures calculated by Boyadgiev, especially for this publication, are given in Table 1.1

Table 1.1 DISTRIBUTION OF GYPSIC YERMOSOLS AND XEROSOLS

Soils	Africa	Southern Asia	Central Asia	Europe	North America	Total	Total
	(km2)	(km2)	(km2)	(km2)	(km2)	(km2)	(%)
Gypsic Yermosols	32215	10850	15165	-	78	58308	89
Gypsic Xerosols	3559	2012	1451	230	-	7252	11
Total (km2)	35774	12862	16616	230	78	65560
(%)	54.6	19.6	25.3	0.4	0.1	100	100

The distribution of Gypsic Yermosols and Gypsic Xerosols by countries is shown in Table 1.2.

The figures in Table 1.2 show that about 45 percent of the gypsiferous soils in the world are found in China, Somalia and Algeria.

In general, gypsiferous soils occupy level undulating land with slopes less than 8 percent (66 percent of the total area). About 50 percent of all Gypsic Yermosols and Xerosols have a petrogypsic horizon and about 5 percent are covered by shifting sands (Table 1.3).

Gypsiferous soils are probably more extensive than estimated in the tables for the following reasons:

1. Only the areas with Yermosols and Xerosols having a gypsic horizon or with petrogypsic phases are shown. The other soils which may have a gypsic horizon are not evaluated. These soils are Luvic and Calcic units of Yermosols, Xerosols, Kastanozems and Chernozems, and Calcic Cambisols and Solonchaks.

2. Several authors have described gypsiferous soils which are not shown on the Soil Map of the World. Gypsiferous soils have been described for example in South West Africa, Australia, Afghanistan, Bahrain, Peru, USA and the USSR (Watson 1979).

	Country	km2	% of total area of country	% of area of gypsiferous soils
Africa	Morocco	1114.3	2.5	1.7
	Algeria	7966.3	3.3	12.2
	Tunisia	1439.8	9.3	2.2
	Libya	3956.8	2.2	6.0
	Egypt	382.2	0.4	0.6
	Sudan	785.0	0.3	1.2
	Somalia	10161.2	16.2	15.5
	Ethiopia	1423.4	1.3	2.2
	Mali	2818.3	2.3	4.3
	Mauritania	396.0	0.4	0.6
	Namibia	5327.7	6.5	8.2
Southern Asia	Syria	3966.6	21.6	6.0
	Jordan	80.5	0.8	0.1
	Saudi Arabia	82.5	0.04	0.1
	Oman	471.6	-	0.7
	Yemen A.R.	2931.0	8.8	4.5
	Kuwait	354.6	-	0.5
	Iraq	4779.2	11.0	7.3
	Iran	4.2	-	-
	Pakistan	9.5	0.01	-
	India	182.0	0.06	0.3
Central Asia	USSR	5074.1	0.2	7.7
	Mongolia	60.9	0.04	0.1
	China	11484.9	1.2	17.5
Europe	Turkey	64.2	0.08	0.1
	Spain	165.5	0.3	0.3
North America	New Mexico	78.0	-	0.1

Table 1.3 SUPPLEMENTARY CHARACTERISTICS OF GYPSIFEROUS SOILS

Gypsiferous soils	Area (km2)
	Africa	Southern Asia	Central Asia	Europe	North America	Total
With petrogypsic horizon	11425.5	9820.6	11545.8	-	-	321791.9
Mobile dunes	3357.2	-	-	-	-	3357.2
Class:
Level to undulating 0-8%	21648.9	9314.5	12236.6	64.2	78.0	43342.2
Level to rolling 0-20%	12457.5	3422.4	1964.7	-	-	17844.6
Rolling 8-20%	666.0	124.8	2418.6	165.6	-	3375.0
Level to mountainous 0-30%	993.3	-	-	-	-	993.3
Strongly dissected to mountainous >30%	8.4	-	-	-	-	8.4

According to the Soil Map of the World, gypsiferous soils are found under the following conditions:

Vegetation:	Artemisia, Atriplex, Stilpa, Lygeum, Ziziphus, Juniperus, Aristida, Solsola, Suaeda, Anabosis, Tamarix, Calligonum, and others.
Climate:	Mediterranean, continental and sub-tropical, desertic tropical and sub-tropical, tropical arid and dry.
Lithology:	Recent alluvium, deltaic and eolian deposits, Quaternary, continental and marine deposits of Cretaceous age, marl, clay marl, gypsic, calcareous and calcareous marl, rocks of various ages (Villefranchian, Neogene, Palaeogene, Cretaceous, Jurassic).

Gypsiferous soils have been described in the field in many countries, for example: in Spain by Carenas and Marfil 1979, Gomez-Miguel et al. 1984, Gumuzzio and Alvarez 1984, Martinez Beltran 1978, Peres-Arias et al. 1984, Altaie 1968, Porta et al. 1977 and Sanchez and Artes 1983; in Tunisia by Pouget 1968, Trichet 1963 and Vieillefon 1976; in Syria by Boyadgiev 1974, Onischenko 1969, Osman and Ilaiwi 1980 and Gibb et al. 1967; in the Soviet Union by Minashina 1956, 1958, Ryding 1978 and Kurmangaliyev 1966a; in Algeria by Boyadgiev 1975 and in many other countries (for example Sanyasi Raju and Venkataraman 1953, Smith and Robertson 1962 and Gonzalez 1978).

Detailed studies of these soils and their properties are relatively few in number. Representative soil profiles selected from Syria, Jordan and USA are given in Appendix 2. Further results of laboratory analyses from typical gypsiferous soil profiles from Iraq and Spain are given in Appendix 3 and 4.