Ife Journal of Agric. Vol. 7 Nos. 1 &' 2 (1 985) Hydromorphism - Its definition and correlation between three major classification systems with reference to West Africa T.A. OKUSAMI Depmtment of Sod S k e , University of Zfe, Zle-Zfe. ABSTRACT Hydromorphic soils may be defined simply as any wetland soil that will not support crops (or plants), with aerobic ed'aphic requirements, during the peak of the groundwater recharge. The influence of the water-table is largely controlled by the texture of the profile. This is of significance because the capillary fringe of any water-table may be as saturated as those horizons under the water-table, and this tends t o increase the height of the groundwater. A depth of 100 cm. t o a saturated zone is suggested as the depth below which a pedon ceases t o become aquic or show features associated with hvdromorohism that could limit the environment to anaerobic crops. For a general landlse purpose, a probable depth of 150 cm. is suggested as the depth beyond which a pedon ceases to become hydromorphic. Because hydromorphism does not preclude those soil forming processes peculiar t o the well-drained upland soils, classification as outlined in Soil Taxonomy (USDA) could be appropriate for the classification of hydromorphic soils. The aquic prefix points to thc influence of the water regime on the pedogenesis and properties of the soils concerned. Attempt at correlating the three systems, viz., USDA, FAO-UNESCO ~ n d French was made. I n t r o d u c t i o n H y d r o m o r p h i c soi ls m a y be s o m e o f t h e m o s t i m p o r t a n t soi l resour - c e s o f t h e w o r l d a w a i t i n g fu l l u t i l i z a t i on f o r f o o d p r o d u c t i o n . T h i s is true o f the w e t t e r t r op i ca l a n d s u b t r o p i c a l z o n e s o f Afr ica w h e r e prec i - p i t a t i o n is n o r m a l l y n o t a l im i t i ng f a c t o r to c r o p p r o d u c t i o n . Howeve r , p r e c i p i t a t i o n is a l im i t i ng f a c t o r t o p r o d u c t i o n i n t h e S u d a n and Sahe l Savannas . The w e t l a n d s in Wes t A f r i c a have b e c o m e c r n t r c s f o r f a rming p o p u l a t i o n s a s is e v i d e n t in t h e Fadarnas , i n t h c i n l and d e l t a o f River N ige r at Mali , a n d i n t h e l o w l a n d s o f Uppe r -Vo l t a . , To e s t i m a t e t h e re la t ive p r o p o r t i o n s o f h y d r o r n o r p h i c a n d o t h e r soils a l o n g West Afr ica , de l i nea t i ng c r i t e r i a h a v e t o b e p r o p e r l y d e f i n e d a n d a c c e p t e d universa l ly or a t l e a s t regional ly. T h i s is necessary s o t h a t technology transfer can be used without reverting~to the trial and error of some agricultural projects. Definition Hydromorphic soils virtually mean the same to all soil scientists in terms of morphological and chemical properties, however the different classification systems are still not very interchangeable because their criteria for classification are based on different premi- ses. In West Africa, the definition of hydromorphic soils can be related to three different schools of classification, viz.; the USDA (1975), FAO-UNESCO (1974), Aubert (1965, 1968) and D'Hoore (1964). Aubert's and D'Hoore's definitions can be jointly called the French system since they are very similar. According to Soil Taxonomy (Soil Survey Staff, 1.975) there are five soil moisture regimes, but these different moisture regimes can be regrouped into three major categories in terms of pedological signi- ficance, that is: (a) the soil is saturated most of the year (b) the soil is saturated for brief periods but the amount of water is sufficient to cause reaching and (c) the soil is never saturated and no leaching occurs. The first category belongs to the aquic moisture regime and defined hydromorphic soils whether this moisture regime affects the whole profile or just the solum or part thereof. The solum is defined by the lowest limit of the rooting depth of the native perennial plant. Soil Taxonomy (USDA, 1975) further defines that a soil or horizon is saturated with water when water stands in an unlined borehole close enough to the soil surface or the top of the horizon so that the capil- lary fringe' reaches the surface or the top of the horizon in question. The soil moisture regimes as used by USDA have the assumpt'ions that the soil is supporting whatever vegetation is capable of supporting, that is, cropq, grass or native vegetation, and the moisture content is not influenced by either fallowing or irrigation. The aquic (L. aqua, water) moisture regime is one of the classes of soil moisture regimes defined by Soil Taxonomy. It implies a reducing regime that is virtually free of oxygen. For differentiation in the highest categories of Soil Taxonomy, the whole profile must be satura- ted, but for subgroups only the lower horizons need be saturated. In situations where ground-water is always at or very close to the surface, which is always the situation for the inland swamps and mangrove swamps, the moisture regime of the latter is called "peraquic." This has only been used in a descriptive sense to aid soil genesis study and has not been used as a formative name in classification. The term "aquic" seems to be adequate when used at several levels in the hierar- chical system of Soil Taxonomy. The FAO/UNESCO (1974) definition and classification of hydro- morphic soil contained in the Soil Map of the World (Volume 1) has much in common with USDA definition. Quantitatively both use the same criteria with regard to whe; a soil is considered saturated (bore hole and stable color dye). Qualitatively they differ. The FA01 UNESCO definition emphasizes the concept of gleization, which is the process that gives the morphological character to profiles with a perma- nent or intermittent saturated condition. Gleysols and the gleyic groups constitute the major hydromorphic soils of the FAO/UNESCO map. Gleysols are those strongly influenced by groundwater and the gleyic groups "constitute soils of which only the lower horizons are influen- ced by groundwater, or which have a seasonally perched watertable within the profile." D'Hoore (1964) classified hydromorphic soils into: (1) mineral hydromorphic soils and (2) organic hydromorphic soils. D'Hoore defined mineral hydromorphic soils as: soils, other than vertisols and similar soils, whose development and characteristics (presence of gley and/or pseudo-gley in at least one of their horizons) are influenced by permanent or seasonal waterlogging. Organic hydromophic soils are insignificant in West Africa. They do however occur mainly in the mangrove swamps. "CLASSE X" of the French system of classification as enunciated by Aubert (1965) consists mainly of "Sols Ilydromorphes" divided into three "sub-classes." While the first two sub-classes are not useful, for the same reason as given for D'Hoore's second unit, the third, "Sous-Classe 3: Sols hydromorphes minerauz ou pel1 humiferes" is very related to the type of hydromorphic soils rrndcr consideration. 1 SSSA glossary (1978) defines capjllary fringe as the zone jus: abovc: the water table, i.e., zero gauge pressure, that rcrnains almost saturated. Gley and pseudo-gley terms play prominent roles in the definition of the French hydromorphism, where pseudogley refers to a condition of fluctuating water-table. I t is said that the French system is almost identical to the Belgian (D'Hoore specifically) though their regions of operation had been different. While D'Hoore got much involved in Central Africa, the French concentrated in West Africa (of the fourteen countries that constitute West Africa, only four are English-speaking, while the rest, except Portuguese Guinea, are French speaking). As far as it is known no extensive work has been done to trace how far the rooting medium of the perennial plants (wild rice inclusive) extends in the wetland soils. The capillary fringe as used by the USDA (1975) is taken into consideration in the definition of hydromorphism. It is difficult t o generalize on the quantitative depth or height of water- table using the capillary fringe, because its extent varies with the soil texture, the density and the distribution of the roots of the locally ada- pted crop. The concept of a saturated zone is being considered as an alternative to 'water table' plus 'capillary fringe' (Rust, personal com- munication). More work is still required on this concept. After working on a toposequence in southwest Nigeria, Moormann et al. (1977) stated that a ground water table below 50 cm. has no .- positive effect on lowland rice yield. FAO/UNESCO (1974) used 100 cm. as the control depth. Arbitrarily using 150&. as the maximum control depth, hydromorphism or hydromorphic soils can then be defined as the properties exhibited in the soil profile, or in the hori- zons, by either the permanent water-table ("peraquic," "gley," 6' ground-water gley") or by a temporary period of saturation ("aquic," "pseudogley," "surface-water gley"). And because Vertisols along West Africa are also sribjpcted to seasonal waterlogging (Pseudogleys), they together with all the qlertic subgroups, subunits, or subclasses exhibit properties of hydromorphism and therefore come under the regime of hydromorphic soils. Whenever excess moisture is the most important limiting factor in land-use for crops requiring aerated soils, such areas will come under the realm of hydromorphism. e All soils that can sustain upland (well-drained) aerobic plants at the peak of the groundwater recharge period are non-hydromorphic. Soils with saline or alkali characteristics are excluded from the definition, since the waterlogged occurrence under these two soil conditions are - more of a result of the chemical nature of the soil components. Classification According to the definitions above, classification systems as given by the proponents of taxonomic concepts will be given here and, where possible, correlation between the systems will be shown. The USDA (1975) taxonomic units applicable to the hydromor- phic soils in West Africa are Aquents, Aquepts, Aquolls, Aquods, Aqualfs, Aquults, Aquox and Uderts, all exhibiting features of gleying. These imply that the soils belong to the order prefixed'by "Aqu," a shortened form of "Aquic." These units are suborders in soil taxo- nomy. Soil taxonomy further explairs that, in the absence of any diagnostic horizon that would reflect the influence of soil moisture and temperature, the regime (either moisture or temperature) defines the taxa. The French classification as enumerated by Aubert (1965, 1968) distinguishes three sub-classes (Sous Classe) of hydromorphic soils, viz:- (1) sols hydromorphes organiques (2) sols hydromorphes mozennement organiques (3) sols hydromorphes mineraux ou peu humiferes. The mangrove swamps along the West African coastline may belong to Aubert's (1965) "Sols Humiques a Gley (parfois denommes "Sols- Torbeux"), which is the only group in "Sous-Classe 2" of the French system. The third class (3) , is the only rel'evant one to West African environment. "Sous-Classe 3" is subdivided into three groups as shown below and then divided into sub-groups. Under this sub-classes, groups (a) and (b) will be particularly relevant while group (c) may occur in some areas of the Sudan and Sahel savannas due to dryness. Generally, this sub-class will consist of hydromorphic soils with a very low content of organic matter, < 10% in the top 10 cm, and generally less than 4 to 5% (Aubert, 1965). A division of subclasse 3 into groups a and b is as follows: * Groupe a- Sols hydromorphes peu humeferes a gley Sous-groupes: * Sols a gley de surface ou d'ensemble * Sols a gley de profoundeur * Sols a gley sales * Sols a gley lessives * Groupe b- Sols hydromorphes peu humiferes pseudogley Sous-groupes: * Sols a taches et concretions * Sols a carapace ou cuirasse. Group c is Sols hydromorphes peu humiferes a redistribution du cal- caire et du gypse. The basic difference between the above groups is that the soils in Group a are due mainly t o a high groundwater table (gley) while the soils in group b are due t o a perched watertable (pseu- L dogley ) . Gleization is a predominant process with great influence in thL French classification of the hydromorphic soils in West Africa. While "Sols a gley de surface ou d' ensemble" will typify a flooded soil or highly saturated soil at least up the base of the A1 horizons, "Sol a gley de profoundeur" typifies a deep groundwater table but still within sufficient range to influence crop growth. "Sols a gley sales" is a salty surface horizon that may not be of sufficient concentration to limit crop growth. "Sols a gley lessives" is an equivalent of Aqualf or Aquult by definition (see Table 1). Vertisols and Paravetisols were previously included in the hydro- morphic class (Classe IV of Aubert, 1965). These soils, due to structural property and topographic position, are seasonally saturated and thus exhibit "gley" and "pseudogley" properties. Soil structure and topo- graphic criteria have been the differentiae used in the division of "Class IV" into sub-classes. "Vertisols et Paravertisols Topomorphes (ou . Topo-Lithomorphes)" is a hydromorphic sub-class because of its posi- tion in the landscape, while the second sub-class, "Vertisols et Paraver- tisols Lithomorphes" inherits its hydromorphic properties due to the presence of expanding 2: 1 clay minerals (the smectite group) though it is usually situated reiativeli high on the landscape. Both FAO/UNESCO (1974) and D'Hoore (1964) classifications do not have as many categories as that of Soil Taxonomy (USDA, 1975) and the French system (Aubert, 1965, 1968). The former two can be said to be monocategorical and may be used as a mapping legend if so dcsired. The word "Gley" (Russian word for mucky soil mass) is very important in the FAO/UNESCO classification of mapping units. Gleysols are all hydromorphic soils while the word "Gleyic" servcs as an adjective to other units and it connotes hydromorphic w properties. According to Soil Taxonomy, the following will be found along West Africa, and, where possible, correlations between the French, USDA and FAO/UNESCO systems are shown (Table 1). D'Hoore's (1964) classification appears very simple for hydromor- phic soils and his classificationschcme is equivalent to his mapping legend. Most related to this discussion are D'Hoore's Mineral Hydro- morphic Soils," which also have many associated units. The one (BoNa) with juvenile soils on riverine and lacustrine alluvium, is the most widely distributed (D'Hoore, 1964). Modrmann and van Breemen (1978) classified hydromorphic soils according to their position on the landscape, fluxial and phreatic soils. This does not however replace the conventional classification systems as discussed earlier. Because of the hierarchical system used in Soil Taxonomy (USDA, 1975), the dominant influence of the soil water regime can be intro- duced in the classification a t several levels. Most of the hydromor- phic soil bodies of the world, in their natural state, alternate between wet and dry cycles, that is, there are pedogenetic processes typical of anaerobic and aerobic soil environments going on throughout the soils' pedologic life cycles. This makes Soil Taxononiy (USDA, 1975) more relevant to the classification of hydromorphic soils. ACKNOWLEDGEMENT This paper was prepared during my stay at International Institute of Tropical Agriculture, Ibadan as a Research Fellow. I acknowledge with thanks the comments of Professor R.H. Rust of the University of Minnesota. TABLE 1 CATEGORIES OF HYDROMORPHIC SOILS AS DEFINED BY FAO/UNESCO(~), FRENCH@) AND USDA SOIL CLASSFICATION SYSTEMS(iii) (i) (i i) (iii) (1) GLEYSOILS (a) Eutric Gleysols Sols a gley peu Haplaquents, Psammaquents, profound peu Tropaquents, Andaquepts, humeferes Fragiaquepts, Haplaquepts, Tropaquepts (b) Calcaric Gelysols S.ols hydromor- ---- phes a redistribu- tion du calcaire (c) Dystric Gleysols Sols a gley peu Haplaquents, Psammaquents, profound Tropaquents, Andaquepts, Fragiaquepts, Haplaquepts, Tropaquepts (d) Mollic Gleysols Sols Humiques a Haplaquolls gley (P) Humic Gleysols Humaquepts (2) VERTISOLS (a) Pellic Vertisols VERTISOLS VERTISOLS Pelluderts, Pellusterts, Pelloxererts Vertisols a drai- nage externe ex- treme nu1 ou redult (b ) Chromic Vetisols Vertisols a drai- nage externe Chromoxererts Chrornusterts, Chromuderts, f orrerts possible (i) (3) SOLONCHAKS Gleyic Solonchaks (ii) Haplaquepts (4) SOLONETZ Gleyic Solonetz Natraqualfs (5) PHAEOZEMS Gleyic Phaeozerns Argiaquous (6) GREYZEMS Gleyic Greyzems Argiaquolls (7) CAMBISOLS (a) Gleyic Cambisols ---- Aquic Dystrochrepts ~ ~ u i d Eutrochrepts Vertic Tropepts (b) Vertic Cambisols Sols Bruns Eutrophes tropi- caux (8) LUVISOLS (a) Vertic Luvisols (b) Gleyic Luvisols Vetic Haplozeralfs Aqualfs Sols a gley Lessives (9) PODZOLS Glcy ic Podzols Aquods Podzols a gley (ii) (iii) (1 0) PLANOSOLS (a) Eutric Planosols Albaqualfs, Paleargids, Palexeralfs, Paleustalfs Al baquults Argiabolls, Mollic Albaqualfs Albaqualfs (b) llystric Planosols (c) Mollic Planosols Pseudogley (d) Humic Planosols (c) Solodic Planosols (1 1) ACRISOLS ---- ---- Gleyic Acrisols ---- Aquults REFERENCES Aubert, G. 1965. 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