PART 1 FERRALSOLS

1.1 Diagnostic properties
1.2 Related geographic distribution
1.3 Kinds of ferralsols

1.1 Diagnostic properties

1.1.1 Differentiating characteristics and definition
1.1.2 Modal concept
1.1.3 Accidental characteristics

1.1.1 Differentiating characteristics and definition

The class of ferralsols has been created by soil taxonomists in order to group the soils which are commonly found at low latitudes and present specific properties related to genesis, geographic location, and management practices. These soils occur mainly under tropical climates, and cover extensive areas on flat, generally well drained land. They are considered as being strongly weathered, and to be associated with old geomorphic surfaces.

The criteria which have been selected to define the ferralsols relate to properties that are characteristic of strong weathering in at least one horizon: an almost complete decomposition of primary weatherable minerals and a clay fraction which is dominated by kaolinite and/or sesquioxides. All soils which have a horizon with such properties which is at least 30 cm thick are grouped in the ferralsols. The diagnostic horizon is called the oxic horizon or the oxic B horizon, and is defined by FAO in accordance with the USDA "Soil Taxonomy" (1970) as follows:

"The oxic B horizon is a horizon which is not an argillic 1/ or a natric B 1/ horizon and:

1/ Argillic and natric horizons are horizons which show significant enrichment in clay which has migrated from overlying horizons. They have usually a blocky structure, a clear upper boundary, and show illuviatlon cutans (clay skins) on horizontal and vertical ped surfaces. There is a marked increase in clay between the A₁ and B_2t horizons over a distance of less than 30 centimeters.

i. is at least 30 cm thick;

ii. has a fine earth fraction (less than 2 mm) that retains 10 meq or less of ammonium ions per 100 g clay from a IN NH₄Cl solution, as follows:

or has less than 10 meq of bases extractable with NH₄OAc and aluminium extractable with IN KCl per 100 g clay;

iii. has an apparent cation exchange capacity of the fine earth of 16 meq or less per 100 g clay by NH₄OAc (pH = 7), unless there is appreciable content of Al interlayered chlorite;

iv. has no more than traces of primary aluminosilicates such as feldspars, micas, glass and ferro-magnezian minerals;

v. has a texture of sandy loam or finer (in the fine earth fraction of less than 2 mm) and contains more than 15 percent clay;

vi. has mostly gradual or diffuse boundaries between its subhorizons;

vii. has less than 5 percent by volume that shows rock structure.

Ferralsols are old soils, or are soils that are developed in strongly weathered parent materials. There is usually no evidence of recent deposition in the profile, such as volcanic ash or fresh alluvium. Thin bedding or rook structure is normally absent, since the material has often been reworked by the soil fauna.

The oxic horizon does not release nutrients by weathering of mineral particles. Weathering has acted upon the soil to destroy all primary alumosilicates (point iv of definition), and soil formation has obliterated the original rock structure (point vii). It is not possible anymore to recognize by the location and arrangement of the individual grains the type of rook from which the horizon was developed, or to detect traces of successive sedimentation by water or wind.

Since kaolinite or oxides are the dominant clay minerals, the capacity of the oxic horizon to retain cations is weak: by definition the cation exchange capacity at pH 7 does not exceed 16 milliequivalents per 100 g of clay. There is no possibility for oxic horizons to contain high amounts of organic matter either, because this would increase the cation exchange capacity above the critical level (see point iii).

Kaolinite has only a low inherent electric charge per unit weight (point ii). In water suspensions, especially in acid conditions, the individual particles cannot absorb a thick cloud of ions which would create repulsive forces and protect them against attraction; most of the clay in oxic horizon is flocculated.

Unless the colloids are protected by organic matter, or when positive charges are present in materials that contain exceptionally high amounts of ironoxides, the typical oxic horizons do not contain water dispersible clay.

Clay which is not dispersed does not move: hence the common absence of clay-enrichment or argillic horizons in ferralsols, and the lack of clear or abrupt textural boundaries between the horizons (point vi). An oxic horizon may therefore not be a horizon which has gained enough clay as to qualify for an argilluvic horizon. Soils with this horizon are usually not ferralsols, unless the illuviation horizon is located under the oxic horizon.

Soil taxonomists have not accepted coarse textured soils within the ferralsols concept. Soils which are rich in sand are not necessarily the product of strong chemical weathering. The dominance of sand particles in the soil material decreases the influence which the clay fraction may have on the physical and chemical properties. An oxic horizon should always have more than 15 percent clay (point v).

The definition of ferralsols, extracted from the "Key to Soil Units for the Soil Map of the World" which was issued by FAO in September 1970, reads as follows:

Ferralsols are mineral soils (the thickness of the organic horizons does not exceed 40 cm), that have an oxic horizon. The upper boundary of the oxic horizon occurs at less than 125 cm depth. They may not show between 25 and 100 cm of the surface intersecting slickensides 1/ or wedgeshaped structural aggregates, and cracks which are at least 1 cm wide at a depth of 50 cm. They should not have a spodic horizon.

1/ Slickensides are polished and grooved surfaces produced on aggregates by one mass sliding past another. They are common in swelling clays that have marked changes in moisture content (U.S.D.A., I960).

According to the present definition, ferralsols may have sandy textures in some horizons; they may also have an argilluvic horizon below the oxic horizon. They may be influenced by water tables as to form either gley, mottling or plinthite (soft laterite). Such double feature profiles are not uncommon. For this reason it is necessary to subdivide the ferralsols into more homogeneous groups.

1.1.2 Modal concept

The broad definition which was given in the previous section, leaves ample margins for variations. It may therefore be helpful to define and describe the central concept of a ferralsol and try to visualize a nodal profile.

Ferralsols are usually deep soils because both the intensity and the duration of weathering have been considerable; the most favourable conditions for the formation of kaolinite are found under free drainage, when silica and bases produced by the weathering of the parent materials can be freely leached out of the profile. These circumstances are conducive to good aeration, under which iron is immobilized in the oxidized stage. It stains the smallest soil particles with yellowish or reddish colours. Most ferralsols are strongly coloured. Ironoxides also contribute to the aggregation of clay and silt which creates porosity and most oxic horizons are friable and have a well aerated structure. Air and water can usually circulate freely though ferralsols; rainfall acceptance by these profiles is usually faster than by most other soils of comparable texture; it also leaches quite rapidly to deeper layers that are beyond the reach of the common rooting of most cultivated crops. Physically, rooting space is abundantly available.

Strong leaching of nutrients and bases is a general property of ferralsols. Downward movement of water is seldom impeded, and the retention of nutrients by the clay fraction which should protect them against losses is not very active. As a result most ferralsols are low in cationic nutrients and the oxic horizons have low pH. The mineral part of a modal profile is considered poor. The amount of available plant nutrients is almost completely dependent cm the amount and the quality of the organic matter.

An acid, strongly coloured, deep profile is called typical under most tropical environments. It has usually favourable physical conditions for plant growth, but is deficient in nutrients. This concept of a modal ferralsol may differ from earlier ideas, which related the ultimate stage of tropical weathering to the irreversible induration of friable soil into ironstone upon exposure to sunlight after clearing the forest or during intensive cropping. Such horizons may occasionally occur in ferralsols. However, they are or were previously associated with wetness of the soil and watertables which fluctuate at the depth of the indurating layer. Remnants of such layers, that have hardened, forming ironstone or ironstone gravel, are frequently found. In this context they are not considered an essential part of the modal concept however, but rather as an inactive soil constituent.

1.1.3 Accidental characteristics

As pointed out before, the definition of ferralsols is sufficiently generalized as to accept within its range of variability a considerable number of soils which have striking differences in the composition and the arrangement of horizons and layers.

Stone-lines may interrupt the diffuse transitions from one horizon to another by the sudden appearance of a layer of gravelly material, either rich in quartz or ironstone. If these gravelly layers are closely packed, thick enough and near the surface, root growth may be severely restricted. The soil volume which is accessible to plant roots may also be limited by the groundwater level, and profiles with impeded drainage may present strong mottling due to oxidation reduction reactions of iron compounds.

The organic matter content may be extremely variable and not all ferralsols are poor in organic carbon; in hot equatorial regions, the A1 horizons are usually thin; even under a dense rainforest, they seldom exceed ten centimeters in thickness. In cool tropical climates, for example at high elevation, the humus content is high, and dark coloured topsoils may cover extensive ferralsolic areas. Other regions where humus tends to be present in considerable quantities are those which at some period have been influenced by basic volcanic ash falls or which contain high percentages of iron oxides.

Other factors, besides climate, may affect soil properties. The nature of the parent rooks may cause marked differences in pedological properties, even in strongly weathered sediments. They are reflected in the particles size distribution, colour and the iron oxide content. Ferralsols from basic rocks have redder hues than those which have developed from acidic rocks, which contain more quartz. The clay content in residual soils depends on the amount of weatherable minerals of the original rook. Geology therefore creates a large variety of subgroups and soil families, that are important to distinguish when evaluating ferralsols for crop production, or when recommending management practices. Hence the necessity to subdivide the ferralsols into several classes some of which are listed in chapter 1.3.1.

For purposes of fertilization the mineralogy class is particularly important, and special attention should be paid to soils which are either ferritic (more than 40% Fe₂O₃ extractable by citrate dithionite), gibbsitic (more than 40% of gibbsite and boehimite) or oxidic (others which have more than 20% ironoxides plus gibbsite in the clay fraction) (SCS, USDA, 1970). The high amounts of oxides in the exchange complex of these soils may indeed have a marked influence on the nutrient supply in ferralsols. These questions will be discussed with more detail in other parts of this review.

1.2 Related geographic distribution

1.2.1 Climatic regimes
1.2.2 Landforms
1.2.3 Vegetation

1.2.1 Climatic regimes

Most ferralsols are found at low latitudes lying between the parallels of the tropics (25°27'). These astronomic lines are not exclusive however and do not set clear boundaries to the distribution of these soils over the world.

The mean annual temperatures may be low and some ferralsolic regions may have as many as ten frost days per year. Precipitation also varies markedly, and the soils may have moisture regimes grading from arid to permanently humid.

Most if not all ferralsols occur under tropical climates and it is worth remembering the unifying characters of the temperature and moisture regimes of the profiles. It is mainly rainfall which determines the cropping period in ferralsols: temperature variations have practically no influence in this respect. The range of monthly temperature changes is narrow, and seasons for plant growth are not defined by a sequence of cold and warm months. The mean annual temperature as it follows changes in elevation (0.6°C/100 m or 3.2° F/1 000 feet) is only important in the selection of crop and crop varieties.

The primary limiting factor of the duration of the growing season is the amount and the distribution of rainfall during the year. In ferralsols the importance of climatic moisture regimes cannot be overemphasized. Precipitation reaches its maximum during the astronomic summer, i.e. in each hemisphere the highest rainfall coincides with the summer months in the same hemisphere. At the beginning of the rainy season, there is generally no stored water available to plants in the top layers of well drained ferralsols. This is a contrasting situation as compared to moisture conditions at higher latitudes where the warm growing season starts with the maximum amount of water stored in the soil during the cold winter, when evapotranspiration was low.

Other effects of the tropical climate on management practices are important. In areas with a dry period, the growing season usually starts at the onset of the rains with a high amount of nitrates or nitrifiable residues in the soil. The formation of easily mineralizable organic matter continues during the tropical dry season; nitrification may also go on provided the soil moisture content is not too low. Since there is no leaching and no plant growth, nitrates tend to accumulate in the soil. This is a contrasting environment compared to conditions in temperate and Mediterranean regions, where the winter temperatures are too low for producing higher amounts of nitrates than can possibly be leached by the percolating rainfall (GREENLAND, 1958).

The climate, more specifically the wet and dry season sequence or the distribution pattern of rainfall during the year, is the key factor for adapting management techniques to local conditions. The duration and the number of dry seasons determine whether one or more cropping cycles will be possible annually, and if irrigation should be considered.

For general purposes it is useful to refer to De Martonne's diagram (fig. 1). Although it is an idealized illustration of the march of rainy periods during the year, it clearly shows the relationship between latitude and rainfall distribution; cloud formation and precipitation reach a maximum in the region of atmospheric convergence which follows the sun's zenithal course between the tropical parallels. This leads to a sequence of wet and dry, or maximum/minimum rainfall periods at given latitudes.

Fig. 1 - Diagram of march of seasons in the intertropical regions (DE MARTONNE, 1958) 1/

1/ Reproduced with the kind permission of the editor, Armand Colin, Paris.

It may be pointed out that the origin of the rains is largely convectional and that they fall in heavy showers. This dense precipitation at the beginning of the rainy season may cause strong erosion on soils which are not covered by vegetation, as is often the case at the end of the dry period.

1.2.2 Landforms

Ferralsolic regions are typically situated in areas which have not been subjected to intensive folding during recent geological periods, but rather went through long periods of broad and gentle upwarping into swells and downwarping into basins. These minor movements in the earth's crust took place on stable continental platforms, usually with crystalline foundations. In places where the crust broke, tectonic rift valleys were formed, with local, usually basaltic intrusions. The form of these platforms have sometimes been compared with cracked pavements.

Typical examples of ferralsols which occur on these old continental stable shields can be observed in Central Africa, the Brazilian and Guinean shields of South America, and the remnants of old surfaces in the Indian peninsula.

The maps of fig. 2 and 3 show the extent of the regions where ferralsols are dominant in South America and Africa.

Outside the continental platforms the ferralsols are rather rare even under hot humid conditions; Asia and Central America have been subjected to the Alpine orogenesis, and as a rule erosion has constantly removed the weathering products of most parent materials. Only those sediments which weather rapidly such as basic and ultrabasic rocks (for example basalts) or basic volcanic glass, have had time enough to form oxic horizons.

Generally speaking most ferralsols occur on horizontal uplifted landscape surfaces. These are not necessarily large but they may be scattered like small islands in a steeply sloping topography. They are then remnants of more extensive flat old surfaces which have been dissected by rivers. This is for example the case in the horst region bordering the rift valley in Africa, which was uplifted without much recent folding of the substratum. Good examples of this type of distribution can also be found in Puerto Rico.

The dominant occurrence of ferralsols in flat topography has attracted the attention of many people, because the level plateaus facilitate access, reduce erosion hazards, and offer considerable possibilities for modern mechanized agriculture. When this type of land utilization is envisaged, flat topography is undoubtedly one of the ferralsols' chief assets.

Figure 2 - Distribution of Ferralsols in South America

Fig. 3 - Distribution of Ferralsols in Africa

The distribution of ferralsols is related to local landscape features. The topographic factors are the slope gradient, its shape, and its length, which all may correlate with soil properties in many different ways. Huge areas with ferralsols are still open for utilization, and the choice of the best available ferralsols may prove essential in future agricultural planning and determine either failure or success of new agricultural operations. Selection of adequate soils is an essential part of good land management.

Only some principles for guidance in the choice of land will be discussed in this chapter. They have mainly local importance. Relationships between topography and soils are studied first: flat topography protects the soils against runoff and erosion; therefore they are mostly covered by old sediments which are more strongly and deeply weathered than the surrounding valley slopes. Soil water may also be at greater depth on the uplifted plateaus. Less runoff means more water penetration into the soil and more leaching. Consequently the chemically poorest soils occur on the high level areas. This is particularly striking in areas where rivers have cut into rich parent rocks which rapidly develop into productive soils on sloping topography (fig. 4a).

Not all the parts of the slopes in ferralsolic areas are necessarily better from the plant nutrient viewpoint than the land on the flat elevated surfaces. Convex parts, where surface water increases its velocity, are often badly eroded, or highly susceptible to erosion. The profiles in these sites may be truncated, and may contain less organic matter (fig. 4b). Humus is the main supplying power of nitrogen to plants, and is of great value in preventing leaching of nutrients. Erosion may also bring gravelly layers and stone-lines closer to the surface, and may create soils which are too shallow for optimum root development.

In areas that are underlain by poor sedimentary rocks, or are covered by thick sediments in which quartz or oxic materials dominate, the dissection of the plateaus by shallow river valleys does not modify the nature of the parent materials. Erosion can only take weathered soil materials at the edge of the plateau, and transport it down into the valleys. During this process, soil particles are sorted. Sands remain usually on the slopes, and the clay is partially removed in suspension in the running water. The resulting topographical soil sequence acquires a textural gradient, where the heaviest soil is found on the highest undissected parts of the landscape, and the sandiest soils on lower laying slopes or concave parts of the valleys (fig. 4c). These sandier soils are more permeable to water and dry out more easily. For these reasons they are less suitable for agriculture than the heavier textured associated soils. In such catenas, which present very often parallel colour changes from red to yellow the plateau soils are to be preferred.

The shape of the declivity may also be important. Concave sites tend to receive more water and erosion products than plans or convex gradients. Small differences in these landforms may give considerable variation in land capability, particularly when soil organic matter is redistributed in various parts of hilly topography. Local farmers have very often taken advantage of privileged sites by placing their most valuable crops in hollows, where humus rich colluvium accumulates.

Fig. 4 - Relief related distribution of Ferralsols

There are probably no rigid rules which in all cases would make it possible to delineate on the basis of topography alone the most suitable ferralsols in a given area. Good management of ferralsols starts with the selection of the most productive ones which will be competitive when they are included into a modern market economy. Of course, all soil imperfections may be corrected but never without additional costs. Therefore all agricultural programmes, at whatever level, may greatly benefit from adequate soil selection based on good soil mapping. Local topography and parent material are usually important factors in differentiating soils. At the same time the relief offers reliable external features which help soil mapping and the design of proper erosion control measures.

1.2.3 Vegetation

There are many kinds of vegetation on ferralsols. None of the diagnostic properties of the oxic horizon which are related to the mineral part of the soil, seem to determine the type of vegetation, or exclude the possible development of any particular plant association.

Vegetation patterns rather follow climatic changes and differences in soil drainage classes. Human influence is capable of modifying the distribution of plant communities in the most drastic way.

As present definitions now stand, no cause to effect relationship seems to exist between the ferralsols as a whole and broad vegetation types. Except in the case of a part of the Humic Ferralsols, which will be defined later and which are mainly associated with mountain vegetation of cool tropical climates, all other classes in the FAO system have no climatic nor vegetation implications.

The plant cover seldom reflects the long-term potential and the qualities of the ferralsols, and no conclusions should be drawn regarding their productivity on the basis of the vegetation alone.

This does not mean however that no attention at all should be paid to the kind of vegetation when evaluating the fertility status. Soils are more than mineral bodies, and the type of soil organic matter, which is closely related to the kind of plant cover, is a major although transient production factor. For example, most agronomists in the tropics are aware of the strong differences in potential between land under savanna and under the rainforest. Some types of secondary regrowth may reflect striking differences in temporary fertility levels which can hardly be detected by accurate chemical analyses and sampling techniques. Most local farmers can evaluate tracts of land on the basis of certain plant species or plant associations. In areas covered by ferralsols small differences in vegetation may be closely related to temporary fertility.

The nature of the soil organic matter at a given time depends on the types of vegetation which recently covered or actually covers the soil; its behaviour when soils are put into cultivation is determined by the desequilibrium in ecological conditions which is caused by clearing, plowing, mulching the soil or its exposure to direct sunlight. Management of ferralsols therefore has to consider the position of a given crop in a rotation, the time after clearing, and the types of fallow, more than the actual amount of organic matter in the soil, if maximum use of the natural fertility is intended.

The vegetation itself may be an asset in the nutrition of plants. Considerable amounts of nutrients are stored in the tissues. Table 1 taken from NYE and GREENLAND (1960) sets figures for estimating the importance of this nutrient reserve which may be released either slowly or quickly, depending cm the practices that are used for clearing land.

Table 1 IMMOBILIZATION OF NUTRIENTS IN THE VEGETATION (KG/HA) (NYE AND GREENLAND, 1960)

		N	P	K	Ca	Mg
Rainforest 40 years		1832	125	819	2527	346
Primary rainforest		1236	123	954	2120	325
Secondary forest
	18 years	560	73	405	562
	5 years	391	24	344	293
Savanna
	grass	27	8	46	35	26
	tress	(100)	(15)	(146)	(235)	(63)
Imperata Savanna
	grass	17	6	35	7	13
	rhizomes	29	13	71	7	10

1.3 Kinds of ferralsols

1.3.1 Main subdivisions
1.3.2 Correlation with order classifications
1.3.3 Description and analysis of typical profiles

1.3.1 Main subdivisions

The key to soil units of the Soil Map of the World (FAO, 1970), subdivides the ferralsols according to the following criteria:

i. Ferralsols having a plinthic horizon at less than 125 cm depth.

Plinthic Ferralsols

ii. Other ferralsols having an organic matter content (weighted average of the fine earth fraction of the soil) of 1.35 percent or more to a depth of 100 cm (exclusive of an 0 horizon if present); having a base saturation of less than 35 percent (by NH₄OAc at pH 7) at least in some part of the B horizon.

Humic Ferralsols

iii. Other ferralsols having an exchange capacity (from NH₄Cl) of 1 meq or less per 100 g of clay in at least some part of the B horizon; having no discernable structure in the B horizon or only very weak blocky or prismatic peds.

Acric Ferralsols

iv. Other ferralsols having a red to dusky red B horizon (rubbed soil has hues redder than 5YR with a moist value of less than 4 and a dry value not more than one unit higher than the moist value).

Rhodic Ferralsols

v. Other ferralsols having a yellow to pale yellow B horizon (rubbed soil has hues of 7.5YR or yellower with a moist value of 4 or more and a moist chroma of 5 or more).

Xanthic Ferralsols

vi. Other ferralsols.

Orthic Ferralsols

The first class is set up in order to separate all ferralsols which have a plinthic horizon with its upper boundary at less than 1,25 meter. These soils undoubtedly suffer from impeded drainage during some periods of the year, and are saturated with water during most months in the plinthic part of the profile. The genetical implications for this class are the presence of a fluctuating water table, and the supply of free iron oxides mainly in the low lying parts of the landscape. There are no climatic implications in the concept. From a practical point of view, these adverse conditions call for special care in order to improve aeration of the soil as root development may be restricted periodically by water excess. The agricultural value of the Plinthic Ferralsols will greatly depend on the thickness and the quality of the horizons which lie above the plinthite. If these are too shallow, there is no interest in bringing them into cultivation as there is no point in draining land that hardens into ironstone after removal of the surplus water.

The Humic Ferralsols are soils without plinthic horizons at shallow depth that are rich in organic matter and have a low base saturation. They are typical for the cooler regions that are either situated in mountain areas or at high latitudes. At high elevation the orogenic precipitation increases the leaching intensity and the soils are poor in cations.

The high organic matter content is considered as being the result of slower decomposition; the lower class limit allows a minimum of 0.78% organic carbon as a weighted average of all horizons down to a depth of 100 cm , not including the litter (0 horizon). An example of a calculation may help to clarify this diagnostic criterion:

Depth	Thickness (t)	C% (c)	t x c	Weighted average
(cm)	(cm.)
0-10	10	3.4	34
10-25	16	2.8	42
25-70	45	1.0	45
70-90	20	0.9	18
90-120	10	0.4	4
		SUM=	143

The profile considered, in this example can be classified as a Humic Ferralsol, provided it has a base saturation which is lower than 35%, and that the profile does not present a plinthic horizon at less than 125 cm depth. Although there is no soil temperature restriction in the definition, it is assumed that most of these soils occur in cool climates. However, careful attention should be paid to the question whether the humic ferralsols present acric properties in addition to the humic ones. In this case management decisions should be based on the acric properties rather than on the richness in humus.

The other four remaining subunits of the ferralsols mostly lie in the warm tropical and equatorial regions. The limit with the previous climatic zones lies somewhere around 22°C (72°F) mean annual temperature. The four classes which have been recognized are separated by criteria related to clay mineralogy, cation exchange capacity and base saturation. These differentiating characteristics are to a certain extent defined by the intensity of weathering and the lithological composition of the parent rook. In this respect the iron and aluminum oxides may play an important role in setting the physico-chemical properties of the soil.

The Acric ferralsols are usually soils that have a high amount of sesquioxides which produces a net positive charge on clay-size particles. Instead of increasing the capacity to retain cations such as Ca, Mg and K, the iron-oxides on the contrary block the existing negative adsorption sites. In extreme cases 100 g soil in the B horizon can only adsorb less than one milliequivalent of bases. These soils are usually clayey, and have normally been subjected to very humid climates during their formation. They are usually very hard to reclaim and to bring into successful types of agriculture. There is not only a strong nutrient shortage, but also a very high capacity of the soil to fix phosphates and to adsorb Ca specifically. Calcium deficiencies are common in these soils. Frequently the pH of the soil measured, in normal KCl is higher than the pH measured, in water.

The Rhodic Ferralsols, are those ferralsols of the warm tropical regions which have no acric properties, and that are mainly formed on basic rocks, such as basalts, diorites etc. The weathering intensity has not reached the advanced stage of the acric group. Dusky red colours which do not change very much upon wetting and drying are common.

The rhodic soils are preferred in ferralsolic areas, as their potential within this class is certainly the highest. Many examples are known where agriculture has been successful on them, and the nature of the original rock often provides possibilities for an almost continuous nutrient supply from layers below the oxic horizon. Their heavy texture retards the downward movement of water. Their high content of total phosphorus and calcium allows the maintenance of high organic matter levels in the topsoil, at least in the absence of erosion.

The other two remaining groups are the common red and yellow ferralsols, which have been called Orthic and Xanthic in the FAO classification system. Yellow latosols are usually the sandier members of the group, or those that are developed from acidic rocks having a high quartz content. They may also have developed in colluvium on lower slopes. These transported parent materials are currently poorer than the autochtonous soils. It is assumed that they may have lost some of their potential value during the translocation of materials. Red and Yellow Ferralsols often occur in catenary association, the reddish members occupying the higher parts of the relief, and the yellow ones covering the sloping land.

Figure 5 gives an idealized cross-section of the distribution of the different kinds of ferralsols in a hypothetical broad landscape. The next section provides complete description and analysis of typical profiles.

Fig. 5 - Distribution of kinds of ferralsols in the landscape

1.3.2 Correlation with order classifications

The name ferralsols is of recent origin, and much of the earlier literature which deals with their management would become unavailable, if no correlation with other taxonomic system would be made.

The concept of ferralsol of the FAO legend is almost synonymous with the term latosol, as defined in Brazil. One should not extend this identity to the latosols of other regions however. Many Hawaiian latosols, and most latosols which have been described in Central America, do not correspond with the FAO concept of ferralsols; they have generally too high a base exchange capacity (pH 7) which is probably due to contamination by volcanic ash and the presence of allophane.

A very close correlation of ferralsols can be made with the Oxisols of the U.S.D.A. Soil Taxonomy. The only minor discrepancy would be that the oxisol order does not allow profiles with a textural B horizon above the oxic horizon, whilst the FAO system is not specific on that particular point. Such cases are relatively rare however.

The ferralsols can also be compared with the Sols Ferralitiques Typiques of the French classification. It should be pointed out that the correlation is best achieved with the "typic" groups that are recognized in the "Classe des Sols Ferralitiques". The other groups, as the "groupes des sols pénévolués, groupes lessivés" do not fall completely within the concept of the ferralsols, and preliminary checking on representative profiles is necessary.

The ferralsols can finally be compared with most Kaolisols of the classification which is used in Zaire. The closest similarity exists with the ferralsols suborder in this national classification system, whilst a certain number of ferrisols, but not all, would fall outside the FAO concept.

At a lower level of generalization the comparisons become more difficult. There are practically no perfect identities between subclasses of different systems. A tentative very loose correlation scheme is given in table 2.

Table 2 BROAD CORRELATION OF SOIL CLASSIFICATION SYSTEMS

F.A.O.	U.S.D.A. Soil Taxonomy	French Classification
Plinthic Ferralsols	Plinthaquex, Plinthic subgroups	Sols à accumulation de fer en carapace ou cuirasse.
Humic Ferralsols	Humex, some Acrorthox and Acrostox	Groupe des sols ferralitiques moyennement désaturés en (B) - humifères. Groupe des sols ferralitiques fortement désaturés an (B) - humifères.
Acric Ferralsols	Acrorthox, Acrustox	Groupe des sols ferralitiques fortement désaturés en (B), typiques, sous-groupe modal.
Rhodic Ferralsols	Orthox and Ustox derived from basic rocks or limestone	Groupe des sols ferralitiques faiblement ou moyennement désaturés en (B), typiques, dérivés de roches basiques ou de calcaire.
Xanthic Ferralsols	Orthox and Ustox which are yellow in the oxic B	Groupe des sols ferralitiques faiblement ou moyennement désaturés en (B), typiques, jeunes.
Orthic Ferralsols	Other Orthox and Ustox	Autres sols ferralitiques faiblement ou moyennement désaturés en (B), typiques.

1.3.3 Description and analysis of typical profiles

i. Acric Ferralsols

Classification: Haplic Acrorthox, clayey, kaolinitic, isohyper- thermic.

Location: Amazonas State, along Highway Am-070, 10.5 km from Cacau-Pirêra toward Manacapuru, on the right side 100 m from the road.

Physiographic position: Top of low, gently undulating plateau.

Topography: Level with gradient, 0 to 2 percent.

Drainage: Well drained.

Vegetation: Evergreen tropical forest.

Parent material: Clayey Tertiary sediments of the Barreiras series.

Sampled by: Team of pedologists of IPEAN.

Soil No.: Profile no. 8

Remarks: Very much biological activity in the A1 and A3 horizons, declining to very little in the B22 and B23 horizons. Current earthworm activity has produced pyramidal hills on the surface that are 15 to 20 cm high and 10 to 15 cm in diameter at the base. The forest litter consists of a few partially decomposed leaves and a very few that have not begun to decompose. Many 1 to 2 mm pores throughout. Concretions occur throughout pedon. Charcoal fragments occur in A3.

Colors are for the moist soil.

A1 0-4 cm (0-2 in). Yellowish brown (10 YR 5/4) sandy loam 5 moderate fine and medium subangular blocky and weak fine granular structure; friable (moist), slightly sticky and plastic (wet); many fine common, medium, and few coarse roots; clear smooth boundary.

A3 4-19 cm. (2-7 in.). Yellowish brown (10 YR 5/6) light clays weak fine and medium subangular blocky and weak fine granular structure; friable (moist), sticky and plastic (wet); many fine and common medium roots; diffuse smooth boundary.

B21 19-87 cm. (7-34 in.). Brownish yellow (10 YR 6/6) heavy clay; weak fine and medium subangular blocky structure; friable (moist), very sticky and very plastic (wet); many fine and common medium roots; diffuse smooth boundary.

B22 87-130 cm. (34-51 in.). Brownish yellow (10 YR 6/8) heavy clay; weak fine and medium subangular blocky structure; friable (moist), very sticky and very plastic (wet); few weakly expressed smooth ped faces; many fine and few medium roots, with medium roots located near the zone transitional to the superjacent B21; diffuse smooth boundary.

B23 130-180 cm. (51-71 in.). Strong brown (7.5 YR 5/8) heavy clay; weak fine and medium subangular blocky structure; friable (moist); very sticky and very plastic (wet); common smooth ped faces; common to many fine roots.

1/ Analysed by the U.S.D.A. Soil Survey Laboratory at Beltsville, Maryland, USA.

Horizon	Extractable bases (meq/100 g)					Extract. (meq)
	Ca	Mg	Na	K	Sum	Al.	Acid.
A1	0.07	0.09	0.03	0.08	0.27	2.3	12.7
A3	0.04	0.05	0.04	0.04	0.17	1.1	8.0
B21	0.01	0.01	0.02	0.01	0.05	0.6	6.0
B22	0.01	0.01	0.02	0.01	0.05	0.2	5.1
B23	0.01	0.01	0.01	0.01	0.03	0.0	4.8

ii. Humic Ferralsol

Location: Zaire, Ituri region, Nioka area 2°15' N, 30°32' E.

Climate: Cf (Koeppen); mean annual temperature: 17°C; annual precipitation: 1450 ram.

Vegetation: Coffee plantation.

Parent material: Weathering products of basic rocks.

Relief: plateau at 2075 m above sea-level.

Drainage class: well drained.

Described by: A. PECROT (1958).

Description

Ap 0-23 cm Clay, 5 YR 3/2, weak medium crumb structure, with some blocks in the lower part of the horizon, very friable, common roots, nonplastic, nonsticky, gradual smooth boundary.

A3 23-40 cm Clay, 5 YR 3/3, moderate fine subangular blocky structure with patchy dark coatings, few roots, friable to firm, slightly plastic, nonsticky, diffuse boundary.

B21 40-70 cm Clay, 5 YR 2.5/2, dark horizon, moderate fine subangular blocky structure with patchy dark coatings on ped surfaces, few roots, friable to firm, slightly plastic, nonsticky, diffuse boundary.

B22 70-100 cm Clay, 5 YR 2/2, dark horizon, moderate fine subangular blocky structure, slightly plastic, nonsticky, diffuse boundary.

B23 100-140 cm. Clay, 5 YR 3/2, dark horizon; moderate fine subangular blocky structure with patchy dark coatings on ped surfaces, friable, slightly plastic, few roots, diffuse boundary.

B24 140-170 cm. Clay, 2.5 YR 3/4, moderate medium subangular blocky structure with discontinuous clay films on ped surfaces, friable, diffuse boundary.

B25 170-200 cm Clay, 2.5 YR 3/5, moderate structure.

Horizon	C (%)	N (%)	pH (H2O)	Exchange with HCL N/20		C.E.C. pH 7	Fo2O3 (%) on clay
				Ca	K
Ap	3.90	0.040	5.1	4.1	1.05	18.0	8.6
A3	1.59	0.013	4.5	1.4	0.30	11.5	7.7
B21	1.80	0.013	4.5	1.3	0.22	13.0	8.1
B22	2.02	0.013	4.4	1.3	0.41	14.2	7.9
B23	1.50	0.012	4.3	1.2	0.23	12.6	9.8
B24	0.73	0.005	4.3	0.9	0.14	7.9	9.4
B25	0.54	0.006	4.4	0.9	0.14	6.0	10.4

iii. Orthic Ferralsols (Commissão de Solos, 1960)

Location: Atibaia, São Paulo, Brazil, 780 m above sea level; 5-10% slope.

Parent rock: Gneiss

Vegetation: Melinis minutiflora and Imperata brasiliensis, with trees.

Drainage class: Well drained.

Description

Ap 0-8 cm Sandy clay, 5 YR 3/2, weak crumb structure, very friable, slightly plastic and slightly sticky, clear wavy boundary, abundant roots.

A3 8-28 cm Clay, 5 Yr 4/3, weak very fine subangular blocky structure, very friable, plastic and sticky, gradual wavy boundary, abundant roots.

B1 28-94 cm Clay, 5 YR 4/8, weak medium crumb structure, very friable, plastic and sticky, diffuse smooth boundary, abundant roots.

B21 94-130 cm Clay, 5 YR 5/6, porous massive which breaks into weak very fine crumb, very friable, plastic and sticky, smooth diffuse boundary, abundant roots.

B22 130-220 cm Clay, 5 YR 5/8, same as above, few roots.

B3 220-310 cm Clay, 5 YR 5/8 - 2.5 YR 6/8, weak fine crumb, very friable, plastic and sticky, diffuse smooth boundary, DO roots.

C 310 + cm Clay, 2.5 YR 6/8, weak fine crumb, very friable, plastic and sticky.

Horizon	Depth (cm)	Gravel %	Particle size distribution (Na OH dispersion)
			2000 200	200 20	20 2	2 0
Ap	0-8	0	37.8	17.9	7.9	36.4
A3	8-28	0.9	30.8	21.4	6.3	41.5
B1	28-94	0.7	27.0	14.7	6.4	51.9
B21	94-130	0.6	25.7	14.4	7.9	52.0
B22	130-220	1.4	23.7	17.0	7.2	52.1
B3	220-310	3.0	21.1	16.7	8.4	53.8
C	310+	4.8	23.6	16.8	15.2	44.4

Location: Zaire, 11°40' N, 27°21' E

Climate: C_w (Koeppen); mean annual temperature: 20°C; annual precipitation: 1250 mm.

Vegetation: Tree savanna.

Parent material: Clay weathered from dolomitic limestone.

Topography: Margin of depression in the end tertiary peneplain.

Drainage class: Moderately well drained.

Described by: C. SYS (1972)

Description

A1 0-4 cm Sandy clay loam, 10 YR 5/2, mixed with ash from burning of plant residues; fine crumb structure, friable, many roots, clear boundary.

A3 4-24 cm Clay, 10 YR 6/4, weak medium subangular blocky structure, friable, few roots, gradual boundary.

B21 24-58 cm Clay, 10 YR 5/6, some mottling (7.5 YR 5/6), weak medium subangular blocky structure, friable, diffuse boundary.

B22 58-93 cm Clay, 10 YR 5/6, some mottling, and ± 15% soft iron concretions having 1 to 2 cm diameter; moderate fine subangular blocky structure, firm, some tree roots.

B23 93-122 cm Clay, 2.5 YR 6/4, with 5 YR 5/8 mottles.

B24 122-170 cm Clay, 2.5 YR 7/2 with 7.5 YR 5/6 mottles.

Horizon	C (%)	N (%)	pH (H2O)	Exchange with HCl N/20		C.E.C. pH 7	Fe2O3 %
				Ca	K
A1	0.75	0.058	5.9	2.3	0.21	4.1	0.6
A3	0.23	0.024	5.4	1.0	0.17	3.6	0.6
B21	0.25	0.020	5.7	0.3	0.26	4.7	0.6
B22	-	-	5.7	0.7	0.21	4.9	0.8
B23	-	-	5.6	0.3	0.11	4.3	1.0
B24	-	-	5.5	0.3	0.11	4.8	1.0

v. Rhodic Ferralsol

Latosol roxo Brazil

Rep. FAO-EPTA 2197 Bennema, 1966, p. 39, Comissão de Solos, 1960, profile 37. p. 287

Location: 15 km N Ituverava, São Paulo State. 20°09' S, 47°47' W

Altitude: 560 m

Physiography: Undulating

Drainage: well drained

Parent Material: Basalt

Vegetation: Second growth forest

Climate: 1.77, humid tierra templada

Description

A1 0-20 cm Dark greyish brown (2.5 YR 3/3) clay; moderate medium granular structure; slightly hard, friable, slightly plastic, sticky; roots abundant; smooth and gradual boundary.

A3 20-40 cm Dark reddish brown (2.5 YR 3/4) clay; weak medium granular structure; soft, friable, slightly plastic, sticky; roots abundant smooth and diffuse boundary.

B1 40-60 cm Dark reddish brown (2.5 YR 3/4) clay; weak medium subangular breaking down into weak fine granular structure; friable, slightly plastic, slightly sticky; few roots; smooth and gradual boundary.

B2 60-120 cm Dark red (2,5 YR 3/5) clay; massive porous breaking down into weak fine granular structure; soft, very friable, slightly plastic, slightly sticky; few roots; clear, undulating boundary.

C 120-130+ Clay loam; horizon comprising rotten rock and material, of B2.

Horizon	Depth	pH		Cation Exchange me. %									CaCO3 %
		H2O	KCl	CEC	TEB	% BS	Ca	Mg	K	Na	Al	H
A1	0-20	5.2	5.0	14.2	8.6	61	5.9	2.1	0.6	0.1	5.6
A3	-40	5.7	5.6	10.0	7.6	76	4.7	2.1	0.7	0.1	2.4
B1	-60	6.0	5.9	8.9	7.2	81	4.2	2.6	0.3	0.1	1.7
B2	-120	6.1	6.3	7.4	6.5	88	3.8	2.3	0.2	0.1	0.9
C	-130+	6.4	6.5	8.2	8.0	98	3.6	3.5	0.8	0.1	0.2

Horizon	Sol. salts		Organic Matter				Particle size analysis % 1/						Flocc index
			% C	% N	C/N	% OM	stones	c. sand f.s.		silt	clay	texture
A1			1.7	0.15	11		1	3	25	16	56	clay	57
A3			1.0	0.08	12		0	2	18	20	60	clay	48
B1			0.8	0.06			0	2	16	17	65	clay	96
B2			0.6	0.05			2	3	20	19	58	clay	100
C			0.4	0.03			0	9	35	21	35	clay loam	99

1/ International size grades

Kaolinitic yellow latosol, very heavy texture, Brazil Rep. FAO-EPTA 2197 Bennema, 1966, p. 55., Sombroek, 1966 profile 24, p. 129.

Location: 247 km S Sãn Miguel do Ouamá, Para State, 3°45' S, 47°45' W.

Altitude: 200 m.

Physiography: Flat top of high terrace

Drainage: Well drained

Parent Material: Pliocene lacustrine sediments.

Vegetation: Primeval tropical forest, dense undergrowth.

Climate: 1.482, hot tropical

Description

A00 8-5 cm Undecomposed plant residues.

A0 5-0 cm Partly decomposed plant residues with many fine roots.

A1 0-2 cm. Dark yellowish brown (10 YR 4/4) heavy clay; moderate medium to fine subangular and weak fine granular structure; many pores; friable, plastic and sticky; locally the horizon is crusty due to the intense activity of insects, especially termites; abundant roots, mostly fine; clear boundary.

A2 2-20 cm Yellowish brown (10 YR 5/6) heavy clay; moderate fine subangular blocky and very fine granular structure; many pores; soft, friable, plastic and sticky; abundant roots, gradual boundary.

B2 20-60 cm Strong brown (7.5 YR 5/6) heavy clay; weak to moderate, fine to medium subangular and weak very fine granular structure; pores; faint clayskins; slightly hard, friable, plastic, and sticky; many roots; diffuse boundary.

B3 (?) 60-150 cm Strong brown (7.5 YR 5/6) heavy clay; weak medium subangular and weak very fine granular structure; pores common; few very weak clayskins; slightly hard, friable to firm, plastic and sticky; many roots; diffuse boundary.

C (?) 150-250+ Yellowish red (5 YR 5/8) heavy clay; massive to weak medium subangular structure; few pores; very few roots.

Horizon	Depth	pH		Cation Exchange me. %									CaCO3 %
		H2O	KCl	CEC	TEB	% BS	Ca	Mg	K	Na	Al	H
A1	0-2	4.0	3.5	14.9	2.2	15	0.9	1.0	0.3	0.1	2.2	10.4
A3	-20	4.2	3.8	6.9	0.7	11	0	6	0.1	0	1.6	4.6
B2	-60	4.7	4.1	4.6	0.6	14	0	5	0.1	0	1.1	2.9
B3	-150	5.2	4.7	2.7	0.6	22	0	5	0.1	0	0.2	1.9
C	-250	5.5	4.9	2.0	0.6	28	0	5	0.1	0	0.2	1.2

Horizon	Sol. salts		Organic Matter				Particle size analysis %						Flocc Index
			% C	% N	C/N	% OM	stones	c. sand f.s.		silt	clay	texture
A1			3.6	0.33	11		0	4	11	10	75	clay	69
A3			1.3	0.13	10		0	2	8	7	83	clay	60
B2			0.7	0.08			0	1	6	5	88	clay	100
B3			0.4	0.05			0	1	13	12	74	clay	100
C			0.3	0.03			0	1	10	14	75	clay	100

Horizon	Solution by H2SO4, d = 1.47 %						SiO2	SiO2	Al2O3	P mg %
	SiO2	Al2O3	Fe2O3	TiO2	MnO	P2O5	Al2O3	R2O3	Fe2O3	Truog	Bray
A1	28.8	25.5	8.3	1.0		0.05	1.9	1.6		0.2	0.5
A3	30.8	29.6	8.7	0.9		0.03	1.8	1.5		0.2	0.2
B2	33.7	32.4	10.0	0.8		0.03	1.8	1.5			0.1
B3	33.9	32.9	10.4	0.8		0.03	1.8	1.5			0
C	32.5	33.4	9.5	1.0		0.03	1.7	1.4			0

Horizon										Moist Equiv
A1										35
A3										32
B2										34
B3										34
C										34