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Technical paper 1: Soil classification and characterization

Main Contributors: B.T. Kang, B. Tripathi


1.0 Performance objectives
1.1 Introduction
1.2 Soils and their classification
1.3 The USDA soil taxonomy
1.4 The FAO/UNESCO system
1.5 The French system (ORSTROM/INRA)
1.6 Characteristics of the major soils of the tropical Africa
1.7 Management of low-activity clays (LAC) soils
1.8 Land capability classification
1.9 Feedback exercises
1.10 Suggested reading
1.11 References


1.0 Performance objectives

Technical Paper 1 is intended to enable you to:

1. Describe the advantages of soil classification and list classification systems used in Africa.

2. Discuss the hierarchy of categories in the Soil Taxonomy classification.

3. Describe the distribution of major soil orders as per the Soil Taxonomy in tropical Africa.

4. Discuss basic features of the FAO/UNESCO and French systems of soil classification and correlate them with the Soil Taxonomy.

5. Describe the main characteristics of the major soils of tropical Africa.

6. Recall major problems of low-activity clays soils and suggest measures to improve these soils.

7. Explain the Land Capability Classification System.

1.1 Introduction

Development of sustainable agricultural systems such as alley farming is an attempt to reduce degradation of natural resources and to find environmentally compatible ways of increasing production and promoting broad-scale development.

Intensification of agriculture on land currently used for traditional farming requires a thorough knowledge of the soil as a resource and attributes of the land. Information on distribution, potential and constraints of major soils is needed, so that the most appropriate soil management systems can be designed. In addition knowledge on land capability and suitability is also essential to determine the best land use for sustained crop production.

This paper reviews current systems used to classify soils and land capabilities. It also provides an introduction to the management requirements of the major soils in the humid and subhumid zones of tropical Africa.

1.2 Soils and their classification

Soil is the thin layer covering the entire earth's surface, except for open water surfaces and rock outcrops. The properties of soil are determined by environmental factors. Five dominant factors are often considered in the development of the various soils: (a) the climate, (b) parent materials (rocks and physical and chemical derivatives of same), (c) relief, (d) organisms (fauna and flora), and (e) the time factor. There are a large number of different soils, reflecting different kinds and degrees of soil forming factors and their combinations.

Figure 1. A hypothetical soil profile.

Scientists have developed different systems of soil classification to group soils of similar properties in one class, allowing them to exchange information on soils found in different areas. Soil classification also helps in determining the best possible use and management of soils. Soil classification is however a controversial subject at both national and international levels. There is lack of agreement for a common classification system, because soil scientists do not agree on the characteristics for differentiating and classifying soils.

Although many soil classification systems exist; however, two system are widely used: The USDA Soil Taxonomy and the FAO/UNESCO legend. The French system (ORSTROM) is also commonly used in France and in Francophone Africa.

The classification of soils starts with examination of soil profiles. Morphologically, soils are composed of a series of horizons. Soil horizons are layers of different appearance, thickness, and properties which have arisen by the action of various soil-forming processes. The horizons are normally parallel to the surface. Collectively, the horizons make up what is called the soil profile or soil "pedon". A soil profile is defined as a vertical section of the soil to expose layering. Figure 1 sketches a hypothetical soil profile having all the principal horizons, with a brief description of the characteristics of each horizon. Individual soils have one or more of these horizons. Very young soils may not yet have started the soil horizonization process.

In soil classification, the item to be classified is the soil profile. The classification or study of the entire profile consists of recognising and naming the horizons which make up the profile. In the study of soil profiles, sub-soil horizons are given greater emphasis than surface horizons which are frequently changed by human activity to such an extent that they bear hardly any relationship with genetic process.

1.3 The USDA soil taxonomy


1.3.1 Hierarchy of Categories in the Soil Taxonomy
1.3.2 Distribution of USDA - Classified Soils in the Tropics


The Soil Taxonomy developed since the early 1950's is the most comprehensive soil classification system in the world, developed with international cooperation it is sometimes described as the best system so far. However, for use with the soils of the tropics, the system would need continuous improvement.

1.3.1 Hierarchy of Categories in the Soil Taxonomy

There are six levels in the hierarchy of categories: Orders (the highest category), suborders, great groups, subgroups, families and series (the lowest category) (USDA, 1978).

Orders

There are ten orders, differentiated on gross morphological features by the presence or absence of diagnostic horizons or features which show the dominant set of soil-forming processes that have taken place. The ten orders and their major characteristics are shown in Table 1. The occurrence of the major soils in the humid and subhumid tropics is shown in Table 2.

Suborders

It is the next level of generalization. It permits more statements to be made about a given soil. In addition to morphological characteristics other soil properties are used to classify the soil. The suborder focusses on genetic homogeneity like wetness or other climatic factors. There are 47 suborders within the 10 orders. The names of the suborders consist of two syllables. The first connotes the diagnostics properties; the second is the formative element from the soil order name. For example, an Ustalf is an alfisol with an ustic moisture regime (associated with subhumid climates).

Great groups

The great group permits more specific statements about a given soil as it notes the arrangement of the soil horizons. A total of 230 great groups (140 of which occur in the tropics) have been defined for the 47 suborders. The name of a great group consists of the name of the suborder and a prefix suggesting diagnostic properties. For example, a Plinthustalf is an ustalf that has developed plinthite in the profile. Plinthite development is selected as the important property and so forms the prefix for the great group name.

Table 1. Brief descriptions of the ten soil orders according to Soil Taxonomy.

SOIL ORDERS

DESCRIPTION

ALFISOLS

- Soils with a clayey B horizon and exchangeable cation (Ca + Mg + K + Na) saturation greater than 50% calculated from NH4OAc-CEC at pH7.

ULTISOLS

- Soils with a clayey B horizon and base saturation less than 50%. They are acidic, leached soils from humid areas of the tropics and subtropics.

OXISOLS

- Oxisols are strongly weathered soils but have very little variation in texture with depth. Some strongly weathered, red, deep, porous oxisols contain large amounts of clay-sized Fe and Al oxides.

VERTISOLS

- Dark clay soils containing large amounts of swelling clay minerals (smectite). The soils crack widely during the dry season and become very sticky in the wet season.

MOLLISOLS

- Prairie soils formed from colluvial materials with dark surface horizon and base saturation greater than 50%, dominating in exchangeable Ca.

INCEPTISOLS

- Young soils with limited profile development. They are mostly formed from colluvial and alluvial materials. Soils derived from volcanic ash are considered a special group of Inceptisols, presently classified under the Andept suborder (also known as Andosols).

ENTISOLS

- Soils with little or no horizon development in the profile. They are mostly derived from alluvial materials.

ARIDISOLS

- Soils of arid region, such as desert soils. Some are saline.

SPODOSOLS

- Soils with a bleached surface layer (A2 horizon) and an alluvial accumulation of sesquioxides and organic matter in the B horizon. These soils are mostly formed under humid conditions and coniferous forest in the temperate region.

HISTOSOLS

- Soils rich in organic matter such as peat and muck.

Table 2. Occurrence of major soils in the Humid and Subhumid Tropics.

Classification (USDA)

Occurrence

1. Alfisols

Savanna and drier forest zones

2. Hydromorphic Soils

Valley bottom of a rolling topography

3. Vertisols

Alluvial plains in savanna

4. Ultisols

Rain forest zone and derived savanna

5. Oxisols

Rain forest and savanna

6. Inceptisols

All regions

7. Andepts (suborder of Inceptisols)

Limited and localized distribution relating to present and past volcanic activities

Subgroups

There are three kinds of subgroups:

1. The typical subgroup which represents the central concept of the great group, for example Typic Paleustalfs.

2. Intergrades are transitional forms to other orders, suborders or great groups, for example Aridic Paleustalfs or Oxic Paleustalfs.

3. Extragrades have some properties which are not representative of the great group but do not indicate transitions, for example, Petrocalcic Paleustalf.

Families

The grouping of soils within families is based on the presence or absence of physical and chemical properties important for plant growth and may not be indicative of any particular process. The properties include particle size distribution and mineralogy beneath the plough layer, temperature regime, and thickness of rooting zone. Typical family names are clayey, kaolinitic, isohyperthermic, etc. There are thousands of families.

Series

The soil series is the lowest category. It is a grouping of soil individuals on the basis of narrowly defined properties, relating to kind and arrangement of horizons; colour, texture, structure, consistence and reaction of horizons; chemical and mineralogical properties of the horizons. The soil series are given local place names following the earlier practice in the old systems in naming soil series. There are tens of thousands of series.

1.3.2 Distribution of USDA - Classified Soils in the Tropics

According to the USDA Soil Taxonomy, Oxisols are the most abundant soils in the humid and perhumid tropics covering about 35 percent of the land area (Table 3). Ultisols are the second most abundant, covering an estimated 28 percent of the region. About half of the Ultisols and 60 percent of the Oxisols are located in humid and perhumid tropical Africa and Asia. In tropical Africa, they are abundant in the eastern Congo basin bordering the lake region; in the forested zones of Sierra Leone; in Ivory Coast; in parts of Liberia; and in the forested coastal strip from Ivory Coast to Cameroon (Figure 2).

The Alfisols, which have high to moderate fertility, cover a smaller area of the humid tropics. In west Africa they are found in Ivory Coast, Ghana, Togo, Benin, Nigeria and Cameroon. They are, however, the most abundant soils in Africa's subhumid and semi-arid zones, covering about one third of these regions. The Alfisols are widely distributed in the subhumid and semi-arid tropical regions of Africa, including large areas in western, eastern, central, and southeastern Africa (Figure 2).

Table 3. Geographical distribution of soils in the humid and semi-arid tropics (millions of hectares).

Soil order

Tropical Africa

Tropical Asia

Tropical America

Total

Percent

Humid Tropics1)






Oxisols

179

14

332

525

35

Ultisols

69

131

213

413

28

Alfisols

21

15

18

54

4

Others

176

219

103

498

33

Total

445

379

666

1490

100

Semi-arid Tropics2)






Alfisols

466

121

107

694

33

Ultisols

24

20

8

52

1

Others

972

178

198

1348

66

Total

1462

319

313

2094

100

1) Data from NAP (1982).
2) Data adapted from Kampen and Burford (1980). Part of the subhumid tropics is included.

Figure 2. Soils of tropical Africa; according to the USDA soil Taxonomy (adapted from Aubert and Tavernier, 1972).

1.4 The FAO/UNESCO system

The FAO/UNESCO system was devised more as a tool for the preparation of a small-scale soil map of the world than a comprehensive system of soil classification. The map shows only the presence of major soils, being associations of many soils combined in general units. The legend of the soil map of the world lists 106 units classified into 26 groupings. The soil units correspond roughly to great groups from the USDA Soil Taxonomy, while larger main grouping are similar to the USDA soil suborder. Table 4 shows the rough correspondence between the Soil Taxonomy and the FAO/UNESCO system.

In 1986 FAO published a soil map of Africa following the FAO/UNESCO system of soil classification. In this map, all the soils of Africa have been grouped into 10 soil associations (Figure 3). Though it is not very precise, the map provides an overview of the soil resources of the continent of the ten major associations, the desert and shallow soil associations (comprising Yermosols, Xerosols and Luvisols) occupy about one-third of Africa's land area. However, only a part of the area occupied by these associations falls in the tropics.

1.5 The French system (ORSTROM/INRA)

The so-called French System of classifying soils is based on principles of soil evolution and degree of evolution of soil profiles. It also takes into account humus type, structure, and the degree of hydromorphism. The system was developed by the Office de la recherché scientifique et technique d'outre-mer (ORSTROM, now Institut français de recherche scientifique pour le développement en coopération). Correlations of Soil Taxonomy with INRA French systems are shown in Table 4.

Figure 3. Principal Soil Associations in Africa

Table 4. Correlation between systems of soil classification: the Soil Taxonomy, FAO/UNESCO legend and the INRA system.

FAO/UNESCO

Soil Taxonomy*

INRA System

Acrisols

Ultisols

Sols Lessive

Andosols

Andepts

Andosols

Arenosols

Psamments

Sols mineraux bruts

Cambisols

Tropepts

Sols bruns eutrophes tropicaux

Ferralsols

Oxisols (Latosols)

Sols Ferraltique

Fluvisols

Fluvents (Alluvial soils)

Sols mineraux bruts

Gleysols

Aquepts and Aquents (Aquic great groups of Entisols, Inceptisols)

Sols a gley peu profond peu humiferes

Histosols

Histosols

Sols hydromorphes organiques

Lithosols

Lithic subgroups

Lithosols

Luvisols

Alfisols

Sols lessives modaux

Nitosols

Tropics, Rhodic great groups of Alfisols and Ultisols


Podzols

Spodosols

Podsols

Regosols

Orthents, Psamments

Sols mineraux bruts d'apport; eolien ou volcanique; sols peu evolves regosolique d'erosion etc.

Vertisols

Vertisols

Vertisols

* = Name in old USDA system.

1.6 Characteristics of the major soils of the tropical Africa

The main characteristics of the soil orders were summarized briefly in Table 1. The following sections provide additional information on the properties and management of the most important soils in the humid and subhumid zones of tropical Africa.

Alfisols

The Alfisols are less leached and have lower acidity than Ultisols and Oxisols, but they exhibit high base saturation and their fertility is low to moderate. The Alfisols and associated soils support a wide variety of cereal crops (maize, rice, sorghum, millet), root and tuber crops (yam, cassava, cocoyam, sweet potato), and grain legumes (soybean, cowpeas, groundnuts, pigeon peas, chick peas).

Distribution of the Alfisols, Ultisols, and Oxisols is shown in the Soil Taxonomy map (Figure 2). Examples of chemical characteristics of Alfisols and Ultisols from Nigeria are given in Table 5 and Figure 4.

The productivity of the Alfisols is limited mainly by their physical characteristics:

· They have low structural stability and are susceptible to surface crusting, soil compaction and erosion.

· They have low water retention capacity and are subject to drought (Lal, 1974, Kang and Juo, 1983).

· Deficiencies of N and P are common while deficiencies of K, Mg, S. Fe, and Zn occur under intensive cultivation (Kang and Fox, 1981; Cottenie et al., 1981).

· Because of their low buffering capacity, Alfisols acidify rapidly under continuous cultivation, particularly with the use of high rates of nitrogenous fertilizers (Kang and Juo, 1983).

Figure 4 illustrates some of the chemical properties of an Alfisols profile from Southwest Nigeria, where the soil is slightly acidic with high base saturation even in the lower soil horizons.

Benefits from N. P. and K application for continuous crop production on the Alfisols have been well documented. With intensive cropping, N is the primary limiting nutrient, followed by P. Potassium is generally needed with long-term continuous cropping, particularly on soils derived from sedimentary rocks. The Alfisols and associated soils have low P-fixation and high residual effects from applied P. In addition, mycorrhiza symbiosis is common and effective on these soils particularly with root crops, resulting in a low P requirement for crop production.

Continuous cultivation and fertilizer application can significantly affect the properties of Alfisols and associated soils. Cropping, and in particular fertilizer application, reduces soil pH, soil organic matter, and extractable cations. Lowering of soil pH on the Alfisols can result in increased toxic levels of Al and Mn (Kang and Spain, 1986).

Table 5. Selected chemical characteristics of surface soils (0-15 cm) of Alfisols and Ultisols collected under natural vegetation from southern Nigeria.

pH-

H2O

Org.C


Exchangeable Cations

Total

ECEC


Bray-P


Ca

Mg

K

Acidity

(%)

me/100g


(m g/g)

Alfisols

Egbeda soil (Oxic Paleustalf), Ibadan (Derived from basement complex rocks)

6.4

1.82

3.80

1.63

0.27

0.04

5.78

7.35

Alagba soil (Oxic Paleustalf), Ikenne (Derived from sedimentary materials)

6.1

1.82

3.90

1.87

0.14

0.12

6.07

8.40

Ultisols

Nkpologu soil (Oxic Paleustult), Nsukka

4.5

1.02

0.40

0.32

0.08

1.44

2.32

9.10

Onne Soil (Typic Paleudult), Onne*

4.3

1.04

0.26

0.09

0.07

2.08

2.50

141.0

* This soil derived from marine sediments has high Bray extractable P level.

Ultisols and Oxisols

The Oxisols and especially the Ultisols are acidic, with low base saturation (Figure 4). Both soil orders commonly have multiple nutrient deficiencies (N. P. K, Ca and Zn), as shown by Kang and Juo (1983). Oxisols are highly weathered and leached, while Ultisols are susceptible to erosion and compaction. The poor productivity of these soils is due to their low capacity to provide nutrients to crops as well as their Al and Mn toxicity. Soils have medium to high P fixation. Chemical characteristics of some Nigerian Ultisols are given in Table 5.

The Ultisols and Oxisols support a lesser variety of food crops than Alfisols, being more suitable for tree crop production. Crops that do well on the Ultisols and Oxisols include some cereal crops (e.g., rice), root and tuber crops (cassava, yam, cocoyam, sweet potato), grain legumes (cowpeas, groundnuts). Plantains and bananas also do well. In traditional system, maize is grown only on newly cleared and burnt plots.

Figure 4. Soil pH, effective cation exchange capacity (ECEC), and degree of base saturation of selected Alfisol and Ultisol profiles under natural-forest vegetation from southwestern Nigeria. (Kang and van den Beldt, 1990).

In many early studies, acid soils in the humid tropics were limed to neutral pH, with generally poor results due to nutrient imbalance. Following the finding in the 1950s that acid soils contain more exchangeable Al3+ than H+, primary consideration has been given to removal of toxic factors which limit plant growth. Research on acid soils in West Africa has confirmed these findings. Low lime rates are needed to reduce toxic levels of Al3+ and application of 0.5 to 1.0 tons of lime per hectare was found to be adequate for highly acid soils (IITA, 1984). These soils are usually deficient in P as well. Rock phosphates can be used on unlimed acid soils as an inexpensive and efficient way of supplying P to acid-tolerant crops.

1.7 Management of low-activity clays (LAC) soils


1.7.1 Problems in Fertility Management of LAC Soils
1.7.2 Integrated Nutrient Management Options
1.7.3 Performance of Woody Species on Alfisols and Ultisols/Oxisols


For purpose of management, the majority of the upland soils in the humid and subhumid tropics is grouped as low activity clays (LAC) soils. A LAC soils has a low effective cation exchange capacity (ECEC) of £ 16 meq/100 g clay in the subsoil (duo and Adams, 1986). The LAC soils are predominantly Alfisols, Ultisols, Oxisols, and associated soils. Vast areas of the rainfed uplands in the humid and subhumid tropics currently used for traditional food crop production are dominated by these "fragile" soils. Observations have shown that the majority of the LAC soils in West Africa have an especially low ECEC of < 8 meq. As the clay fraction of these soils are composed mainly of kaolinite, halloysite, and oxides of Fe and Al, the soil ECEC depends mainly on the soil organic matter level, which controls nutrient absorption and release.

1.7.1 Problems in Fertility Management of LAC Soils

One of the major problems associated with extended cultivation of LAC soils is the maintenance of favorable soil physical conditions and the control of soil erosion. Significant changes in soil chemical and biological properties also occur following forest or bush fallow clearing and cropping. Soil organic matter declines sharply during the first few years under cropping and the effect is more pronounced with intensive continuous cropping.

The loss of organic matter and acidification resulted in a decrease in the effective cation exchange capacity (ECEC) and the loss of Ca and Mg (Kang and Juo, 1983). The arbitrary application of exotic, high -input food crop production technologies on these fragile soils therefore often leads to rapid chemical, physical, and biological degradation of the soil.

Although soil fertility problems on the LAC soils can be corrected by liming and appropriate fertilization, socioeconomic constraints often limit the application of these crop production technologies in many areas of tropical Africa. Currently, sub-Saharan Africa's per capita and per hectare fertilizer use is very low compared with that of other regions. There is a need to develop integrated soil fertility management systems for the region based on better utilization of local nutrient sources. Such systems should be supplemented with external inputs wherever that is feasible and affordable.

For sustained crop production in addition to adequate supply of plant nutrients, the LAC soils also require continuous addition of organic matter.

1.7.2 Integrated Nutrient Management Options

Integrated soil fertility management for LAC soils can be achieved by various methods including:

· promoting maximum recycling and more efficient use of nutrients from plant residues,
· increasing contribution of biological nitrogen fixation,
· improving efficiency of use of mineral nitrogen fertilizers and local sources of phosphate fertilizers,
· using organic residues to reduce soil acidity problems, and
· using acid-tolerant cultivars.

Use of low levels of chemical inputs in combination with fallowing and agroforestry systems has shown varying degrees of success. Fallowing and addition of organic mulches may correct chemical soil degradation resulting from continuous cultivation; at the same time, it may also increase efficiency of fertilizer use.

Crop residue management and seed bed preparation methods can play an important role in sustaining the productivity of these soils for crop production. This can be achieved in reduced tillage systems through the use of crop residue mulches, in situ mulches from cover crops, and/or hedgerow prunings from alley farming. The presence of adequate amounts of mulch cover helps maintain high soil nutrient status and high biological activity. Mulch also protects the soil against high temperatures, soil erosion, and run-off, thereby preventing the breakdown of soil structure and the resultant soil compaction and decreased permeability. Furthermore, mulching increases soil moisture retention and reduces runoff and soil erosion (Lal, 1974; Kang and Juo, 1986).

Results of long-term field experiments carried out on Alfisols have also shown that with judicious fertilizer use and crop rotation, high and sustained crop yields can be obtained (Kang and Juo, 1986). Similar principles also apply for managing the Ultisols/Oxisols. For sustained crop production, the Ultisols and Oxisols additionally require judicious liming (IITA, 1984; Nicholaides et al., 1984).

1.7.3 Performance of Woody Species on Alfisols and Ultisols/Oxisols

The integration of food crops and forages with multi-purpose tree species (MPTs) in agroforestry and alley farming systems have received much attention in recent years as an alternative, low chemical input management possibility for LAC soils. However, little information is available on the soil requirements for growing the MPTs.

As with crops, the capacities of MPTs for biomass production and nutrient recycling are affected by soil and climatic conditions. Under the same climatic regime, growth and biomass production of MPTs is expected to be higher on the more productive Alfisols than on the less productive Ultisols/Oxisols. Additions of nutrients may be needed for good growth of MPTs.

MPTs for alley farming such as Leucaena leucocephala and Gliricidia sepium do well on non-acid or slightly acid Alfisols. Both species perform poorly on acid soils. On the low pH soils, MPTs such as Acioa barteri, Calliandra calothyrsus, and Flemingia macrophylla perform well.

1.8 Land capability classification


1.8.1 The USDA Land Capability Classification System


The technique which allows determination of the most suitable use for any area of land is called land classification. A great number of systems of land classification are in use, varying mainly according to the purpose for which the land is classified. Land may be classified according to its present land use, its suitability for a specific crop under the existing forms of management, its capability for producing crops or combinations of crops under optimum management, or its suitability for non-agricultural types of land use. A good knowledge of the land capability and suitability combined with good understanding of the soil characteristics and management aspects are the keys to more productive and sustainable agriculture.

The purpose of land capability classification systems is to study and record all data relevant to finding the combination of agricultural and conservation measures which would permit the most intensive and appropriate agricultural use of the land without undue danger of soil degradation.

1.8.1 The USDA Land Capability Classification System

The best known of these systems is the United States Department of Agriculture system (Klingebiel and Montgomery, 1961). The USDA land classification system is interpretative, using the USDA soil survey map as a basis and classifying the individual soil map units in groups that have similar management requirements. At the highest of categorization, eight soil classes are distinguished, namely:

Class I soils have few limitations restricting their use. Erosion hazards on these soils are low; they are deep, productive and easily worked. For optimum production, these soils need ordinary management practices to maintain productivity, as regards both soil fertility and favorable physical soil properties.

Class II soils have some limitations that reduce the choice of plants or require moderate conservation practices. Limitations of soils in Class II include (singly or in combination) the effect of gentle slopes, moderate susceptibility to erosion, less than ideal soil depth, somewhat unfavorable soil structure, slight to moderate correctable salinity, occasional damaging overflow, wetness correctable by drainage, slight climatic limitation. Soils in this class require more than ordinary management practices for obtaining optimum production and for maintaining productivity.

Class III soils have severe limitations that reduce the choice of plants or require special conservation practices. The limitation of soils in this class are those of Class II, but in higher degree; including additional limitations such as shallow depth, low moisture-holding capacity, and low fertility that is not easily corrected. Class III soils require considerable management inputs, but even so, choice of crops or cropping systems remains restricted because of inherent limiting factors.

Class IV soils have very severe limitations that restrict the choice of plants and or require very careful management. Restrictions, both in terms of choice of plants and or management and conservation practices are greater than in Class III to such an extent that production is often marginal in relation to the inputs required. Limiting factors re of the same nature as in the previous classes but more severe and difficult to overcome. Several limitations such as steep slopes are a permanent feature of the land.

Some of the limitations due to sloppiness and erosion hazards in classes II to IV can be reduced by biological terracing as practiced in agroforestry and alley cropping.

In the USDA system, soils of classes V to VIII are generally not suited for cultivation, although certain of them may be made suitable for agricultural use with costly measures.

Class V soils have few or no erosion hazards but have other limitations, impracticable to remove, that restrict their use to pasture, range, woodland, or wildlife food and cover. Although they may be level or nearly level, many of these soils are subject to inundation or are stony or rocky.

Class VI soils have severe limitations- that make them generally unsuited to cultivation and limit their use largely to pasture or range, woodland, or wildlife food cover. This class is a continuation of Class IV, with very severe limitations that cannot be corrected. They may serve for some kinds of crops, such as tree crops, provided unusually intensive management is practiced.

Class VII soils have very severe limitations that make them unsuited to cultivation and also, restrict their use largely to grazing, woodland, or wildlife. The limitations are such that these soils are not suited for any of the common crops.

Class VIII soils and land forms have limitations that preclude their use for commercial plant production.

In the second level of generalization of the USDA land capability classification system, sub classes specify the kind of limitations. Four kinds of limitations are recognized at this level, namely, risk of erosion; wetness, drainage or overflow; rooting zone limitations, and climatic limitation. The third level, that of the capability unit, provides more specific and detailed information for application to specific fields on a farm.

A new standard framework for land evaluation by means of land suitability classification has been developed by FAO (1983). As in other systems, the land suitability component of land evaluation is based on the survey of the physical attributes of the land (soils, climate, vegetation, topography, hydrology, etc.), and consequently requires interpretation of these attributes. The proposed FAO land suitability classification integrates relevant social and economic factors with the technical suitability classification. At the present stage, the system mainly concentrates on the classification of land based on technical suitability.

1.9 Feedback exercises

All answers can be found in Technical Paper 1.

1. Provide brief answers to the following questions:

i) What is soil horizon? What is soil profile?
ii) Name the factors that are often considered in the development of soil.
iii) What is land capability classification?
iv) In the USDA system for classifying land capability, what lauds of criteria are used to assign soils to a particular class?

2. a) Name the three soil orders that are most abundant in Africa's humid tropics, with approximate percentages, and FAO/UNESCO names.

Soil Order

Percentage of Land Area in African humid tropics

FAO/UNESCO Name(s)

1. _______

____ %

______

2. _______

____ %

______

3. _______

____ %

______

b) Draw lines to connect the names of soil order (left) to their characteristics (right). The names of soil order are from the USDA Soil Taxonomy.

ALFISOLS

Soils rich in organic matter such as peat and muck.

ULTISOLS

Young soils with limited profile development.

OXISOLS

Strongly weathered soils with very little variation in texture with depth.

VERTISOLS

Dark clay soils containing large amounts of clay minerals.

INCEPTISOLS

Soils with a clayey B horizon and exchangeable cation saturation greater than 50.

HISTOSOLS

Acidic, leached soils from humid areas of the tropics and subtropics.

3. Answer by circling T for true or F for false:

i) Shortening of the fallow period in traditional farming results in a decline in soil organic matter

T

F

ii) Alley farming is a low chemical input technology and is not appropriate for low activity clays (LAC) soils

T

F

iii) Acid Ultisols and Oxisols are better suited to tree crop production while Alfisols can be used for a wider variety of crops.

T

F

iv) In situ mulches and hedgerow prunings are two options for sustaining productivity. Alley farming makes use of the second option.

T

F

v) On acid soils, Leucaena and Gliricidia perform better than other hedgerow tree species.

T

F

1.10 Suggested reading

Boul, S.W., Hole, F.D. and R.J. McCrackens. 1979. Soil genesis and classification (2nd Edition). Ames, Iowa: Iowa State University Press.

Kalpage, F.S.C.P. 1974 Tropical Soils; Classification, Fertility and Management. New York: St. Martin's Press.

1.11 References

Aubert, G. and R. Travernier. 1972. Soil survey. In: Soils of the humid tropics. U.S. Washington: National Academy of Sciences.

Cottenie, A., Kang, B.T., Kiekens, L. Sajjapongse, A. 1981. Micronutrient status. pp. 149-163. In: Greenland, D.J. (ed.). Characterization of soils in relation to their classification and management for crop production: Examples from some areas of the humid tropics. London: Oxford University Press.

FAO (Food and Agriculture Organization). 1983. Guidelines: Land evaluation for rainfed agriculture. Soils Bulletin No. 52. Rome: FAO.

FAO (Food and Agriculture Organization). 1986. Atlas of African Agriculture. Rome: FAO.

International Institute of Tropical Agriculture (IITA). 1984. Farming system program research highlights 1981-1984. Ibadan, Nigeria. IITA.

Juo A.S.R. and Adams, F. 1986. Chemistry of LAC Soils. pp. 37-62. In: Proceedings of Symposium on Low Activity Clays (LAC) Soils. SMSS Technical Monograph No. 14. Washington DC.

Kampen, J., and Burford, J. 1980. Production systems, soil related constraints and potentials in the semi-arid tropics with special reference to India. pp. 141-165. In: International Rice Research Institute (IRRI) (ed.). Priorities for alleviating soil-related constraints to food crop production in the tropics. Los Banos, Philippines: IRRI.

Kang, B.T. and Fox, R.L. 1981. Management of soils for continuous production and controlling nutrient status. pp. 202-213. In: Greenland, D.J. (ed.). Characterization of soils in relation to their classification and management for crop production. Examples of some areas of the humid tropics. London: Oxford University Press.

Kang, B.T. and Juo, A.S.R. 1983. Management of low activity clay soils in tropical Africa for food crop production. pp. 450-470. In: Beinroth, FH, Neel H., Eswaran H. (eds.). Proceedings of the Fourth International Soil Classification Workshop, Kigali, Rwanda. Brussels, Belgium: ABOS-AGCD.

Kang, B.T. and Juo, A.S.R. 1986. Effect of forest clearing on soil chemical properties and crop performance. pp. 383-394. In: Lal, R. Sanchez, P.A., Cummings, R.W. (eds.). Land clearing and development in the tropics., Rotterdam, Netherlands: A.A. Balkema.

Kang, B.T. and Spain, J.M. 1986. Management of low activity clays with special reference to Alfisols, Ultisols and Oxisols. pp. 107-131. In: Proceedings of Symposium on Low Activity Clays (LAC) Soils. SMSS Technical Monograph No. 14. Washington DC.

Kang, B.T., and VandenBeldt, R. 1990. Agroforestry systems for crop production in the tropics with special reference to West Africa. pp. 13-34. In: Moore, E. (ed.). Agroforestry land-use systems. Waimanalo, Hawaii: NFTA.

Klingebiel, A.A. and Montgomery, P.H. 1961. Land capability classification. Agriculture handbook no 210. Soil conservation service, Washington D.C. US Department of Agriculture (USDA).

Lal, R. 1974. Role of mulching techniques in tropical soils and water management. Technical Bulletin I. Ibadan, Nigeria: IITA.

NAP (National Academy Press). 1982. Ecological aspects of development in the humid tropics. Washington D.C.: National Academy Press.

Nicholaides III, J.J., Bandy D.E. Sanchez, P.A., Villachienea, J.H., Coutou, A.J., Valverde, C. 1984. From Migratory to continuous agriculture in the Amazon basin. pp. 141-168. In: Improved production systems as an alternative to shifting cultivation. Soils Bulletin No. 53. Rome: FAO.

UNESCO (United Nations Educational, Scientific and Cultural Organization). 1974. FAO/UNESCO Soil map of the world, 1:5,000,000 Vol. 1 Paris: UNESCO.

USDA. 1978. Soil Taxonomy. Agriculture Handbook no. 436. Washington D.C.: USDA, Soil Conservation Service.


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