Soil resources

Regional overview

The soil resources of an area are an important factor in managing the effects of drought and climate variability. Those soil properties that relate to water storage (texture, soil depth and internal drainage) are particularly critical in semi-arid environments experiencing drought conditions. Soils also reflect environmental changes, and monitoring such changes is important in assessing the impacts of land use.

The soils in the Limpopo River Basin may be categorized broadly into two main groups: (i) old soils formed on deeply weathered parent materials, influenced by earlier erosional surfaces; and (ii) relatively young soils, formed on the more recent erosional surfaces, or on alluvial deposits. Deeply weathered ancient soils occur mainly on the plateaus (highveldt) of South Africa and Zimbabwe, and in some protected areas of the escarpment zone. These soils have formed over long periods on the weathering mantle or saprolite, and have developed under warm and humid climate conditions needed for intense chemical weathering. Younger and less weathered soils characterize the denuded hills and mountain ridges, the lowveldt, the coastal plains of Mozambique, and also large parts of the higher plains within the Limpopo River Basin where recent and subrecent erosion has removed any deeply weathered soils. Recent and subrecent climate conditions have not been conducive to strong weathering and new formation of saprolite in the eroded areas. This applies also to the highveldt; higher rainfall and higher temperatures than those occurring at present are required for progressive saprolite formation.

Extensive work on soil mapping has taken place in the last 20-30 years in the subregion, including the four countries of the Limpopo River Basin. A wealth of soil information is available, but it is not easily accessible. Different systems of mapping and classification are evident, and still in use.

FAO, in cooperation with the SADC countries and the International Soil Reference and Information Centre (ISRIC), has produced a seamless, generalized soils coverage (scale: 1: 2 million) of those countries with soil and terrain digital databases (SOTER) (Figure 22). The World Reference Base for Soil Resources (WRB) classification (FAO-ISRIC-ISSS, 1998) was used as a unifying medium of communication. However, the beta version of the CD-ROM released does not contain information on critical soil attributes such as soil depth and texture, apart from what may be inferred from a number of soil profiles for which data are given (not included here). Figure 22 shows the following:

• Arenosols (recent and preweathered sands) occur on the Mozambique coast, in a zone adjacent to the Lebombo range, and in the southern half of the Botswana part of the basin.

• Solonetz soils (sodium-affected soils) cover the bulk of the Mozambique coastal plain.

• In the Zimbabwean and northern Botswanan parts of the basin, Luvisols (soils with a clay increase, not highly leached) and Leptosols (shallow soils) dominate.

• In the South African part of the basin, Regosols (weakly developed soils) dominate the lowveldt between the eastern escarpment and the Lebombo range. Leptosols occur wherever the terrain is hilly.

• Vertisols (swelling clays) and associated Nitisols (red, fertile clays or clay loams) are found on mafic rocks.

• Acrisols, soils with a low cation exchange capacity (CEC) and low base status, dominate the highveldt of the southern basin.

A message that might not be conveyed effectively by overview maps, presenting taxonomic information only, is that the soil cover is highly variable and mostly thin except for the areas covered by sandy blanket surface deposits in the southwest, the coastal plain in the east and the highveldt plateau in the south. This is due to the hard and variable geology, the dry climate and the process of basin incision.

Status of soil mapping in the four basin countries

Botswana

Extensive soil mapping in the 1980s covered most of the country at a reconnaissance scale of 1:250 000, including the Limpopo River Basin. Systematic soil description, classification and analytical methods were developed, as were computerized systems for storage and retrieval of soil information, in which 3 500 soil profiles were captured (Remmelzwaal, 1988). The soil database (SDB) developed in Botswana has become the FAO SDB standard (in terms of quantity and quality, it is one of the most comprehensive and reliable databases in Africa). An ongoing soil survey (1:50 000) is taking place within the agricultural areas. The general soil map of Botswana (De Wit and Nachtergaele, 1990) provides information and spatial distribution on a national basis.

The Limpopo River Basin is covered by a series of soils and land suitability reports and maps which include the following areas: southeast central district (Remmelzwaal, 1989), northern central district (Moganane, 1990), Gaborone (Moganane, 1989), Lobatse (Mafoko, 1990), and northeast district (Radcliffe, Venema and De Wit, 1990).

Mozambique

Soil distribution in Mozambique generally follows the physiographic characteristics (Figure 23). The southern region and the coastal plains have sandy soils, except for the rich alluvial deposits along the major rivers and streams.

Soil survey activities in Mozambique started in the 1940s with the compilation of the first soil map of Mozambique (Schokalsky, 1943). In the same decade, exploratory studies covered large areas, done principally by the Centro de Investigacoes Cientifica Algodoeira, which compiled the second soil map of the country at a scale of 1:6 million (CICA, 1948). Soil studies continued over the years, and in the early 1970s various foreign consulting firms (e.g. Loxton-Hunting, COBA and ETLAL) completed reconnaissance surveys of large areas. During this period, two published soil maps covered the whole country: (i) Carta dos Solos (scale: 1:4 million); and (ii) the Soil Map of the World (scale: 1:5 million) (FAO-UNESCO, 1974a; 1974b). In Mozambique, 34 different soil units of the FAO-UNESCO system occur, comprising 16 major soil units. However, for areas defined as belonging to the Limpopo River Basin (with the exception of a narrow coastal strip north of Maputo), the reliability of soil units is reported as poor for the upper reaches of the Limpopo River in Gaza Province to the Zimbabwe border, extending north to the Save River; and fair for the middle and lower zones of the Limpopo River to the coast. This map served as the inventory of soil resources, providing essential phase data until its revision in 1984 (Voortman and Spiers, 1984). In 1991, a revised national soil map based on descriptions of 800 soil profiles was produced at scale of 1:1 million (digitized), using the 1988 FAO-UNESCO-ISRIC legend (INIA, 1995). The National Institute for Agronomic Research (INIA) maintains computerized soil records using the FAO-ISRIC SDB. Specifically, the soils of the Limpopo River Basin were covered by a general reconnaissance survey as part of the Limpopo Master Plan studies by Selkhozpromexport (1983), covering an area of approximately 4.17 million ha.

South Africa

South Africa has a vast database on soil and terrain conditions, the national land type database, containing the data of a complete coverage of land type maps at a scale of 1:250 000. This database contains soil classification, soil attributes, terrain and climate data (Turner and Rust, 1996). A second database contains descriptive and analytical data of a vast number of representative soil profiles. These databases are archived at the Institute for Soil, Climate and Water of the Agricultural Research Council (ARC). Eleven 1:250 000-scale land type map sheets cover the Limpopo River Basin. Detailed soils maps are also available for a 2-km-wide strip along the Limpopo River and for most irrigated areas in the South African part of the basin. The Generalized Soil Patterns of South Africa is the most recent national soil map compilation (GOSA-Land Type Survey Staff, 1997). The main soil groupings of the legend are subdivided according to properties such as base status, topsoil development and texture (in brackets, the number of subdivisions):

A red-yellow well-drained soils lacking a strong textural contrast (4);
B soils within a plinthic catena (2);
C soils with a strong textural contrast (2);
E soils with high clay contents (1);
F soils with limited pedological development (5);
G Podzolic soils (1);
H rocky areas (1).

The map is useful for an overview of the spatial distribution of the main soil types in the Limpopo River Basin and its relationship with the main landscape units. The dominant soil groups are A and B, with C, E and H subordinate or occurring in complex. The oldest and most-weathered soils (B with low base status) are found on remnants of the African erosion surface. This surface is of Tertiary to Cretaceous age (Partridge and Maud, 1987) and, within the Limpopo River Basin, constitutes the highveldt southeast of Pretoria. Soils from group A with high base status are dominant in the eroded areas nearer to the Limpopo River.

Zimbabwe

The most comprehensive map available is the Soil Map of Zimbabwe, at a scale of 1:1 million (GOZ-DRSS, 1979). Although there is large uncertainty with respect to the accuracy of the information for some areas, this map is widely used. The legend to the soil map reflects primarily the degree of weathering and leaching of the soils, and the influence of geology. Nyamaphene (1991) provides a relevant summary of the soils of Zimbabwe, with details on the properties and distribution of the major soil groups.

The soils of the communal lands of Zimbabwe, which cover 42 percent of the country, were mapped at a scale of 1:250 000 in a physical resource inventory (Anderson et al., 1993). The information is presented as land units, which represent a combination of features such as geology, erosion, soils and land use. Typical soil profiles are classified according to the Zimbabwe soil classification system, and have been correlated with the legend of the Soil Map of the World (FAO-UNESCO-ISRIC, 1990) and soil taxonomy (Soil Survey Staff, 1975).

A project under the auspices of FAO, the ISRIC and the UNEP aims to update the 1979 soils map with information from the communal lands study. Information will be digitized and contribute to the revision of the 1:1 million Soil Map of the World.

Soil classification and correlation

The sustainable use of major soil groupings and specific soil types requires differential management. Soil classification is an important element of soil science, and allows transfer of information relevant to soil resources in comparable environments. Efficient transfer requires correlation of the different classification systems.

Botswana and Mozambique use the FAO soil classification system (FAO-UNESCO-ISRIC, 1990). The South African binomial system of soil classification (MacVicar et al., 1977) is applied to all major mapping programmes in South Africa. The revised South African soil classification (GOSA-Soil Classification Working Group, 1991) introduced new soil forms for the arid and semi-arid regions, which were underrepresented in the first edition. Thompson and Purves (1978) developed the classification system used in Zimbabwe, based on the inter-African pedological system of the 1960s.

Soil correlation between the first version of the legend of the Soil Map of World (FAO-UNESCO, 1974a) and the South African, Zimbabwean and Botswana soil classification systems was undertaken in the 1980s (SARCCUS Standing Committee for Soil Science, 1984). Although the results of this first effort are still relevant, there have since been two revisions of the FAO system: (i) the revised legend of the Soil Map of the World (FAO-UNESCO-ISRIC, 1990); and (ii) development of the WRB (FAO-ISSS-ISRIC, 1994, 1998; ISSS-ISRIC-FAO, 1998; ISSS Working Group RB, 1998a, 1998b).

In preparation for developing the SOTER, Remmelzwaal (1998) correlated the South African soil forms with the soil units of the WRB (FAO-ISSS-ISRIC, 1998), and between the South African soil forms and the legend units of the generalized soil map of South Africa (GOSA-Land Type Survey Staff, 1997). These classification efforts contain several useful elements for the 1:1 million South African SOTER soils definition currently under preparation, but require further analysis, and probably a development towards a larger and more precisely defined set of soil groups. Remmelzwaal (1998) also proposed the transfer of soil series information from the South African land type maps to the new SOTER system. The FAO classification and description systems are the ones most commonly used in the SADC region. The SOTER system has close links with the FAO approach, and its wider introduction would promote the standardization of soil and terrain description in southern Africa. The SOTER should be organized in such a way that it readily provides the basic information needed for land evaluation and AEZ.

The Africa volume of the Soil Map of the World (FAO-UNESCO, 1974b) shows major deficiencies as its compilation took place when insufficient soil information was available from southern Africa. The new SOTER standardized soil map covering the Limpopo River Basin (Figure 22) would facilitate land evaluation and appraisal of land suitability for various uses, once the necessary soil attributes have been provided.

Major soil units of the Limpopo River Basin

This section contains an overview of the major soil units in the Limpopo River Basin, based on available soil maps and reports and using soil classification terminology defined in the World Reference Base for Soil Resources (FAO-ISSS-ISRIC, 1998). Their occurrence is linked to the physiographic units applied to the Limpopo River Basin (above).

Soils of the high plateaus and escarpment

Highveldt

Non-incised plateau areas in the southeastern part of the basin, the highveldt east of Pretoria in particular, are mainly covered by Acrisols and Ferralsols. These deeply weathered and highly leached red-yellow soils reflect long periods and cycles of soil formation. They are characterized by an acid soil reaction, high or moderate clay contents, low CEC of the clay, and low base saturation. Ferralsols and Acrisols also occur in watersheds adjacent to the Limpopo River Basin, in South Africa and in the northern highveldt and eastern highlands of Zimbabwe. Within the Limpopo River Basin, these soils also occur as relicts in southeast Botswana, in North West Province in South Africa and at the Northern Divide in Zimbabwe. Associated soils include Leptosols, Regosols and Histosols on incised topography.

In the western highveldt areas of the basin, Arenosols and Regosols are dominant on sandstone and sandy surface deposits. These occur extensively in Botswana, and also in the western parts of South Africa, and to some extent in Zimbabwe towards the Mozambican border. Occasionally, Fluvisols and Gleysols occur on alluvial deposits.

Incised highveldt

Parts of the highveldt within the basin consist of incised topography. Examples occur in relatively close proximity to the Limpopo River in Botswana, Zimbabwe and South Africa. The soils in these areas reflect the rejuvenating effects of stream incision on the landscape (mostly in the form of shallow profiles). They also reflect the current relatively dry climate. Dominant soils on the granite/gneiss Basement Complex are Lixisols and Luvisols, with slightly acid to neutral soil reaction extending to alkaline in poorer drained conditions. These yellowish-red soils are of sandy loam to sandy clay loam texture, with medium to relatively high CEC and medium to high base saturation. Associated soils are Regosols, Arenosols and Leptosols. Calcisols, Planosols and Solonetz may occur in the lower positions of soil catenas. Most Calcisols and Solonetz occur in the driest parts of the basin.

The soil sequences found on basalt and other basic rock include Vertisols, Regosols, Luvisols and Calcisols. These predominantly dark-coloured clayey soils generally have a high base status and CEC. Basalt occurs at the border near Gaborone, and eastwards in a strip from the extreme northeastern part of Botswana across Zimbabwe towards Mozambique. Soil patterns on basalt are variable, often dominated by shallow Regosols. Vertisols occur predominantly in lower and alluvial positions, such as on the Springbok Flats north of Pretoria. Luvisols, Lixisols and Nitisols are the main soils in Zimbabwe, particularly in areas with mafic rocks (greenstone belts) around Bulawayo (highveldt) and Gwanda (middleveldt).

Hills, mountains, and higher parts of the Escarpment

Hills and mountains exhibit a larger variety of rock and weathering materials than the relatively level plateaus and plains. On the lower and middle slopes, a variety of soils occur such as Regosols, Luvisols, Cambisols and Lixisols. Leptosols dominate the higher and most-eroded hills and mountain slopes.

Soils of the lowveldt and coastal plains

Lowveldt

Soils of the escarpment foot slopes and the lowveldt itself are at best moderately weathered and show a wide range of soil characteristics, depending on parent material, position, erosion, etc. They include Vertisols, Planosols, Solonetz, Lixisols, Luvisols, Phaeozems, Cambisols, Arenosols, Regosols and Leptosols. All these soils have a neutral or alkaline soil reaction, a high base status and medium or high CEC values. However, textures and some other properties such as soil depth, colour and structure show a wide variation.

The interior plains and low plateaus of the basin in Mozambique consist almost entirely of Solonetz, associated with Solonchaks and Arenosols in secondary occurrence across large areas to the east of the Limpopo River. These soils coincide with the Mananga landscape and exhibit characteristics of coarse texture, very low water retention capacity, and low inherent fertility (especially nitrogen and phosphorus). Coupled with a low rainfall environment, these areas impose severe limitations on rainfed agriculture.

There are two distinct belts of soils running north-south on the western side of the Lebombo Ridge. Soils formed on basalt occur immediately west of the ridge. These consist of dark brown Luvisols in high landscape positions and dark grey and black Cambisols and Vertisols in lower landscape positions. Further to the west is a belt of soils mainly developed on shales and sandstones, with Lixisols, Luvisols and Arenosols dominant.

Nitisols are found in some specific locations, such as on the Lebombo Plateau. This soil type shows more intensive weathering and soil formation than generally found in the low plains, and is characterized by intermediate CEC, relatively high base saturation and high clay contents. This belt extends towards the south outside the basin.

Coastal plains and alluvial areas

The dominant soils of the coastal dunes and coastal plains of Mozambique are Arenosols, with Gleysols found in secondary occurrence. Alluvial deposits upstream from Messina are found mainly in narrow strips along the Limpopo River and its main tributaries. The most common soils are Cambisols, Luvisols, and Arenosols on terraces and levees, with some Fluvisols on recent deposits. Downstream from Messina and into Mozambique, Fluvisols dominate the extensive floodplains along the Limpopo, Changane and Elephant Rivers. Cambisols are characteristic soils of the hills and minor scarps bordering the Limpopo and Elephant Rivers, extending north from their confluence.

Problem soils and environmental aspects

Some of the soils of the Limpopo River Basin may be regarded as problem soils. The constraints may be inherently present or caused by unsustainable use (Barnard et al., 2000; Van Der Merwe et al., 2000; Nzuma, Mugwira and Mushambi, 2000). Depletion of soil resources may result from a range of interrelated natural and anthropogenic factors, whose processes and causes are elaborated more fully in the section on land degradation (below).

Restricted water-holding capacity

Although the rainfall of the basin is mostly low and erratic, large rain events occur periodically. The best soils have the ability not only to absorb and make available to plants small rainfall events of 5-10 mm, but also to absorb, store and make available the water from rain events of 50-70 mm. Three common restrictions are: inadequate soil depth (restricting the plant water reservoir); high clay content (causing runoff and low water availability); and excessively low clay content (causing excessive drainage and restricting the plant water reservoir). The presence of slowly draining material beneath a permeable rooting zone may add considerably to the profile water-holding capacity (Box 6). Figure 24 shows some examples of problem soils in the South African part of the basin.

Erodibility and crusting/surface sealing

Four relatively permanent land characteristics determine the susceptibility of land to water erosion. These are slope gradient and length, rainfall erosivity and the susceptibility of the soil to water erosion. The latter is of concern here. Solonetz and Planosols generally have low structural stability, resulting in adverse macrostructure conditions in the subsoil and susceptibility to crusting of the surface horizon. These conditions stem from the presence of relatively easily dispersible clay minerals or clay-size quartz (Bühmann, Rapp and Laker, 1996; Bühmann, Van Der Merwe and Laker, 1998; and Bühmann, Beukes and Turner, 2001) and may be aggravated severely by sodicity. These soils are rendered susceptible to erosion and require adequate management. Southern African soils in general are susceptible to crusting/surface sealing owing to a low organic matter content, high rainfall energy and sparse vegetation cover in places.

In the western arable districts of the basin, wind erosion is an acknowledged problem. This is caused by the prevalence of sandy soils (Arenosols), cultivation practices and low rainfall resulting in low plant biomass production and soil organic material. Smit (1983) and Hallward (1988) have pointed out that the main danger of wind erosion is the loss of fine materials (fine silt and clay) from topsoils in the form of dust. By losing fine material, the soil loses much of its ability to provide plants with water and nutrients.

Textural contrast

The strong textural contrast displayed by Solonetz, Planosols and some Luvisols renders them problematic from a plant-extractable water viewpoint. Some members (mostly Solonetz or Planosols) display an abrupt transition between the topsoil (or sandy layer beneath the topsoil) and the subsoil with respect to texture, structure and consistence. The material above the transition is usually of light texture, permeable and can be penetrated readily by water and roots. The material below the transition is usually clayey, dense, very slowly permeable and can be exploited by roots to a very limited extent. The subsoil is characterized by very low water stability and, thus, is highly susceptible to water erosion, particularly deep gullying, when exposed. In other members (certain Luvisols), the textural contrast is less prominent. A clear transition is found between the topsoil and the subsoil in respect of texture, structure and consistence. The topsoil is relatively sandy in relation to the subsoil, and the subsoil is clayey and dense, but commonly not to the extreme. Nevertheless, these soils are less water stable than the norm, and water infiltration is slower than the norm, rendering them prone to water erosion. Sodium is often present in the subsoils of both types, contributing to water instability and erodibility. Because of the severely limited effective depth and plant extractable water-holding capacity, textural contrast soils have become known as “droughty” agricultural soils.

Vertic properties

Vertisols and soils with vertic properties are common in South Africa and Botswana. They are reported to suffer from crusting, runoff, erosion and other forms of degradation (Van Der Merwe et al., 2000). This rather negative picture is not generally applicable, but depends rather on specific occurrence, development, and management. For example, crusting is not typical of Vertisols. On the contrary, well-developed Vertisols have a surface mulch layer and cracks rather than crusts. Infiltration rates are impeded after the first rains close the cracks and, hence, are susceptible to waterlogging, especially after heavy rains. Although Vertisols have a high water-holding capacity, they also have high water retention unavailable to plants. Nonetheless, vertic soils belong to the better and most fertile soils, but they require good management with low tillage and stubble cover.

Acidity

Acid problem soils are those most frequently reported, often without indicating their specific occurrence in the basin. Most acid soils are found in areas with relatively high rainfall, e.g. on the northern and southern fringes of the basin, in particular the South African highveldt areas occupied by Ferralsols and Acrisols. Acrisols and other inherently acid soils also occur locally in the more central parts of the basin, in southeast Botswana, in some of the central higher rainfall parts of Limpopo Province in South Africa, and in similar areas of Zimbabwe and Mozambique.

Salinization of irrigated soils

Inappropriate irrigation methods have led to saline soils, and in the worst cases result in the formation of Solonchaks, characterized by high salinity. Commercial irrigation normally applies improved management systems to control and monitor salinity levels, but under small-scale irrigation salinity is not always well managed. According to Barnard et al. (2000) about 10 percent of all irrigated soils in South Africa suffer from salinity or sodicity in one form or another, and this problem is likely to increase with the expected scarcity of water. Mashali (1997) discusses causes of salinization, its impact on production, and options to improve management.

Salinity is a major factor limiting the use of land developed for irrigation in this basin in Mozambique. Saline soils occupy 8 percent of the total productive area in the upper Limpopo River Valley, 30 percent in the middle Limpopo River Valley and as much as 70 percent in the lower Limpopo River Valley, where the Chokwé irrigation scheme is located.

The Chokwé irrigation scheme is the largest in Mozambique, and dates back to plans initially drawn up in the 1920s to irrigate the Limpopo River Valley. It was constructed in the early 1950s. It has supported intensive irrigated agriculture in the Limpopo River Valley, but has suffered badly from gross negligence in maintenance. By 1992, it was able to irrigate barely half of its design command area of about 33 000 ha (Tanner, Myers and Oad, 1993). In the past 40 years of irrigation, groundwater has risen to within about 30-50 cm of the soil surface, and the already significant land area out of production because of high soil salinity is increasing annually. The irrigation scheme suffered serious physical damage during the large-scale flooding in February-March 2000.

The problem of salinity is aggravated by the lack of adequate water management skills and by the poor drainage systems, resulting in soil fertility loss from waterlogging and salinization. Saltwater intrusion into deep-seated “soil” materials in the dry season exacerbates the risk of salinization, particularly during high tides. This is observed in the coastal areas of Xai-Xai District, from where the Limpopo River enters the sea, extending up to 50 km inland. The view is held that some Limpopo River Basin areas are too saline for complete reclamation to be economic.

Organic matter and nutrient depletion

Intensively cultivated soils in the basin generally undergo serious decline in organic matter. This results in structural and biological degradation and contributes to acidification. Organic matter contents are reported to have dropped to unacceptably low levels, leading to undesirable changes in soil structure and sharp yield declines. Folmer, Geurts and Francisco (1998) assessed the loss of soil fertility in Mozambique from agricultural land use, producing a map at scale 1:3 500 000. Negligible loss in soil fertility was reported for the middle and upper zones of the Limpopo River Basin, compared with moderate to high fertility loss south of Chokwé, particularly in Xai-Xai District and areas on both sides of the Limpopo River extending to the coast. Another study by Tique (2000) in the northern district of Chicualacuala links farmer observations to soil fertility declines associated with land pressure and reduced fallow periods in the district.

Soils with a serious decline in fertility as a result of cultivation are commonly reported from Zimbabwe and South Africa, but their extent is not known owing to the lack of monitoring on a wider scale (Van Der Merwe et al., 2000). However, the negative impact on production is well documented from experimental plots and other observations. Their main occurrence is associated with soil types that are already of low inherent fertility (Box 7). Nabhan (1997) discusses general aspects of these soils and presents management options for addressing the problem. Low fertility soils, together with soils with a low organic matter content, are the main focus of a large number of soil fertility programmes, which have been implemented throughout the region. Chapter 4 discusses the results of these programmes.

Resilience and sensitivity of soils in relation to erosion and drought

Erosion-induced loss in soil productivity is a major threat to food security. There is sufficient evidence of a relationship between changes in productivity and cumulative water erosion, following a negative exponential curve. This means that initial yield decline is severe but that after prolonged erosion the yield decline lessens (Tengberg and Stocking, 1997).

Resilience and sensitivity of soils are important factors relating to changes in productivity (Table 4). Resilience describes the property of a soil to withstand an external force; it is site specific and relates to the erosion rate or the ease of restoring the land. Solonetz (sodic soils) are one of the least-resilient soil types, while Vertisols are one of the most resilient. Sensitivity describes the degree to which the soil changes when subjected to an external force, such as erosion; and relates to the relationship between soil loss and yield or how easy it is to degrade the land. Good, productive soils are more sensitive than eroded soils. The combination of the two factors is important. Tengberg and Stocking (1997) analysed a number of major soil units with respect to these factors. The sensitivity to yield decline from erosion was found to be highest in Phaeozems and lowest in Luvisols. Of the soils studied, resilience to erosion was highest in Phaeozems and lowest in Ferralsols and Acrisols. Of key importance are: soil organic carbon, erosion-induced acidity, and soil-water relationships.

Food security issues are related strongly to soil resilience and sensitivity as these factors determine critical production levels of a soil. The number of years required to reach this level varies considerably, and is dependent on management and soil type. The time taken for the major soil groups to reach their critical production level is, in increasing order: Ferralsols, Acrisols, Luvisols, Phaeozems, Cambisols and Nitosols.

It is concluded that Acrisols and Ferralsols are unsustainable under any continuous use without rest, and that Luvisols and Cambisols allow continuous use only under good management. Management is related to soil cover. Bare or poor soil cover can result in productivity declines within 5 years, moderate cover indicates a period of 20-50 years, and a good cover 100-200 years (Tengberg and Stocking, 1997).

TABLE 4
Factors affecting soil resilience and sensitivity

S = sensitivity; R = resilience; OF = determined by combination of other factors; SOM = soil organic matter.

Source: Stocking and Murnaghan (2000).

Although not always based on sufficiently long records of monitoring soil conditions, the importance of soils, their management and erosion risk in relation to food security is evident. This may also be extended to drought as a major factor in food security, and to drought as a factor in accelerating erosion. The results of soil resilience studies are very relevant to land use planning, in particular in drought sensitive areas.