Soil, Water and Nutrients

Abstract

This Chapter provides an introduction to basic elements of soil science, from an understanding of the soil profile, its develop and its importance to plant growth. The processes of weathering and the development of laterites, calcretes, salinised and other major soil types and their distribution in Angola are described. Soil water relations and soil chemistry and thus the availability of water and nutrients are fundamental determinants of plant growth, species composition and productivity. The differences between dystrophic (low base status) and eutrophic (high base status) soils and the distribution of the mesic/dystrophic savanna biome and the arid/eutrophic savanna biome, which dominate Angolan landscapes (totaling over 90% of the vegetation mantle of the country) are emphasised. The Key Soil Groups of Angola are mapped and their characteristics summarised. Sandy arenosols cover 53% of Angola, mainly comprising the Kalahari sands of the eastern half of Angola. Ferralsols cover 23% of Angola, occupying the spine of crystalline rocks along the western highlands. Both are of low nutrient status but carry dense miombo woodlands where they have not been transformed by human activities. Richer soils occur along the escarpment and hot coastal lowlands. The processes of land degradation, due to inappropriate soil management threaten the livelihoods of communities living on these fragile soils, are described.

FormalPara Key Concepts and Questions: This Chapter Explains

• How soils develop and what their critical roles are as the sources of moisture and nutrients for plants and in turn for animals.

• How soils vary in physical and chemical properties and in the vegetation that they support.

• How the main soil types are classified and where they occur in Angola.

• Why over half of Angola’s land surface is covered by infertile Kalahari sands, yet these nutrient-poor soils carry dense grasslands and woodlands.

Context: Soils, Plant Growth and Soil Types

Soil is the substrate in which plants grow, where water and nutrients are cycled, and where the remains of plants and animals are transformed into their basic elements through the process of decomposition and mineralisation, mediated by fungi and bacteria that live in the soil. An understanding of the processes of soil development, and of the interactions of soil chemistry and physics to yield soil properties, is a necessary basis to answering questions about why plants grow where they do, and what root, stem and shoot structures they develop. This chapter examines the physical and chemical weathering processes, and the mechanisms that determine the availability of water and nutrients to the root hairs and leaf stomata of plants—where the transfers of moisture, nutrients and gases take place.

Soils are typically formed in situ, but in Angola, erosion and transport of soil and mineral particles by water and wind results in the current distribution of vast areas of sand that covers over one half of the country. These are known as the Kalahari sands. Like vegetation, soils have been classified into several dozen main Soil Groups. These serve as points of reference in any discussion of the ecology of Angola’s ecosystems, and how they relate to the soils of other regions of Africa and the globe.

1 Soil Structure

A first step in understanding soils is to examine their profile—exposed on a roadside cutting or by digging a soil pit (Fig. 6.1). Soils display a succession of layers, or horizons, that might or might not be present in any one soil type. From top to bottom these are the O—organic horizon, comprising accumulated organic material; A—top soil, with high biological activity, comprising a mix of organic and mineral material; B—subsoil, with a concentration of silicate clay, sesquioxides (oxides of iron and aluminium) and organic matter; C—the unconsolidated weathered regolith extending down to the bedrock. Once a soil pit has been dug, one can sample the horizons and, using simple criteria such as texture and colour, identify the type, and proceed to analyse the important properties of soil moisture holding capacity and ion-exchange capacity.

Fig. 6.1

A typical soil profile illustrating the main horizons. O = organic horizon; A = top soil; B = subsoil; C = weathered regolith. Creative Commons Attribution 4.0

The soil profile is influenced by several processes relating to drainage. Eluviation refers to the removal of mineral or organic soil material in suspension or solution from part of, or the whole horizon. Leaching refers specifically to removal of nutrients in solution. Illuviation is the deposition of soil material removed by percolating water from one part of the soil profile to another. The importance of these processes is discussed in Box 14.1.

Different soil colours result from their organic content (black), oxides (red, purple), minerals (white, grey) and drainage (mottled). While colour is useful in soil description, it has little influence on plant growth and survival. However, soil colour can be an excellent indicator of soil drainage which is a major factor influencing vegetation structure and composition in many African landscapes:

• Red soils indicate free drainage.

• Yellow soils indicate slightly impeded drainage, implying more available moisture than red soils, but generally not waterlogged.

• Grey subsoils indicate poor drainage, producing anoxic conditions which limit rooting depth.

• Mottled soils indicate frequently changing depths of a water table with both oxygenated and reduced soil atmospheres.

Soil texture is an important soil characteristic as it influences water flow and nutrient availability. Soil texture is defined by the relative proportions by weight of three main constituents—sand, silt and clay. It is the pore space within soil that determines the movement of air and water in the soil through its porosity and permeability, and access to these by plant roots. Soils with 50% of their volume as pore space provide optimal conditions for root access to air, water and nutrients. Coarse sandy soils allow rapid water infiltration and drainage, and very fine clayey soils might be poorly aerated and easily compacted. Soil scientists (pedologists) use a simple model to classify texture (Fig. 6.2), and a standard Munsell chart to describe colour.

Fig. 6.2

A soil texture chart showing the percentages of clay (less than 0.0002 mm diameter), silt (0.002–0.05 mm) and sand (0.05–2.00 mm) in the basic soil textural classes. For example, a soil with 60% sand, 30% silt and 20% clay would be classified as sandy loam. Creative Commons Attribution 4.0

A further characteristic of soil structure relates to the type of clay minerals found in them. Two types are of particular importance: Shrink/swell clays (2:1 montmorillonite or smectites), and non-shrink/swell clays (1:1 kaolin). The 2:1 and 1:1 sign relates to the atomic structure of their minerals. Kaolin, produced by the chemical weathering of aluminium silicate minerals in moist tropical climates, has low cation-exchange capacity and often has reddish colouration from iron oxides, typical of ferralsols. Montmorillonites develop in more arid climates, have high sodium content, and are dark coloured. On wetting, montmorillonite clays can expand to three times their dry structure, characterising vertisols. This swelling and shrinking limits woody plant growth due to the shearing of roots, and accounts for the extensive treeless Setaria welwitschii grasslands of the ‘terras de Catete’ that occupy much of Quiçama National Park (Figs. 3.35 and 15.11).

2 Development of Soil: Weathering and Pedogenesis

Soil is comprised of mineral and organic constituents, water and air. It is the product of the weathering of unconsolidated rocks and minerals, with the addition of nutrients and chemical components derived from organic materials. Soil has been appropriately described as 'the epidermis of the Earth'.

Weathering comprises physical, chemical and biological processes. Five factors influence the weathering of the geological substrate and the development of soil.

• Parent material. The weathering of the geological bedrock in hot wet climates, such as Angola’s, forms saprolite. The weathered saprolite layer is referred to as the regolith. It can be very shallow or absent as on steep rocky slopes, or up to 100 m in depth on flat relief. Geological substrates determine the mineral constituents from which the soil develops, and the associated soil properties such as soil structure, texture, porosity and permeability. As examples, granites produce acidic, low-nutrient soils, while basalts produce basic soils with higher nutrient status.

• Climate is the driving force of most soil formation processes. Across the 1270 km of Angola’s latitude, the climate varies dramatically, and therefore the end product of the soil development process is strongly influenced by local temperature and precipitation. The impact of leaching of soluble nutrients through the soil is particularly important in the higher rainfall regions of Angola. In general, lower temperatures and drier climates have slower soil development; warmer, wetter climates have faster rates and deeper soil development.

• Biological composition of soils, from micro-organisms to the plants and animals occupying the soils, is influenced by and in turn influences the nature of soil developed, particularly in forest/grassland mosaics. Plant roots actively mine soils for phosphorus and can increase weathering rates by an order of magnitude. Vegetation cover also influences weathering rates, with faster rates under forests and slower rates under grassland.

• Topography plays a critical role in the movement and nature of soils—from shallow gravels on steep, rapidly eroding mountain slopes—to deep alluvial soils that accumulate on bottomlands and river valleys.

• Time is required for the slow physical, chemical and biological processes needed to develop individual soil types. Soils typically require from 2000 to 20,000 years to form, yet a landscape’s soil mantle can be swept away within decades by erosion due to injudicious land management.

The products of weathering develop further through soil formation processes (pedogenesis) which vary according to substrate, climate, vegetation and hydrology. Five main soil types, resulting from distinctive pedogenetic processes are recognised in Angola. Some of the terms introduced here are defined in Sect. 6.7.

• Laterite soils are common across the humid tropics of the world where precipitation exceeds evaporation. Rapid chemical weathering of rocks and minerals results from a hot, wet climate, with leaching of nutrients out of the soils. Lateritic soils are characterised by low organic matter, high eluviation and a lack of soil structure. Eluviation is the movement of dissolved mineral and organic material from one layer in a soil profile to another layer. Oxides of iron and aluminium (sesquioxides) are not leached, and they accumulate, as displayed by the red colour (through the oxidation of iron) in the ferrallitic soils (ferralsols—that include lateritic soils) that dominate much of the Angolan plateau. Over the millions of years in which soils have formed on the extensive planation surfaces of the African plateau, thick hard crusts—duricrusts—have formed by chemical processes. These occur at or just below the soil surface and may be from centimetres to several metres thick. Duricrusts are resistant to water penetration because they are indurated (resistant to crumbling or powdering). The three most common duricrust types are laterites, calcretes and silcretes.

• Calcified soils occur in semi-arid subtropical climates, where evaporation exceeds precipitation and where the water table fluctuates through the seasons. Evaporation causes an upward movement of dissolved alkaline salts, typically calcium carbonate, from the groundwater. The upward movement is countered by a downward movement as rain water moves the salts down the soil profile. This results in gentle leaching and eluviation, resulting in a light-coloured soil. A hardpan or calcrete layer may be formed in the subsoil. Calcrete pavements occur across much of the coastal peneplains of Benguela and Namibe provinces. Silcrete is formed in a similar manner, when sand and gravel are cemented together by dissolved silica. Silcretes are found in both coastal and interior sandy arenosols.

• Salinised soils follow the evaporative process described for calcification, but with the salt deposits accumulating on the surface. This typically occurs in arid regions, where evaporation exceeds precipitation, with a negative water balance, especially where internal drainage (endorheic) basins receive mineral-rich water, such as Etosha Pan in Namibia. Examples of salinization can be seen at Gunza in Quiçama National Park (Fig. 6.3) and along the Curoca valley in Namibe.

  Fig. 6.3

  

  A soil profile at Gunza, Quiçama National Park, illustrating the formation of soils under changing climatic conditions. The upper profile is most probably a Cambisol, formed through colluvial deposits of material washed down from the nearby hills. The white lower profile comprises sodium chloride, rising by capillary action from an adjacent stream during long dry seasons. The deep salt horizon might have accumulated during an arid period of the Pleistocene, with the colluvial horizon forming during a wetter period. Photo Antonio Martins

• Podzol soils are typically found in the cool, moist climates of temperate zones, where precipitation exceeds evaporation, and where coniferous forests dominate, resulting in acidic, strongly leached soils with a white/grey E horizon and a dark brown B horizon. The accumulation of organic matter creates a strongly acidic soil solution that results in heavy leaching of cations, including the sesquioxides of aluminium and iron, from the topsoil. Podzols typically have a pale grey to white layer in the A horizon with a dark iron-humus (ferrihumic) B horizon. In Angola, podzols occur on the wetlands of cool moist valleys of higher regions of Bié and Huambo.

• Gley soils occur in poorly drained, clay-dominated areas such as wetlands and footslopes in areas of high rainfall, where the decomposition of abundant organic matter is very slow, and organic acids accumulate. Under anaerobic conditions, iron oxides are in their reduced ferrous form which is black (rather than in oxygenated soils with red ferric iron oxide), resulting in black to bluish colouration and a characteristic ‘rotten egg’ smell. Extensive areas of gleysols occur in the Moxico and Cuando Cubango provinces, such as the Bulozi Floodplain.

3 Soil Water Relations

Water Availability

Water is essential for all life on Earth. Too much water, or too little water, can be fatal. Most plants (and animals) are largely composed of water, but even at close to 95% of plant mass, the water within a plant at any time represents a minor fraction of that which passes through the plant during growth, carrying nutrients and providing a moist surface for the transfer of gases in photosynthesis. Carbon dioxide enters a plant through the tiny pores (stomata) on leaves, simultaneously allowing moisture and oxygen to escape during the respiration phase of photosynthesis. A fine balance must be reached between gas and moisture exchange processes during transpiration, photosynthesis and respiration. Soil is the medium that holds and makes water available to living organisms.

Rain falling on the soil surface drains through the soil, but much is held against gravity by capillary forces. The size and distribution of soil pores determines the passage of rainwater through the soil profile. Penetration by infiltration might be rapid in sandy soils, with large pore spaces, but slow in clayey soils. Clay soils have very fine pores and hold much more moisture per unit volume than do sandy soils. However, the higher water content of clay soils does not mean higher water availability to plants. The moisture in clay soils is held tightly by the surface tension of the fine grains and by chemical bonding, and it is thus not as accessible to the root hairs of plants as is the moisture in sandy soils. Plants growing on clayey soils can suffer from physiological drought, unable to access the moisture which is held tightly by the strong capillary forces of the fine clay particles.

Pore Spaces in Clays and Sands

If all pore spaces are filled to overflowing, the soil is saturated. If all pore spaces remain full after excess rain has drained off, with capillary water being held within the profile by capillary forces, the soil is said to be at field capacity—measured as the weight of water held in soil expressed as a percentage of the oven-dried soil weight. As a general rule, pore space increases as soils become finer. Clayey soils, with up to 60% pore space by volume, have higher field capacity than sandy soils, with 30% pore space. Water is lost from the soil profile by gravitational drainage to the water table, by the transpiration of growing plants and by evaporation.

Evaporative loss from sandy soils is restricted by its coarse texture, and water below 30 cm depth in sandy soil is reasonably protected from such evaporative loss. In contrast, the higher water-holding capacity of fine textured (clayey) soils results in the water being held nearer the soil surface, which leads to more water being lost from the soil due to evaporation, with less available for plant uptake. Clayey soils are thus generally drier and hold less oxygen than sandy soils. Swelling clays (common in drier climates, such as the ‘terras de Catete’ of Quiçama), restrict water infiltration more than the ferralsols of the more humid climate of miombo mesic savannas.

Once capillary water is no longer available to a plant, it has reached its wilting point. The difference in the amount of water held in soil at field capacity and wilting point is known as the plant available water capacity of the soil. Soil texture thus determines available water capacity. Clays have a higher surface area to volume ratio than do sandy soils, and thus hold moisture more tightly than do sands (Fig. 6.4).

Fig. 6.4

Plant available water of different soils at permanent wilting point and at field capacity. The soils differ in texture from coarse-textured sand to fine-textured clay. Plant available water is defined as the difference between field capacity and wilting point. Both field capacity and wilting point increase from coarse- to fine-textured soils, and the highest available water capacity is in the intermediate-textured soils

Water Movement from Root Hairs to Leaf Stomata

Within the plant, the water status is expressed in terms of the turgor pressure—which is the pressure exerted by the water in the cell against the cell wall. Wilting occurs when the turgor drops in response to dehydration within the plant xylem. Plants must draw water upwards from the soil, via their roots, stems and shoots, to retain turgor. The gradient in water along the soil/plant/atmosphere continuum is measured as water potential. In simple terms, if the atmosphere surrounding the plant leaf is saturated (relative humidity 100%) and the soil is at field capacity, the gradient of water potential is zero. If the gradient increases, with water loss via the leaf stomata, a transfer of water will occur along the gradient from the area of higher water potential to the area of lower water potential from soil to root hairs to leaves. As the available water in the soil declines, the water potential gradient between soil/plant/atmosphere increases. Eventually the leaf stomata close until the soil water potential again rises. The ability to maximise photosynthesis while minimising water loss via stomata (transpiration) varies greatly between plants growing in different environments. The ratio of carbon fixed through photosynthesis to water lost by transpiration is called water use efficiency. As described in Sect. 10.2, C4 grasses are more water efficient than are C₃ grasses, accounting for the success of C₄ grasses in seasonally dry tropical savannas.

Responses to Rainfall Seasonality

Available water regulates a plant’s access to moisture and nutrients. Seasonality of rainfall is a feature of Angola’s climate, and is a key characteristic of deciduousness, whether in the closed forests of the Maiombe or the mesic and arid savannas that make up the major proportion of the country’s vegetation. The pattern of hot wet summers and mild dry winters brings with it several months during which the moisture essential for plant growth is limited or even absent in the soil, which might drop below wilting point for extended periods. Furthermore, nutrients cannot be transported in the absence of available water. Trees grow tap roots for accessing water at deep levels, and fine roots close to the soil surface for nutrient uptake. This dimorphic rooting strategy also provides for the use of shallow roots for water uptake in the rain season, and for tap roots to draw on deeper resources during the dry season. As a further conservation strategy, trees in mesic savannas (miombo) conserve nutrients by withdrawing them from their leaves into their terminal branches before leaf fall, which usually occurs before the first rains. In mesic savannas leaf fall is immediately followed by a flush of new leaves, made possible by drawing on water reserves held within the plant, and with the new growth appearing before the rains.

Rainfall Interception

It is important to note that trees, grasses and litter have important impacts on the amount of water reaching the root zone of soils. The process whereby plants redistribute rain falling onto them (through evaporation or stem-flow) is known as interception. An example of the effects of interception on the water budget of mesic savanna on sandy soils is drawn from the Savanna Ecosystem Project at Nylsvley (Scholes & Walker, 1993). In this field study, of the total incoming rainfall: 47% evaporated from the soil surface; 36% was lost via transpiration (trees 21%, grass 15%); 15% was lost by evaporation from interception (by trees 6%, grass 3% and litter 7%); and only 1% infiltrated to the water table. No water was lost to runoff on the sandy soils. The study demonstrated that in broadleaf savannas, only one third of incoming rainfall is lost through the plant metabolic processes via transpiration, with the rest lost to evaporation from plant, litter and soil surfaces.

4 Soil Chemistry and Nutrient Status

Macro- and Micro-nutrients

The water in soil provides the medium in which chemicals dissolve to form an exchangeable nutrient solution, available for uptake by plants. Both water and mineral-derived nutrients are extracted from soil by the root hairs. Root systems are designed to forage for both water and nutrients according to their distribution in the soil profile. Some nutrients are needed in large amounts. These are called macronutrients and include carbon (C), hydrogen (H), oxygen (O)—(the basic constituents of all organic matter), nitrogen (N), calcium (Ca), phosphorus (P), magnesium (Mg), sulphur (S) and potassium (K). Nitrogen is fixed by living organisms from atmospheric nitrogen and is not derived from the geological parent material of soils. Micronutrients (trace elements) are needed in very small quantities for plant growth, and include chlorine (Cl), iron (Fe), manganese (Mn), boron (B), copper (Cu), molybdenum (Mo), zinc (Zn) and nickel (Ni). At high concentrations, some micronutrients such as copper and aluminium can be toxic for plant growth.

Ions: Positive Cations and Negative Anions

Nutrients comprise oppositely charged particles known as ions: those with a positive charge are cations (such as calcium Ca²⁺, magnesium Mg²⁺, potassium K⁺, sodium Na⁺ and ammonium (NH₄⁺); those with a negative charge are anions (chloride Cl⁻, nitrate NO₃⁻, phosphate PO₃⁻, carbonate CO₃⁻ and sulphate SO₄⁻). These ions bind with negatively and positively charged sites on soil particles, the availability of which is called the ion exchange capacity. Negatively charged mineral or organic soil aggregates are called colloids. Colloids are the most active constituents of soils and determine the physical and chemical properties of the soil. The total number of negatively charged sites on clay and organic matter particles is referred to as the cation exchange capacity (CEC) and the CEC of the soil is closely linked to its total clay content. Most soils contain more negatively charged sites than positively charged sites, resulting in anions such as nitrate and phosphate being leached if not absorbed by plants. Like soil texture and water retaining capacity, the CEC is an essential measure of soil agricultural quality. The process of cation exchange in soils is illustrated in Fig. 6.5.

Fig. 6.5

The process of cation exchange in soils. Cations occupying the negatively charged particles in the soil are in a state of dynamic equilibrium with similar cations in the soil solution. Cations in the soil solution are continuously being replaced by or exchanged with cations on clay and humus particles. Cations in the soil solution are also taken up by plants and leached to ground and surface waters. From Smith and Smith (2015) Elements of Ecology (9th Edition). Pearson, Boston

Bases, Base Status and Leaching

The importance of the negatively charged particles is that they prevent leaching of positively charged nutrient cations (e.g. calcium, magnesium, potassium and sodium). The cations are collectively known as 'bases' (derived from the base or alkali metals) and are lost from the ecosystem through leaching. The 'base saturation' is the percentage of the soil cation exchange capacity filled with bases. This measure decreases with increasing rainfall and increasing weathering intensity, thus the soils of arid savannas have a higher base saturation than those of the mesic savannas and forests. In some arid savanna soils, the bases exceed the CEC and the excess of salts can precipitate as calcrete, saline or sodic horizons.

Dystrophic, Mesotrophic and Eutrophic Soils

A general measure of soil nutrient properties is its base status (also referred to as the S-value)—essentially the sum of its exchangeable Ca, Mg, Na and K. Dystrophic (low base status) soils have a sum of exchangeable (as opposed to soluble) Ca, Mg, Na and K, of below 5 milli-equivalents/100 g clay. These are highly leached soils, such as those of much of the miombo systems (mesic/dystrophic savanna) of Angola’s planalto. Eutrophic soils have a high base status (exchangeable cations of more than 15 milli-equivalents/100 g clay). Eutrophic soils are typically found in the arid lowlands of southwest Angola and along the coast—the arid/eutrophic savannas. Mesotrophic soils have exchange capacities of between 5 and 15 me/100 g clay (MacVicar et al., 1977). Much of the Escarpment has mesotrophic soils, which are good for agricultural production.

Soil pH: Measures of Acidity and Basicity

An important soil characteristic is that of pH, a measure of acidity or basicity, which is controlled by its CEC. Rainwater and acids from organic matter in the soil increase the concentration of hydrogen ions in the soil, and displace other cations such as calcium on the soil exchange sites. Increasing hydrogen ions relative to other cations result in the soil becoming increasingly acidic. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. Soil pH ranges from pH 3 (extremely acid), pH 7 (neutral) to pH 9 (strongly alkaline). Acid soils, such as most of the soils of the Angolan plateau and interior, have high levels of exchangeable aluminium cations, which can reach concentrations toxic to agricultural crops. Aluminium toxicity damages plant root systems, reducing their ability to take up nutrients. Neutral or slightly alkaline soils are developed from limestones, such as those along parts of the coastal belt, where marine sediments of the Cuanza and Benguela basins occupy large areas. Soil acidity has a very strong influence on the species of plants found at any one place, and on the ecological processes involved in shaping Angola’s ecosystems.

Mineralisation

Within terrestrial ecosystems, the most important nutrients to plant growth, ecosystem structure and production, are nitrogen and phosphorus. These are found in organic form (as part of carbon-based molecules) in dead plant and animal matter. This bonding with carbon needs to be broken (mineralised) for N and P to be released as inorganic N and P, and thus become available for uptake by organisms in ionic form. Mineralisation dominates both the nitrogen and phosphorus cycles in savannas and is in turn controlled by water availability and temperature. Scholes and Walker (1993) consider that the key linkage between rainfall and primary production in savannas such as the broad-leaf Burkea africana ecosystem at Nylsvley (which is a species-poor form of miombo) probably operates via the influence of water availability on the nitrogen and phosphorus cycles, which in turn determine the carbon cycle and hence photosynthesis and primary productivity. The interactions between light, water, soil, chemicals, physical properties, plant structure and growth, and the microbial communities that drive mineralisation processes, demonstrate the complexity of even the simplest ecosystem. The role of micro-organisms in the mineralisation process is discussed in Sect. 10.6.

5 Soils and Tree Growth

Soils are a fundamental necessity for the growth of all but highly specialised plants such as epiphytes and parasites. Trees require soils deep enough to ensure sufficient root volumes to provide water, nutrients and anchorage. Trees, on average, require a rooting depth of 7.5 m, shrubs require 5.1 m, and grasses 2.6 m. Many Angolan ferralsols have hardpan (duricrust) horizons within a metre of the soil surface, which limit the size of trees that can grow on them due to the alternation of waterlogging during the rains and excessive dehydration during the dry season. Some vertisols comprise swelling-shrinking clays which damage tree roots but do not damage the shallow, fibrous roots of grasses. Gleysols are seasonally waterlogged, creating anoxic conditions that prevent tree growth. In areas receiving less than 250 mm annual rainfall, dominated by arid tropical calcisols, water constraints prevent anything more than scattered trees becoming established, except in the deep sands of ephemeral rivers which can support dense groves of tall acacias, figs and tamarisks. But for most Angolan soils, conditions are favourable for tree growth in the absence of recurrent fires. The role of soils in determining the patterns of vegetation in Angola will be discussed more fully in Part lV, specifically in Box 14.1. Most important for Angola’s human wellbeing is the maintenance of soil fertility and productive potential. The loss of this potential over much of Angola’s most productive land is a cause for serious concern, and a topic that requires fundamental ecological knowledge to reverse. The problems of soil deterioration and land degradation are discussed in Box 6.1.

Box 6.1: Human–Environment Interactions: Livelihoods and Land Degradation

Almost everything that man eats and wears is derived ultimately from plant food retained in the thin envelope of topsoil … The good health of this layer is therefore of vital importance to the prosperity of mankind. Elspeth Huxley (1937).

Huxley’s words, and her description of the problems and processes relating to poor land management, are as pertinent today as they were over 85 years ago. Indeed, the IPCC Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security and Greenhouse Gas fluxes in Terrestrial Ecosystems (IPCC, 2019) echo her words.

The IPCC report concludes that land degradation is rampant across Africa, and occurs on 46% of the total land area. Land degradation and climate change reinforce each other, creating serious implications for food security, biodiversity and livelihoods in Africa. The main drivers of land degradation include demographic growth, social displacement due to conflicts, inappropriate soil management, deforestation, shifting cultivation, insecurity of land tenure, and intrinsic features of fragile soils. Further, land use change through clearing of woodlands is a major source of greenhouse gas emissions from both plant biomass and soil organic carbon. All these IPCC conclusions apply to Angola.

Land degradation refers to the loss of the productive capacity of soils characterised by loss of soil fertility, biodiversity and overall deterioration of natural resources. Soil erosion is a key component of land degradation, and involves the relocation and loss of soil within or from a field, a decline in organic matter, in soil structure, in nutrient content and in soil fertility. It is one of the greatest threats to the production capacity and sustainability of agriculture and of rural livelihoods. A distinction can be made between deforestation (where trees are clear-felled over a given area) and woodland or forest degradation (where trees are selectively removed for construction timber, charcoal production or other uses).

Few county-wide studies of land degradation and soil erosion and their socio-economic impacts have been conducted in Angola. However, the use of satellite imagery has made some general measures possible. Schneibel et al. (2018) and Mendelsohn (2019) provide syntheses of available knowledge, and demonstrate the widespread deterioration of forest and woodland cover, and of trends in soil erosion, in Angola. While much of the Huambo, Bié and Huíla planalto was cleared in the colonial era for crop farming, the rate of deforestation has accelerated since independence, especially since the peace accords of 2002, after which many displaced rural people returned to their original homesteads. Most alarming is the finding that woodland cover in Huambo fell from 78% in 2002 to 48% in 2015 (Palacios et al., 2015). Similar trends are being witnessed across northern, central and southeastern provinces.

Small-scale subsistence farming by rural communities is based on the shifting cultivation (slash-and-burn) process. Due to land pressures on the heavily populated planalto, cultivated fields are no longer being given time to recover their nutrient and soil carbon reserves between planting seasons. The change from the tradition of shifting cultivation to semipermanent to permanent rainfed cultivation is contributing to the exhaustion of soil nutrients and decrease in food production potential. Furthermore, regular fires remove the vegetation cover that protects the topsoil from erosion by wind and rain. The result is a rapid loss of food production, loss of cash crop income and heightened levels of rural poverty.

In a detailed time-step analysis of Landsat imagery from 1989–2013, Schneibel et al. (2016) found a steady increase in the area of woodland cleared, mainly for maize production, in central Angola. The reduction of ecosystem services provided by miombo woodland is serious. Schneibel et al. (2016) found that during the period 2009–2013, the removal of 961 000 t of woodland biomass per year resulted in a maize crop of 1240 t per each succeeding year during the production lifetime (4 years) of cultivated fields. The negative trade-off between deforestation and food production, using unsustainable farming methods, is cause for concern.

The general problem of rural land degradation is not as intense as it is in areas of urban settlements, infrastructure developments and opencast mining. Many Angolan cities face regular crises when poor stormwater management results in rapidly growing gullies and canyons. High-density informal settlements (musseques) are the most seriously affected by flooding, erosion and landslides. However, the most extreme levels of soil erosion are found in the diamond mining areas of Lunda-Norte, where a century of uncontrolled opencast mining and river diversion has created vast areas of erosion and the annual spillage of millions of tonnes of sand and silt into the Congo tributaries, destroying vulnerable aquatic ecosystems and placing human communities at risk.

The socio-economic consequences of weak legislation, poor land management and a general lack of knowledge and information on the processes of soil conservation and sustainable agriculture amount to a national crisis. They point to the urgent need for agro-ecological research at the landscape level and the training of all users and administrators of Angola’s soil resources.

6 Soil Classification

Soils, like rocks, plants and animals, have been classified into a hierarchy of types. Soils have been classified on universally agreed criteria into Forms and Series—vaguely analogous to biological Orders and Families. As is the case with biomes and ecosystems, a somewhat confusing array of terms have been used by pedologists from different countries for similar soils. Fortunately, the Food and Agricultural Organisation (FAO) of the United Nations has developed a standard international soil classification system and a soils map of Africa (Jones et al., 2013). The FAO recognises 32 Reference Soil Groups (RSGs) as the highest level of classification. The next level of classification uses a set of principal and supplementary qualifiers. Angola has representatives of 17 RSGs, mapped in Fig. 6.8, and outlined in Box 6.2.

7 Key Soil Groups of Angola

The geological history and soil genesis of Angola is complex and interrelated, and as previously described, is influenced by rainfall, drainage, evaporation, wind, and time, as well as ecosystem properties and dynamics. At a regional scale, it is important to recognise the sharp contrasts in age, stratigraphy and composition of the geological structure of Angola, as presented in Chap. 4 on geology and in Fig. 4.6. The predominance of a broad belt of Proterozoic Precambrian (2700–541 Ma) crystalline rock systems along the western margin of the country, with Cenozoic (66 Ma to present) Kalahari sand systems occupying most of the eastern half, is striking. Geology determines the major soil divisions. Over three-quarters of the country is covered by two main soil groups (arenosols and ferralsols), an understanding of which provides an essential introduction to Angolan soil classification.

The characteristics of the FAO Reference Soil Groups of Angola, mapped in Fig. 6.8, are summarised in Box 6.2. A broader profile of the main groups follows.

Arenosols

The main soil group in Angola comprises sandy arenosols (solos psamíticos) that cover more than 53% of the country. These sands are dominant features of three major landscapes: the dunes of the Namib Desert; the red ‘terras de musseque’ of the coastal belt northwards from Sumbe (Fig. 6.6); and the vast Kalahari Basin. The great majority of the arenosols lie to the east of approximately 18° longitude, comprising the aeolian (wind deposited) sands of the Kalahari Basin—which cover nearly 50% of Angola and which hide nearly all of the underlying geological formations. The Kalahari Basin, extending across 2500 km from the Cape in the south to the Congo Basin in the north, and up to 1500 km in breadth, is reputedly the largest continuous body of sand in the world. The sands of the Kalahari Basin have been deposited by wind and water over the past 66 million years, with the upper layers being of more recent age, perhaps as recent as the last 5–2 Ma. Composed of quartz grains that have very little accumulated organic matter, they are of very low fertility, are acidic and of low water-holding capacity. Waters passing through the vast catchments of the Congo, Cubango and Zambezi basins that drain the Kalahari sands are therefore extremely pure. As nutrient minerals are all concentrated in the humus of the tree and grass rooting zone of the upper 20–50 cm of the soil, removal of the vegetation by cultivation inevitably results in loss of fertility and of ecological and economic value (Ucuassapi & Dias, 2006).

Fig. 6.6

Photo Antonio Martins

A typical representative of the ‘terras de musseque’ arenosols of the Luanda Plateau. Note the absence of clear diagnostic horizons in the soil profile

Ferralsols

The higher ground of the western half of Angola (the Ancient Massif) is dominated by the second major soil group, the ferralsols (solos ferralíticos) derived from underlying crystalline rocks (gneisses, granites, metamorphosed sediments of the Precambrian Basement Complex; and schists, limestones and quartzites of the West Congo System). Ferralsols are deep, infertile and dominated by 'low activity' clays such as kaolinites and other aluminium oxides. Ferralsols cover approximately 23% of Angola. The soils are mostly of low water-holding capacity. Ferralsols are usually several meters thick, with diffuse soil horizon boundaries (Fig. 6.7). Quartz is the primary mineral. Most ferralsols are clayey, a consequence of advanced weathering. The presence of micro-aggregates (particles composed of mineral, organic and biotic materials bound together during soil formation) explain the excellent porosity, good permeability and high infiltration rates of ferralsols. Their good permeability and stable microstructure make ferralsols less susceptible to erosion than most other tropical soils. Dissolved minerals are absent and cation retention is weak. Under natural vegetation, some nutrient elements taken up by the roots are eventually returned in leaves and other plant debris falling to the surface. But the bulk of all nutrients taken up is held in the living trees. Thus almost all plant nutrients available in the soil (and living plant roots) are concentrated in the upper 10–50 cm soil layer. While ferralsols have good physical properties, their low natural fertility is a serious challenge to plant nutrition and to crop production in the absence of agricultural fertilisers. The addition of lime is used to raise the pH-value of the surface soil, to combat aluminium toxicity, and to raise the soil cation exchange capacity. Ferralsols are characteristically reddish, orange or yellow (Fig. 6.7) due to oxidation of their high iron and aluminium content, which also accounts for the presence in many areas of ferricrete/laterite hardpan horizons a metre or two below the surface, impeding root and water penetration and resulting in the formation of extensive areas of laterite.

Fig. 6.7

A ferralsol profile illustrating a senile soil, with the presence of pisoliths (concretionary grains), and with a dark laterite formation at the base. Wako Kungo, Cuanza-Sul Province. Photo Antonio Martins

Fig. 6.8

Outline of the main soil types of Angola (from the FAO Soil Map of Africa), illustrating the dominance of arenosols in the eastern half of the country, and ferralsols across the western and central plateau. From Huntley (2019) based on Jones et al. (Eds) (2013). Soil Atlas of Africa. European Commission, Publications Office of the European Union, Luxembourg

These two low-fertility soil groups (arenosols and ferralsols) cover over 76% of the country. Thus, despite the adequacy of rainfall over most of Angola, agricultural production faces the challenges of low soil fertility. The soils are acidic and have high levels of silicon which is not a plant nutrient. The natural vegetation types that cover both arenosols and ferralsols—predominantly miombo woodlands—are well adapted to these soil conditions, and the untransformed landscape gives the deceptively misleading appearance of great vitality and luxuriance. These soils dominate the old, leached African Planation Surface of the Central African Plateau, dating from the long period of stability following the breakup of Gondwana. For the rest of Angola, uplift and incision by rivers during the Post-African erosional cycle rejuvenated the soil substrates, producing potentially richer soils, usually in the lower, younger landscapes of hotter, drier valleys. The erosion processes exposed rocks of volcanic origin, such as basalts, gabbros and dolerites which produce soils with the mineral elements that support plant growth, with 'high activity' clays such as montmorillonite. The younger soils are generally of higher nutrient status, but mostly occur in the lower, drier landscapes of Angola and therefore require irrigation for agricultural production.

Leptosols, Calcisols and Cambisols

The remaining 24% of Angola’s soil cover include a wide range of soil types, on specific geological substrates or positions in the landscape. In terms of landcover, the third largest soil group, occupying 6% of Angola, are the shallow leptosols or regosols (litosolos) of rocky hills and gravel plains, most extensive in the arid southwest. Other important soil types, described in Box 6.2, include luvisols, calcisols and cambisols (solos calcários, solos calcialíticos), which provide fertile loam soils for crops (including the ‘coffee forests’ of the Escarpment Zone); alluvial fluvisols (solos aluvionais) in drainage lines with high organic content and high water retaining capacity, suitable for crops if not waterlogged; gleysol clays (solos hidromórficos), typically acidic and waterlogged and occasionally very extensive, as on seasonally flooded plains such as the Bulozi Floodplain.

Box 6.2: FAO Reference Soil Groups (RSGs) Represented in Angola

(Portuguese names according to Diniz, 1991).

Leptosols (Litossolos). Soils with a very shallow profile depth (indicating little influence of soil-forming processes). They often contain large amounts of gravel. Extensive in southwest arid zone.

Solonetz (included with Arídicos tropicais). A dense, strongly structured, clayey soil that has a high proportion of adsorbed Na and in some cases also Mg ions. Solonetz soils that contain free soda (Na₂CO₃) are strongly alkaline (field pH > 8.5). Also known as sodic soils.

Vertisols (Barros). These heavy clays are known as Terras de Catete in the Sedimentary Basin of the Cuanza, north and south of Luanda. They comprise heavy alkaline or saline shrinking/swelling montmorillonite clays. These soils form wide cracks from the surface downward when they dry out. Extensive areas are found in Quiçama, where they are covered by a tall, dense monospecies grassland of Setaria welwitschii.

Gleysols (Hidromorphícos). Gleysols cover the extensive Bulozi floodplains of Moxico and other floodplains of Angola’s vast network of major rivers. They are much more abundant and widely distributed than the mapped data indicate because they occur as narrow strips at the bottom of the undulating landscapes that cover most of the Angolan interior. Such areas are too narrow to be mapped at national scales. Hydromorphic soils can be saturated for long periods, leading to reducing conditions through oxygen depletion and results in an anaerobic soil. These reduction and oxidation (redox) processes remove iron, leaving the gleyic layer devoid of the brown and red colouration normally derived from iron. The uppermost A horizon layer is rich in organic matter and may be up to 50 cm thick. This upper layer is also relatively fertile and well-suited to small-scale crop production.

Podzols. These infertile soils are usually found in moist temperate regions of the world. In Angola podzols are found in the upper sources of the great river systems on Kalahari sands. These podzols have developed under the influence of acidic humus of high water-table grasslands and wetlands, in Bié, Moxico and Cuando Cubango provinces. The soils have a bleached eluvial horizon and low cation exchange capacity. They typically have sub-surface impermeable laterites formed by the vertical migration of iron and aluminium oxides.

Nitisols (Paraferralíticos). Deep, well-drained, red tropical soils predominantly found on level to hilly land under forest or savanna vegetation in Zaire and Uíge. Among the most productive soils of the humid tropics. Ferralsols (Ferralíticos). The dominant soil group of the western plateau typical of the deep-weathered red and yellow soils of the tropics, with a low base status and kaolin clays and high content of sesquioxides and limited fertility. Laterites are common.

Phaeozems Young soils of wet grasslands within miombo, porous, dark, rich and fertile.

Calcisols (Calcarios; Arídicos tropicais). Soils characterised by a layer of translocated calcium carbonate—soft and powdery or hard and cemented. Covering much of the Cuanza Sedimentary Basin, plus the coastal plains and interior of Namibe (often as calcrete pavements, interspersed with gypsum) and lower Cunene, and northwards along the Escarpment. The translocation of calcium carbonate from the surface horizon to an accumulation layer at some depth is the most prominent soil-forming process in Calcisols, and it is common to find the surface horizon wholly or partly de-calcified. Calcisols contain only 1–2% organic matter but are rich in plant nutrients. The pH is near-neutral in the surface soil, but slightly higher at a depth of 80–100 cm where the carbonate content may be up to 25%.

Acrisols. Strongly weathered acid soils with low base saturation, in miombo. These soils generally form on old land surfaces with undulating topography, and in regions with a wet tropical climate. Acrisols are characterised by a dominance of kaolin clays, a general deficiency of base or cation minerals, as well as most other minerals that have been leached away. Exceptions are iron and aluminium. Levels of plant nutrients are low and aluminium toxicity limits plant growth. Some regeneration of Acrisols is possible if fields are used for short periods (one to three years only) and then left fallow for long periods of up to 10 years.

Lixisols (Fersialíticos). Soils developed on old landscapes. Their age and kaolin clay mineralogy have led to low to moderate levels of nutrients and high erodibility along the Escarpment. Indurated lateritic layers are frequent. Lixisols are strongly weathered soils in which clay has been washed down from the surface layers to an accumulation horizon at some depth. These soils occur in areas with a warm climate and a pronounced dry season, and are very similar to Acrisols. Lixisols have low levels of available nutrients and low nutrient reserves. However, their chemical properties are generally better than those of Ferralsols and Acrisols because of their higher pH and the absence of severe aluminium toxicity.

Alisols (Oxissialíticos). Soils with a low base saturation, highly acidic, poorly drained and prone to aluminium toxicity and water erosion. Alto Cuanza, Bié and Malange.

Luvisols (Calsialíticos). Moderately fertile and moderately base-saturated soils of younger landscapes of the Baixa de Cassange.

Cambisols (see Calcisols). Poorly developed young soils of steep slopes of the Escarpment, with moderate to high fertility.

Arenosols (Psamíticos) The deep sandy acidic and infertile soils with low organic content that occupy most of eastern Angola on the Kalahari sands, Namibe desert dunes and the red musseque sands of the Luanda Plateau.

Fluvisols (Aluvionais) The young fertile alluvial soils of mixed sediments of river valleys and well-drained floodplain margins as along the Cuanza, Longa and Luando rivers. Fluvisols form from alluvial sediments of silt and clay deposited by the periodic flooding of floodplains along rivers, and in deltas and alluvial fans and lakes. Permanent or seasonal saturation with water preserves the stratified nature of the original deposits. Over time, the sediments change (or mature), becoming stratified by chemical processes associated by drying (oxidizing) and wetting (reducing). Fluvisols generally have neutral or near-neutral pH values, which do not impair the availability of nutrients.

Regosols (see Leptosols). Weakly developed soils in unconsolidated material of eroding landscapes along escarpments such as that above the Baixa de Cassange.