Soils and Food Security

By R. E. Hester, R. M. Harrison

The Royal Society of Chemistry

Copyright © 2012 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-426-4

Contents

Editors, xiii,
List of Contributors, xv,
Soils and Food Security: Challenges and Opportunities Peter J. Gregory, 1,
Global Soils: Preserving the Capacity for Food Production Luca Montanarella, 31,
Soil Natural Capital and Ecosystem Service Delivery in a World of Global Soil Change David A. Robinson, Bridget A. Emmett, Brian Reynolds, Ed C. Rowe, Dave Spurgeon, Aidan M. Keith, Inma Lebron and Neal Hockley, 41,
The Evaluation and Reporting of Soils in Sustainable Agriculture and Food Systems Alfred E. Hartemink, Amanda L. Raster and Molly M. Jahn, 69,
Agrobiodiversity and Potential Use for Enhancing Soil Health in Tropical Soils of Africa Fredrick O. Ayuke, Nancy K. Karanja, Julius J. Okello, Peter M. Wachira, Gerald K. Mutua, David K. Lelei and Charles K. K. Gachene, 94,
Organic Matter Availability and Management in the Context of Integrated Soil Fertility Management in sub-Saharan Africa B. Vanlauwe, 135,
Climate Change and Forest Dynamics: A Soils Perspective Wendy Peterman and Dominique Bachelet, 158,
Plant Nutrients David A. C. Manning, 183,
Soil Physical Degradation: Threats and Opportunities to Food Security Paul D. Hallett, Kenneth W. Loades and Julia Krümmelbein, 198,
Subject Index, 227,

CHAPTER 1

Soils and Food Security: Challenges and Opportunities

PETER J. GREGORY

East Malling Research, New Road, East Malling, Kent, ME19 6BJ, UK, and Centre for Food Security, School of Agriculture, Policy and Development, University of Reading, Reading, RG6 6AR, UK

E-mail: peter.gregory@emr.ac.uk

ABSTRACT

Soils most obviously contribute to food security in their essential role in crop and fodder production, so affecting the local availability of particular foods. They also have a direct influence on the ability to distribute food, the nutritional value of some foods and, in some societies, the access to certain foods through local processes of allocation and preferences. The inherent fertility of some soils is greater than that of others, so that crop yields vary greatly under semi-natural conditions. Husbandry practices, including the use of manures and fertilisers, have evolved to improve biological, chemical and physical components of soil fertility and thereby increase crop production.

The challenge for the future is to sustain soil fertility in ways that increase the yield per unit area while simultaneously avoiding other detrimental environmental consequences. This will require increased effort to develop practices that use inputs such as nutrients, water and energy more efficiently. Opportunities to achieve this include adopting more effective ways to apply water and nutrients, adopting tillage practices that promote water infiltration and increase of organic matter, and breeding to improve the effectiveness of root systems in utilising soil-based resources.

1 The Role of Soils in Food Security

There are many definitions of food security, but one that is most commonly employed is that food security is the state when “all people, at all times, have physical and economic access to sufficient, safe, and nutritious food to meet their dietary needs and food preferences for an active and healthy life”. Food security is, then, a social construct in which availability, accessibility and utilisation all contribute to its achievement (Figure 1). Food security is underpinned by effective food systems, which constitute a set of dynamic interactions between and within biogeophysical and human environments. Food systems comprise a number of activities (producing food; processing, packaging and distributing food; and retailing and consuming food) that lead to a number of associated outcomes, some of which contribute to food security (i.e. food availability, access to food and food utilisation), and others which relate to environmental and other social welfare concerns. Because food security is diminished when food systems are disrupted or stressed, food security policy must address the whole food system.

Soils most obviously contribute to food security in their essential role in crop and fodder production, thereby markedly influencing the availability of food. The inherent properties of different soils have marked effects on crop productivity (see, for example, the writings of Cato and Pliny the Elder) and, while interventions to improve fertility can over-ride these properties, some soils are inherently more fertile and productive than others. However, soils also have a direct influence on the ability to distribute food, the nutritional value of some foods and, in some societies, the access to certain foods through local processes of allocation and preferences. An obvious, if slightly extreme, example of the influence of soils on the ability to distribute food is seen in the behaviour of soils containing large amounts of swelling and shrinking clays (vertisols). These soils are frequently inherently fertile but are often very wet or waterlogged in one season making it impossible to harvest crops or to move easily across their surface, while in the dry season the shrinking of the soil induces large cracks so that engineered structures such as houses and irrigation ditches fail. The combination of shrinkage in the dry season followed by considerable swelling in the wet season means that roads are also difficult to sustain and the distribution of food can be affected.

The nutritional value of many foods is markedly influenced by the soils on which they are grown, although processed foods are often supplemented with essential minerals and vitamins to make good any deficiencies. Crop production depends on the availability of sufficient quantities of the 14 essential mineral elements required for plant growth and reproduction. These essential nutrients include the macronutrients required in large amounts by plants (nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S)) and the micronutrients (boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn)) which are required in smaller amounts. Deficiency in any one of these elements restricts plant growth and reduces crop yields, so that they are often applied to crops as inorganic or organic fertilisers to increase crop production. Humans require many more mineral elements for their wellbeing than plants. In addition to the 14 elements essential for plants, humans also require significant amounts of cobalt (Co), iodine (I), selenium (Se) and sodium (Na) in their diet and, possibly, small amounts of arsenic (As), chromium (Cr), fluorine (F), lead (Pb), lithium (Li), silicon (Si) and vanadium (V). The majority of these mineral elements are supplied to humans by plants.

Unfortunately, the diets of over two-thirds of the world’s population lack one or more of these essential mineral elements, with over 60% being Fe-deficient, over 30% Zn-deficient, almost 30% I-deficient, and about 15% Se-deficient. Dietary deficiencies of Ca, Cu and Mg are also prevalent in many countries. This mineral malnutrition is attributable to either crop production on soils with low phytoavailability of the mineral elements essential to human nutrition, or consumption of staple crops, such as cereals, or phloem-fed tissues, such as fruit, seeds and tubers, that have inherently low tissue concentrations of certain mineral elements, or both. Soils that are low in phytoavailable minerals include:

• alkaline and calcareous soils that have low availabilities of Fe, Zn and Cu; these comprise 25–30% of all agricultural land;

• coarse-textured, calcareous or strongly acidic soils that have low Mg content;

• mid-continental regions that have low I content; and

• soils derived mostly from igneous rocks that have low Se content.

In contrast, excessive concentrations of potentially toxic mineral elements may also compromise both crop production and human health. On acid soils occupying about 40% of the world’s agricultural land, toxicities of Mn and aluminium (Al) may limit crop production, while on sodic or saline soils (5–15% of agricultural land) sodium (Na), B and Cl toxicities frequently reduce crop production, and toxicities of Mn and Fe can occur in waterlogged or flooded soils. Excessive concentrations of Ni, Co, Cr and Se can limit growth of plants on soils derived from specific geological formations (e.g. serpentine) and toxic concentrations of As, cadmium (Cd), Cu, mercury (Hg), Pb and Zn have accumulated in agricultural soils in some areas due to mining and industrial activities. These toxic elements, contained in plants and animals that graze on them, can accumulate in the food chain with detrimental consequences for human health.

Soils also directly influence elements of accessibility and social preferences for certain foods. An obvious European example is the importance of “terroir” in the perceived quality of certain wines and the social cachet attached to them. In a multi-factorial experiment, it was demonstrated that the effect of soil appeared to be principally via effects on vine water status rather than effects on mineral nutrition. Similarly, in Asia, different rice types associated with different soils and growing systems have assumed positions of political, social and commercial importance.

2 Key Soil Constraints to Crop and Fodder Production

The concept of soil fertility is widely used as a framework for exploring the relationships between crop productivity and soil characteristics. It is an expression that synthesises chemical, physical and biological properties of soils and their effects on the growth and activities of root systems and the shoot. Soils can be inherently fertile because of combinations of high mineral nutrient availability, good soil structure, high available water contents and appropriate microbial and faunal communities that facilitate good root and shoot growth, or be managed to promote soil fertility through, for example, cultivation techniques that do not destroy structure or through additions of manures and fertilisers. More recently, crop genotypes have been developed to overcome some key soil constraints to fertility.

In essence, the task of farmers and their advisers is to identify the soil constraints to crop production, and then to ameliorate these with inputs and/ or management practices that minimise them so that the potential yield determined by genetic and climatic properties can be approached (Figure 2). The main factors limiting yields on many soils are: depth of soil, soil compaction, water supply, nutrient supply, erosivity, and soil reaction, including pH and salinity. These key soil constraints vary between soils and their past histories of use and management. For example, salinity is often a constraint to production on soils irrigated with low quality water and with no or limited drainage, whereas human-induced compaction may constrain production on soils cultivated with heavy machinery.

2.1 Soil pH

Many aspects of soil chemistry, and hence soil fertility, are influenced by soil pH, including the bioavailability of plant mineral nutrients, microbial activity and root growth. Nearly all natural soils have a pH between 4 and 10. Soil pH at a given location is a function of soil composition (the relative proportions and types of organic and mineral constituents) and the consequent ion-exchange and hydrolysis reactions. Generally, soil pH values of <4 are uncommon because in such acid soils aluminosilicate and oxide minerals dissolve and buffer the pH. However, in soils recently reclaimed from marine sediments, the presence of sulfur in appreciable quantities can give rise to very acidic soils on drainage (acid sulfate soils) that severely curtail plant growth. Slight-to-moderately alkaline soils are typically associated with calcareous parent materials or accumulations of calcium carbonate, although some may contain magnesium carbonate. Highly alkaline soils (pH 8.5 to 10) are usually associated with the presence of dissolved sodium carbonate, which results in the greater production of OH- ions than calcium carbonate. Such soils are typically associated with irrigated soils in arid regions.

The optimum soil pH for most crops is typically 6.5 to 7, a pH at which the availability of most plant mineral nutrients is optimal. Because rain is naturally slightly acidic, and many industrial processes result in acid deposition on vegetation and land surfaces, soil acidification is the norm. In many regions, soil pH is frequently managed by the application of materials such as lime and dolomite that act to neutralise the acidity. The growth of a wide range of crop and pasture species is enhanced by this practice, with responses ranging from 5% to 200% relative to an unlimed control. This wide range of response is due to a combination of soil and crop factors which mean that there is no unique relation between observed response to lime applications and meaningful soil parameters.

Naturally occurring acid soils with pH <5.5 in the upper soil layers occupy about 30% (4000 million ha) of the world’s ice-free land area. They are found predominantly in two geographical regions: one in the humid northern temperate zone and the other (the majority) in the humid tropics; these are areas where high rainfall leads to intensive leaching of basic cations such as calcium and magnesium, so that less soluble and acidic minerals, such as aluminium and iron oxides, become more dominant. The largest areas of acidic soils are located in North and South America (41% of the world’s soils by area), Asia (26%) and Africa (17%). Because of the difficulties of growing crops on such soils without inputs of liming materials and phosphatic fertilisers (aluminium and iron oxides absorb phosphate, strongly limiting its bioavailability), many of the world’s acidic soils are still under natural vegetation. A notable exception to this has been the recent development of the Cerrado region of Brazil in which amelioration of subsoil acidity with lime and the use of phosphate fertilisers has allowed productive agriculture to occur.

In addition to natural processes of soil acidification, some agricultural practices can also enhance the process. In particular, the use of ammoniacal fertilisers is well known as a factor leading to acidification because the nitrification of ammonium by microorganisms results in the production of protons. Similarly, the process of nitrogen fixation by free-living bacteria and bacteria living in the nodules of leguminous crops also results in acidification through the production of protons.

2.2 Saline and Sodic Soils

About 7% of the world’s total land area (930 million hectares) is salt-affected. Salt accumulation, as a consequence of irrigation accompanied by limited or no drainage, has been associated with the decline of several past civilisations, including those of Babylon, Carthage and the Hohokam Indians. Salts, particularly of sodium, magnesium and calcium, accumulate in soils because plant roots are selective in the ions that they allow into the plant, and these ions are required in smaller quantities than many others. In many circumstances this is not a problem if there is sufficient rainfall (or irrigation) to leach the accumulated salts from the soil, but in arid regions, or in areas where irrigation with water-containing dissolved salts is practised, salts build up.

Saline soils are those for which the electrical conductivity of a saturated paste extract exceeds 4 dS m-1, corresponding to an osmotic potential of about -145 kPa or a total cation concentration of about 40 mmol l-1. This approximates to values at which the growth of many plants is reduced by salt accumulation. In practice, measuring electrical conductivity of saturated paste extracts is laborious, so a soil–water (1 : 5) extract is normally used and a conversion factor applied. In contrast to saline soils, there is no universally applied definition of what constitutes a sodic soil, although the key factor is a high proportion of sodium relative to other cations. Sodicity is typically defined in relation to the exchangeable sodium percentage, which expresses the sodium on exchange sites as a percentage of the total exchangeable cations. In the USA, a value of 15% is typically used to define a sodic soil, but in Australia a value of 6% is common. The difference arises because the property of agricultural importance most frequently associated with sodicity, the dispersion of soil, varies considerably with soil type and is not uniquely related to exchangeable sodium percentage.

Naturally occurring saline soils are widespread in arid and semi-arid regions where low rainfall and no, or limited, leaching occurs to remove salts from soils (in many ways, these are the opposite conditions from those that lead to the formation of the acidic soils described in section 2.1). In many Middle Eastern countries, saline soils have formed as a consequence of irrigation with water over many hundreds of years, but in Australia dryland salinity is a more complex phenomenon. Salt has accumulated deep in soils of many parts of Australia for prolonged periods, but this was not a problem until large-scale land clearance and deforestation replaced deep-rooted perennial species with shallow-rooted annual crops, allowing increased drainage of water to the watertable. This led, in turn, to rising watertables and the upward movement of salts through the soil profile into the rooting zone of the annual crops and pastures, with severe consequences for productivity.

Irrigation-induced salinity is increasing along with the introduction of new irrigation schemes. Most commonly, it arises because of inattention to the need for adequate drainage systems without which the consequences are combinations of rising watertables, upward movement of salts from naturally-occurring saline soil layers, and accumulation of toxic concentrations of salts. Even when drainage is adequate, inadequate leaching can result in the build-up of high salt concentrations, with deleterious effects on plant growth. With pressure to use effluents and “grey water” for irrigation increasing around the world, many of which contain relatively high concentrations of salts, including sodium, there is an increased risk of salinisation of soils.

(Continues…)Excerpted from Soils and Food Security by R. E. Hester, R. M. Harrison. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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