The region of the world stretching from Morocco to Pakistan has been variously referred to as the “Middle East” or the Near East; the term WANA is more embracive of more recent vintage. That area of the world is great anthropological, social, cultural, political, scientific, and economic significance for mankind. It is the center of origin of some of the world’s food crops, notably cereals and pulses, and the site where sheep (Ovis aries) and goats (Capra hircus) were domesticated and where settled agriculture began in the “Fertile Crescent” (Harlan, 1992), an arc-shaped swathe of the Near East that comprises Lebanon, Syria, Turkey, Iran, and Iraq. The region subsequently supported the Roman Empire with cereal supplies and led to a flowering of western civilization, and became the source of some of the world’s great religions. Yet, as in other areas of the world defined by the constraints of an arid to semi-arid climate (Stewart and Robinson, 1997), the Mediterranean region is beset with many biophysical obstacles to its agricultural sector.
One of the great ironies of history is that this once—bountiful areas of the world is now largely a food-deficit one, mainly due to rapid population growth that has characterized the region’s development, with serious water and land constraints to support agricultural production (Khuriet al.,2011).
According to the Global Hunger Map (IFPRI, 2010), the situation of many countries in the WANA region is currently not critical (less than 5% of under-nourished population). However, the expected population increase in the next few decades will generate greater food demand and the need for of increased agricultural output (Roy et al., 2006). While current food deficits can be met by importation, the projected deficits of virtually all food staples in most countries of WANA are disquieting (Rosegrant et al., 2002). Any discussion of constraints to sustainable agricultural
production—and therefore societal wealth—has to center on its low rainfall and the inherent restriction of the bimodal (wet, dry) climate, and limited water resources as well as cultivable land (Harris, 1995; Kassam, 1981). Any meaningful examination of the potential of fertilizers to contribute to agricultural output under such circumstances must consider the limitations in climate, soils, and water resources (Ryanet al., 2006a).
Land use in the Mediterranean region is extremely diverse, with a range of agro-climatic zones (Fig. 5), and a wide variety of crops being grown under rainfed and irrigated conditions. Livestock plays a key role in this region and, is in most cases, characteristically interrelated with other land uses, through residue and stubble grazing or use of marginal lands, in parti- cular rocky shrubs, or woodlands, and the more arid rangelands, most of which are overgrazed. Irrigation is practiced on only a small proportion of the land, usually 10% or less, but accounts for a disproportionate percentage of the region’s agricultural production. Although the area under irrigation is still expanding, supply constraints are likely to increase for several reasons;
limitations on the total size of the extractable water resources, continued population growth coupled with increasing urbanization, and competition between communities, industrial and service sectors, and agriculture for increasingly scarce water. As expounded later, rainfed agriculture is the dominant form of crop production; wheat, barley (Hordeum vulgare), food legumes are the dominant crops.
3.1. Climate and environmental conditions
Water availability is a serious constrain for agricultural in the world dryland regions, where drought and related biophysiological factors create a fragile and uncertain environment for production (Rosegrantet al.,2002). From
Figure 5 Land use and land cover in the Mediterranean region (from Ryan et al., 2006a).
time immemorial, lack of rainfall has dominated the fortunes’ of the WANA or Middle East region, with numerous references in the Bible to “famine”
and “drought.” The climate has not perceptibly changed in the past two millennia; the only difference is that now drought has severe implications for vastly greater numbers of people. Unfortunately, with the current manifestation of global climate change, the WANA region is likely to become even drier than it already is (IPCC, 2008). While the Mediterra- nean climate of the region has been described in detail by authors such as Kassam (1981)and more recently in the context of cropping systems and N use (Ryanet al., 2008a, 2009a), some brief mention of key climatic features are pertinent in the context of P use.
The defining feature of the typical Mediterranean climate is its bi-seasonal nature, that is, a relatively cool, moist winter-spring season (November–April) and a dry hot summer season (Kassam, 1981). The onset of rain in the fall coincides with a decrease in temperature and consequently in evapotranspiration; rainfall normally peaks in January–
February, along with minimum temperature as illustrated graphically in Ryan et al. (2008a). Seasonal rainfall varies in space and time in the WANA region (Harris, 1995); there is no one pattern that describes the entire region. Highland areas have higher rainfall and colder conditions, including severe frosts and snow, while lowland areas are milder with lower rainfall (Harris, 1995). Similarly, rainfall is dictated by landforms; inland from the Mediterranean basin, the climate increasingly assumes more con- tinental features, with lower rainfall and greater extremes of temperature.
Thus, climatic conditions of the region can be described as semi-arid, where rainfall is normally sufficient to support dryland or rainfed agricul- ture, and arid where rainfall is too low (<200 mm) to support crops and where rangeland or steppe predominates, with deserts in the extremely low rainfall areas (<100 mm). Notwithstanding the variability of rainfall across the WANA region, rainfed agriculture is generally practiced in areas having 200–600 mm rainfall. While rainfall can vary from year to year, fluctuations are greater in the lower range (200–350 mm). Similarly, considerable within-season variation can occur (Harris, 1995). While rainfall is obviously linked to crop growth and yields by satisfying its physiological need for moisture, it influences the behavior of P in soils with indirect effects on crop P uptake. As an immobile element, P diffusion from the soil to plant roots is enhanced by favorable moisture conditions, which also promotes greater root exploitation of the soil volume and greater P uptake. Favorable soil moisture conditions also promote mineralization of P from the soil’s reserve of organic matter.
Besides rainfall limitations, salinity is often a relevant problem in dryland areas (Sakadevan and Nguyen, 2010; Szalbocs, 1989). Scarcity of water, in terms of quantity and quality, leads to high risk of salinization of agricultural land that is increased under dryland conditions (Keren, 2000), which
determines that salinity problems are most extensive in the irrigated arid and semi-arid areas (FAO, 1988). Thus, main limitations to agricultural produc- tion in these areas derive from low water potential in soil, due to physical—
drought—or chemical reasons —low osmotic potential. In WANA, these reasons contribute to explain that 10% of the land area being amenable to rainfed cropping (Matar et al., 1992), the most extreme case being Egypt, with less than 3.5% of the land used for agriculture and with very high risks of salinization in irrigated areas (Royet al., 2006).
3.2. Landscape features
The WANA region displays a wide range of landscape features, that is, high mountains, some of which still have forest cover, plateaus, coastal plains, river valleys, and dry deserts and scrubland (Clawsonet al., 1971). While major centers of population were sited in river valleys and fertile plains, where water sources were available to sustain the population, most of the WANA region reflects centuries of cultivation and land abuse. Most of the original forests have been cleared for fuel and buildings and to make way for cultivation. The presence of terraces in some parts of the region has allowed cultivation on hillsides (e.g., Lebanon, Yemen). The bare rocky landscape that describes most of the region bears mute testimony to centuries of soil erosion and land degradation (Carter, 1964). While some efforts are being made to redress the situation with reforestation, such efforts are limited and do not address issues that impinge on land use such overgrazing and common grazing lands; nowhere is the “tragedy of the commons” in evidence than in the Middle East region. A large but generally unrecognized problem for agriculture in the WANA region is the relative absence of zoning regulations and effective implementation. Thus, the limited amount of agricultural is further reduced by urban expansion which is occurring at an alarming role in the WANA region. Such expansion, mainly onto the most productive land pushes the boundary of cultivation toward more marginal land (Keyzer, 2010; Young, 1999). Even with curtailment of urbanization, the small percentage of land for cultivation well decrease further, with corresponding demand to produce more from that land.
3.3. Soils and soil components
Many of the world’s major Soil Orders are represented in the WANA region. Given the diversity in topography, climate, and vegetation; the distribution of such soils in relation to rainfall and landforms has been depicted by Ryanet al. (2006a). Cropland in the region is dominated by eight Soil Groups according to the World Reference Base for Soil Resources (IUSS Working Group WRB, 2007): Calcisols, Cambisols,
Fluvisols, Kastanozems, Leptosols, Luvisols, Regosols, and Vertisols.
Solonchaks, Gleysols, Gypsisols, and Arenosols are also common, even though they are largely confined to uncultivated areas or pastureland. The pedo-diversity of the region is reflected, among other properties, in the effective soil depth, particle size distribution—and hence water-holding capacity—and mineralogy of the clay fraction. The soil organic matter (SOM) content, an important factor in modifying soil physical properties and serving as potential nutrient reserves, usually ranges from 0.5% to 1.5%, higher values being often recorded in Kastanozems and Vertisols; in eroded areas, SOM rarely exceeds 0.5%. The carbonates are represented by calcite (CaCO3), dolomite [CaMg(CO3)], and Mg-calcite. Of these carbonates, dolomite is lithogenic, whereas calcite can be either pedogenic or lithogenic.
Calcic horizons with high concentrations of carbonates are common- place in the region; in many areas, the calcic horizons have been exposed at the surface by plowing or as the result of erosion by water. In contrast to its lithogenic counterpart, pedogenic (secondary) calcite occurs mainly in the fine-sized (silt and clay) fractions and thus accounts for most of the surface area pertaining to the mineral (Borrero et al., 1988). Many authors use
“active lime,” defined operationally as the carbonate capable of reacting with a neutral NH4oxalate solution (Drouineau, 1942), as a measure of the reactivity of soil carbonate because of its correlation with the surface area of this constituent (del Campillo et al., 1992). The specific surface area of carbonate is negatively correlated with the total CaCO3content of the soil, probably because secondary calcite crystals tend to become larger, and thus less reactive, as more carbonate is precipitated in the calcic horizon (del Campilloet al., 1992). Although dominant soils in the area are not classified has high P-fixing (Delgado and Scalenghe, 2008), P reactions in carbonate- dominated soils negatively affect P solubility (Delgadoet al., 2002a), there- fore, the efficiency of applied P fertilizers.
Iron (Fe) oxides (a term used here to designate both oxides and oxy- hydroxides of Fe) are generally concentrated in the clay fraction, and typically account for less than 10% of the clay (Torrent, 1995). Because the degree of weathering of most soils in the region is limited, only part of the Fe oxides present is the soils are actually pedogenic, the rest of them being inherited from the parent material. The moisture and temperature regimes of soils in the region generally favor the crystallization of ferrihydrite (the initial, poorly crystalline oxyhydroxide arizing from the weathering of Fe-bearing primary minerals) into crystalline goethite (a-FeOOH) and hematite (a-Fe2O3) (Schwertmann, 1985). Thus, the ratio between the contents in poorly crystalline and total Fe oxides, as represented by the Feo (acid oxalate-extractable Fe)/Fed (citrate/bicarbonate/dithionite- extractable Fe) ratio, is, as in many soils of semi-arid areas, less than 0.2 (Ryan et al., 1985a,b; Singer, 1978). However, the poorly crystalline Fe
oxides have a disproportionate effect on the reactivity of Fe oxides because their specific surface area, for example, 600 m2 g1 (Cornell and Schwertmann, 2003) is much greater than that of their crystalline counter- parts, for example, 50–100 m2g1(Pen˜a and Torrent, 1984).
The most abundant silicate clay in the region’s soils is illite, followed by smectite and kaolinite. Illite is mostly inherited from the parent material, whereas smectite often results from neoformation processes taking place in soils with a non-leaching regime, as is the case with many Vertisols. Due to the limited rainfall in the Mediterranean region, a notable feature of most soils is an absence of leaching and its influence on profile development and nutrient distribution. The clay fraction of most soils in the WANA and other regions with a semi-arid climate exhibits a mixed mineralogy, which results in relatively large values of specific surface area and cation exchange capacity, for example, 40–60 cmolc kg1(Torrent, 2005). The implications of clay mineralogy for P reactions are dealt with in a subsequent section.
3.4. Agriculture and cropping conditions
Since the dawn of settled communities in the Middle East, agriculture has been dominated by rainfed cereal production (Gibbon, 1981), wheat and barley, in association with small ruminant livestock production, mainly sheep and goats. Olives (Olea europea), vines (Vitis spp.), fruit trees, and vegetables are widely cultivated. In terms of area, cereals dominate; wheat tends to be grown in the more favorable and stable rainfall zones and barley, which is more drought tolerant, is more common in drier areas. Kassam (1981)estimated that dryland cereals could be grown in about half the land area of North Africa and on over 60% of the area of West Asia. While much variation occurs in the region with respect to cropping systems, the practice of alternative food and forage legumes with cereals is common; chickpea (Cicer arietinum) and lentil (Lens culinaris) are the most widely grown legumes (Cooper et al., 1987a). While fallow has traditionally been practiced to conserve moisture to combat drought and to secure harvestable yields once every 2 years, the practice is declining due to land-use pressure and giving way to cereal monoculture, which is deemed unsustainable due to disease buildup (Jones and Singh, 2000). Consequently, research efforts have focused on various rotations and in cropping diversity (Ryan et al., 2008a). While irrigation has always been practiced in the region’s river valleys, allowing for a greater range of crops and cropping intensification, supplemental irrigation based on groundwater is increasing in rainfed crop- ping areas in order to increase and stabilize cereal yields (Oweiset al., 1998).
All such developments have implications for fertilizer demand and use.
3.5. Soil fertility and fertilizer use
Besides water availability to plants, low productivity in dryland agroecosys- tems is also related to low nutrient availability (Singh and Singh, 1994). As a consequence of soil properties, due to pedogenic factors associated with dry regions, together with environmental influences, the soils of WANA region are inherently poor in N (Ryan et al., 2009a) due to the low levels of mineralizeable SOM, which is attributed to limited root and residue bio- mass inputs to soils because of low crop yields and offtake of residues by grazing animals, in addition to favorable environmental conditions for SOM mineralization. Available P is invariably low (Matar et al., 1992) due to adverse soil chemistry (Delgadoet al., 2002a), but soils are generally well supplied with available potassium (K; Johnston, 1997). Prior to 1970, despite widespread N and P deficiencies, little fertilizer was used in Syria and most other countries of the WANA region. Pakistan is an exception as fertilizer use was already common in irrigated rice and wheat in the 1960s.
However, since that time, there was a rapid expansion in fertilizer N and P use in all the main agricultural countries of WANA (Fig. 6). Total nutrient is dominated by N, which continues to increase, while P use has plateaued in the past two decades. In all countries, almost without exception, consump- tion of K fertilizers is low by global standards of fertilizer use.
While the general calcareous nature of most soils in the region ensures that supplies of calcium and magnesium are adequate for crop growth, there has been a recent realization of the potential importance of micronutrient deficiencies, such as Fe and zinc (Zn), as constraints to crop yields (Rashid and Ryan, 2004). Indeed, evidence of fairly widespread Zn deficiency in some areas of the WANA region such as the Anatolian plateau in Turkey (Cakmak, 1998). A less widely known nutrient constraint is due to toxicity due to inherently high levels of boron (B) in the soil (Yau and Ryan, 2008), a phenomenon of local importance. Breeding for adaptation or tolerance to
0 1 2 3 4 5
1960 1970 1980 1990 2000 2010 Consumption (Tg yr–1)
Total N Total P
Figure 6 Nitrogen and phosphorus fertilizer consumption of the WANA countries from 1961–2008 (IFADATA, 2011; Total P recalculated from original data expressed in P2O5).
such toxicity is the only feasible solution, with some landraces and locally adopted varieties having some genetic tolerance.
The commercial fertilizers used in the WANA region vary from country to country, but are mainly conventional sources such as ammonium nitrate, ammonium sulfate, diammonium phosphate, calcium ammonium nitrate, urea, single superphosphate and increasingly triple superphosphate, and various N, P, or K formulations. The amount of K fertilizers on the market as potassium sulfate and potassium chloride is limited. Notwithstanding the emerging awareness of micronutrient constraints, the use of micronutrient fertilizers, except in intensive protected and high-value agriculture, is rare.
Regardless of the element in question, the removal of a crop growth constraint has indirect implications for the efficiency of P, the subject of the review.