Agro-edaphic suitability analysis

Adequate agricultural exploitation of the climatic potentials and maintenance of land productivity largely depend on soil fertility and the management of soils on an ecologically sustained basis. Soil fertility is concerned with the ability of the soil to retain and supply nutrients and water in order to enable crops to maximally utilize the climatic resources of a given location. The fertility of a soil is determined by both its physical and chemical properties. An understanding of these factors and insight in their interrelations is essential to the effective utilization of climate, terrain and crop resources for optimum use and production.

For assessing the suitability of soils for crop production, soil requirements of crops must be known. Also, these requirements must be understood within the context of limitations imposed by landform and other features which do not form a part of the soil but may have a significant influence on use that can be made of the soil (FAO, 1978-81a). The basic soil requirements of plants have been summarized under the following headings, related to internal and external soil properties:

Internal requirements:

External Requirements:

In addition to the above internal soil requirements of plants, a number of external soil requirements are of importance, such as:

From the basic soil requirements of crops, a number of soil characteristics are directly related to crop yield performance. For most crops, optimal, sub-optimal, marginal and unsuitable levels of these soil characteristics are known and have been quantified. Beyond critical ranges, crops cannot be expected to yield satisfactorily unless special precautionary management measures are taken. Soil suitability classifications are based on knowledge of crop requirements, of prevailing soil conditions, and of applied soil management. In other words, soil suitability classifications quantify in broad terms to what extent soil conditions match crop requirements under defined input and management circumstances. For a global study this necessitates expert judgment and a semi-quantitative approach.

Agro-edaphic suitability analysis is described in terms of:

1.	Soil suitability evaluation for rain-fed crop production .
2.	Terrain suitability evaluation for rain-fed crop production .
3.	Soil and terrain suitability evaluation for irrigated crop production.

 

Soil suitability evaluation for rain-fed crop production

FAO’s agro-edaphic suitability classification used in AEZ is to a large extent based on experience documented by Prof. C. Sys and others (FAO, 1978-81a; Sys and Riquier, 1980; FAO, 1984b; FAO, 1985; Nachtergaele, 1988; Sys, 1990; Sys et al ., 1993). The agro-edaphic suitability classification has been intensively used by FAO and other organizations, at various scales in many countries and regions; it passed through several international expert consultations, and hence it constitutes the most recent consolidation of expert knowledge. In this system a suitability rating is proposed for each soil unit, by individual crops at three defined levels of inputs and management circumstances. The agro-edaphic suitability rating is based on a comparison of soil requirements of crops and prevailing edaphic conditions. Data available from various sources have been summarized by Sys et al . (1993).

The source of soil information is primarily the digital version of the FAO/UNESCO Soil Map of the World (FAO, 1995c). A tabulation of the soil ratings by FAO ‘74 soil unit for all crop/LUTs considered in Global AEZ is presented in . Soil phase suitability ratings are listed in . Modifications of soil suitability ratings for soil units with coarse textures are treated according to procedures presented in FAO (1978-81a).

Terrain suitability evaluation for rain-fed crop production

The influence of topography on agricultural land use is manifold. Farming practices are by necessity adapted to terrain slope, slope aspect, slope configuration and micro-relief. For instance, steep irregular slopes are not practical for mechanized cultivation, while these slopes might very well be cultivated with adapted machinery and hand tools.

Sustainable agricultural production on sloping land is concerned with the prevention of erosion of topsoil and decline of fertility. Usually this is achieved by combining special crop management and soil conservation measures. Slopes cultivated with crop/LUTs providing inadequate soil protection and without sufficient soil conservation measures, cause a considerable risk of accelerated soil erosion. In the short term, cultivation of slopes might lead to yield reductions due to loss of applied fertilizer and fertile topsoil. In the long term, this will result in losses of land productivity due to truncation of the soil profile and consequently reduction of natural soil fertility and of available soil moisture.

Rain-fed annual crops are the most critical to cause topsoil erosion, because of their particular cover dynamics and management. The terrain-slope suitability rating used in the Global AEZ study captures the factors described above which influence production and sustainability. This is achieved through: (i) defining for the various crop/LUTs permissible slope ranges for cultivation, by setting maximum slope limits; (ii) for slopes within the permissible limits, accounting for likely yield reduction due to loss of fertilizer and topsoil, and (iii) distinguishing among farming practices ranging from manual cultivation to fully mechanized cultivation.

Ceteris paribus, i.e., under similar crop cover, soil erodibility and crop and soil management conditions, soil erosion hazards largely depend on amount and intensity of rainfall. Data on rainfall amount is available on a monthly basis in the 0.5 degree latitude/longitude climate databases. Rainfall intensity or energy, as is relevant for soil erosion, is not estimated in these data sets.

To account for clearly existing differences in both amount and within-year distribution of rainfall, use has been made of the modified Fournier index ( Fm ), which reflects the combined effect of rainfall amount and distribution (FAO/UNEP, 1977), as follows:

where:

P _i = precipitation of month i

Pann = total annual precipitation

When precipitation is equally distributed during the year, i.e., in each month one-twelfth of the annual amount is received, then the value of Fm is equal to Pann . On the other extreme, when all precipitation is received within one month, the value of Fm amounts to twelve times Pann . Hence, Fm is sensitive to both total amount and distribution of rainfall and is limited to the range of Pann Fm 12 Pann . The Fm index has been calculated for all 0.5 degree grid-cells of the climatic inventory. The results have been grouped in six classes, namely: Fm < 1300, 1300-1800, 1800-2200, 2200-2500, 2500-2700, and Fm > 2700.

Slope ratings are defined for the seven slope range classes used in the land resources database, namely: 0-2% flat, 2-5% gently sloping, 5-8 % undulating, 8-16% rolling, 16-30% hilly, 30-45% steep, and > 45% very steep. The following suitability rating classes are employed:

Soil and terrain suitability evaluation for irrigated crop production

The evaluation procedures for gravity irrigation suitability cover the dryland crops and wetland rice, at both intermediate and high levels of management and input circumstances. Three important assumptions have been made in setting up the procedures: [1] firstly, water resources of good quality are available; secondly, irrigation infrastructure is in place; and thirdly, the crop-specific soil limitations for rain-fed production (such as limitations imposed by soil rooting conditions, soil nutrient availability and soil nutrient retention capacity, soil toxicity, soil salinity, soil alkalinity, and calcium carbonate and gypsum content) also apply to irrigation. For irrigation these limitations are assumed to be similar or more severe. Note, however, that the Global AEZ assessment does not provide a quantification of irrigation water availability. Nevertheless, it can generate useful information for integrated analysis at the watershed level.

The following land and soil characteristics have been interpreted specifically for the irrigation suitability classification: topography; soil drainage; soil texture; surface and sub-surface stoniness; calcium carbonate levels; gypsum status; and salinity and alkalinity conditions. The main literature sources used in the interpretation include Sys et al. (1993), Sys and Riquier (1980), FAO (1985), FAO (1996), FAO (1976b), FAO/UNESCO (1974), and FAO/UNESCO/ISRIC (1990).

Topography

The dominant topographic factor governing the suitability of an area for gravity or sprinkler irrigation is the terrain slope. Other topographic factors, such as micro-relief, have partly been accounted for in the soil unit and soil phase suitability classifications. Permissible slopes for irrigation depend on type of irrigation systems and assumed level of inputs and management.

Gravity irrigation (basin, border, and furrow systems) is suitable for a large range of crops, provided it is managed properly. It is used for terrain slopes up to 5 percent. For ‘non-row crops’ such as wheat, barley, pasture and forage legumes, slopes up to 10 percent can be used with special systems such as corrugations. At these steeper slopes irrigation efficiency is diminished due to poor uniformity of the water distribution, leading to irregular stands of crops. Therefore, slopes between 5 and 10 percent are classified as sub-optimal for all types of gravity irrigation.

Sprinkler irrigation systems include many types. They are generally more efficient than gravity systems but also much more expensive, and they require special management skills. Sprinklers can be used on somewhat steeper slopes than the gravity systems. However, some of the larger central pivot systems can only be used on flat or almost flat terrain. Small-scale systems are more suitable on sloping land. For perennial crops or well established pastures adapted systems may be used on slopes up to 24 percent. For annual crops, serious erosion risk starts at about 10-12 percent slopes, depending on soil erodibility, ground cover, and management. Sprinkler irrigation is obviously not suitable for wetland crops.

Terrain-slope suitability ratings, respectively for gravity and sprinkler irrigation systems are presented, for eight groups of crops at high and intermediate levels of inputs. The suitability rating classes areas are as follows:

Soil texture

Soil texture provides a measure for permeability and to some extent, for water retention capacity. Soils with potentially high percolation losses and soils with low water retention capacity, e.g., Vitric Andosols, Arenosols, Podzols and all soils with coarse textures have been considered not suited for gravity irrigation. For medium and fine textured soils excessive percolation and low water-retention capacities are less relevant. However for Acrisols, Nitosols and Ferralsols the irrigation suitability ratings are slightly different as compared to rain-fed conditions, because of their specific clay mineralogy, resulting in relatively low water-retention capacity and slightly higher percolation losses. The modifications related to texture/clay mineralogy are summarized by major soil units.

Major Soil Unit			Suitability
		Soil Unit	Dryland Crops	Wetland Rice
Acrisols (A)		all units	S1/S2	S2
Ferralsols (F)		all units	S1/S2	S2
Nitosols (N)		all units	S1/S2	S2
Podzols (P)		all units	N	N
Arenosols (Q)		all units	N	N
Andosols (T)		Tv	N	N
n.a.		all units	S1/S2	S2
n.a.		All units	S1/S2	S2

Soil drainage

Irrigation of dryland crops requires well drained soils to assure aeration and to avoid the danger of secondary salinization. Drainage conditions depend on depth and quality of groundwater. At present, this can not be assessed on regional and global scales due to lack of systematic data. Soil drainage quantification, however, is available for the soil units of the FAO ‘74 legend (FAO, 1995c). For wetland rice and dryland crops drainage requirements under irrigation are quite different as compared to rain-fed conditions. Modifications to rain-fed suitability ratings for different drainage classes were applied.

Soil drainage limitations	Drainage Classes:
LUT Soil Drainage Class Suitability Wetland Rice P S1   VP, I, MW, W S2 SE, E  N Dryland crops W S1   MW S1/S2   I, P S2   VP, SE, E  N	VP very poor P poor I imperfectly MW moderately well W well SE somewhat excessive E excessive

Soil depth and soil stoniness

Under irrigated conditions soil depth affects drainage, aeration and water retention properties. Deep soils favor drainage and are therefore optimal for irrigation of dryland crops. Soils with impermeable layers favor maintenance of flooding conditions for wetland rice. Shallow soils such as Rendzinas and Rankers (Umbric Leptosols) and soils with phases implying a reduction in soil depth have been reviewed and adjusted for irrigated conditions.

Surface stoniness affects soil workability. In addition, subsurface stoniness reduces water-holding capacity and increases infiltration rates. It is assumed that a level of more than 40 volume percent of coarse materials will markedly influence the water-balance in the soil profile (Sys and Riquier, 1980). To cater for these constraints specifically affecting irrigation suitability, the soil phase suitability ratings for petric and stony phases in FAO legends have been adjusted from the rain-fed ratings. The soil phase ratings for irrigated crop production are presented in .

Calcium carbonate

Calcium carbonate in the form of free lime in the soil profile affects soil structure and interferes with infiltration and evapotranspiration processes. It influences both the soil moisture regime and availability of nutrients. This, however, applies equally to rain-fed and irrigated cropping. Therefore, no changes are required to the crop-specific limitations as established for rain-fed cropping.

Gypsum

Gypsum interferes with water absorption and availability. As consequence of the solubility of gypsum in water, so-called dissolution depressions can be formed as a result of the application of irrigation water to gypsiferous soils. This renders soils with high gypsum content unsuitable for irrigation.

Gypsiferous Soil Units		Gypsiferous Soil Phases
Yy, Yl, Xy, Xl, Kl, Kk, Ck, Cl, and Bk		Petrogypsic

 

Salinity and alkalinity

Irrigation in semi-arid and arid regions requires careful soil drainage (natural and/or artificial) to avoid irrigation-induced secondary salinization. It is assumed that, where so required, appropriate drainage systems are in place and that irrigation water is non-saline. Alkalinity, expressed as sodium saturation, influences the structure stability of soils, which in turn affects infiltration rates and aeration of soils. However, the alkalinity (sodicity) constraints are equally important for rain-fed and irrigated conditions. Therefore, the crop-specific soil unit and soil phase ratings evaluated for rain-fed conditions under both saline and alkaline conditions remain unchanged for irrigated cropping.

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