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Chapter 3: Soil and terrain suitability for surface irrigation
The evaluation of soil qualities and terrain conditions to predict the performance for specific crops is an essential part of a land evaluation and land use planning exercise applied to agriculture. In the framework of the study, emphasis was placed on the suitability of soils and terrains for irrigation development.
In order to compare soil and terrain conditions with specific crop requirements for optimum growth and production, soil and terrain qualities or characteristics, as derived from the FAO - UNESCO soil map of the world [1], were matched against specific crop requirements derived from agricultural experiments and literature review.
Given the small scale of the soil map of the world (1:5000000), the approach to evaluating soil and topography effects is the one used in the continental agro-ecological zones study [16]. This method expresses in a qualitative way, using three suitability classes, the estimated performance and suitability of a given land use type, for specific soil and terrain conditions, assuming climatic conditions to be optimum. It proceeds in four steps:
1. Matching crop requirements with the inherent fertility and physical characteristics of each soil unit in the FAO - UNESCO legend of the soil map of the world.
2. Downgrading, if necessary, the soil unit by a factor which takes account of its texture.
3. Downgrading, if necessary, the suitability class obtained after steps I and 2 for terrain influences, such as slopes.
4. Downgrading, if necessary, for soil phases such as effective soil depth, the presence of gravel and stones, the presence of high levels of sodium, depending on the specific crop requirements in this respect.
Soil requirements for irrigation
Qualitative land evaluation for irrigation is generally based on interpretation of environmental characteristics, of which slope, soil and groundwater are the most important.
The evaluation criteria adopted here consider surface irrigation using water of good quality (Table 3). Accordingly, some soils considered not suitable for such a development could be suitable for sprinkler irrigation or micro-irrigation. It was also assumed that the irrigation infrastructure is in place and that an adequate level of inputs is applied.
TABLE 3: Criteria used in the evaluation of soil and terrain suitability for irrigation
CRITERIA |
CONDITION |
UPLAND CROPS |
FLOODED RICE |
Topography: slope |
Optimum |
< 2% |
< 2 % |
2-8% |
2-8% |
||
Drainage (1) |
Optimum |
W |
P |
Marginal/Range |
MW-1 |
VP-W |
|
Texture (2) |
Optimum |
L-SiCL |
CL-MCm |
Range |
SL-MCs |
SL-MCm |
|
Soil depth |
Optimum |
> 100 cm |
> 50 cm |
Marginal |
50 - 100 cm |
20 - 50 cm |
|
Surface stoniness |
no stones are acceptable |
no stones are acceptable |
|
Subsurface stoniness |
Optimum |
< 40 % |
< 40 % |
Marginal |
40- 75 % |
40- 75 % |
|
Calcium carbonate |
Optimum |
< 30 % |
< 15 % |
Marginal |
30- 60 % |
15 - 30 % |
|
Gypsum |
Optimum |
< 10 % |
< 3 % |
Marginal |
10 - 25 % |
3 - 15 % |
|
Salinity (3) |
Optimum |
< 8 mmhos/cm |
< 2 mmhos/cm |
Marginal |
8 - 16 mmhos/cm |
2 - 4 mmhos/cm |
|
Alkalinity (3,4) |
Optimum |
< 15 ESP |
< 20 ESP |
Marginal |
15 - 30 ESP |
20 - 40 ESP |
(1)
Drainage:
W = Well drained;
MW = Moderately Well drained;
I = Imperfectly drained;
P = Poorly drained;
VP = Very Poorly drained.
(2) Texture:
L = Loamy
SiCL = Silty Clay Loam
SL = Sandy Loam
CL = Clay Loam
(3)
Salinity and alkalinity:
The criteria refer to salinity and alkalinity conditions that can be accepted for irrigation and possibly improved by irrigation management. The choice of crops has to be made with regard to the local salinity and alkalinity situation.
(4)
Alkalinity:
ESP = Exchangeable Sodium Percentage.
Two main land utilization types have been considered: the whole group of upland crops and rice under irrigation. It was decided that, where the soil is suitable for both upland crops and rice, priority be given to rice. This choice is necessary to avoid counting the same land in both categories, thereby artificially increasing the area suitable for irrigation. This approach differs from the 1987 study [20] where the same land was accounted for in both categories.
The attributes of the FAO-UNESCO Soil Map of the World which were used for irrigation appraisal are: topography, drainage, texture, surface and subsurface stoniness, depth, calcium carbonate level, gypsum status salinity and alkalinity conditions. Criteria were established for evaluating each of these characteristics in relation to the specific requirements for upland crops and flooded rice.
The evaluation of the soil units for irrigation is performed in the same way as the evaluation for rainfed crops [16], namely:
• evaluation of soil units;
• texture modifications;
• slope modifications;
• phase modifications.