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4.2 Land Suitability

The suitability of a given piece of land is its natural ability to support a specific purpose. According to the FAO methodology (1976), this is strongly related to the "land qualities" such as erosion resistance, water availability, and flood hazard that are not measurable. As these qualities derive from the "land characteristics", such as slope angle and length, rainfall and soil texture which are measurable or estimable, it is advantageous to use these later values to study the suitability. Thus, the land characteristics parameters were used to workout land suitability for irrigation, crops and forest.

The land suitability classification consists of assessing and grouping the land types in orders and classes according to their aptitude.

The order defines the suitability and is expressed by:

The classes (1, 2 and 3 for suitable order; 1 and 2 for unsuitable order) express the degrees of suitability or unsuitability. Thus, there are 5 classes according to Table 17. The areas that were not assessed are allocated to an extra class "NR" meaning not relevant.

Order 

Class 

Description 

Suitable 

S1 (Highly suitable) 

Land having no, or insignificant limitations to the given type of use 

S2 (Moderately suitable) 

Land having minor limitations to the given type of use 

S3 (Marginally suitable) 

Land having moderate limitations to the given type of use 

Not suitable 

N1 (Currently not suitable) 

Land having severe limitations that preclude the given type of use, but can be improved by specific management 

N2 (Permanently not suitable ) 

Land that have so severe limitations that are very difficult to be overcome 

Table 17 - Land suitability classes.

4.2.1 Land suitability for irrigation

The study area belongs to a semi arid region globally characterized by a strong spatial-temporal rainfall variability. The irrigation can be adopted as a solution for crop production. It can be used only during the dry season, as they do in the Niayes and the open depressions or in a complementary mode during the rainy season when a water stress occurs.

Figure 17 - Land capability maps.

Methodology

To evaluate the land suitability for irrigation, the parametric evaluation system of Sys et al., (1991) was applied, using the soil characteristics. These characteristics concern environmental factors, drainage properties, soil physical properties and soil chemical properties. They are rated and used to calculate the capability index for irrigation (Ci) according to the formula:

Where Ci = capability index for irrigation;

A = soil texture rating;

B = soil depth rating;

C = CaCO3 status;

D = electro-conductivity rating;

E = drainage rating;

F = slope rating.

Suitability classes are defined considering the value of the capability index (Table 18).

Capability Index 

Class 

Definition 

Symbol 

>80 

Highly suitable 

S1 

60-80 

II 

Moderately suitable 

S2 

45-60 

III 

Marginally suitable 

S3 

30-45 

IV 

Currently not suitable 

N1 

<30 

Permanently not suitable  

N2 

Table 18 - Suitability index for the irrigation capability indices (CI) classes.

For our study area, gravity or surface irrigation that require impermeable soil with drop and localized irrigation and are less regarding to these characteristics, were analyzed. The soil characteristics were used according to different ratings specifying each type of irrigation (Table 19).

Textural class 

Rating for gravity irrigation 

Rating for drop and localized irrigation 

Fine gravel 

Fine gravel 

Coarse gravel 

Fine gravel 

Fine gravel 

Coarse gravel 

<15% 

15- 40% 

40-75% 

15- 40% 

40-75% 

<15% 

15- 40% 

40-75% 

15- 40% 

40-75% 

Clay Loam (CL) 

100 

90 

80 

80 

50 

100 

90 

80 

80 

50 

Silty Loam (SiCL) 

100 

90 

80 

80 

50 

100 

90 

80 

80 

50 

Sandy Clay Loam (SCL) 

95 

85 

75 

75 

45 

95 

85 

75 

75 

45 

Loam (L) 

90 

80 

70 

70 

45 

90 

80 

70 

70 

45 

Silty Loam (SiL) 

90 

80 

70 

70 

45 

90 

80 

70 

70 

45 

Silt (Si) 

90 

80 

70 

70 

45 

90 

80 

70 

70 

45 

Silty Clay (SiC) 

85 

95 

80 

80 

40 

85 

95 

80 

80 

40 

Clay (C) 

85 

95 

80 

80 

40 

85 

95 

80 

80 

40 

Silty Clay (SC) 

80 

90 

75 

75 

35 

95 

90 

85 

80 

35 

Silty Loam (SL) 

75 

65 

60 

60 

35 

95 

85 

80 

75 

35 

Loamy Sand (LS) 

55 

50 

45 

45 

25 

85 

75 

55 

60 

35 

Sandy (S) 

30 

25 

25 

25 

25 

70 

65 

50 

35 

35 

Table 19 - Textural classes rating.

Soil depth (cm) 

Rating forgravity irrigation 

Rating for drop and localized irrigation 

<20 

30 

30 

20-50 

60 

70 

50-80 

80 

90 

80-100 

90 

100 

>100 

100 

100 

Table 20 - Soil depth rating.

CaCO3 (%) 

Rating for gravity irrigation 

Rating for drop and localized irrigation 

<0.3 

90 

90 

0.3-10 

95 

95 

10-25 

100 

95 

25-50 

90 

80 

>50 

80 

70 

Table 21 - CaCO3 status rating.

Ece (dS/m) 

Rating for gravity irrigation 

Rating for localized and drop irrigation 

C, SiC, S, SC textures 

Other textures 

C, SiC, S, SC textures 

Other textures 

<4 

100 

100 

100 

100 

4-8 

90 

95 

95 

95 

8-16 

80 

50 

85 

50 

16-30 

70 

35 

75 

35 

>30 

60 

20 

65 

20 

Table 22 - Electro-conductivity rating.

Drainage class 

Rating for gravity irrigation 

Rating for localized and drop irrigation 

C, SiC, SC textures 

Other textures 

C, SiC, SC textures 

Other textures 

Well drained  

100 

100 

100 

100 

Moderately drained 

80 

90 

100 

100 

Imperfectly drained 

70 

80 

80 

90 

Poorly drained 

60 

65 

70 

80 

Very poorly drained 

40 

65 

50 

65 

Drainage status not known 

70 

80 

70 

80 

Table 23 - Drainage classes rating.

Slope class (%) 

Rating for gravity irrigation 

Rating for localized and drop irrigation 

Non terraced 

Terraced 

Non terraced 

Terraced 

0-1 

100 

100 

100 

100 

1-3 

95 

95 

100 

100 

3-5 

90 

95 

100 

100 

5-8 

80 

95 

90 

100 

8-16 

70 

85 

80 

90 

16-30 

50 

70 

60 

70 

>30 

30 

50 

40 

50 

Table 24 - Slope rating.

Results and discussions

The processing of the parametric evaluation system for gravity and drop/localized irrigation gave the results that are represented in Figure 18 and summarized in Table 25. It should be highlighted that the land units 16 (Tanma Lake) and 23 (natural vegetation corresponding to the managed forest) are not used for agriculture.

Suitability  

Gravity irrigation 

Drop and localized irrigation 

Land units 

Area(km2) 

Area(%) 

Land units 

Area(km2) 

Area(%) 

     

12, 19, 24, 27, 30, 31, 32, 34 

161.90 

25.03 

S2 

24, 30, 31, 34 

50.02 

7.73 

3, 5, 6, 7, 8, 9, 14, 16, 17, 18, 20, 21, 25, 28, 33, 35 

292.64 

45.25 

S3 

16, 17, 20, 21, 27, 33, 35 

80.89 

12.51 

2, 4, 10 

18.31 

2.83 

N1 

3, 5, 9, 12, 18, 19, 32 

98.52 

15.23 

11, 13, 26 

37.71 

5.83 

N2 

2, 4, 6, 7, 8, 10, 11, 13, 14, 22, 23, 25, 26, 28, 29 

372.91 

57.66 

23 

91.78 

14.19 

NR 

 

44.38 

6.86 

 

44.38 

6.86 

Total 

 

646.71 

100 

 

646.71 

100 

Table 25 - Absolute and relative classes of gravity and drop/localized irrigation suitability.

For the surface irrigation, there is no area classified as highly suitable (S1). Only 20.24% of the study area is suitable (S2, 7.73%) or slightly suitable (S3, 12.51%) and it is located in the backslope, accumulation glacis and outliers of the Thiès cuesta, and in the valley bottom 15.23% is almost suitable. Most of the study area (57.66%) is classified as unsuitable (N2). The limiting factor to this kind of land use is mainly the soil drainage status and texture that is mostly sandy, while surface irrigation requires heavier soils. In some cases, the slope class also handicaps the use for irrigation purposes.

For drop and localized irrigation, a good proportion (45.25%) of the area is suitable (S2) and 25.03% is classified as highly suitable (S1). Only a few proportion is almost suitable (N1, 5.83%) or unsuitable (N2, 5.83%) and mainly concerns the backslope of the Thiès cuesta. In these later cases, the handicap is given by the shallow soil depth, bad texture due to a large amount of coarse gravel and /or poor drainage.

The comparison of the two types of irrigation, revealed that it would be more benefit to irrigate by drop or localized type, as the later mode improves all the suitability to the irrigation purposes (Table 26). Only the land unit 23 remains unsuitable, but it is already not used for agriculture.

Moreover, because of the insufficiency of surface water, and the aridity of the climate, only the drop irrigation is recommended for a sustainable use of this natural resource.

Type of improvement 

Land units 

From N1 to S1 

19, 12, 32 

From N1 to S2 or S3 

3, 5, 9, 18 

From N2 to S2 or S3 

2, 4, 6, 7, 8, 10, 14, 25, 28 

From N2 to N1 

11, 13, 26 

No amelioration 

23 

Table 26 - Improvement of suitability classes of the land units from gravity (surface) to drop type of irrigation.

4.2.2 Land suitability assessment for crops and fruit trees

Land suitability assessment for agriculture is meant to evaluate the ability of a piece of land to provide the optimal ecological requirements of a certain crop variety. In other words, assessing the capability of land in enabling optimum crop development and maximum productivity. This evaluation needs a specification of the respective crop requirements and calibrating them with the terrain and soil parameters.

The identified limiting factors could be managed to suite the various crop requirements and improve crop productivity. This is a pre-requisite to productivity maximization in the agricultural sector.

In the case study, the land suitability assessment for the study area has the following objectives:

There are also other common crops in the area, but due to lack of data they could not be included in the analysis.

Optimal crop growth and productivity is based amongst others on soil conditions, the climate and agricultural practices. However, this chapter will only focus on the land parameters and from our analysis, it was realized that the following soil parameters:

are amongst the main factors that influence crop adaptability to a given land in the study area.

Some conservative farming practices could as well accelerate soil chemical and physical degradation and create some of the unfavorable soil conditions as mentioned above.

As indicated in the chapter of agriculture, the common tree crops cultivated in the study area are mangos (Mangifera indica) and cashew (Anacardium occidentale) while millet, maize, sorghum, groundnuts, beans and cassava that are widely grown as annual crops. Fruit, root and leafy vegetables are wide spread as well, and key amongst these are onion, cabbage, carrot and tomato, eggplant etc. Vegetable cultivation is more concentrated in the depressions along the coastal zones but due to unavailability of data, the analysis focused mainly on mango, cashew, groundnuts, cassava, beans and up-land cereals in the study area.

Methodology

The land suitability assessment for annual crops and fruit trees was carried out with the use of the Sys et al., parametric methodology (1991). This is based on the Food and Agricultural Organization (FAO) land evaluation framework (1983) for rainfed agriculture.

Figure 18 - Gravity irrigation suitability (A) and drop/localized irrigation suitability (B) classes.

This methodology requires a climatic data analysis focusing more on specific parameters like radiation, relative humidity, temperature and rainfall. However, as mentioned above, due to the limitation in acquiring climatic data, it was impossible to perform this analysis. Also, owing to the small size of the study area (646km2), the climate was considered homogeneous and suitable for the crops under review.

The adopted method consists of matching individual crop requirements as specified in the land evaluation manual (part III), Sys et al., (1993), with the actual land characteristics. A suitability rating is attributed to each land characteristic. Terrain factors (slope gradient and coarse fragments), and both measurable soil chemical and physical parameters were taken into consideration as shown in Table 27.

The crop requirements were further adjusted to suite the conditions of the study area.

The soil depth requirement of individual crops was chosen according to the usual root depth (in cm) of the crop. This is referred to as the control section in the evaluation process. The following table shows the specification per crop that was used in this case study.

Crop 

Control section (cm) 

Mango 

150 

Cashew 

150 

Millet 

30 

Maize 

40 

Sorghum 

40 

Groundnuts 

30 

Beans 

30 

Cassava 

75 

Table 27 - Crops and control sections.

The terrain and soil parameters used in the land suitability assessment for the crops commonly grown in the study area, are shown in Table 28.

Terrain parameters 

Soil parameters 

Slope depending on traditional or mechanized system (%) 

Depth 

Surface coarse fragments (%) 

Drainage 

 

Texture 

 

Soil coarse fragments 

 

Calcium carbonate (CaCO3

 

pH 

 

Organic carbon (%) 

 

Salinity or EC (ds/m) 

 

CEC (cmol+)/kg of soil) 

Table 28 - Terrain and soil characteristics used in the land suitability assessment.

The final score is given by multiplying the single parameters.

The above mathematical formula was adopted for land suitability evaluation. This formula consists of calculating products of the different parameters according to their respective weights.

A model was used for the simulation of the evaluation per crop.

Land suitability index

This index is calculated using the crop requirements and the actual land data. The final value obtained is matched with a pre-defined score rating which gives the final suitability class of each relevé. The Table 29 below is the score table used for the land suitability classification.

Suitability class 

Index 

Description 

S1 

0.8-1 

Highly suitable 

S2 

0.8-0.6 

Moderately suitable 

S3 

0.6-0.45 

Marginally suitable 

0.45-0 

Unsuitable 

NR 

Not relevant 

Table 29 - Land suitability index classes.

For easier analysis, the study area was classified into five main land systems, a suitability analysis is then done per land unit and for each of the crops concerned. Crops were grouped according to life cycle, as shown in Table 30.

Crop class 

Crop 

Tree crops (perennials) 

Mango and cashew 

Annual crops 

Millet, sorghum, maize, groundnut, beans and cassava 

Table 30 - Analyzed crops.

Results and discussion of the evaluation

The following figures (from Figure 19 to Figure 22) show the result. First, it should be noted that some of the results obtained from the land suitability assessment for certain crops precisely groundnuts, maize and cashew don’t seem to correspond with the realities of the study area. These irregularities are mainly due to the inadaptability of the model used to the conditions of the study area. This model was developed, tested and validated for the temperate and Mediterranean areas. It was used for the first time in sahelian conditions.

From a general point of view, the area reviewed (study area) is suitable for the cultivation of most crops analyzed.

Even though the agricultural systems slightly change with landform systems, the form of the terrain doesn’t seem to have much influence on crop suitability for the area.

As a general comment, the other land systems out side the cuesta of Thiès are more suitable for crop production purposes and this could be explained by its specific soil characteristics such as the pH, fertility and texture.

According to the results obtained, the model showed a high suitability for mango and cassava cultivation, while cashew and sorghum are not highly suitable in the entire study area.

The northern part of Thiès is highly or moderately suitable for all the crops under review.

The extreme north-western part of the study area, along the beach and coastal zone, is unsuitable for crop cultivation due to the predominant sandy soils.

Along the dune-interdune successions (fixed dunes) and the accumulation glacis (land unit 6 and 7), the land is marginally suitable for millet, maize, sorghum, cashew and groundnut, while it is moderately suitable for mango and beans and highly suitable for cassava.

The gently dipping slope with shallow sand coverage on the back slope and the depression bottom (land unit 9, 15 and 16) on the Thiès cuesta are unsuitable for all the crops. Despite the fact that the gently dipping slope with thick sand coverage classified as ideal for rain-fed agriculture, the evaluation model has reported it as unsuitable for maize, groundnuts and cashew cultivation.

On both, the sub-level summit area and the accumulation glacis (land unit 33, 34 and 35) of the cuesta outlier, it was found suitable for all crops analysed only with the exception of cashew.

The alluvial-colluvial flat area located in the valley bottom (land unit 19, 20 and 21) is also suitable for all crops.

Land system 

Land unit 

Surface area (ha) 

Lagoonal depression 

16

11

17

14 

1085

555

56

84 

Present-day beach and coastal dune 

1

4

161

985

730 

Thiès cuesta 

23

29

26

33

35

22 

7943

1172

2876

110

515

57 

Table 31 - Unsuitable land units for all crops.

The unsuitability of the above land units for the cultivation of the analyzed crops could be explained by their type of soil which varies from very sandy to heavy clayey soils. Land units 1 was classified as Arenosols that are characterised by a very high percentage of sand (poor organic matter content). Vertisols, Plinthosols, Cambisols and Gleysols characterise the other land units. However, this unsuitability is not entirely based on the soil type but on many other soil parameters associated with local soils conditions and water availability. The model has classified some land units as unsuitable but they are currently exploited for crop cultivation. This is due to the constraints of the model as stated earlier in this chapter.

Figure 19 - Suitability maps for groundnut, beans, cassava and maize.

Figure 20 - Suitability maps for mango and cashew

Figure 21 - Suitability maps for Millet and Sorghum

Figure 22 – Suitability scenarios: market-oriented vs. food security production.

Below are the results of the land suitability rating per land unit for some of the commonly cultivated crops of the region of Thiès. Some of the land units are not suitable for any of the crops considered as specified above, while some land units show suitability for some and unsuitability for others. On the other hand, land unit number 3, 5, 18, 24, and 27 are suitable for all the analyzed crops. The beach, the Thiès city and the bare soils were not been taken into account during the evaluation.

Land unit  

Mango 

Cashew 

Cassava 

Millet 

Maize 

Sorghum 

Groundnut 

Beans 

Area (ha) 

730 

S2 

S3 

S1 

S3 

S3 

S3 

S3 

S2 

3383 

985 

S1 

S3 

S1 

S3 

S3 

S3 

S3 

S2 

497 

S1 

S3 

S1 

S3 

S3 

S1 

84 

S2 

S3 

S1 

S3 

9536 

S2 

S3 

S1 

S3 

S3 

1570 

S3 

481 

10 

S3 

116 

11 

555 

12 

S3 

S3 

S2 

S2 

S3 

S1 

S2 

1317 

13 

S2 

340 

14 

84 

16 

1085 

17 

56 

18 

S1 

S3 

S1 

S3 

S3 

S3 

S3 

S2 

42 

19 

S1 

S2 

S3 

S2 

S2 

1029 

20 

S2 

S3 

S2 

S2 

S3 

S3 

75 

21 

S2 

S3 

S2 

S2 

S3 

S3 

509 

22 

57 

23 

7948 

24 

S1 

S2 

S1 

S1 

S2 

S2 

S2 

S2 

1256 

25 

S1 

S3 

S2 

S3 

S3 

S2 

6108 

26 

2876 

27 

S1 

S3 

S1 

S2 

S3 

S3 

S3 

S2 

5740 

28 

S1 

S2 

S3 

S3 

S2 

5128 

29 

1172 

30 

S1 

S1 

S1 

S1 

S3 

S1 

1842 

31 

S2 

S3 

S2 

S1 

S2 

S2 

S1 

S1 

1604 

32 

S1 

S1 

S2 

S2 

S2 

S3 

3103 

33 

110 

34 

S1 

S1 

S1 

S1 

S3 

S1 

301 

35 

515 

Table 32 - Land unit suitability for the common crops in the study area.

In conclusion, the case study has shown that 60, 395 hectares (ha) of the study area is exploitable for agricultural purposes. From this surface area:

Corresponding percentages are shown in Figure 23. Suitability ranking for market oriented and food security production shows that certain land units has more potentialities for either commercial or food security purposes. Details are indicated in Table 33 and Table 34.

Crops 

Land units 

Beans 

3, 7, 8, 6, 5, 18, 25 and 28 

Millet 

19, 34, 12, 20, 21, 24, 27, 32, 31 and 30 

Groundnut 

9 and 13 

Mango 

10 

Table 33 - More suitable land units for food security oriented production.

Crops 

Land units 

Mango 

3, 7, 8, 6, 19, 5, 34, 18, 10, 20, 21, 25, 24, 28, 27, 32, 31 and 30 

Groundnut 

9, 12, 13,  

Table 34 - More suitable land units for market oriented production.

Most of the land units are suitable for both market and food security production.

Figure 23 - Suitability percentage of the arable land in the study area.

4.2.3 Land suitability for forest plantation

The main constraints in the study area are rainfall and soil fertility. They limit the potential of establishing forest plantations. The study area also, is under a strong human pressure for social and economic reasons. Many villages are located near the managed forest or natural formations. So, the wood collection is intense and the grazing compromises the natural regeneration of the species. The same situation happens in the agricultural areas where, farmers clean the fields in the beginning of the rainy reason.

Coinciding with the change in associated economic environment and the necessity to implement an integrated approach to increase the soil fertility or to protect cultivated areas, there is an expansion of plantations. The main source of motivation is the new forestry law (93-06), passed by the national assembly in February 1993. It reinforce the right of ownership of private persons, whether physical or juridical, of the trees they plant as well as their right to exploit these resources to their advantage.

The main use of this land suitability classification is to be considered as an aid to decision-making in terms of forestry potential, to strengthen support, to combat land degradation and drought, which are the most important constraints in the study area.

The applications of geomatics for forest resources management and planning activities are investigated in this case study to define suitable areas for plantation. The system is an interpretation derived from several sources and, as with all such approaches, it will be subjected to some degree of arbitrary decision. It is based on an assessment of the increasing degree of limitation imposed by the physical factors of soil and topography on the growth of trees and on silvicultural practices.

Methodology

The methodology used is based on the FAO approach for land evaluation for forestry (FAO, 1984). The main stages as the following:

According to this methodology, a determined value ranging from 0 (worst case) to 1 (optimum conditions) was given to each class of the parameter (from Table 35 to Table 41). All the classes received a weight corresponding to the degree of limitation. The weight ranges from 1 (limitation) to 5 (indifferent to the limitation).

DRAINAGE 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

Well-drained 

1  

1  

1  

1  

1  

1  

Rarely saturated 

0.8  

0.5  

0.1  

0.5  

0.8  

0.5  

Saturated for short periods in most years 

0.5  

0.3  

0.5  

0.5  

0.5  

0.3  

Saturated for long periods every year 

0.3  

0.8  

0.3  

0.3  

0.3  

0  

Always saturated 

0  

0.5  

0  

0.1  

0.1  

0  

Weight 

2  

2  

1  

2  

3  

2  

Table 35 - Rating and weights for soil drainage.

SLOPE (%) 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

0-1 

1  

1  

1  

1  

1  

1  

1-5 

0.8  

0.8  

0.5  

0.3  

0.5  

0.8  

5-15 

0.1  

0.5  

0.5  

0.5  

0.3  

0.3  

Weight 

Table 36 - Rating and weights for slope.

SOIL DEPTH 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

0-50 

0  

1  

0.8  

0  

0.5  

0.3  

50-100 

0.3  

0.5  

0.8  

0.1  

0.8  

0.8  

100-150 

0.5  

0.3  

1  

0.3  

1  

1  

>150 

1  

0.8  

1  

1  

1  

1  

Weight 

1  

1  

2  

2  

3  

2  

Table 37 - Rating and weights for soil depth.

Ph 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia

albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

0-4 

0  

0  

0  

0  

0  

0  

4-5 

0.3  

0.1  

0.3  

0.3  

0.3  

0.3  

5-6 

0.5  

0.5  

0.3  

0.5  

0.5  

0.5  

6-7 

1  

1  

0.5  

1  

0.8  

1  

7-8 

0.3  

0.8  

0.8  

0.5  

0.8  

0.8  

>8 

0  

0.5  

0.3  

0  

0.3  

0.5  

Weight 

2  

2  

2  

2  

1  

2  

Table 38 - Rating and weights for pH.

SALINITY (ECE) 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acaciaalbida 

Ziziphus mauritiana 

Balanites aegyptiaca 

0-2 

1  

1  

0.5  

1  

1  

1  

2-4 

0.5  

0.8  

0.5  

0.8  

0.8  

0.5  

4-6 

0.3  

0  

0  

0  

0.5  

0.3  

>6 

0  

0  

0  

0  

0  

0  

Weight 

2  

1  

2  

1  

1  

1  

Table 39 - Rating and weight for soil salinity.

TEXTURE 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acaciaalbida 

Ziziphus mauritiana 

Balanites aegyptiaca 

Clay 

0  

1  

0.5  

0.3  

0.5  

0.8  

Loam 

1  

1  

1  

0.5  

0.8  

0.8  

Clay loam 

1  

0.8  

1  

0.5  

0.8  

0.8  

Silty clay 

0.8  

0.8  

0.5  

0.5  

1  

0.8  

Silty clay loam 

0.8  

1  

1  

0.8  

1  

1  

Silty loam 

0.8  

0.8  

1  

0.8  

1  

1  

Sandy clay 

1  

0.8  

1  

1  

0.5  

1  

Sandy clay loam 

1  

1  

0.8  

1  

0.8  

1  

Sandy loam 

1  

0.5  

0.8  

1  

0.8  

1  

Loamy sand 

0.8  

0.5  

0.8  

0.8  

0.8  

1  

Sand 

0.5  

0.5  

0.8  

0.5  

0.5  

1  

Weight 

4  

3  

5  

2  

2  

4  

Table 40 - Rating and weights for soil texture (USDA classification).

COARSE FRAGMENTS (%) 

Eucalyptus

camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia

Albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

0-2 

1  

1  

1  

0.8  

1  

0.8  

2-5 

1  

1  

1  

0.8  

1  

1  

5-15 

0.8  

1  

0.8  

0.5  

0.8  

1  

15-40 

0.8  

0.8  

0.8  

0.3  

0.8  

0.8  

40-80 

0.5  

0.5  

0.5  

0.1  

0.5  

0.5  

>80 

0.3  

0.1  

0.3  

0.1  

0.3  

0.3  

Weight 

4  

3  

3  

3  

3  

2  

Table 41 - Rating and weights for coarse fragments.

Evaluation

The evaluation was done for each specie. It can be divided in three steps:

.

This score was converted to a suitability class, as shown in Table 42.

Score 

Suitability classes 

>0.8 

S1 

0.6-0.8 

S2 

0.3-0.6 

S3 

<0.3 

Table 42 - Suitability classes.

The suitability class obtained for each relevé was used to be generalised to the land unit according to the importance of the facet it belongs.

The provisional land suitability obtained is based upon comparison of plants requirements. The final stage is to revise this classification from three scenarios derived from environmental, economic and social analysis of the behavior of the species (Table 43).

Scenarios 

Eucalyptus 

A. nilotica 

Prosopis 

A. albida 

Ziziphus 

Balanites 

environmental 

80 

30 

50 

60 

70 

50 

economic 

80 

40 

50 

20 

90 

50 

social 

25 

15 

10 

50 

25 

Table 43 - Scenarios for the final classification.

The values represented the importance of each specie according to the scenario chosen. The results of each land suitability class are converted to that scale (Table 44), in order to calculate the final suitability.

Land suitability classes 

Conversion 

S3 

S2 

S1 

Table 44 - Conversion of the land suitability classes.

Results and discussion

Provisional land suitability classification.
Eucalyptus camaldulensis

It is highly suitable on the open scrub, forest plantation in the dunes and in open scrub on the lagoonal depression terraces (Figure 24). Fallows located in the interdunal succession or in the gently dipping slope with shallow sand coverage are moderatly suitable for Acacia albida. It is marginally suitable for 59% of the study area. Acacia albida is unsuitable mainly in the saline area (Tanma Lake), the beach and in the area occupied in the back slope by the open tree and scrub woodland.

Acacia nilotica

It is highly suitable in the fallow located in the gently dipping slope with shallow sand coverage (Figure 24). It is one of the three species (with Acacia albida and Prosopis juliflora) that are moderately suitable in the back slope open tree and scrub woodland unit. The coastal area, mainly the wet interdunal depression present good ecological conditions to this specie. It is also possible to find it near (Tanma Lake).

Acacia nilotica is the specie which has the most area classified as unsuitable. It needs more requirements for growing than that given by the soils.

Prosopis juliflora

It is one of the most suitable species for the study area (57%). The percentage of the unsuitable areas is also important (19%). It is not very well indicated in the the lakes or the depressions in Thiès cuesta (Figure 24).

Acacia albida

This tree is almost suitable in 94% of the study area (Figure 25). It is highly suitable in 65% of the lands. It prefers alluvial, riverine zones and depressions with water present below the surface and drier areas with a high water table.

Moderately suitable in 29% of the study area. It is still one of the species that can tolerate a slight salinity.

Ziziphus mauritiana

Ziziphus mauritiana is the most plastic specie. This tree performs well, even on marginal and inferior lands where most other fruit tree species either fail to grow or give poor performance.

There is no area classified as highly suitable for that specie (Figure 25). But most of the area (86%) is moderately to marginally suitable for Ziziphus mauritiana. It is only unsuitable in the lagoonal depression and along the dunes (mostly the part which is in front of the beach).

Figure 24 - Suitability map for Eucalyptus camaldulensis, Acacia nilotica and Prosopis juliflora.

Figure 25 - Suitablity map for Acacia albida, Ziziphus mauritiana and Balanites aegyptiaca.

Balanites aegyptiaca

It is highly suitable in the open scrub and the swales of the coastal dunes, the open shrubs and the tree savanna in the dry areas. It is also highly suitable along watercourses and in woodlands. It borders seasonally flooded black clay soil and grows well in valley bottoms (colluvial flat areas), in the lagoonal depression (terraces and swamps) and in the accumulation glacis (Undissected). The back slope with thick sand coverage is highly suitable for Balanites. It is still moderately suitable in (54%) and unsuitable in 19% (back slope, Tanma Lake, forest plantation on the dunes) of the study area.

Final land suitability classification

Table 45 presents the results obtained for Eucalyptus camaldulensis, Acacia nilotica, Prosopis juliflora, Acacia albida, Ziziphus mauritiana and Balanites aegyptiaca.

Land suitability for environmental forestry

Acacia albida is the most suitable specie for the 78% of the study area (Figure 26). It can develop a massive root system with a deep tap root, allowing good growth in areas if roots have access to the water table. It grows slowly but accelerates once the taproot reaches ground water. A. albida is important as a source of nitrogen, and the fallen leaves are important source of humus. It is good species to use in agroforestry for its ability to improve the soil because it drops its leaves at the beginning of the rainy season, providing nutrients to newly established crops. Since it is leafless at this time, shading of crops is not a problem.

Plantations of A. albida must be carefully done in the areas where A. albida is moderately suitable. Mainly in the back slope with shallow sand coverage (land unit 23) and the depression bottom with herbaceous (Land unit 16). The availability of water combined with salinity should be a constraint.

E. camaldulensis is suitable for 11% of the study area, mainly in the open scrub of the coastal dunes. It can be associated with P. juliflora; highly suitable in the area closed to the beach, as windbreaks. In this situation they can protect the cultivated areas located in the small depression against the wind effects.

Z. mauritiana and B. aegyptiaca are each, suitable for 1%. The lagoonal depression terrace with sparse scrub is more suitable for Z. mauritiana plantation.

Land suitability for economic forestry

Eucalyptus camaldulensis is a fast-growing tree. It the most profitable specie in wood production. The optimal rotation for E. camadulensis wood production is between 5 and 8 years, where later the farmers can cut the trees. The sticks can be used as material for house construction. But an attention should be payed to not allowing land degradtion. The farmer should make a choice between the income generated and the environmental impact, because E. camaldulensis is the most suitable specie for soil conservation in the dunes.

Fast-growing, drought resistant, and with remarkable coppicing power, Prosopis is a natural fuelwood candidate. This wood, because of its high heat content, burns slowly. It is highly suitable in most of the study area. It should be an important source for generating incomes for the farmers.

B. aegyptiaca is highly suitable for the study area, grows slowly and requires protection. It is easily worked and takes a good polish. The wood is durable and resistant. It is good for tool handles and domestic items such as spoons. Some of these products may be found in the local markets. The fruits constitute one of the principal sources of income.

Z mauritiana as B. aegyptiaca is a commercial fruit crop. The fleshy drupes of several species are rich in sugars and vitamins, and this fact has made Ziziphus species important fruit trees.

Currently, most cash crop fruit production in semi-arid regions relies on species such as peach, which require relatively intensive management and high irrigation inputs for successful establishment and fruit development. Compared to other more commonly cultivated fruit tree species like mango, Ziziphus species have several physiological and morphological characteristics that may contribute to their ability to adapt to arid environments.

The Senegalese institute for agronomic research try to improve the yield and fruit quality which routinely grafted on to the vigorous rootstocks of wild species to provide a reasonable cash crop on land which is unsuitable for other forms of cultivation. The result selected for Ziziphus correspond to those in which it is marginally suitable in our study. It should be a good starting point for the further research.

Land suitability for social forestry

The social forestry related to these elements: fuelwood, domestic timber, fruits, roots, medicines, grazing, hunting etc. (FAO, 1984). Z. mauritiana (72% of the area) and B. aegyptiaca (5%) are the most important food. E. camaldulensis (12%) is important for medicine and building materials. A albida, only 4% of the area classified, gives fruits for animal feeding.

Figure 26 - Land suitability for environmental, economic and social forestry.

Land unit 

Provisional land suitability classification 

Final land suitability classification 

Eucalyptus camaldulensis 

Acacia nilotica 

Prosopis juliflora 

Acacia albida 

Ziziphus mauritiana 

Balanites aegyptiaca 

Suitability for environmental forestry 

Suitability for economic forestry 

Suitability for social forestry 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

S1 

S2 

P. juliflora 

P. juliflora 

A albida 

S1 

S2 

S1 

S1 

S3 

S1 

E. camaldulensis 

E. camaldulensis 

E. camaldulensis 

S1 

S2 

P. juliflora 

P. juliflora 

A. albida 

S1 

S2 

S1 

S1 

S2 

S1 

E. camaldulensis 

E. camaldulensis 

Z. mauritiana 

S3 

S1 

S1 

S3 

S2 

A. albida 

P. juliflora 

Z. mauritiana 

S3 

S1 

S1 

S3 

S2 

A. albida 

P. juliflora 

Z. mauritiana 

S2 

S2 

S1 

S1 

S3 

S2 

A. albida 

E. camaldulensis 

E. camaldulensis 

S3 

S1 

S2 

S2 

S2 

P. juliflora 

Z. mauritiana 

Z. mauritiana 

10 

S3 

S2 

S2 

S2 

Z. mauritiana 

Z. mauritiana 

Z. mauritiana 

11 

S3 

S2 

S2 

S2 

Z. mauritiana 

Z. mauritiana 

Z. mauritiana 

12 

S3 

S2 

S2 

A. albida 

B. aegyptiaca 

B. aegyptiaca 

13 

S3 

S2 

S1 

B. aegyptiaca 

B. aegyptiaca 

B. aegyptiaca 

14 

S2 

S2 

S2 

S1 

E. camaldulensis 

Z. mauritiana 

Z. mauritiana 

15 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

NR 

16 

S3 

S2 

A. albida 

A. nilotica 

A. albida 

17 

S3 

S2 

A. albida 

A. nilotica 

A. albida 

18 

S1 

S2 

S1 

S1 

S2 

S1 

E. camaldulensis 

E. camaldulensis 

Z. mauritiana 

19 

S3 

S1 

S2 

S2 

A. albida 

Z. mauritiana 

Z. mauritiana 

20 

S3 

S3 

S1 

S2 

S1 

A. albida 

Z. mauritiana 

Z. mauritiana 

21 

S3 

S3 

S1 

S2 

S1 

A. albida 

Z. mauritiana 

Z. mauritiana 

22 

S2 

S2 

S2 

S3 

A. albida 

P. juliflora 

Z. mauritiana 

23 

S2 

S2 

S2 

S3 

A. albida 

P. juliflora 

Z. mauritiana 

24 

S3 

S1 

S1 

S3 

S1 

A. albida 

P. juliflora 

B. aegyptiaca 

25 

S3 

S1 

S1 

S3 

S2 

A. albida 

P. juliflora 

Z. mauritiana 

26 

S2 

S1 

S2 

S3 

S2 

E. camaldulensis 

E. camaldulensis 

E. camaldulensis 

27 

S3 

S1 

S1 

S2 

S1 

A. albida 

Z. mauritiana 

Z. mauritiana 

28 

S3 

S1 

S1 

S3 

S2 

A. albida 

P. juliflora 

Z. mauritiana 

29 

S2 

S2 

S2 

S3 

A. albida 

P. juliflora 

Z. mauritiana 

30 

S3 

S1 

S2 

S2 

A. albida 

Z. mauritiana 

Z. mauritiana 

31 

S3 

S3 

S1 

S1 

S2 

S1 

A. albida 

Z. mauritiana 

Z. mauritiana 

32 

S3 

S1 

S2 

S2 

A. albida 

Z. mauritiana 

Z. mauritiana 

33 

S3 

S2 

S3 

S2 

A. albida 

B. aegyptiaca 

Z. mauritiana 

34 

S3 

S1 

S2 

S2 

A. albida 

Z. mauritiana 

Z. mauritiana 

35 

S3 

S2 

S3 

S2 

A. albida 

B. aegyptiaca 

Z. mauritiana 

Table 45 - Land evaluation for forestry.


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