Zambezi

Integrated Agro-Commercial Development Project Soil Survey

Detailed soil survey


The soil survey was carried out in the Zambezi Integrated Agro-Commercial Development Project in North Botswana, about 40 km south of Kasane and 50 km north of Pandamatenga, extending on a 30,000 hectares area. The aim of the survey was to create a set of recommendations for agricultural management and modern methods implementation, irrigation, fertilizers and crop adaptation. The survey was carried out in full cooperation with the department of soil survey of the Ministry of Agriculture in Gaborone which was involved in all aspects of the work and contributed to the success of the project.


Description of Project Area


The surveyed area in north-east Botswana is part of a region consisting of heavy clayish soils that originated in lacustrine deposits. These heavy clay soils spread over 28,000 hectares in an area dominated mainly by sand stones and sandy soils. There are two large plots of heavy clay soils. The northern plot is adjacent to the Kasane Forest Reserve, while the southern plot, known as Pandamatenga, contains large cultivation areas, as well as a settlement of about 2,000 people.

Survey goals


  • An analysis of the soil uses in the entire area. 

  • A general investigation of the area’s natural conditions. 

  • A general investigation of the area’s natural drainage system. 

  • A detailed account of the landscape units which make up the area. 

  • Detailed soil survey and recommendations of agricultural management.


Existing literature and studies


The information sources consist of general land maps, a number of profiles, soil descriptions, and soil analysis results from the Land Survey Department of the Ministry of Agriculture in Botswana. The data sources at our disposal were:


  • Erick Van Waveren, (1988), Botswana Soil Database Manual, Soil Mapping and Advisory Services Botswana.

  • P.V De Wit, R.P Bekker, (1990), Explanatory Note on the Land Systems Map of Botswana, Soil Mapping and Advisory Services Botswana.

  • Soil surveys in Botswana 1989-1990 Chobe and Pandamatenga.

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Slopes


The surveyed area is a plateau with a very moderate slope to the east of 0.15%, running down to the stream flowing northwards to the Zambezi River. the difference between the height of the ridges (the sandy dunes) and the low clay plains may be no more than 1-2 meters.


Geomorphology


The project area is divided into two main land units:

  • The western land unit is part of a much larger clay basin, spreading west and south, characterized by flat land with heavy clay soils, low slopes and poor natural drainage.

  • The eastern part consists of alternating ridges in the form of stabilized sand dunes running east-west, with flat clay surfaces between them. The width of the ridges (both the stabilized sand dunes, and the clay surfaces between them)


ranges between 200 and 1,000 meters, and their length may reach and even exceed 15 km. All in all, 5-6 series of alternating sand dunes and clay surfaces were identified, occupying most of the eastern part of the project area. The appearance of the stabilized sand dunes on the clay, and the resulting interfingering in the landscape, are the dominant causes to the creation of landscape units. The landscape units are areas with homogeneous morphology, environment, and ecology. Soils develop on the landscape units linked to their unique natural conditions.

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Detailed soil survey


The soil survey took place between 2008-2009 more than 300 boreholes and ditches up to 2 meter to the bedrock. The average borehole density amounted to one borehole for every 330/20,000 hectares, which is appropriate for highly-detailed surveys. The soil properties for each borehole were photographed and described in detail according to the guidelines provided by the USDA Soil Survey Manual (Soil Survey Staff, Soil Survey Manual, United States Department of Agriculture, 1962). Samples from selected boreholes were sent to the laboratory for chemical and physical testing purposes. The soil properties which were observed in the field include: 


  • Depth

  • Color

  • Structure

  • Stoniness

  • Texture

  • Drainage


In addition, general observations in the field were made relating to topography, relief, geomorphologic features, vegetation and hydrology.

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H- The northern plateau

Relief: Slope gradient to the west of 1% Depth and bedrock: Relatively shallow soils carbonate rock on a 70 -100 cm depth. Texture: Upper crust with a high percentage of sand. Drainage: Moderately to well drained


SH – The eastern sand blocks

Relief: moderate slope, 1% gradient. Depth and bedrock: Deep soil, no rocks on the lower crust. Texture: Sandy loam on the upper crust. Drainage: well drained to excessive


SH soil units

Relief: moderate platform and slopes. Depth and bedrock: Soils with variable depth, the horizon appears worn fragments of basalt at a depth Texture: Sandy loam on the upper crust, sandy loam – sandy clay loam on the lower crust. Drainage: well to moderately well drained.


T- Sand dunes

Relief: east west elongated Extension, 0.1% gradient towards the East and 0.3% moderate gradient north to south. Depth and bedrock: varying soil depths 70-180 cm. on the lower crusts might be found clusters of chalk and iron. Texture: Sandy loam, fine sandy loam. Drainage: well drained.

L- Northern plains

Relief: Inclined plain to the west, 0.1% gradient. Depth and bedrock: Shallow soils, worn carbonate rock at 70-80 cm depth. Texture: Sandy clay loam on the upper crust, clay loam on the lower crust. Drainage: moderately to well drained.

M- The central block

Relief: plains, 0.1% gradient. Depth and bedrock: deep soils, worn carbonate rock at 180 cm deep. Texture: Sandy clay loam on the upper crust, clay loam on the lower crust. Drainage: poor to imperfect.

B- Basin

Relief: wide plains, moderate gradient toward the west less than 0.2%. Depth and bedrock: deep soils, limestone and brittle clay appear on a 150 cm depth. Texture: clay loam on the upper crust and clay on the lower crust. Drainage: imperfect.


Lab analysis

About 30% of the boreholes were submitted to a series of lab analysis such as:

1. SP - Saturation Percentage

2. Ec - Electrical conductivity

3. pH - Acidity/Basicity Absorption 

4. NO3 – Nitrate

5. P – Phosphorus

6. K – Potassium

7. Ca+Mg - Calcium and Magnesium 8. Na – Sodium

9. Cl – Chloride

10. SAR - Sodium

11. Soil texture - Sand, Clay and Silt 

12. Calcite – CaCo3

13. B – Boron

Location of Agricultural


Land Agricultural development of the project area consists of a combination of different types of agriculture: orchards, vegetables, oil crops (seeds), together with fish ponds and produce processing plants. The following lists the current development program and the required soil conditions

Recommended: 

  • Vegetable crops, sandy soils

  • Orchards, sandy soils

  • Orchards, medium texture soils

  • Oil crops, light to medium clayish soils

  • Fish ponds, extremely heavy soils

  • Area of produce processing plants Recommendations

  • Defining plot limits and borders.

  • Salination problems: In the clay soils on the western area salination problems are expected. The recommendation is to over irrigate in order to decrease salinity in the soil.

  • Monitoring water levels that might arise after irrigation. 

  • Crop suitability: Orchards, plantations and crops that need well drained soiled will be located in the sandy areas to the east. Other crops not sensitive to drainage will be located in the western part. Fish ponds should be located in plains with heavy clay soils.

  • Irrigation methods: Determined by soil conditions; on sandy soils sprinklers are recommended and on clay soils, it is advised to conduct infiltration tests to avoid over salination of the soil.

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