Glen Valley

Soil Survey Report

Objectives

The main objective of the soil survey was to determine the distribution and characteristics of the different soil types in the Glen Valley project area. Identification and characterization of the soil properties is essential for intense agricultural projects such as this one. Certain soil characteristics may limit the variety of crops which may be cultivated or influence decisions regarding agricultural management. The survey also aimed to assess the layout of the planned agricultural plots, irrigation and drainage systems, and offer recommendations concerning cultivation and irrigation management.

General Background of the Project

One of the main undertakings of the National Master Plan for Arable Agriculture and Dairy Development (NAMPAADD) project is to create production and training farms (PTF) that will serve as examples for advanced and modern agricultural practices for Botswana’s agricultural sector. The purpose of these farms is to test and demonstrate the relevant technologies to the farmers which cultivate the pilot farms surrounding the PTF plots as well as to other farmers in the region. The PTFs will operate towards the development of three agricultural practices: rainfed farming, irrigated farming and dairy farming. The Glen Valley Irrigation Project encompasses solely the field of irrigated farming.

Location and Description

The Glen Valley Irrigation Project is located approximately 2.5 km north and east of the town of Gaborone, on the west bank of the Ngotwane River. The Botswana Defense Forces base is located northwest of the project area, and to the south lies Gaborone’s sewage treatment plant and ponds.


The planned location for the National Museum is among the agricultural plots in the northern section of the project area.


The Marapoathuttwa River, a tributary of the Ngotwane River, transects the project area in the north, and the Segoditshane tributary flows along its eastern boundary before joining the Ngotwane River.

Objectives The main objective of the soil survey was to determine the distribution and characteristics of the different soil types in the Glen Valley project area. Identification and characterization of the soil properties is essential for intense agricultural projects such as this one. Certain soil characteristics may limit the variety of crops which may be cultivated or influence decisions regarding agricultural management. The survey also aimed to assess the layout of the planned agricultural plots, irrigation and drainage systems, and offer recommendations concerning cultivation and irrigation management. The project extends over an area of approximately 200 hectares and is divided into over 80 separate cultivation plots with an average area of 2-3 hectares each. The NAMPAADD PTF lies on a 13 hectares plot.


The ridges are comprised of steeper slopes as well as flatter, plateau areas. Moving east towards the banks of the Ngotwane River, a steeper slope is once again present before flattening into the basin lining the banks of the Marapoathuttwa and Ngotwane rivers. Heavy and consistent rains cause the Marapoathuttwa River to inundate the flat area surrounding it.

Infrastructure in the Glen Valley Project Area

Dirt roads, power lines and drainage canals transect the project area. The drainage canals were constructed in the project area to ensure adequate drainage of the agricultural plots. These canals were designed in a grid-like manner, along the outside borders of the agricultural fields. Three types of drainage systems were constructed:


  •  Main canals: cemented, three running northeast, one to the north and one in the southeast direction

  • Secondary canals: made of cement or dirt, running along each of the plots;


  •  Subsurface drainage system

SURVEY METHODS

For this high intensity survey, 61 boreholes were dug using a JCB tractor, which allows a large number of boreholes to be dug in a short period of time. The average borehole density amounted to one borehole for every 3.3 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, Structure, Texture, Color, Stoniness, Drainage In addition, general observations in the field were made relating to topography, relief, geomorphologic features, vegetation and hydrology.


These observations assist in accurately identifying the different soil types and provide clues to determining the origin and boundaries of each soil unit.

FINDINGS

Soil Types and Distribution

Landscape Units

In general, the project area can be divided into a few landscape units. 


  • River beds and the valleys surrounding them;

  • Elevated plateaus;

  • Slopes leading from the plateaus to the river.


Map E – Landscape Units

Soil Categories


Two dominant categories of soil types can be distinguished by their texture and color, which also coincide with the landscape units of the area.


  • Upland soils characterized by their coarse texture and good drainage conditions are found in the higher elevated areas. The source of this soil is weathered granite rock in the area. This category includes the gravelly soils which were formed by ancient streams as well as the luvic calcisol soil which contains a higher clay content and therefore a more compact structure.


  • Lowland soils found in the river basin. These soils are heavy, compact and dark and lie along the Ngotwane River and its tributaries. The source of this soil is alluvial material transported by the streams from distant areas.

RECOMMENDATIONS

Stoniness and Depth The soils which pose the greatest difficulty for agricultural cultivation are those which possess high stoniness or granite outcrops (Soils R and R2). The recommendations for these plots are as follows:


Areas marked by high stoniness and shallow soil should be limited to the cultivation of fruit trees.


The areas with excessively shallow soil, high gravel content and granite outcrops are not suitable for any agricultural cultivation whatsoever.


Fertility


Low soil fertility levels (low CEC) are found in the soils located in elevated areas, mainly in AF and Ar soils. Fertilizer products must be applied to these soils.


Water Holding Capacity and Drainage


Well-drained soils with a low water holding capacity can be found in the relatively light soils of the elevated areas. The area host to areni-hapliclixisol (Ar) soil is characterized by a relatively steep slope. In order to avoid problems such as soil erosion, the surface runoff generated in this area must be dealt with appropriately. In the lower, alluvial soils (VL and VC), and in the plane host to luvic calcisol (LC) soil, drainage problems are expected. In these areas, measures must be taken to ensure that excess water is removed efficiently. Frequent monitoring and proper measures must also be taken to ensure that long water retention periods do not cause the accumulation of salt in the soil. Intervals between irrigations cycles must be planned in accordance with the water holding capacity of each soil type. For the lighter soils, the intervals should be shorter, and for the heavier soils, irrigation cycles should occur less frequently. Soil Compaction The LC, VL and VC soils contain large percentages of clay, and thus are heavy and compact by nature. Deep and intensive plowing will assist to aerate the soil.


Use of heavy machinery should be avoided however, especially during wet or dry conditions to avoid damage to the soil structure and further compaction of the soil. Salinity Results from the laboratory tests indicated high electric conductivity for some of the samples from the heavier soils (LC and VC). In the future, these areas will be cultivated under intense irrigation, which raises some concern regarding water retention, capillary action and salt accumulation in the subsurface of the soil. Therefore constant monitoring for salinity is required, at depths of 50-100 cm beneath the surface in order to prevent saline soil conditions.

Soil Compaction


The LC, VL and VC soils contain large percentages of clay, and thus are heavy and compact by nature. Deep and intensive plowing will assist to aerate the soil. Use of heavy machinery should be avoided however, especially during wet or dry conditions to avoid damage to the soil structure and further compaction of the soil.


Salinity


Results from the laboratory tests indicated high electric conductivity for some of the samples from the heavier soils (LC and VC). In the future, these areas will be cultivated under intense irrigation, which raises some concern regarding water retention, capillary action and salt accumulation in the subsurface of the soil. Therefore constant monitoring for salinity is required, at depths of 50-100 cm beneath the surface in order to prevent saline soil conditions.

The Drainage Canals

As mentioned previously, many problems exist regarding the drainage canals, mainly due to the fact that the canals do not follow the natural drainage paths and obstruct the natural flow of water in the area. This causes the inundation of fields following heavy rains.


It is problematic to suggest a drastic overhaul of the drainage system in order to mitigate these problems. However, the more severe problems can be dealt with by making some minor changes in the current layout of the drainage network.


  • The northern canal blocks the flow of the Marapoathuttwa River, causing cracking in the sides of the canal and potential blockage as was depicted in the pictures. The canal should be adjusted to allow a smooth entrance of the river into the canal.


  • A main canal also blocks the path of the Marapoathuttwa River downstream, causing inundation of the fields in the area. The route of this canal should be relocated to the natural flow path of the River which would take advantage of the natural drainage system as well as prevent flooding of the fields.


  • Soil erosion is visible above the subsurface drainage pipe outlets into dirt canals. This erosion poses a threat of soil loss to adjacent fields. These problematic areas must be located, and the proper measures taken to prevent further erosion.

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