Soils of the central Orange River basin
Van Rooyen, Theodorus Hermanus
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The work reported in this thesis is based on a reconnaissence soil survey of the Central Orange River' Basin with the object to identify and select soils suitable for irrigation under the Orange River Development Project. The demarcated area constitutes 3,1 million hectares and is situated between latitudes 28° 45' and 30°, 45 and Iongitudes 22° 37' and 25° 15' in, the South Western Orange Free State and Northern Gape. Province , This is an arid to semi-arid region according to climatic classification. During the soil survey particular attention was paid to those factors of the environment which have a bearing on soils found in this area, and especially on their morphology" genesis and distribution. The environment, apart from the soils, is dominated by the general aridity" Iow relief with isolated hilly features and Karroe System rocks. Soils identified, fall in, two general classes, viz. clayey soils of colluvial , alluvial and sedentary origin and sandy soils of aeolian origin. The former are mostly of a saline nature and have poor physical properties with respect to irrigability. The sandy soils contain little or no soluble salts, are well, drained and well suited towards irrigation. Identification and mapping of the soils were carried out in accordance with a current classification system of the Soil and Irrigation Research Institute. Seventeen soil series of six soil forms and various land classes and complexes were identified and mapped. A soil map on a scale of approximately 1:320 000 accompanies this thesis. Soil samples of representative profiles were collected and analysed in order to further characterize soils. Chemical analyses substantiated field observations that the clayey soils were, in most cases, highly saline and the sandy soils non-saline. Clayey soils were furthermore alkaline in reaction, and rich in-exchandeable cations, notablyCa++,Mg ++ and Na+. High CEC values indicated a dominance of 2:1. clay minerals, whereas X-ray diffractograms gave evidence of mixed layering of these minerals. The aeolian sands generally had lower pH values low clay contents and hence Iow CEC and exchanqeable cations. Clay mineral suites appeared to be of a similar nature to those of the clayey soils. The prominence of large Aeolian deposits in the landscape and their importance towards irrigation prompted an investigation on the origin of the Aeolian sandy parent materials. Morphological and mineralogical studies proved that two types of Aeolian sandy deposits, distinctly different in origin, occur. It is postulated that the yellowish sands were blown from the Orange River bed in situations favourable for westerly winds. The red sands are of Vaal River origin and were blown from the river bed towards the east and south-east by westerly to north-westerly winds.