Chemical characterisation of the soils of East Central Namibia

[摘要] ENGLISH ABSTRACT: A number of chemical and physical features of Namibian soils in a 22 790 km2, two degree-square blockbetween 17 – 19 oE and 22 – 23 oS in eastern central Namibia, had been investigated, and the fertilitystatus established.In 80 % of samples the nitrate, nitrite, and sulfate concentrations of the saturated paste extract are 0 – 54.6mg l-1 NO3-, 0 – 24.7 mg l-1 NO2-, and 5.4 – 20.9 mg l-1 SO42- respectively. In 90 % of samples the plantavailablephosphorus is below 12 mg kg-1, which is low for a soil under natural grassland, but in line with theprevailing semi-arid climate and low biomass production of the study area. In 80 % of samples theextractable calcium, magnesium, potassium, and sodium concentrations are 0.61 – 5.73 cmolckg-1 Ca (122 –1 146 mg kg-1 Ca), 0.12 – 2.28 cmolckg-1 Mg (15 – 278 mg kg-1 Mg), 0.13 – 0.54 cmolckg-1 K (51 – 213 mgkg-1 K) and 0.05 – 0.38 cmolckg-1 Na (11 – 87 mg kg-1 Na) respectively, while the exchangeable calcium,magnesium, potassium, and sodium concentrations are 0.21 – 6.02 cmolckg-1 Ca, 0.12 – 2.01 cmolckg-1 Mg,0.12 – 0.49 cmolckg-1 K and 0 – 0.13 cmolckg-1 Na respectively. The mean ± 1 standard deviation is 3.57 ±3.57 cmolckg-1 for cation exchange capacity, 3.48 ± 3.61 for sum of exchangeable bases and 4.53 ± 4.39 forsum of extractable bases. The cation exchange capacity and the sum of exchangeable bases are virtuallyidentical, which indicate the almost complete absence of exchangeable H+ and Al3+ in the soils of the studyarea, as expected from a semi-arid climate. None of the profiles were classified as saline or sodic. In 80 % ofsamples the plant-available iron, manganese, zinc and copper concentrations are 7.2 – 32.8 mg kg-1 Fe, 13.6– 207.5 mg kg-1 Mn, 0 – 1.80 mg kg-1 Zn and 0 – 4.0 mg kg-1 Cu respectively. Soil organic matter content ofthe study area soils ranges between 0.05 – 2.00 %, with most (80 % of samples) containing 0.25 – 1.20 %organic matter. This is considerably lower than values reported in literature, even for other southern Africancountries. The reason lies with the hot, semi-arid climate. The pH distribution is close to normal, with 80 % ofsamples having pH (H2O) of 5.54 – 8.18, namely moderately acid to moderately alkaline. Sand, silt and claycontent of most (80 %) samples varies between 60.3 – 89.7 % sand, 4.6 – 25.2 % silt and 3.5 – 19.1 % clay.The soils of the study area are mainly sandy, sandy loam and loamy sand. In 80 % of samples the coarsesand fraction ranges from 3.5 – 34.5 %, the medium sand fraction from 20.5 – 37.3 %, the fine sand fractionfrom 38.7 – 54.5 % and the very fine sand fraction from 0.0 – 12.9 % of all sand. The fine sand fraction, thus,dominates, with very fine sand being least abundant. The topsoil contains relatively more coarse sand andless very fine sand than the subsoil. Instances of sealing, crusting and hardening occur sporadically in thestudy area. Cracking is only found in pans, while self-mulching is not evident. No highly instable soils wereencountered in the study area. The water-holding capacity is generally low, with depth limitations in thewestern highlands, the Khomas Hochland, and texture limitations in the eastern Kalahari sands. The centralarea has soils with a somewhat better water-holding capacity, but it is still very low when compared to arablesoils of temperate, sub-humid and humid zones elsewhere in southern Africa.Soil characteristics are perceptibly correlated with climate, parent material, topography, degree of dissectionof the landscape and position in the landscape. The most obvious differences are between soils formed inschistose parent material of the Khomas Hochland in the west and those of the Kalahari sands in the east.The soils of the study area are unsuitable to marginally suitable for rainfed crop production, due to lowivfertility. The study area is climatologically unsuited for rainfed crop production, so the present major land useis extensive livestock production on large farms. The natural vegetation is well adapted to the prevailingconditions.The methodology followed to delineate terrain units, with a combination of procedures involving digitalelevation data and satellite imagery, seems to work well in the Namibian landscape. This study thus servedas a successful proof-of-concept for the methodology, which can in future be rolled out for the remainder ofthe country. The site and analytical information is available in digital format as spreadsheets and in ageographical information system, as well as in a variety of digital and printed maps.

[发布机构] Stellenbosch University