Soil nutrient status in the different land use systems and soil profiles of Seronga, Botswana
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The status of soil physico-chemical properties as governed by land use systems at local level is necessary for determining soil productivity and appropriate management options for sustainable land use. A study investigating soil nutrient status in the different land use systems and soil profiles of Seronga, Botswana was conducted. A total of 597 soil samples were collected at different soil profile depths (0-10, 10-20, 20-30 and 30-40cm) in three seasons (i.e., onset of rains, peak of the rains and dry season) between 2011 and 2012. Both descriptive and comparative analyses were conducted to identify potential agricultural sites and assessing the effects of cultivation on soil properties. Seronga soils contained high sand (>90%) and low clay (0.4% to 2.8%), with pH (CaCl2) being acidic (woodland) and close to neutral (cropped land, fallow land and grassland). The soils have low moisture content (<9%) and electrical conductivity (<1mS/cm). On average (0-40cm soil profile), the highest (0.78±0.10%) and lowest (0.32±0.03%) soil organic matter content was recorded in tall mopane woodland at the onset of rains and in the dry season, respectively. Soil organic matter content was highest in the soil surface (0-10cm) samples. However, isolated instances of soil organic matter migration to the lower horizons were observed. Soil total nitrogen (N) was very low (<0.03%) at the onset of rains and dry season, showing a decrease with depth. Total N increased at the peak of rains; with highest in Ximenia woodland (0.39±0.03%), grassland (0.37±0.01%) and Dichrostachys woodland (0.36±0.01%). Total phosphorous (P) was below detectable limit (<0.0001g/kg) at the onset of rains and in the dry season in all the land use systems and soil profiles. At the peak of rains, total P was high in tall mopane woodland (0.08±0.06g/kg) and in grassland (0.07±0.02g/kg). Available P ranged from 22.16±1.09mg/kg in grassland at the peak of rains to 1.98±0.09mg/kg in Terminalia woodland during the dry season, decreasing with soil depth. Generally very low levels (<0.02%) of total sulphur (S) were recorded in all land use systems and soil profiles. However, total S differed significantly (P<0.05) among the land use systems. Base cations (calcium, magnesium, sodium and potassium) varied significantly (P<0.05) among the land use systems but did not differ within the soil profiles except at the peak of rains in Ximenia woodland. Copper, iron, manganese and zinc highly differed significantly (P<0.01) among the different land use systems and soil depth. Overall, the results indicated that Seronga soils are low in soil nutrients and deficient in DTPA-Cu and Zn. Land use influenced soil nutrient status of Seronga; with the effect varying with profile depth and season. Generally, the results indicate that grassland have high capacity to retain nutrients whereas Terminalia woodland had the least nutrient retaining capacity except in the case of DTPA-Fe. The results also indicate that native soils (non-cultivated Mopane woodland) retained less nutrients compared to the cultivated soils, attributed to human intervention by introducing organic materials in the fields. Cultivated soils and fallow soils did not differ significantly (P<0.05) in organic matter (dry season), total N, available P, Ca, Mg, K, Fe and Zn, suggesting no justification for shifting cultivation. Fallowing had minimal impact on nutrient levels; increasing only total S in the wet season. Recently cultivated soils had significantly high (P<0.05) Na content compared to continuously cultivated and non-cultivated soils in the wet season. Seronga soils have very low fertility and productivity level.