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sur le développement des zones arides et semi-arides

Titre : The effects of long-term tillage and crop rotation practices on nutrient stratification in the Western Cape

Auteur : Van der Merwe, Annemarie

Université de soutenance : Stellenbosch University

Grade : Master of Science (MScAgric) 2022

Résumé partiel
The Western Cape is one of the most successful provinces in South Africa in converting to Conservation Agriculture (CA), with an adoption rate of around 80%. CA is a production system that promotes minimal soil disturbance, maintaining crop residues on the soil surface combined, with crop rotation with different species, including legumes. The absence of soil mixing in CA systems can lead to the stratification of immobile nutrients at the surface of the soil profile. Rapid drying of top soil layers may prevent roots from absorbing nutrients from these layers. Up until now, little was known regarding the extent of nutrient stratification in CA systems in the Western Cape. The first objective of this study was to determine the vertical distribution of plant-available nutrients under different tillage practices and rotation sequences at Tygerhoek (34 ̊29’32” S, 19 ̊54’30”E) and Langgewens (33 ̊16’34” S, 18 ̊45’51” E) Research Farms. Soil samples were collected at 0-5, 5-10, 10-15, 15-20 and 20-30 cm depth intervals in zero-till (ZT), no- tillage (NT), minimum tillage (MT) and conventional tillage (CT) treatments combined with 4 crop rotation sequences - wheat monoculture (WWWW), wheat and medics rotation (WMWM and MWMW) and canola/wheat/lupine/wheat (CWLW). Crop rotation and its interaction with tillage and soil depth did not influence (p < 0.05) the distribution of nutrients in the soil at Tygerhoek, but the distribution of K and S in the soil at Langgewens was influenced. Tillage significantly influenced nutrient stratification at both sites. The amount of extractable Ca, Mg, P, K and C were significantly higher in the surface 0-5 cm of the soil under ZT and NT compared to CT. The higher soil organic carbon (SOC) in the topsoil under CA (ZT, NT and MT), may be due to reduced soil disturbance and retention of crop residues. The organic C in the 0-5 cm layer decreased as degree of soil disturbance was increased (ZT2.6%>NT 2.23%> MT 2.15% > CT 1.96%). The percentage difference in soil exchangeable K at Langgewens between 0-5 cm (308 mg kg-1) and 5- 10 cm (172 mg kg-1) layers were the most at 79% for ZT. The second objective was to evaluate the extent of soil nutrient stratification in a wide range of CA systems on commercial farms. Stratification was observed in both the natural veld and the cultivated soil. The cultivation and addition of ameliorates (fertiliser and lime) in the cultivated soil have accentuated the stratification compared to the natural veld.

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