Long-term changes of soil chemical characteristics and maize yield in no-till conservation agriculture in a semi-arid environment of South Africa

Soil management practices may change soil chemical properties and thus fertility. The magnitude of change varies depending on soil type, cropping system, climate, fertilizer application and management practice. The objectives of this study were to evaluate the effects of no-till (NT), rotational tillage (RT), conventional tillage (CT) treatments, and three N-fertilizer application rates (0, 100 and 200 kg ha(-1)) on soil chemical properties and maize yield in a semi-arid environment, Haplic Ferrasol in South Africa, Bergville. This was evaluated in 0-10, 10-20 and 20-30 cm depths. Soil chemical properties were measured 13 years after the implementation of the trial. Marginal differences (p > 0.05) were found in total soil organic carbon (SOC) with NT having the highest concentration (27.1 t.ha(-1)) compared to RT (26.0 t.ha(-1)) and CT (26.5 t.ha(-1)). The concentration of Nitrogen (N) followed the same trend (p = 0.024) where it was found to be higher under NT (1.54 t.ha(-1)) than RT (1.30 t.ha(-1)) and CT (1.42 t.ha(-1)), respectively. SOC and N were found to be highly concentrated in the 0-10 cm depth in both NT and RT treatments. Phosphorus was significantly higher (p < 0.001) under NT (0.0213 t.ha(-1)) than RT (0.0127 t.ha(-1)) and CT (0.00704 t.ha(-1)). A large amount of phosphorus was in the 0-10 cm depth in NT and it was distributed more uniformly under RT and CT. Potassium was also higher (p < 0.05) under NT (9.73 t.ha(-1)) than RT (9.52 t.ha(-1)) and CT (8.00 t.ha(-1)). It was found to be uniformly distributed across the soil depths in all tillage treatments. No significant differences were found in the concentration of calcium across the tillage treatments, however, it was observed to increase with depth under NT and RT and to decrease with increase in depth in CT. The soil of NT and RT treatments had lower pH values (5.80 and 5.86) than CT (6.68) at 0-10 cm depth while in the lower depths, 10-20 and 20-30 cm depth was observed to increase significantly, by 1.2 and 0.9 units in NT and RT respectively. Similar trends were observed in CEC. Yield across the years averaged at 12.3, 12.4 and 11.8 t.ha(-1) in NT, RT and CT treatments respectively, under 200 kg.ha(-1) N-application. However, yields gains under NT diminished significantly than in RT and CT treatments in 100 (10.6, 11.3 and 11.4 t.ha(-1), NT, RT & CT, respectively) and 0 kg.ha(-1) N-application rate (6.6, 7.6 and 8.3 t.ha(-1), NT, RT & CT, respectively). This suggested the application of correct fertilizer proportion to increase yield. The results of this study indicated that SOC takes time to improve in conservation agricultural practices in a semi-arid environment and its increase is only confined to the soil surface. This increase in the soil surface is also associated with increased availability of the soil important nutrients which may reduce the application of these nutrients as a starter.