Soil fertility constraints of irrigated wheat production in South Africa.
There is a gradual decrease in the number of wheat (Triticum aestivum L.) producers in South Africa due to low profitability of the crop. As producers target higher yields to improve profits, nutrient demand and removal increases, and poor soil fertility management is inevitably a major limiting factor for wheat production. This study was aimed at exploring the soil fertility constraints of irrigated wheat producers in South Africa. The objective of this study was to determine the influence of geographical regions and crop management strategies such as tillage and crop rotation, on soil fertility status of irrigated wheat fields in relation to wheat requirements. The study was conducted across the major South African irrigated wheat production regions namely: KwaZulu-Natal, the Eastern Highveld, the Warmer Northern and the Cooler Central areas. Soil were sampled from the fields of representative producers (n = 130) in all the irrigated wheat production regions at 0 - 20 and 20 - 40 cm depths. The samples were analysed for soil organic carbon (SOC), sulphur (S), phosphorus (P), zinc (Zn), calcium (Ca), magnesium (Mg), potassium (K), cation exchange capacity (CEC), acidity, exchangeable sodium percentage (ESP), texture and electrical conductivity (EC). Wheat crops were also scored for visual symptoms of nitrogen (N) deficiency. The majority of the producers (63.85%) practised conventional tillage. Conservation tillage combined with a legumes-wheat crop rotation was adopted by 88.37, 13.89, 0 and 0% of the producers in in KwaZulu-Natal, Warmer Northern, Cooler Central, and Eastern Highveld regions, respectively. Soil pH, ESP and plant available P, K, Mg and Ca varied considerably across geographical regions. Soils in KwaZulu-Natal were the most acidic (pH [KCl] 4.51 ± 0.05), followed by Eastern Highveld (pH 4.97 ± 0.08), Cooler Central (pH 5.75 ± 0.09) and Warmer Northern (pH 6.32 ± 0.12). Significant positive Pearson’s correlations between pH and Mg, ESP, CEC and Ca indicated that acidity was a major factor that influenced the availability of nutrients. The Eastern Highveld and Warmer Northern regions had a slightly lower soil pH (KCl) for rotations in which wheat was preceded by a legume than by a non-legume. Mean plant available P was below the minimum range in KwaZulu-Natal (27.49 ± 2.04 mg/kg) and Warmer Northern (36.35 ± 3.65 mg/kg) regions. Conservation tillage and rotations where wheat was preceded by legumes generally had higher acidity levels and P deficiency problems than either conventional tillage or non-legume - wheat rotations. Plant available Zn, Mg, K and S were generally adequate across all geographical regions. Electrical conductity and ESP were also acceptable across all geographical regions. Calcium to Mg ratio was low in the Warmer Northern region (1.91 ± 0.11), but was within the acceptable range of 2-8 in the other regions. Soils of the Warmer Northern region had very high Mg (634.30 ± 54.31 mg/kg). There was an indication of P stratification on all all farms, hence generally adequate P levels at 0-20 cm (mean 45.57 ± 2.54 mg/kg), but deficiency at 20 – 40 cm (34.36 ± 2.28 mg/kg). Soil organic carbon varied considerably across the irrigated wheat production regions in the country, ranging from 0.13% to 6.02%, with a mean of 1.55% and 65.60% coefficient of variation. Overall, 42.31% farms had SOC below the critical limit of 1% at 0 – 20 cm soil depth. Geographic region, tillage and soil depth had a significant (p<0.05) effect on the SOC. Soils in the KwaZulu-Natal region, where the majority of producers (88.37%) use conservation practices had the highest SOC (2.00 ± 0.09%), followed by Warmer Northern (1.65 ± 0.14%), Cooler Central (0.84 ± 0.08%) and Eastern Highveld (0.82 ± 0.07%) where soils had lower SOC. Farms where conservation tillage (2.15 ± 0.10%) is practiced had more SOC at both sampling depths of 0-20 cm and 20-40 cm than farms that used conventional tillage (1.02 ± 0.05%). In Warmer Northern region, fields where wheat was preceded by a legume crop had more SOC than those where it was preceded by a non-legume crop. The opposite was true for the KwaZulu-Natal. In the Cooler Central and Eastern Highveld, crop rotation systems had no significant difference in SOC. Geographical region × soil depth (p<0.05) interaction was also significant and showed that in KwaZulu-Natal, the SOC was higher in the 0 - 20 cm depth than in the 20 - 40 cm while depth did not affect SOC in the other three regions. No significant correlation (r = 0.0) was observed between SOC and clay content in KwaZulu-Natal, while moderate positive correlations were obtained for SOC and clay in other regions. Cation exchange capacity, Ca, Mg and S were significantly related to SOC in the the Cooler Central and Eastern Highveld regions. In the Warmer Northern region, there were significant negative relationships between SOC, P and pH. Zinc did not have a significant relationship with SOC in all the geographical regions. The findings suggested that conservation tillage and inclusion of legume crop rotation systems could be important strategies for increasing SOC and soil fertility on irrigated wheat fields. Sustainable approaches for effectively enhancing P availability and addressing pH problems under these systems need to be sought.