Long-term effects of micro-fine and class A calcitic lime application rates on soil acidity and rooibos tea yields under Clanwilliam field conditions

Previous research on the effect of soil liming on rooibos tea growth was limited to short-term greenhouse trials. This is the first long-term field study to investigate the effect of lime application on rooibos tea production. Micro-fine and class A calcitic lime with similar levels of purity were applied at rates of 0, 0.5, 1.0, 2.0 and 5.0 tonnes ha−1 to a depth of 15 cm on an acid (pHKCl 4.4), sandy soil in Clanwilliam, prior to planting. Soil pH and electrical conductivity (EC), rooibos mortality and yields were then measured over a period of 4 years. Foliar nutrients were determined in Year 4. Micro-fine lime resulted in significantly higher soil pH values (0.5 pH units) than class A lime. Both limes achieved maximum soil pH and EC values in Year 2, but these values declined significantly in Years 3 and 4. This is most likely due to leaching and exhaustion of the most soluble lime fractions. Although micro-fine lime had positive effects on foliar nitrogen and calcium concentrations and bush survival, liming appeared to have a negative effect on rooibos tea yields. Yields tended to decline with increasing lime application rate. Thus, it appears that under the Clanwilliam field conditions in the present study, liming is unnecessary.