Soil clay mineralogy and potassium buffer capacity as potential wine quality determining factors in Western Cape vineyards

The potassium (K) supply characteristics and clay mineralogies of a population of Western Cape soils were investigated to determine their potential effects on vine K uptake and wine quality. The total K contents of granite-, shale- and sandstone-derived soils varied, averaging 33.7, 26.1 and 4.5 cmol(+)/kg, respectively. Corresponding M NH4Cl exchangeable soil K levels were: 0.172, 0.042 and 0.035 cmol/kg. Ability to fix applied K also varied, decreasing from 0.350 in the shale-, to 0.188 in the sandstone- to -0.177 cmol/kg in the granite-derived soils. Potential buffering capacity for K was pH / liming dependent, particularly in the shale soils. Potassium uptake by Italian rye grass correlated negatively with K fixation. The K contents of Italian rye grass grown on the sandstone, shale and granite soils were, respectively, 2.32, 2.12 and 5.56 dry mass %. These results were explicable in terms of soil mineralogy. The presence of kaolinite in the clay fraction, with mica and K-rich feldspar cores in the silt fraction enabled the granite soils to release primary K, but conferred little power to fix, or to buffer K against luxury uptake or loss through leaching. In contrast, the shale soil clay fractions consistently contained vermiculite and interstratified 2:1 minerals. These conferred marked pH / liming dependent K buffer capabilities. The shale soils also contained K in micas in the non-clay fractions. The sandstone soils varied in terms of both mineralogy and clay content. Sandstone soils, in which the sand fractions were quartzitic were unable to deliver primary K. Similarly, sandstone soils having low clay contents had severely limited K buffering capabilities. The observed differences in the abilities of sandstone-, shale- and granite-derived soils to supply and buffer K may be sufficient to affect grape vine performance and wine quality in Western Cape vineyards.