Spatial Variability of Soil K Fixation within Vineyards of California's Lodi Winegrape District.

Potassium (K) is a key nutrient in winegrape (V. vinifera) production, with both K deficiencies and excessive K in fruit being undesirable. The main factors controlling soil K supply and retention are cation exchange capacity (CEC), exchangeable K levels (XK), and K fixation (Kfix) potential. Data are presented from three vineyards (V1, V2, and V3) in the northern San Joaquin Valley of California where some soils fix K due to granitic parent material and presence of vermiculite. Soils are mapped as (V1) fine-loamy, Typic Palexeralf/Haploxeralf (old dissected terrace); (V2) coarse-loamy Typic Haploxeroll (low fan terrace) and (V3) fine- and coarse-loamy Aquic Xerofluvent (flood plain). Kfix was measured by a procedure using a 1-hr incubation in 2 mmol KCl (1:10 soil solution). XK was measured in 1M neutral ammonium acetate extracts. Profiles were sampled from pits and auger holes to maximum depths of 0.9 to 1.8 meters at 17, 5, and 9 locations in V1, V2, and V3. Generally, Kfix increased with depth to a maximum of 1.5-1.8 cmol/kg soil and was low or zero in the surface 10-20 cm. Spatial variability of CEC, Kfix, and XK was characterized using depth-weighted averages with the surface sample excluded. In V1 and V3, most profiles showed significant Kfix below a depth of 30-60 cm. In V2, two of five profiles showed no Kfix above 90 cm but displayed Kfix in deeper samples. Coefficients of variation for Kfix were 73, 35, and 16% and for XK were 14, 39, and 11%, respectively, for V1, V2, and V3. Kfix was weakly related to XK, but was more strongly related to XK expressed as a fraction of CEC. Mapping of soil Kfix within vineyards appears feasible and may be useful at the time of vineyard establishment for selection of the appropriate grape rootstock, assessing the need for high basal application rates of K fertilizer, and design of the drip irrigation system to allow for zone-specific K fertigation.