## 3 Results
### Grape Composition  
Total soluble solid content (TSS) in the study period was never significantly affected by the K fertilization treatments (Table 4). The 4-year values for the total content of titratable acids (TA) in the grape must range between 7.4 and 7.9 g/L, with K60 treatment showing values significantly higher than K0 and K15, but not different from K30 treatment. As shown in Figure 3, the decline of TA displayed an almost linear trend in all the vintages. In 2015 and 2016, significant differences were found only one week before harvest, with K30 and K60 displaying higher acidity values than K0 and K15. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Variable** & **Treatment** & **2013** & **2014** & **2015** & **2016** &
\begin{tabular}{c} **Average** \\ **2013–2016** \\ \end{tabular} \\ \hline \multirow{4}{*}{Pruning weight (kg/vine)} & K15 & 2.39 & 2.53 & 1.7 & 2.35 & 2.24 \\  & K30 & 2.50 & 2.20 & 2.21 & 2.20 & 2.28 \\  & K60 & 2.67 & 2.52 & 2.00 & 2.28 & 2.37 \\  & K0 & 2.37 & 2.52 & 1.94 & 2.06 & 2.22 \\  & F test sign & ns & ns & ns & ns & ns \\  & Treatment \(\times\) year & & & & & ns \\ \cline{2-6}  & K15 & 9.0 & 5.8 & 5.8 & 6.8 & 6.9 \\  & K30 & 8.1 & 6.5 & 5.2 & 7.4 & 6.8 \\  & K60 & 8.4 & 6.7 & 4.5 & 7.3 & 6.7 \\  & K0 & 8.5 & 6.3 & 5.4 & 8.0 & 7.1 \\  & F test sign & ns & ns & ns & ns & ns \\  & Treatment \(\times\) year & & & & & ns \\ \cline{2-6}  & K15 & 36 & 21 & 20 & 33 & 28 \\  & K30 & 29 & 24 & 16 & 32 & 25 \\  & K60 & 28 & 21 & 16 & 36 & 25 \\  & K0 & 32 & 21 & 19 & 39 & 28 \\  & F test sign & ns & ns & ns & ns & ns \\  & Treatment \(\times\) year & & & & & ns \\ \hline \multirow{4}{*}{Cluster weight (g)} & K15 & 247 & c & 282 & b & 285 & b & 212 & 257 \\  & K30 & 284 & ab & 278 & b & 329 & a & 229 & 280 \\  & K60 & 303 & a & 320 & a & 287 & b & 202 & 278 \\  & K0 & 263 & bc & 299 & ab & 287 & b & 201 & 263 \\  & F test sign & * & * & * & & ns & ns \\  & Treatment \(\times\) year & & & & & & ns \\ \hline \multirow{4}{*}{Berry weight (g)} & K15 & 1.76 & & & & & & \\  & K30 & 1.96 & & & & & \\ \cline{1-1}  & K60 & 1.88 & & & & & \\ \cline{1-1}  & K0 & 2.04 & & & & & \\ \cline{1-1}  & F test sign & ns & & & & & \\ \cline{1-1}  & Treatment \(\times\) year & ns & & & & & ns \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Yield components and pruning weight of Glera vines in response to different potassium fertilization. Means followed by different letters differ significantly by [PERSON]’s least significant difference test (* = \(p\leq 0.05\); ns: non-significant). The pH was not affected by potassium applications, with mean values almost equal for all treatments. On the 4-year average, K60 showed a significantly higher malic acid content than K15 and K0, whereas K30 was not significantly different from the treatments receiving lower or zero K rates. Comparing the values of tartaric acid, similarly to what observed for malic acid, in the 4-year average K60 showed significantly higher contents than K15 and K0, while K30 showed intermediate values, not significantly different from all the other treatments. Finally, analyzing the results for single years, the K content in the berries did not show a clear relation with the amount of K supplied with the fertilization. Considering the 4-year average, the only significant difference was recorded between the treatments receiving the highest K rate (K30 and K60) and the control K0, while K15 showed intermediate values.