Leaf analysis as a decision tool for wine-grape fertilization

Tissue analysis is used in annual and woody crops as a tool to aid decision making about plant needs, usually in reference to a determined threshold. The two basic requirements for reliable analysis are that they 1) are reproducible and not dependent on climatic factors, and 2) reflect differences between plants with different mineral status (different levels of fertilization). A third requirement involves human error – the sensitivity of the result to the exact timing and tissue (leaf position) used. Leaf blades and petioles of ‘Cabernet Sauvignon’ vines were sampled for 7 years in the center and north of Israel. Each vineyard was divided into two parcels receiving high (90N:40P:100K ha^-1) or low (Ca. 45:0:0) levels of fertilizer each year and each parcel was divided to three sub-plots with marked vines serving as replicates. Sampling was done at full flowering, end of veraison and at a brix level of 21%. Success index (SI) for a tissue × timing × analytical method combination was calculated based on the percentage of times that the results reflected the difference in fertilization levels of the plots. Petioles at veraison best reflected mineral status: SI for NO3-N was 83% in both vineyards. The results for P and K differed between the two vineyards: the best SI for P (67% for water extraction of petioles at veraison) was found in the central vineyard while in the north, high natural soil P masked any added effects. In contrast, K was very low in the northern vineyard soil and demonstrated an SI of 100% at both veraison and 21% brix, while there was no reaction in the central vineyard, which had excess levels even after 7 years with no K fertilization. Our results and the smaller day to day changes around veraison compared with flowering suggest veraison petioles are preferred for leaf sampling.