Potassium Fertilization of Thompson Seedless Grapevines Using Fertilizer Sources Under Drip Irrigation

¹ Department of Viticulture and Enology, University of California, Davis, and Kearney Agricultural Center, 9240 S. Riverbend Avenue, Parlier, CA 93648
² University of California Cooperative Extension, Agricultural Building, County Civic Center, Visalia, CA 93291.

Drip irrigation is suitable for injection of soluble fertilizer due to ease in uniformity of application and efficiency of placement. A potassium (K) fertilization trial was initiated in 1984 to determine the influence of rate, frequency of application, and K source on correcting K deficiency in a drip irrigated vineyard. Own-rooted Thompson Seedless vines on a K deficient sandy loam soil were used for study. The eight treatments compared two rates (0.1 and 0.2 kg K/vine) of two K sources (K₂SO₄ and KCl) under two application methods--single treatment (1x) and weekly partitioned treatment (8x) over 8 weeks. Single treatment (1x) of higher rates of K₂SO₄ at 0.4 and 0.8 kg K/vine as well as 0 fertilizer were also compared. Vine response as measured by bloom petiole K levels was most immediate with the lx applications but by veraison in the year of treatment the partitioned 8x applications resulted in higher petiole K. Thereafter, in 1985 and 1986, application method (lx vs. 8x) did not show differences in influencing petiole K. Fertilizer rate treatments increased petiole K and reduced visual K deficiency symptoms linearly. All fertilizer treatments showed response in petiole K increase and visual symptom reduction for two seasons, 1984 and 1985. However, by 1986 only the lx treatments continued to show higher petiole K than 0 fertilizer. Petiole Cl was increased with Cl as a K source but without apparent Cl toxicity problems. No other differences due to K source (SO₄ vs. Cl) were shown except for higher petiole K from SO₄ in one year of measurement, 1985. Soil samples taken in December of 1984 at incremental distances from the emitter showed greatest concentration of K directly below the emitter for the low lx and all 8x treatments. Highest 1x treatment rates increased K levels at the 0.5 m distance and depth. The study demonstrates the feasibility of K fertilizer application to vineyard under drip irrigation as lx or partitioned treatment at varying rates of K₂SO₄ and KCl. Higher rates of K₂SO₄ at 0.4 kg K and above can provide correction beyond two years in the soil type studied here. Rates of KCl above 0.2 kg K/vine were not used in the study to avoid possible Cl toxicity problems.

Key words: potassium, irrigation, drip, Thompson Seedless

Submitted on June 28, 1989