Long-Term Responses of 'Thompson Seedless' Vines to Potassium Fertilizer Treatment

¹ University of California Agricultural Extension Service, Fresno County, 1720 South Maple Avenue, Fresno, California 93702.

A 10-year study was conducted in 3 Fresno County 'Thompson Seedless' vineyards on the effect of K fertilizer placement methods on vine K uptake, correction of deficiency symptoms and K movement in the soil.

In 2 trial vineyards on fine sandy loam soils, an in-furrow application of 1 Kg K/vine as K₂SO₄ provided significant vine response for the 10-year study period. In the more severely deficient vineyard, however, the response declined after 7 years. The response was briefer in a 3rd trial vineyard on a loamy sand soil: a 4-year period of good response declined to no measurable response after 7 years.

Surface placement of 1 Kg K/vine gave a more immediate vine K uptake response than in-furrow placement. However, by the 2nd or 3rd year both placement methods gave similar responses except in the loamy sand soil, where responses were briefer for surface placement. Vine response was not improved by the addition of 0.5 Kg P as CaH₄ (PO₄)₂, 2.3 Kg CaSO₄, or 2.3 Kg commercial organic amendment/vine to the K furrow application.