Use of Boundary Lines to Determine Effects of Some Salinity-associated Soil Variables on Grapevines in the Breede River Valley

Abstract

The boundary line concept was used to assess grapevine responses to salinity-associated soil variables. Soil chemical status and grapevine responses were measured in 13 vineyards in the Breede River Valley during the 2001/2002 season. Chardonnay grafted onto 110R and 101-14 Mgt, as well as Ruby Cabernet on the same two rootstocks, was included. The selected vineyards were representative of the variation in salinity-associated soil variables, as well as of leaf and juice element contents previously reported for South African vineyards. Under the prevailing conditions, the four scion-rootstock combinations responded similarly to the salinity-associated variables. The results confirm that soil pH(KCl) should be at least 6.0 for grapevines. The salinity threshold for vineyards in the Breede River Valley should be between 0.7 dS/m and 1.5 dS/m to avoid growth and yield reductions. To reduce the risk of Na toxicity, the SAR should be below 3, and the soluble Na content in the soil should not exceed 5 mg/kg. If gypsum is used to reduce soil Na, it should be applied judiciously to avoid soil SO4 accumulation, thereby reducing the risk of K and Mg deficiencies. Under the prevailing conditions, B and Cl toxicity apparently contributed to reduced vegetative growth. Therefore, soil Cl and B should be kept as low as possible, but care should be taken that B is not reduced to deficient levels. The boundary line concept proved to be useful for determining the effect of a single salinity-associated soil variable on grapevine response.