Definition - What does Alkaline Soil mean?
Determined by the pH scale that spans from 0-14, Alkaline soil is categorized by having a pH above 7.0 (anything below 7 is considered to be acidic). Grapevines prefer a soil with a pH level between 5.0 – 7.0; if the soil is too alkaline, it will prevent the absorption of nutrients and may have nutrient deficiencies which will affect the growth of the grapevine. Soils should be tested a year before planting in order to issue methods to balance the pH of the soil and so that the soil has time to adjust its pH level accordingly.
WineFrog explains Alkaline Soil
Alkaline soils can directly affect plant growth, as most plants prefer soils that are slightly below neutral (7.0), including grapevines. The “sweeter” soil prevents the uptake of nutrients like phosphorus, potassium and nitrogen, in addition, alkaline soil can prevent certain nutrients like phosphorus, calcium and magnesium from being dissolved into water. Iron and zinc are also less available in soils that have a high alkalinity, and areas of drought are inclined to have soils that are alkaline. There are ways to adjust the higher pH in alkaline soils which include, the addition of organic materials like pine needles, sulfur, sawdust, peat moss and shredded leaves. Grapevines that are left in untreated alkaline soil will have poor root growth, reduction in pollination rate, lower yields and/or smaller berries.