Wine Arcana

Concept

Soil pH

Also known as Soil acidity

Soil pH measures acidity or alkalinity in the soil's aqueous environment. It influences the chemical forms of nutrients, their availability to roots, and the activity of soil organisms. A vineyard can contain abundant iron yet supply too little usable iron to its vines because the soil chemistry limits uptake.

Values below pH 7 are acidic and those above it are alkaline. The scale is logarithmic: a decrease of one unit corresponds to a tenfold increase in hydrogen-ion activity. Laboratory soil tests measure a prepared suspension, commonly using water or a salt solution such as calcium chloride. Values from different methods should be compared with their methods stated.

What makes a soil acidic or alkaline?

Parent material supplies an initial chemical setting. Carbonates in limestone and chalk consume acidity as they dissolve and can maintain alkaline conditions. Weathering and leaching remove base cations such as calcium and magnesium. Some nitrogen fertilizers, nutrient removal in harvested crops, and other farming processes can contribute to acidification over time.

The profile can vary with depth and position. A surface sample may miss an acidic subsoil or carbonate-rich layer. Irrigation and fertilizer placement can also create differences between vine rows and the ground between them.

Buffering capacity describes resistance to a change in pH. Two soils with the same measured pH can require different amounts of amendment to change it. Clay, organic matter, and carbonate reserves contribute to that resistance. A single pH number therefore cannot determine a liming rate.

Effects on vines

Strongly acidic conditions can increase soluble aluminium to levels that restrict root growth. Nutrient availability also changes. In alkaline, carbonate-bearing soils, iron can become insufficiently available even when the soil contains plenty in total. The resulting iron chlorosis reduces the leaf's green colour and can impair growth.1. Pacific Northwest Plant Disease Management Handbook, “Grape—Iron Chlorosis,” revised March 2026, cause and diagnostic limitations.

Rootstocks differ in adaptation to these conditions, alongside their responses to drought and root pests. Grafting makes it possible to combine a fruiting variety with a root system suited to the site. Nutrient assessment then needs evidence from the vines as well as a soil test. Leaf symptoms can have several causes, and an apparently normal total leaf-iron concentration does not always exclude iron chlorosis.

Growers use agricultural lime to raise the pH of acid soil. Calcitic lime supplies calcium carbonate; dolomitic lime also supplies magnesium. Changing an established vineyard's deeper layers is harder than treating the surface. Choice and quantity depend on laboratory assessment, soil depth, and the condition being corrected.

Soil pH and wine acidity

Wine pH describes juice or wine, where organic acids, potassium, and other constituents establish a different chemical balance. Vines synthesize grape acids and regulate ion uptake. Soil acidity does not pass unchanged into the fruit.

Nutrient uptake supplies an indirect connection. Potassium in grapes can raise must pH, but its uptake depends on rootstock, water, nutrient availability, and vine physiology. Climate, ripeness, and cellar treatments also affect the wine. Alkaline vineyard soil can consequently support grapes with substantial acidity, as the carbonate-rich ground of Chablis illustrates. A soil pH result cannot predict a bottle's pH or sourness.

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