How Soil pH Influences Grapes
When we talk about the influence of the environment on grapes, what we usually mean are things like air temperature, humidity, and sunlight — the things that influence the parts of the vine that are above ground. But the environment where the roots are located — the soil — can be just as crucial to successfully growing grapes. And one of the most important aspects of the soil environment is pH — the measure of how acidic or alkaline the soil is.
Nutrient problems in grapevines are often a result of soil pH, because the nutrients are bound up in the soil or are in a form that the roots are unable to take up. For example, in acidic soils (pH less than 5.5) high amounts of free aluminum and iron precipitate phosphorus (P) out of the soil solution, making P unavailable to the plant. High amounts of aluminum can also impact root growth by inhibiting cell division at the root tip. In alkaline soils (pH greater than 8.0), micronutrients like zinc, iron, and copper become less available to the vines, and it is not uncommon to see deficiency symptoms in these cases.
The best time to influence the pH of a vineyard is before a single vine goes into the ground. The first option is to find a vineyard site with a native soil pH in the desirable range to begin with, and which needs no adjustment at all. The second option is to adjust the soil pH to a desirable level before the vineyard is planted. In most cases in the Eastern U.S., this will mean needing to raise the pH by adding lime. Unlike some nutrients, limestone does not move through the soil profile very quickly, and therefore the best option to change the soil pH down near the root zone is to apply the amendments to the surface and then use a plow to get the lime down deeper into the profile. Relying on the lime to move through the top 12 to 14 inches of soil and into the root zone on its own would take many years.
Even if the vineyard has a proper soil pH at the time of planting, it will be necessary to occasionally conduct soil tests to determine if it has changed over time. Soils will gradually acidify over time due to the removal of cations like calcium, potassium, or magnesium either by leaching or uptake by plants, acid rain, or the reaction of certain nitrogen fertilizers in the soil (e.g., urea and ammonium nitrate). We generally recommend that soil samples be collected every four to five years in a particular location, unless the grower is in the process of changing pH or nutrient levels. Soil pH is a routine measure that should be included in any basic soil analysis. These soil analyses generally cost around $15 to $20 per sample.
If test results indicate that an adjustment to the pH is necessary, most labs will make a recommendation for how much lime to add to the soil to achieve a certain pH (if that desired pH level is not clear, ask the lab what it is). This recommendation is based on the difference between the actual and desired pH level and the buffering capacity of the soil. The targeted pH level will depend on what varieties are being grown. Labrusca-based varieties are better adapted to more acidic soils, while hybrids and vinifera tend to perform better in more neutral pH soils. Soils with higher clay content or higher levels of organic matter will require more lime to change the pH than sandy or low organic matter soils.
Growers may need to adjust the rate recommended from their samples, depending on the material that they choose to use. Any liming material that is purchased should state the “effective neutralizing value” (ENV) that is used to determine the actual amount of that material to apply. The calculation is straightforward:
Actual lime required x Recommended rate (tons per acre) / ENV (as an integer) x 100
Having the soil pH in a range that is appropriate for the varieties being grown will go a long way to making the environment below ground hospitable for the roots, and can help to minimize the need for supplemental fertilizers. The best time to ensure a proper pH is before planting, by choosing a good site and making any necessary adjustments when it is easiest to do so. But regular soil testing will help to ensure that the soil pH remains in the proper range over the life of the vineyard.
Targeting Soil pH
Soil pH levels will range depending on the type of grape variety. Even if the vineyard has a proper soil pH at the time of planting, it will be necessary to occasionally conduct soil tests to determine if it has changed over time.
Type of grape variety and target soil pH:
- Natives: 5.5-6.0 pH
- Hybrids: 6.0-6.5 pH
- Vinifera: 6.5-7.0 pH