Delving into detail about soil pH

Soil pH can significantly impact vineyard productivity and grape quality. Hutchinsons agronomists Rob Saunders and Chris Cooper explain why, and show how to address any issues.

Although vines are generally quite resilient to varied growing conditions, soil pH is an important factor to monitor regularly, due to its influence on the plant availability of nutrients.

The Liebig principle applies here: The least available nutrient comes to be the growth limiting factor, even if all other nutrients are available in luxury amounts. It is best visualized as a wooden barrel with variable height staves, each representing a different nutrient. The lowest stave will dictate the maximum amount of water the barrel can hold.

In the cool UK climate, higher soil pH (above 7.3) makes vines more prone to iron deficiency, while boron, phosphate, manganese, magnesium and to a lesser extent, potassium availability can all be reduced beyond pH 8 to 8.5.

The nutrients are still present, but chemical interactions in more alkaline soils cause them to be unavailable for plant uptake. In extremes, this causes visible chlorosis, but more frequently just sub-optimal growth and lower grape quality. Where nutrients are sub-optimal, physiological disorders can occur. For example, millerandage risk increases where boron and zinc are limited just before flowering.

Low pH causes problems too. Below pH 6.0, problems with nitrogen, phosphorous, potassium, calcium and molybdenum are often encountered.

Generally, few growers establish vineyards on very acidic soils, however waterlogging and/or poor drainage can cause acidity to increase, and sandier soils are prone to acidity increasing over time due to their lower cation exchange capacity (CEC). It therefore pays to test soils regularly, and is a legal requirement under DEFRA’s farming rules for water for England.

Terramap analysis often reveals interesting and sometimes significant variations in soil pH, even within a relatively small area. In one six ha field recently mapped in Kent, for example, pH ranged from 6.2 to 8, which was thought to be impacting on crop performance.

Remember, pH is measured on a logarithmic scale, so the physical impact of a single point shift may be much greater than it sounds.

Buffer pH

When discussing pH, we must differentiate between the soil’s normal water pH and its buffer pH, essentially the natural ‘resting' pH. Surface interventions, such as applying ammonium fertilisers (especially ammonium sulphate), which lowers pH, or lime, which raises pH, will push any soil away from its buffer pH, often resulting in pH differences through the profile.

The extent of this variation depends on past land use and how much soil mixing occurred from tillage practices, but generally a larger pH gradient is likely where soil was previously undisturbed (e.g. after old permanent pasture).

Remedial actions

On sites destined for planting new vines, rootstock selection should be tailored to the soil’s active calcium level.

The SO4 rootstock is one of the most widely grown in the UK, and generally favours soil of pH 6.5-7.5, while on more alkaline soils above 7.5, Fercal may be better. In more acidic soils, Gravsac is a consideration.

For established vineyards requiring lime to nudge from acid towards alkali, agricultural liming equipment is too large, so the alternative is a granulated limestone product such as Calcifert. The material can be applied with a regular fertiliser spreader, and has been finely ground, giving a larger surface area in relation to its volume, making it available to soil and plants faster than agricultural lime products with large aggregates.

Lime is best applied in early spring, but avoid applying to frozen ground, or when there is risk of material being washed off before it can work its way into the soil. The same applies to any granular fertiliser.

The quantity of lime required is related to soil texture (relative proportions of clay, silt and sand). Generally, sandy soil requires less material to move the pH by a given amount than a clay soil, due to inherent differences in CEC. However, sandier soils are less able to hold onto lime and other nutrients, so are at greater risk of pH slippage.

On high pH chalky soils, most growers will apply an iron sulphate or iron metalosate type product in spring to address potential deficiencies, supported with foliar applications of key nutrients where required through the growing period.

In-season tissue testing, typically at early flowering, and possibly again at veraison, is worthwhile for tailoring nutritional programmes to soil and crop requirements.