Acidic soil pH values, in the range of 4.5 to 5.5, are considered fundamental for blueberry production, but this seems to be insufficient to describe optimum soil environment on its own.
The reason underlying the need for acidic soil pH values is sometimes forgotten: In acidic soils, insoluble, oxidized ferric iron is often converted to soluble, reduced ferrous iron, which plants can absorb more easily. And blueberries require the high levels of iron that in most soil types are more available at low pH values.
In addition to high iron requirements, blueberries respond positively to higher concentrations of manganese in its plant sap than most other crops. Elevated manganese concentration is expressed in stronger reproduction, larger bud sizes, increased number of reproductive buds, and more uniform blossoming. This effect can result in a condensed harvest window, particularly important for machine-harvested blueberries.
More Than Just Acidic Soils
In blueberry production, you want to have the largest number of the biggest, firmest fruit possible on each plant. Inadequate absorption by blueberry plants of calcium, iron, and manganese are limiting factors in reaching that objective. The highest quality fruit is often not produced on the most acidic soils; there are additional factors which need to be considered.
In some soils with high iron concentrations and the most acidic soil pH values, iron and manganese absorption was the poorest of all. On a number of occasions, blocks with comparably high pH values substantially outperformed nearby blocks of the identical soil type/variety/management combination with more acidic pH values.
These blocks outperformed comparison blocks in fruit size and firmness, flavor, sugar content, overall yield, and in measured plant absorption of iron and manganese from the soil. A number of top performing blocks had soil pH ranges from 5.7 to 6.2 — still acidic, but not as acidic as might have been anticipated.
Another factor affects iron and manganese absorption, and possibly to a greater degree even than soil pH. The reduction of iron and manganese is not only pH dependent. Of equal importance is the eH value, sometimes referred to as oxidation/reduction or redox potential. Soil amendments and fertilizers affect eH in much the same manner they can affect pH; however, one of the more substantial contributing factors to soil redox potential is soil water/oxygen ratios.
Wet soils usually have low oxygen levels and are a reducing environment, which translates to converting iron and manganese to the reduced plant-available form, and increased plant absorption. Dry soils with high oxygen content are usually very oxidized environments, and maintain iron and manganese in the oxidized, unavailable form even in the presence of acidic soil pH values.
When soils are excessively saturated for an extended period, root respiration generates various alcohols in the rhizosphere, and these are damaging to the root system.
Soil amendments and fertilizers can also be categorized based on whether they have an oxidizing or reducing effect. For example, nitrate fertilizers have a very strong oxidizing effect, whereas ammonium fertilizers have a reducing effect. Limestone has a very strong oxidizing effect; gypsum does not.
Look at Plant Structure
When we consider blueberries’ native habitat and their nutritional requirements, it becomes clear they are specifically adapted for a reduced soil environment. Blueberries have a relatively shallow root system, which means they are adapted to adequately hydrated soil with a shallow water table.
They are dependent on ammonium (the reduced form of N) as their primary form of nitrogen, and cannot absorb nitrate (the oxidized form of N) well. They also have a high requirement for iron and manganese, which are most available in reduced environments. These characteristics point to blueberries being adapted to soil that is in a reduced state as well as being acidic.
Fruit firmness and fruit size can be directly correlated with calcium levels in the plant sap which, in many cases, is very low in acidic soils — too low to produce the premium crop we are working for. Top performing crops are those where soils are maintained in the reduced state with consistent watering, and soil pH is maintained in a range from 5.2 to 5.8 with regular calcium applications.
Obviously, a solid nutritional system that addresses all of these elements will give the best results. When this foundation is in place, plant absorption of iron, manganese, and calcium correlates directly to fruit quality and yield.