The management of soil pH is important throughout the life of a blueberry planting. Blueberry plants are more vigorous, have more uniform color, and are more productive when growing in acid soil conditions – below pH 5.0. Since pH is frequently between 7 and 8, or even higher in many potential California blueberry growing areas, careful attention to soil pH management is important.
In those instances where soil pH is above 8.4, excess sodium may also be a problem, and these soils require additional special management. Poor drainage also limits soil acidification, and the installation of special drainage measures will be needed before soil acidification should be attempted.
To lower the soil pH, acid – usually sulfuric acid – may be applied directly to the soil. Sulfuric acid application is a rapid and effective way to lower the pH but requires specialized equipment and applicators specially trained to handle the potentially hazardous acid. Other acids, such as nitric, phosphoric, etc., can also be used, but they should be used sparingly and paying close attention to the potential for over-application of nitrogen and phosphorus that could result.
More often, elemental sulfur is applied and incorporated in the soil. When elemental sulfur is mixed with moist soil, soil bacteria convert the sulfur to sulfuric acid.
Sulfur is oxidized by bacteria to form sulfuric acid
S + O2 + H2O . . . . H2SO4
(elemental S + oxygen + water + thiobacillus + time . . . . . sulfuric acid)
Soil pH in fields can be quite variable initially following application of sulfur to lower pH. The desired lower pH may not be completely uniform in the bed for several months. It is difficult to get the sulfur uniformly distributed in newly sulfured fields, and the reaction of the sulfur granules can be slow, depending upon the type of sulfur applied and the field conditions. Acidified soils will require continuing ph management for optimum blueberry growth and production throughout the life of the planting.
Apply the sulfur to the soil as early as possible prior to planting and use only finely ground sulfur as opposed to traditional “popcorn” sulfur. The sulfur products that have shown good results in California are Tiger Sulfur 90 and Dispersul, both of which are finely ground sulfurs formed into pellets with bentonite clay for ease of application. The clay expands as it absorbs moisture and breaks apart to disperse the fine sulfur particles.
The amount of sulfur to apply can be determined in a soil laboratory by an analysis for the soil in question or can also be determined by using the current pH and the soil texture chart in Table 1. Normally 2 million pounds representing an acre slice to a depth of 6 inches is used as the basis for calculation of these rates. If beds are raised to 8-12 inches and additional soil volume is moved to establish the beds, this figure may actually be 50-100% higher than the values indicated. If the soil has high levels of bicarbonates and/or free limestone (calcareous) – also common in California – then the S-requirement may also be higher than that indicated by the chart. Try to correct the figures with an accurate estimate of the total soil volume moved. The sulfur can just be applied to the 4-foot-wide band centered over each row to decrease the costs of acidification. The sulfur should be incorporated uniformly with the soil and any other added ammendments.