- Highbush blueberry (Vaccinium corymbosum L.) is adapted to soils with high organic matter and acidic pH. Composts provide organic matter and nutrients for growing blueberry in mineral soils, but many composts are high in pH and soluble salts. Acidification with elemental sulfur (S°) can remedy high pH, but the process further increases compost salinity. The objectives of this study were to: i) determine the effects of diverse composts on blueberry growth and nutrient uptake, ii) determine whether S° acidification can ameliorate the negative effects of high compost pH, and iii) develop and validate a simple titration method to determine pH buffering capacity of compost.
In our first study, ten composts were evaluated under greenhouse conditions with one-year-old blueberry. Each compost was either acidified or not with S° and then mixed 30% by volume with silt loam soil before planting. Controls were sawdust incorporation or soil only. Plants were managed with a low rate of fertilization to emphasize compost effects on nutrient supply. Compost inorganic nitrogen (N) ranged from 0.1 to 4.1 g∙kg⁻¹, and NO₃-N to NH₄-N ratios were greater than 20:1. Aboveground plant growth and nutrient uptake, including N, phosphorous (P), potassium (K), and boron (B) were higher in soil with compost than in soil with sawdust or soil only. Composts with pH < 7.5 produced more shoot growth than those with higher pH, and composts with EC > 2 dS∙m⁻¹ produced less root growth than those with lower EC. Adding S° reduced compost pH by an average of 1.9 units, and acidification increased shoot and root growth in the composts as well as uptake of many nutrients. Plant growth and N uptake were not correlated with compost N supply. Composts with greater amounts of plant-based feedstocks produced more total shoot growth than manure-based composts.
In our second study, 3.7 L blueberry transplants were grown outdoors for 119 d in pots containing compost and soil. Manure feedstocks included separated dairy solids or horse stall cleanings. Plant feedstocks included urban yard trimmings, leaves from street sweeping, conifer bark conditioned with municipal biosolids, or peppermint distillation residue. Control treatments were sawdust amendment or soil only. Plant-based compost treatments had less effect on soil pH and produced 20% greater plant growth than manure-based compost treatments. Plant growth was not different with plant-based composts vs. the sawdust amendment control. Elemental S addition at potting did not acidify soil enough to overcome the increase in pH resulting from compost addition.
In our third study, we titrated eight composts with dilute H₂SO₄ to predict response to S° acidification. Our objective was to develop and validate a simple method to determine the pH buffering capacity of compost. Compost pH decreased linearly with laboratory acid addition. Compost pH buffering capacity (linear slope of titration curve) was 0.16 to 0.39 mol H⁺ per kg dry compost per pH unit. To determine correlation between titration and S° acidification, composts samples were incubated with S° at 22° C for 70 d. The majority of the decrease in pH from S° addition occurred within 28 d. Compost pH at 28 d was closely predicted by the laboratory titration. Because of the linearity of compost response to acid addition, a 2-point titration method (one rate of acid addition) is an effective alternative to the 7-point method we used.
We conclude that compost pH is the primary limiting factor affecting blueberry response to compost. Compost incorporation provided benefit to blueberry, especially under low N management, and generally increased plant growth and nutrient uptake when compared with plants grown in mineral soil only. Composts made from plant-based feedstocks, rather than manure-based feedstocks, tend to have characteristics which favor blueberry growth, including low buffering capacity, pH, and soluble salts. Feedstocks which showed promise include deciduous leaves, yard debris, bark, and horse bedding with low manure content. Acidification of composts with S° increased plant growth and nutrient uptake, and is a promising technology for eliminating the negative effects of high compost pH. Our method of laboratory titration with dilute acid addition was effective in determining pH buffering capacity of compost, and predicted the compost acidification by S° and the effect of compost on soil pH.