Custom Compost for Acid-Loving Plants: Chemistry Matters.

We have been investigating compost suitability for an acid-loving plant, highbush blueberry (Vaccinium corymbosum L.) for several years.  We developed a quick test (3-d equilibration with incremental rates of dilute sulphuric acid) to determine compost capacity to buffer pH, and to estimate H⁺ addition required to reduce compost pH.  Compost buffering capacity (across 20 composts) ranged from 0.08 to 0.36 mol H+/kg compost/pH unit.  Compost acidification to pH 5 (ideal for blueberry) required an average of 10 g S^o/kg across all compost feedstock categories (manures and plant materials).  The most variable compost acidification requirements (within a feedstock) were found among dairy manure composts (8 to 28 g S/kg; n = 5) and horse manure composts (4 to 12 g S/kg; n = 4).

Compost acidification, as a technology for producing compost suitable for acid-loving plants like blueberry has practical limitations.  The first limitation is the reactivity of the elemental S product used for acidification.  We have had success using fine S dust to acidify compost, but have been unsuccessful in achieving rapid acidification with pelleted S products (S + bentonite; commonly sold as “split-pea” S).   The second limitation is that compost acidification increases soluble salts in compost, especially in highly-buffered manure composts.  Acid-loving plants do not thrive in high EC soil, and the high level of plant-available K in manure-based composts can cause nutritional imbalance in the plant (Mg deficiency induced by excess plant K uptake).  So, after acidification, compost use must be managed to avoid plant injury from excess salt and K (may be impractical for growers of high-value crops).  The  third limitation involves routine compost testing procedures. Composts are typically evaluated for total nutrient content by agronomists, and for saturation extract nutrient content by horticulturalists.  It is uncommon for compost analyses to be performed via soil testing methods (determination of exchangeable basic cations).  So, confusion exists in the compost industry about how to interpret analyses for cations.  When we have acidified compost with dilute H2SO4 from pH 8 to 5, we observe increased solubility of almost all nutrients.  Large simultaneous increases in compost Ca and P suggest that some of the soluble salts found in acidified compost may originate from dissolution of the mineral fraction of compost (not all cations are not held on compost CEC).  

In the future, we recommend that soil scientists work in teams with horticulturalists to develop improved compost testing protocols.  Standardizing methods for determination of basic cations and  H+ buffering capacity of compost will assist in choosing composts that meet edaphic needs of acid-loving plants.