Influence of Recent Acidification on Soil Test Phosphorus and P Sorption

H ighbush blueberry (Vaccinium corymbosum L.) is adapted to soils with high organic matter and pH of 4.5-5.5. Coniferous sawdust is the blueberry industry standard soil amendment and mulch in the Pacific Northwest, USA. However, sawdust prices are increasing, and its availability is becoming limited. Therefore, many blueberry producers are interested in compost as an alternative to sawdust, which might also reduce the need for nitrogen (N) fertilizers. Our objectives were to: (i) estimate elemental S (S o ) rate needed to acidify compost to target pH for blueberry, and (ii) evaluate diverse composts vs. sawdust as soil amendments in plant growth trials under low and adequate N fertilization regimes. The compost feedstocks were animal manures (horse or dairy manures; solids + bedding) or plant materials [yard debris (grass + woody prunings from urban landscape maintenance), deciduous tree leaves (from street sweeping), mint (leaves and stems recovered after steam distillation of peppermint oil) or bark (aged conifer bark composted with fine sawdust and municipal wastewater treatment biosolids)]. Finished compost pH was usually 7.5-8.5, except for the bark compost, which had a pH of 5.2. Composts were titrated with dilute sulphuric acid to determine their capacity to buffer pH (Fig. 1). Compost acidification to pH 5 required an average of 10 g S o /kg across all feedstock categories. Dairy and mint composts had the highest S requirement, and the most variable compost acidification requirements (within a feedContinued on page 2