Organic Amendments and Sulfur in Combination Reduce Soil pH

Crop plants such as blueberry and landscape species such as rhododendron prefer relatively low soil pH; they often fail to thrive in alkaline soils found in California.  Sulfur, added to reduce soil pH, is slow to mineralize and dependent on soil microbial processes.  A single application of sulfur (5600 kg/ha) resulted in a 5-fold increase in extractable sulfate after 2 weeks; sulfate content roughly doubled at each subsequent sampling during the first year and was 760 ppm after 2 years.  Tall fescue (Festuca arundinaceae) was seeded 4 months after sulfur was applied.  Sulfate content in plots with fescue was 61% lower than in plots without fescue after 1 year and 85% lower after 2 years, indicating significant uptake by the grass cover. Both sulfur and fescue increased organic matter content by 0.1% during 2.5 years. pH was reduced from 7.8 to 7.0 and 6.3 after 1 and 2 years, respectively, in plots with sulfur.  pH was reduced to 6.7 by application of commercial coffee grounds (25% by volume in 1 m² plots) after 1 month; after 2.5 years, pH was 7.2, the lowest of any organic amendment.  Application of culled lemon fruit ground in a wood chipper caused an immediate soil pH reduction to below pH 5; pH ranging from 6.6 to 7.5 persisted for 2 months after application.  Peat moss and Pinus canariensis (canary island pine) needles reduced pH to 6.7 and 7.4, respectively, for about 2 weeks, while Quercus agrifolia (coast live oak) leaves and municipal yardwaste had no effect.  The combination of sulfur with each amendment, however, resulted in a synergistic pH reduction measureable after 1 year.  pH for coffee, lemon, oak, pine and yardwaste treatments ranged from 5.2 to 5.5 with sulfur compared to 7.5 to 7.8 without sulfur.  Peat moss had the largest range in soil pH after 2 years: pH 5.1 with sulfur compared to 7.6 without.  Organic matter content in coffee plots ranged from 4.5% at the start to 2.4% after 2 years, the highest of any amendment.  Organic matter content after application of peat and culled lemons averaged 2.9% and 2.5%, respectively, compared to 1.5% for unamended soil. Although increased organic matter content and reduction in soil pH was measured using sulfur and some organic amendments alone, coffee, peat, and culled lemons in combination with sulfur achieved the greatest and most sustained pH reduction.