Limestone Particle Size and Residual Lime Concentration Affect pH Buffering in Container Substrates

The objective was to quantify how the concentration and particle size of unreacted “residual” limestone affected pH buffering capacity for ten commercial and nine research container substrates that varied in residual calcium carbonate equivalents (CCE) from 0.3 to 4.9 g CCE·L. The nine research substrates contained 70% peat:30% perlite (by volume) with dolomitic hydrated lime at 2.1 g·L, followed by incorporation of one of four particle size fractions [850 to 2000μm (10 to 20 US mesh), 250 to 850μm (20 to 60 US mesh), 150 to 250μm (60 to 100 US mesh), or 75 to 150μm (100 to 200 US mesh)] of a dolomitic carbonate limestone at 0, 1.5 or 3.0 g·L. Substrate-pH buffering was quantified by measuring the pH change following either (a) mineral acid drenches without plants, or (b) a greenhouse experiment where an ammoniumbased (acidic) or nitrate-based (basic) fertilizer was applied to Impatiens wallerana Hook. F. Increasing residual CCE in commercial substrates was correlated with greater pH buffering following either the HCl drench or impatiens growth with an ammonium-based fertilizer. Research substrates with high applied lime rate (3.0 kg·m) had greater pH buffering than at 0 or 1.5 g·L. At 3 g·L, the intermediate limestone particle size fractions of 250 to 850 μm and 150 to 250 (20 to 60 or 60 to 100 US mesh) provided the greatest pH-buffering with impatiens. Particle fractions finer than 150 μm reacted quickly over time, whereas buffering by particles coarser than 850 μm was limited because of the excessively slow reaction rate during the experimental periods. Addition of acid from either an ammonium-based fertilizer or HCl reduced residual CCE over time. Dosage with 40 meq acid from HCl per liter of substrate or titration with HCl acid to substrate-pH of 4.5 were well-correlated with pH buffering in the greenhouse trials and may be useful laboratory protocols to compare pH buffering of substrates. With nitrate fertilizer application, residual CCE did not affect buffering against increasing pH. Residual limestone is an important substrate property that should be considered for pH management in greenhouse crop production under acidic conditions. Introduction Substrate-pH and pH buffering capacity are important chemical properties for successful nutritional management in crop production. Substrate-pH measures the active acidity or alkalinity in soil solutions, whereas pH buffering capacity refers to the ability of a substrate to maintain a stable pH following addition of acid or base. Peat moss sources differ in buffering capacity in the pH range of 5.4 to 6.2 (Rippy, 2005). These differences in buffering capacity, which occur over the pH range in which most horticultural crops are grown, could contribute to problems of rapid substrate-pH changes during the course of crop production. Major substrate properties that contribute to the buffering capacity of peat are cation exchange capacity (CEC) and base saturation [fractional calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na)](BS) (Rippy and Nelson, 2007). Limestone materials are incorporated into substrates to neutralize acidity and to buffer pH. When limestone is added to a substrate, it can be separated into two fractions, the reactive fraction and the residual fraction. The reactive fraction is the limestone that reacts initially after mixing to increase the substrate pH to an acceptable level for plant growth. The residual fraction is the limestone that remains undissolved in the substrate. The ratio of reactive:residual lime at the equilibrium pH is dependent on particle size, chemistry, crystalline structure, reactivity rate, and the acidity of substrate components such as peat.