Effect of fertilizer potential acidity and nitrogen form on the pH response in a peat-based substrate with three floricultural species

The potential of a water soluble fertilizer (WSF) to raise or lower substrate-pH is estimated in calcium carbonate equivalents (CCE) of acidity or basicity per unit mass of fertilizer. The CCE is currently estimated using Pierre's Method, PM, which is based on assumptions as to the effects of nitrogen and other ions in field soils that may not apply in container substrates. In a greenhouse experiment, the substrate-pH change was measured with 18 WSFs that varied in the concentration of NH4-N, NO3-N, urea-N and other nutrients. 'Ringo Deep Red' Pelargonium × hortorum (Bailey. L.H.), 'Super Elfin Bright Orange' Impatiens wallerana (Hook. F.), and 'Ultra Red' Petunia × hybrida seedling plugs were grown in 70%:30% (v:v) peat:perlite substrate amended with dolomitic hydrated limestone. Plants in 900 mL, 6-celled containers were top-irrigated with a total of 3.07 L over 4 weeks at 100 mg L−1 N without leaching. Plant species varied in their pH effect, in the order from acidic to basic of Pelargonium, Impatiens, and Petunia. Fertilizer CCE was positively correlated with substrate-pH, with r2 between 0.54 and 0.80 depending on the species. Multivariate regression also quantified NH4-N, NO3-N, and urea-N concentration effects on substrate-pH and CCE of applied fertilizer. Estimated mequiv. of acid (negative values) or base (positive values) per mmol of each nitrogen form applied were NH4-N −0.6678, −0.6143, −0.8123; NO3-N 0.0713, 0.2746, −0.1296; and urea-N −0.2038, −0.1445, −0.2711 for Impatiens, Petunia, and Pelargonium, respectively. Ammonium-N therefore had a strong acid effect, nitrate-N was a weak base or acid, and urea-N was a weak acid. Calculation of CCE based on PM or nitrogen alone provided a similar R2 with observed pH, despite a wide range in concentrations of macronutrients other than N in the fertilizer blends. Pierre's Method and nitrogen estimates of CCE for fertilizer blends were similar to each other (R2 = 0.97). However, PM estimates were biased compared with experimental results, over-predicting acidity of high-ammonium fertilizers, and over-predicting basicity of high-nitrate fertilizers. Results indicate that nitrogen form and concentration may provide a simple estimation of the acidity or basicity of blended fertilizers, although research under other growing conditions would be required. Accurate estimation of CCE is important to help growers formulate appropriate fertilizers to balance other factors such as water alkalinity and plant species.