Low substrate pH can affect plant growth without inducing leaf symptomology of petunia and poinsettia

Diagnosing plant nutritional disorders requires knowing the critical pH ranges and visual symptoms. Two experiments were conducted incorporating increasing rates of dolomitic lime to an 80% peat + 20% perlite substrate (v:v) to establish substrate pH ranges for assessing plant growth. In Experiment 1, petunia ‘Single Picobella Rose Star’ (RS; Petunia milliflora) and ‘Prostrate Easy Wave Neon Rose’ (NR; Petunia multiflora) were grown in substrates with 0, 2.97, or 5.93 kg m³ of incorporated dolomitic lime. At termination, the corresponding substrate pHs were 3.3, 4.7 and 5.1 for RS and 3.4, 4.6, and 4.9 for NR, respectively, as lime rate increased. No visual symptomology was observed, but plant growth was significantly lower (59 and 42% less, respectively, for RS and NR) when comparing the lowest to the highest lime rates. In Experiment 2, poinsettia (Euphorbia pulcherrima) ‘Premium Red’ (PR) and ‘Viking Red’ (VR) plants were grown in substrates with 0, 4.45, or 8.90 kg m³ of incorporated dolomitic lime. The corresponding substrate pHs were 3.3, 4.3 and 5.4 at termination. Plant growth was 30.4% less for PR grown at pH 3.3 than 5.4. There were no visible symptoms of iron (Fe)/manganese (Mn) toxicity at pH 3.3, and the corresponding leaf tissue values were all within the acceptable range. Therefore, smaller plant size appears to be the only observation of low pH for these two species. Growers must rely upon monitoring the substrate pH to diagnose low pH disorders, because leaf symptomology and leaf tissue analysis are not adequate indicators for accessing problems.