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The pH of peatmoss generally ranges from 3.0 to 4.0 and limestone is typically added to raise pH to a suitable range. Compost is also used as a substrate component and typically has a high pH of 6.0 to 8.0. When using compost, lime rates must be reduced or eliminated. The two objectives of this study were to determine the resulting pH of substrates created with varying amounts of limestone and compost and assess the impact of the various amounts of limestone and compost on pH buffering capacity. Compost was created from a 1:1:1 weight ratio of a mixture of green plant material and restaurant food waste:horse manure:wood chips. The first experiment was a factorial design with five compost rates (0%, 10%, 20%, 30%, and 40% by volume), four limestone rates (0, 1.2, 2.4, and 3.6 g·L⁻¹ substrate) with five replications. The experiment was conducted three times, each with a different batch of compost. With 0 lime, initial substrate pH increased from 4.5 to 6.7 as compost rate increased. This trend occurred at all other lime rates, which had pH ranges of 5.2–6.9, 5.6–7.0, and 6.1–7.1 for rates of 1.2, 2.4, and 3.6 g·L⁻¹ substrate, respectively. Substrate pH increased significantly as either compost or lime rates increased. The second experiment was a factorial design with four compost rates by volume (0%, 10%, 20%, and 30%), the same four limestone rates as Expt. 1, and five replications. Each substrate treatment was titrated through incubations with six sulfuric acid rates (0, 0.1, 0.2, 0.4, or 0.7 mol of H⁺ per gram of dry substrate). Substrates with a similar initial pH had very similar buffering capacities regardless of the compost or limestone rate. These results indicate compost can be used to establish growing substrate pH similar to limestone, and this change will have little to no effect on pH buffering capacity.

The cause of sudden substrate pH decline by geranium (Pelargonium ×hortorum Bailey) is unknown. Published reports indicate that this response can be influenced in other plants by temperature and light extremes. The first of five experiments compared plants with all flowers removed to plants that were allowed to flower. Experiment 2 compared plants grown at four light levels (105, 210, 450 and 1020 μmol·m^–2·s^–1). Experiment 3 compared plants grown at four temperatures (14/10, 18/14, 22/18 and 26/22 °C day/night). Experiment 4 was a repeat of Experiment 1 and Experiment 5 was a factorial combining the three highest light levels and the three highest temperature levels. Plants allowed to form flowers had a final substrate pH of 5.7 compared to 6.3 for plants where flowers were removed. With increasing increments of temperature, substrate pH declined from 6.8 to 4.6 and with increasing light intensity from 6.1 to 4.8. There was no effect of flower removal in Experiment 4. Light and temperature had no consistent effects in Experiment 5 throughout 46 days after planting, with most pH values remaining in the acceptable range of 5.6–6.1. By 60 days, temperature treatments began to segregate, with pH being highest in the low-temperature treatments and lowest, down to 5.5, in the highest-temperature treatments. High temperature stimulated geranium acidification in both experiments, with the effect more severe in the first experiment. The flowering and high light effects were not duplicated in the second trial. This indicates that an additional factor is involved in expression of the light, temperature, and flowering control of acidification.

The cause of sudden substrate pH decline by geranium is unknown. Low Fe and low P have been shown to cause many plant species to acidify the substrate. Research was done to determine if low Fe or P stresses caused four geranium (Pelargonium ×hortorum Bailey) cultivars to acidify nutrient solution. Two cultivars were susceptible and two resistant to substrate acidification based on a grower survey. Rooted geranium cuttings were transferred to 4-L containers containing modified Hoagland's solution with N supplied as 15% NH₄ and 85% NO₃. The plants were grown in a greenhouse for 44 days. Treatments consisted of a complete nutrient solution and two similar solutions devoid of either Fe or P. Solutions pH was set at 5.8, changed weekly, and tested 3 and 6 days after each change. Because all cultivars showed similar responses, results were combined. Twenty days after transplanting (DAT), plants in all treatments, including control, caused solution pH to fall below 5. At 37 DAT, the solution pH levels for control, minus Fe, and minus P treatments were 4.1, 3.7, and 3.6, respectively. Results indicated that geranium is an acidifying plant when N is supplied as 15% NH₄ and 85% NO₃. Additionally, low Fe and low P stresses increase the acidification rate. Total dry weights of minus-P plants were about half that of minus-Fe plants. This indicated that plants under P stress had a higher specific rate of acidification than plants under Fe stress.