Determination of limestone requirement of soilless root media for a specific pH

Pauline Helen Kaufmann Andrews, Purdue University


A rapid, accurate laboratory procedure to determine the limestone requirement of soilless greenhouse root media for a specific pH was developed. Although limestone requirement tests are routine in agronomic soil laboratories, no such test exists for soilless root medium. The SMP buffer procedure for mineral soils was modified through laboratory and greenhouse studies to accurately predict limestone requirements for soilless media. 40 mL of SMP buffer was added to 6 g ground media, equilibrated for 48 hours, and pH measured. Media-buffer was plotted against the incubation limestone requirement for pH 5.5. Thirty-four media formulations were amended with increasing rates of limestone and incubated in the greenhouse to develop calibration tables. Pots were irrigated with RO-water, pH recorded at week 5, and limestone requirement for pH 5.5 determined. Separate regression equations and calibration tables were developed for non-coir media and coir media. The correlation was -0.97 for non-coir media (limestone requirement = -1.19 * media-buffer pH + 7.52) and -0.94 for coir media (limestone requirement = -1.12 * media-buffer pH + 6.14). The procedure was verified using 15 media and 3 greenhouse potting treatments of incubated media, geranium, and petunia. Media-buffer pH was determined using the modified SMP buffer procedure and limestone requirement calculated using the non-coir or coir media regression equations. The incubated media treatment was irrigated with RO-water. Rooted cuttings of geranium 'Candy Lavender' and petunia 'Easy Wave Blue' were transplanted into pots containing the media treatments. Geraniums were irrigated with non-acidified fertilizer water and petunias with acidified fertilizer water. Media pH was determined at days 18, 35, 53, and 70. Media pH among media formulations was not significantly different (P ≤ 0.0001) for non-coir or coir media. Average media pH over time for incubated media, geranium, and petunia treatments was pH 5.72, 5.67, and 5.93 for non-coir media and pH 5.32, 5.42, and 5.81 for coir media, respectively. These results indicate the modified SMP buffer procedure can be used to accurately determine the limestone requirement of soilless root media. However, results show medium pH can be significantly affected by plant species and water quality.




Hammer, Purdue University.

Subject Area

Plant propagation

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