Research matters ... sudden acidification of growing media

Dr Ken Cockshull, Emeritus Fellow at Warwick HRI, highlights the latest findings from horticultural research.

Sudden drops in the pH of growing media sometimes occur for no apparent reason.

This acidification increases the availability of many micronutrients and it frequently causes plants to show symptoms of micronutrient toxicity.

In the experiments described here, cuttings of a zonal Geranium were rooted and planted in a peat:perlite mixture. The plants were then placed in growth rooms where the main treatments were day/night temperatures of 14 degsC/10 degsC, 18 degsC/14 degsC, 22 degsC/18 degsC and 26 degsC/22 degsC.

Although the pH of the growing medium initially rose because of a faulty component, it started to fall after 35 days in the warmer treatments and fell most steeply in the 26 degsC/22 degsC treatment.

The three warmer treatments were then combined with five concentrations of phosphorous in the growing medium. As before, the pH fell most steeply in the highest temperature treatment, but the fall was worst when the concentration of phosphorous in the medium treatment was low.

Finally, young Geranium plants were grown in water culture, which showed that the roots of plants grown at 26 degsC/22 degsC produced more acid than those grown at 22 degsC/18 degsC, especially if the nutrient solution was deficient in phosphorous. Evidently, the sudden acidification of the growing medium was due to acids produced by the roots.

Substrate Acidification by Geranium: Temperature Effects by Taylor, Nelson & Frantz (2008). Journal of the American Society for Horticultural Science 133 (4): 508-514.Members of ISHS can view the Journal of the American Society for Horticultural Science from the website www.ishs.org.


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