An African Marigold being grown at the proper pH. Photo: Tom Ford, Penn State
As a best management practice, we encourage growers to monitor the pH of the growing media in greenhouses at regular intervals so that the astute grower can make adjustments to their fertility program to mitigate the downward or upward movement of media pH.
For growers who don’t track the subtle fluctuations in media pH and who don’t tweak their fertility programs in response to these fluctuations, there may be a point where you need to take draconian measures and apply flowable lime or potassium bicarbonate in solution to the growing media to adjust the pH upwards. As a rule, growers looking to adjust media pH apply 4 qt./100 gal. flowable lime (split applications) or 2 lb./100 gal. potassium bicarbonate. Please note, that potassium bicarbonate can cause phytotoxicity in the roots and on the foliage of some plants so please wash it off the foliage after application.
While the use of flowable lime and potassium bicarbonate are the primary materials utilized by growers to raise media pH, there have been attempts made by some rural growers to utilize milk to raise media pH. Milk has a pH of approximately 6.7 when it is fresh and coming out of the cow or carton. When a grower sees this high pH, they view it as a base so they reason that they can apply it to greenhouse media to possibly increase media pH. While well-intentioned, many growers don’t realize that the bacteria that feed on the sugars (lactose) in milk convert the lactose into lactic acid which then acidifies the media making the low pH problem potentially worse. Under no circumstances should growers apply milk to increase media pH.