Understanding Plant Nutrition: Managing Media pH

Managing the pH of container media is a challenge in the greenhouse and nursery industry. Many growers face problems associated with their media pH either drifting up or down to levels that result in loss of crop quality and sales. In this article, we will discuss how the factors that we have discussed in previous articles (media, lime, water, fertilizer, etc.) interact to affect pH management.

Balancing Factors

Consider pH management as a balance (Figure 1). One side of the balance has the basic reactions commonly found in container media (i.e. the reactions that make the media pH increase). The four main basic reactions are: nitrate (NO3-N) fertilizers, irrigation water alkalinity, reactive lime and residual lime.
On the other side of the balance are the acidic reactions (i.e. the reactions that make the media pH decrease). The three main acidic reactions are ammoniacal (NH4-N) fertilizers, media lime requirement and plant species.

If the media pH is to remain stable over time, then the strength of all the basic reactions has to equal the strength of all the acidic reactions.

Why Media pH Changes

Media pH changes when the strength of the reactions on one side of the balance becomes greater than those on the other side. For example, consider growers who had a nutrition program that maintained a stable pH (all factors are balanced) that included a water source with 300 ppm alkalinity and a fertilizer with 59 percent ammoniacal nitrogen (20-20-20). If the growers switched to a fertilizer that was high in nitrate nitrogen like 13-2-13 (6 percent NH4-N), not only would they lose the acidic effect of the ammoniacal nitrogen, but the basic reactions caused by the higher concentration of nitrate nitrogen in the fertilizer would also increase.

Going back to the balance analogy, switching to a high nitrate fertilizer caused a decrease in strength to the acidic reactions and an increase in strength of the basic reactions. The overall effect would be that the medium pH would go up.

In contrast, consider growers who had a nutritional program that maintained a stable pH that included a water source with 20 ppm alkalinity and a fertilizer with 6 percent ammoniacal nitrogen (13-2-13). If they switched to a fertilizer that was high in ammoniacal nitrogen like 20-20-20 (59 percent NH4-N), they would not only lose the basic affect of the nitrate nitrogen, but the acidic affect from the higher ammoniacal nitrogen fertilizer would increase.

The net result would be that the acidic reactions are greater than the basic reactions, and the media pH would decrease over time.

Many times, growers will change the strength of the reactions on the pH balance without knowing. For example, residual lime can play a key role in counteracting the acidic affect of ammoniacal nitrogen. The amount of residual lime contained in a media will depend on the chemical composition, particle size distribution and incorporation rate of the limestone, as well as the lime requirement of the media. So anything that changes the lime rate, like changing peat sources, changing the ratio of components in the media or even year-to-year variability in the peat from the same company, will influence the amount of residual lime contained in the media after the pH has stabilized. So changes to residual lime rates will influence the amount of ammoniacal nitrogen fertilizer that can be applied to a crop before pH problems occur.

Always remember that changing any of the factors on the pH balance can change how all the factors interact to affect media pH.

Having A Plan To Manage pH

Fertilizer action plans are a good way of having a systematic approach to pH management. An added benefit is that everyone in the greenhouse will use the same methods for managing pH.

In general, fertilizer action plans adjust only two factors, fertilizer nitrogen form and water alkalinity. Once the crop is planted, water and fertilizer are the two factors that growers can most easily manage.

A simple fertilizer action plan is given in Table 1 for a 4-inch geranium crop. The alkalinity of the water was reduced through acid injection from 250 ppm to 80 ppm. The primary fertilizer was 17-5-17 at 200 ppm N, with a more acidic 20-10-20 being used when needed.

As long as the media pH and EC are within the acceptable range for the crop (6.0 to 6.6), as measured with weekly soil testing, then the primary fertilizer solution (17-5-17 with acidified water) is used (Table 1).

If the media pH is at or below the acceptable level, then the acid is turned off yet the 17-5-17 is still used. The resulting fertilizer solution is more basic, causing the media pH to increase back up to the acceptable level. If the media pH gets too high, then the acid remains but the fertilizer is changed to 20-10-20. The resulting fertilizer solution will be more acidic, causing the media pH to decrease. In either case, once the media pH is back within the acceptable range, then use of the standard fertilizer solution (17-5-17 with acidified water) is resumed.

Similarly, if the EC in media is above or below the acceptable level, then the fertilizer concentration is changed to affect media EC, but not necessarily media pH. However, once the media EC is back within the acceptable range, the standard concentration (200 ppm N) is used.

The action plan can be different for different crops. For example, a petunia crop will have a lower acceptable pH range than a geranium crop, but the same basic framework can be used.

Some crops are particularly sensitive to the media pH being outside the acceptable range. For example, low pH-induced iron/manganese toxicity is a common problem with geraniums. Typically, the risk of geraniums having iron/manganese toxicity increases dramatically when the media pH is less than 5.8.

A column could be added to the action plan to apply some basic chemical (either potassium bicarbonate or flowable limestone) to the crop if a media pH of 5.8 were reached. That way, everyone knows when to apply the basic chemical, and it is applied to the crop before significant problems typically occur.


The goal of any nutritional program is to keep the media pH and nutrient concentrations within acceptable ranges. Decisions about the type and concentration of fertilizer and the water alkalinity used to grow the crop should be based on regular testing (every one to two weeks). Simply measuring medium pH, medium EC and the EC and alkalinity concentration of the fertilizer solution, and acting on that information can solve most pH or nutritional problems by alerting growers to problem trends before plants are stressed. Don’t forget to monitor other factors (e.g. root diseases, greenhouse temperatures, pest problems, high or low medium EC) to help rule out these problems, because many factors other than medium pH can cause problems in the crop.

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One comment on “Understanding Plant Nutrition: Managing Media pH

  1. I have high alkaline water in my area (280-320 ppm)with no quick way to get better water so i must manage the balancing act as explained above. Thank you for helping me better understand.

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