Commonly used limestones for adjusting pH in greenhouse mixes

Part 2 of a series on adjusting substrate pH will discuss commonly used lime products growers are using to adjust (raise) substrate pH.

Michigan State University Extension developed a short survey to determine the types of limestone that are commonly being used in Michigan greenhouses to adjust substrate pH when needed. Adjusting pH of the substrate prior to transplant is much easier than adjusting the pH after transplant. Prior to planting, growers can amend their substrate with either superfine or pulverized limestone.

Adjusting substrate pH after transplanting is substantially more challenging. Some growers are decreasing the amount of acid injected into their water to use the naturally high alkalinity in their water to increase the substrate pH. Another method of increasing the substrate pH after transplanting is by switching to a more basic fertilizer that has a higher percentage of nitrate (NO3-) nitrogen and less ammonium (NH4+) nitrogen. Growers of diverse crops need to be careful when using either of these methods since you are changing the irrigation water’s properties. Raising the pH of the substrate of petunias, for example, will lead to iron (Fe) deficiencies when the pH rises above 6.4.

Growers who need to rapidly increase the substrate pH with lime can do so by applying flowable lime directly to the pots or via an injector. There are numerous lime products currently being used to adjust substrate pH after transplanting. According to the online survey, greenhouse growers are using the following products to increase the substrate pH after planting: CalOx, Cal-Flo and Limestone F.

Products commonly used to adjust substrate pH after transplanting

CalOx (74 percent calcitic limestone; 26 percent inert ingredients)

  • Suspended liquid calcitic lime.
  • Will be more reactive than using a dolomitic limestone product.
  • Product label says to use immediately after opening and for best results, use within one month of purchase.
  • According to information provided by BioSafe Systems LLC, a drench with 250 gallons per 1,000 square feet caused the following increases in pH units 24 hours after application:

Starting pH

Ending pH

pH units increased

Injector ratio (Concentration)

4.05

4.44

0.4

1:100

4.05

5.28

1.23

1:50

6.15

6.65

0.5

1:1000

6.15

7.02

0.87

1:500

6.15

7.97

1.82

1:100

Growers should note that the injector ratio or rate of lime being applied varies significantly with the starting pH of the substrate. For example, starting with a pH of 4.05, an injector ratio of 1:100 was needed to get a 0.4 unit increase in pH. In contrast, when the pH started at 6.15 an injector ratio of 1:1000 was needed to get a similar increase in pH units. Why? Remember that pH is a logarithmic scale and it is 10 times more difficult to raise the pH from 4.0 to 5.0 than it is to raise it from 5.0 to 6.0.

Cal-Flo  (62 percent calcium carbonate)

  • Suspended liquid calcitic limestone.
  • Particle size: superfine suspended (100 percent passing through a 200 mesh screen).
  • Will be more reactive than using a dolomitic limestone product.

LimestoneF (50 percent dolomitic limestone; 50 percent inert ingredients)

  • Liquid flowable limestone.
  • Particle size: superfine (89 percent passing through a 200 mesh screen).
  • Less reactive than calcitic lime products, but the superfine particle size will make it more reactive than pulverized limestone products.

The recommended rates of each of these products vary. Developing strict recommendations for growers is challenging since the pH of the incoming substrate varied between batches so widely. Growers will need to do small scale, in-house trials with their substrate in order to develop corrective procedures for their situation.

For more information

Thank you to Drs. Roberto Lopez and Brian Whipker for their reviews.

Related Events

Related Articles

Related Resources