pH got you down?

Does your substrate pH have you on a rollercoaster with the unwanted ups or downs? This article is for the greenhouse or nursery manager responsible for substrate mixing. Below is a simple procedure for more accurately predicting the necessary lime rate to reach a target pH for any substrate. The A+ method is a modification of a procedure developed by James Atland at Oregon State University, refined by Erin Lowe, biotechnician extraordinaire, and further modified by Drs. Owen and Altland at the USDA Agricultural Research Service Application Technology Research Unit Laboratory in Wooster, Ohio.

This procedure works with any combination of substrate components and amendments. It starts by accurately preparing 8 liters of your amended substrate. That 8-liter batch is moistened before being divided into 1-liter batches and amended with a range of lime rates. Finally, pH is measured allowing you to accurately predict any lime rate to reach a target pH. With no further ado, below is the step-by-step process to optimizing your pH using the “A+ method”:

Making and measuring unamended substrate using your components (2 options):

Option (1) Premade substrate: Loosely fill a 4-liter pitcher (Fig. 1) to the 4-liter (or 4000 mL) mark using unamended substrate comprised of your components (i.e. peat, bark, coir) in their correct proportions. Lightly tap or drop the container to set bulk density before filling again to the 4-liter mark. Repeat filling the 4-liter pitcher so that you have a substrate pile containing a total of 8 liters of substrate.

Option (2) Making your own substrate: Construct a pile of your substrate in the correct component proportions (i.e. peat, bark, coir). For example, a 30% (by vol.) peat amended bark substrate would be 3 scoops of loose, moist peat and 7 scoops of moist bark. Then mix the substrate until it visually appears and feels the same as your typical substrate before measuring the 8 liters of substrate as described above.

Amend your substrate with pre-plant fertilizer amendments (e.g. micronutrient packages, water soluble nutrients, etc.).

1. Dump your container on the potting table or concrete.
2. Spread out substrate into an approximately six by four-foot square.
3. Using any digital scale, weigh out 4.7 grams of your amendment for each pound you would add to a cubic yard (27 cubic feet). For example, if you normally add 1.5 pounds per cubic yard of granulated micronutrients, you will add 7.1 g of micronutrients to 8 liters of substrate (1.5 x 4.7 = 7.1 grams).
4. Apply the pre-measured amount of each amendment (e.g. mineral nutrients, aluminum sulfate), except lime or dolomite, by “salt and peppering” the surface of your six by four-foot square of substrate.
5. Incorporate your amendments by table mixing; this is accomplished by moving the pile three times from one location to another in small scoops, pouring atop the pile to let it homogenize.
6. Add irrigation water to substrate and continue mixing until moist, but no free water is visible. The substrate will feel relatively moist in your hand, but not make your hand wet when handling.

Adding multiple rates of lime or dolomite

1. From your 8-liter pile of amended substrate, accurately measure 1-liter sub-batches by dropping and refilling to 1 liter volume. Keep sub-batch separated.
2. Label one bag 0 pound per cubic yard with a Sharpie, fill with one liter of substrate or sub-batch, and set aside. This will reveal the baseline pH of this substrate blend without added lime.
3. Label an additional four, or up to seven of the remaining bags, with a Sharpie to indicate the lime rate you will apply and place with the corresponding pile.
4. Add the corresponding lime or dolomite for each individual rate to each substrate sub-batch using Table 1 (for example, add 1.2 grams of dolomite to 1 liter of substrate to have an effective rate of 2 pounds per cubic yard)
5. Mix in the substrate for 30 seconds to a minute or until thoroughly blended.
6. Further moisten substrate with irrigation water as needed to ensure you have consistently moist, not wet amended substrate mix.
7. Place each sub-batch of substrate into the appropriate 1-liter bag of amended substrate into individual, labeled 1-gallon Ziploc bags.
8. Let sit for 72 to 96 hours.

Measuring pH

1. Add 1 liter of your irrigation water to each bag.
2. Let sit for a minimum of 60 minutes and no more than 2 hours.
3. Calibrate your pH probe while waiting.
4. Stick your pH probe in slurry of one part substrate and one part water.
5. Record pH from each Ziploc bag, noting rate of lime or dolomite added.
6. Rinse pH probe before moving to next Ziploc bag and repeat steps 18 to 20.

Identifying your lime or dolomite rate to achieve your desired pH

1. Enter data into Microsoft Excel (or freeware Google Sheets) using two columns.
2. Plot data in Excel by first clicking on “Insert” at the top left, highlighting data, choosing charts icon with only dots. Then, right click and add trendline. Choose a linear or polynomial trendline from format trendline on the right side of your screen. Data will typically “best fit” with polynomial. If wanted, you can also add the equation from the trendline.
3. Find our optimal pH on the y-axis (left vertical axis) and follow parallel to the x-axis (horizontal bottom axis) to where it intersects your curve on the graph. Then move straight down to the x-axis to determine the optimal lime rate. An example is provided in spreadsheet image (Fig. 2) with intersecting dashed lines indicating an optimal pH of 5.8 using 3.9 pounds of lime per cubic yard. Algebraically solving the equation for X can also provide the same answer.

Utilizing the data

Drs. James “Jim” Owen Jr. and James E. Altland have worked together for two decades to understand and improve soilless substrates utilized in greenhouse and nursery production. Together, they run one of the most advanced soilless culture labs in the U.S., located at the USDA ARS Application Technology in Wooster, Ohio, where they optimize existing or create new methods to measure and monitor the physical, chemical, biological and hydrological properties of soilless substrates. USDA ARS Bio-technicians Erin Lowe, Hannah Blice, and Leslie Morris are integral to employing these methods and their ultimate success.