Authored by Tejendra Chapagain, PhD, PAg (OMAFA)
Soilless media has largely replaced soil-based mixes over the past few decades in nursery plant production. These media mixes are available to purchase from suppliers as prepared, specialized mixes or some may purchase separate components that can be made into custom blends used for propagation, container growing, etc.
The pH of soilless growing media must be monitored on a regular basis for nursery container crops because it directly controls nutrient availability to the plants. If the pH drifts outside the ideal range for crops, plants cannot absorb essential elements even if they are abundant in the root zone. This leads to nutrient deficiencies, and if left unmanaged, plant death can occur – especially if paired with other plant stressors.
Whether you’re learning about soil pH for the first time or need a refresher, read on to explore why regular pH monitoring is important and what practices OMAFA’s Soil Fertility Specialist recommends.
Key Reasons to Monitor pH
Monitoring pH of a soilless growing media is critical for several reasons as listed below:
- Prevents nutrient lockout or toxicity: A pH of 5.5 to 6.5 is considered optimal for most nursery and greenhouse crops grown in soilless mixes with more than 50% organic matter. If pH rises above 6.5, critical micronutrients like iron, manganese, and zinc become insoluble and unavailable to roots. If the pH drops too low (typically below 5.5), micronutrients can become highly soluble and reach toxic levels for susceptible crops. In other plants such as azalea and hydrangeas, a low pH is required to ensure maximum uptake of iron and aluminium essential for proper leaf and bloom colour.
Table 1: Nutrient availability by pH range.
| Media pH Range | Nutrient Status | Risk to plants |
| Below 5.5 (Too acidic) | High micronutrient solubility | Iron and manganese toxicity; calcium deficiency |
| 5.5-6.5 (Ideal range) | Balance availability of all elements | Optimal growth and maximum fertilizer uptake |
| Above 6.5 (Too Alkaline) | Micronutrients become insoluble | Iron chlorosis (yellowing leaves); stunted growth |
- Low buffering capacity: The buffering capacity of soilless media (like peat moss or coir) is very low because they lack the natural mineral reserves found in field soils. They are treated with limestone to buffer pH initially, but frequently altered by irrigation water alkalinity, fertilizer reactions, and natural root respiration.
- Root health: Extreme pH levels physically damage sensitive root hairs. This reduces water uptake and stunts overall plant growth.
- Disease suppression: Some root pathogens thrive in specific pH ranges. Keeping the correct pH helps the plant naturally resist infection.
- Fertilizer efficiency: Incorrect pH stops plants from utilizing applied fertilizers. This wastes money and increases chemical runoff.
Recommended Practices for pH Management in Soilless Growing Media
- Test daily or on a regular schedule: Use a calibrated pH pen to test the drainage runoff (leachate) or use the 1:2 dilution method. pH pens are reasonably priced, quite accurate and easy to handle. They are simply dipped into the solution to provide a digital readout of the pH.
Also, Litmus paper, which is the cheapest (but also the least accurate method), involves dipping the paper into the solution and comparing the colour change in the paper to establish colour codes for different pH ranges.
Electrode-type pH meters are the most accurate for measuring pH, but they may be the most expensive and require the most maintenance. Placing the pH probe directly into the solution until the reading stabilizes gives a quick reading. Rinse the probe between samples. Also, calibration is very important.
Laboratory analysis of soilless potting media includes samples collected from only the bottom two-thirds of the potting substrate (do not include the top layer, particularly when the crop is being sub-irrigated). Bulk together 15-20 samples; from this take a representative sample of approximately 600 g, place in a plastic bag and send to one of the laboratories accredited by OMAFA (Soil, leaf and petiole tissue, and forages and feed testing labs | ontario.ca). - Check source water: Monitor the alkalinity of your irrigation water, as it acts as a permanent pH driver.
- Inject acids/bases: For many growers, injecting acid (e.g., nitric, phosphoric, or sulfuric acid) to lower pH through their fertilizer injector is the method of choice. Be sure that the injector is designed to handle acids. For other growers, installation of in-line pH probes controlling an acid pump is an option. Both methods work well.
- Choose the right fertilizers: If the pH of a soilless growing media is at the higher end (e.g., 6.2 to 6.5), use ammoniacal nitrogen to lower media pH. However, if the pH is too low, it is important to discontinue acidifying water supply and shift to a basic-reaction fertilizer (high in nitrate nitrogen) to raise it over time.
In conclusion, when pH falls outside the optimal range, the chemical availability of nutrients changes, and plants may be unable to absorb elements that are physically present in the substrate which is one of the primary causes of poor crop quality and plant losses in commercial greenhouses and nurseries. To prevent undetected nutrient disorders from creeping up and causing significant crop losses, growers should regularly test the root-zone environment for pH and other available nutrients.
Dr. Chapagain is the Soil Fertility Specialist with the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) based in Guelph. He also coordinates OMAFA’s agronomic soil test labs accreditation program and can be reached at Tejendra.Chapagain@ontario.ca
