Fred Hulme, Ph.D.,
Technical Services – ICL Specialty Fertilizers,
Why do we care about root zone pH?
As a measurement of root zone hydrogen ion concentration, pH is a major concern in container production. Root zone pH influences essential element availability (e.g. iron or manganese), and an excess or deficiency of these nutrients can dramatically affect crop quality. At high pH levels (> 6.5), the solubility of some micronutrients is restricted, causing some crop types like petunias, ornamental grasses and roses to develop deficiency symptoms. Conversely, when root zone pH levels are too low (< 5.5), some micronutrients are more soluble and can cause toxicities in some crop types, including geraniums, marigolds and New Guinea impatiens.
How do you keep root zone pH levels in acceptable ranges?
Typically, growers strive to maintain growing media pH between 5.5 and 6.5 – the “sweet spot” where nutrient availability is optimized. However, there are a number of growing system factors that impact root zone pH:
- Irrigation water quality. Irrigation water’s total alkalinity (or bicarbonate level) provides much of the pH buffering in the container during the crop cycle. Think of alkalinity as dissolved lime that is applied each time the plant is watered. Ideally, irrigation water should run 50 to 120 ppm total alkalinity.
- If alkalinity is too low, root zone pH can drop over time because acidic influences from fertilizer build up.
- If it is too high, root zone pH can rise as excess bicarbonates accumulate. This is especially a problem with highly alkaline waters (above 150 ppm) where some sort of acidification may be needed to adjust the water.
- Growing media components (e.g. sphagnum peat moss and green bark) can have very low pH levels. Adjust starting mix pH upward by adding limestone. Keep in mind that, depending on the type and rate of limestone used, liming is a temporary fix. Not enough lime will result in low starting pHs and excess lime will create high pH issues. Limestone will leach and neutralize over time, eventually removing it as a factor, especially in long-term crop cycles.
- Fertilizers can be acidic, neutral or basic in nature depending on the raw materials used to make a specific formulation. Typically, fertilizers with a high percentage of ammonia and urea N are acidic in nature, and fertilizers with a higher percentage of nitrate N are more neutral or basic. It is critical to match fertilizer to water chemistry and growing media to keep things in balance and avoid pH swings. Fertilizer company experts should be able to help growers systematically select optimal fertilizer programs.
What can you do if root zone pHs drop too low?
It is always best to avoid low pH levels, but this sometimes occurs due to media issues or improper fertilizer choice or application. Remember, pH is a logarithmic value. A pH of 4.0 is 100 times more acidic than a pH of 6.0. Once a crop’s pH falls to 4.0, growers must do a lot of heavy lifting to raise it back up. While low pHs are difficult to completely mitigate quickly, there are steps a grower can take:
- Use fertilizers with higher nitrate N percentages (with potential basicity values) to help root zone pH levels trend upward; however, this can be a slow and gradual process.
- Add buffering to the irrigation water by injecting potassium bicarbonates, applying liquid limestone or even topdressing plants with lime. These practices can help, but may not completely solve the problem.
- Switch to fertilizers with lower micronutrient percentages to minimize iron and manganese build-up in sensitive crop tissues.
What can you do if root zone pH levels rise too high?
Typically, this happens with highly alkaline water or when too much lime is added to the growing media. High pHs are can be mitigated with these steps:
- Use fertilizers with higher ammonia percentages or urea N fertilizers (with high potential acidity values) like Peters Excel® pHlow® 21-7-7 Acid Hammer, which will push root zone pHs down.
- Inject acids – nitric, sulfuric and citric – into the irrigation line to reduce media pH. The key is to constantly monitor pH during treatment and not overshoot the mark, which can lead to excessively low root zone pHs.
- Treat the symptoms. Since high pH will restrict availability of iron and manganese, switch to products with higher micronutrient content or fertilizers containing chelates. Products that are more efficient at higher root zone pHs, like Peters Professional black iron products, provide iron at extremely high pH levels. You can also green up chlorotic plants by supplementing with foliar sprays or micronutrient drenches (e.g. Peters Professional Liquid STEM™ or iron chelates).