pH-induced micronutrient deficiency
Photo: Sinclair Adam

Many different potting media are utilized in greenhouse production, but pH will affect the crop nutrient availability and uptake in all media. Regulating media pH is therefore an important aspect of nutrient management in greenhouse crops.

In perhaps 50 percent of all crop nutritional problems, media pH was not maintained in the desired range for that crop. As greenhouse growers produce many different species and cultivars of plants, knowledge of the crop requirements in terms of nutrient levels and pH is an important aspect of good management. This information is readily found from the breeding company, and much of it can be easily obtained online, or from a company’s sales team. Fertilizer providers and state extension service personnel can also be a useful source of information on crop requirements. Once obtained, this information will help growers stay on target in producing a good-quality crop.

Test for success

The other aspect of management growers should be implementing is a frequent testing procedure using a pH and EC meter at the greenhouse. If the crop has a bench time of six weeks or more this is very important. The frequency of testing would actually depend on the crop bench time, and plant growth rate. Most crops would be tested weekly, but some species that have short crop times may benefit from more frequent testing.

Potting media ingredients such as peat, coir and pine bark are acidic, and media manufacturers will adjust this acidic pH of the ingredients with dolomitic and calcitic limestone when blending. Compost, hardwood bark and vermiculite are typically basic in comparison. The blended media formed from these ingredients typically comes to a grower from the manufacturer, in the correct pH range for good growth, but it is always a good idea to test the media pH and EC before the crop is planted. Custom media blends can also be obtained, which have a lower limestone level, if that serves to improve finished crop quality.

The greenhouse water and nutrients applied may change the media pH over time and so correct choices in fertilizers and management of the irrigation water. Water pH and alkalinity can change in a season, so testing the water at least annually is worth doing. It will only be a matter of time until the water pH has altered the media pH. It is important to know how alkaline the water actually is. In some cases as the water table ground water level drops then the pH can increase, and so can alkalinity adversely affecting the crop. As most greenhouse crops prefer a pH range of 5.4-6.6, it is important to stay within this range. Using acidic fertilizer products can be a way to maintain the desired pH range, when alkalinity is lower than 150 ppm, but high NH4 formulas may not be the best choice of nitrogen form for a particular crop. High alkalinity can also be a problem for injectors, and clog the system with deposits.

pH meter with calibration solutions
Photo: Sinclair Adam

Adjusting pH

Plants such as calibrachoas and petunias would prefer pH in the low side of this range, but other plants such as geraniums would prefer the higher side of the range. Calibrachoas and petunias may also be termed iron inefficient, as they benefit from extra iron in their fertility program. Petunia feed fertilizers may help to compensate, but a good management plan is to adjust the water pH to the crop needs. This can be done using acid injection systems that can adjust the water pH to the desired range. (An excellent tool for calculating the required acid injection rate based on a water test is the University of New Hampshire’s Alk Calc website: Putting in the water alkalinity level, from the water test, and choosing the target pH as well as an acid type (and concentration) will enable the grower to properly adjust the acid injector.

Choice of acid will also result in some additional nutrient added to the fertility provided to the crop. If you elect to use sulfuric acid additional sulfur will be in the growing medium; phosphoric acid will add phosphorous to the medium. If an operation is growing crops that require a pH range of 5.5-6.0, and the water is pH 7.2 or greater, this acid injection system will provide an improved crop quality. The cost of such a system is not prohibitive, and will doubtlessly improve sales of products produced compared to operations where there is no acid injection. Reduced labor costs will also be a benefit, as additional sprays of iron chelate would not be required. Newer pH low fertilizer products are also available, and these formulas can reduce the amount of acid needed in the injection system. Despite the options in fertilizer products selected, a highly alkaline water source is best managed by an acid injection system.

As the crop cycle progresses frequent pH and EC testing is a grower’s tool to measure the media changes accurately. Using a reliable meter that is properly calibrated is critical to sound information. If the meter has difficulty being calibrated, it is probably time for a new pH electrode. Some meters will need replacement electrodes after two years of use. If the meter is poorly maintained the electrode may need replacement sooner.

Annual water testing is also important to monitor the water pH and alkalinity. In one operation testing the water every month showed a pretty significant pH change after the snow-melt was complete, and spring rains diminished in early summer. This downturn in ground water caused the water table to be lowered, and as the water table dropped in level, the pH came up and so did the alkalinity. With increased rainfall in the autumn the pH came back down to former (spring) levels.

It can be helpful to get assistance in interpreting test information from your extension representative or fertilizer technical representative. Good crop quality, and staying on your production schedule, are directly connected to managing pH and EC properly throughout the entire crop cycle.

Sinclair is an extension educator at Penn State University specializing in floriculture, propagation, breeding and nutrition.