How to Balance Plant Nutrition and pH in Ornamentals
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Grower Question: “I have trouble managing plant nutrition and pH. Do you have any advice for managing this issue effectively for optimal results?”
Jessica Tatro: Nutritional management goes hand-in-hand with pH management because nutrient availability varies with substrate pH. Nutritional issues may not necessarily stem from too little or too much fertilizer applied, but from the nutrients not being readily available for plant uptake. The first step in pH management is to understand the main factors that affect substrate pH. These include water alkalinity, lime rate in the substrate, and type of fertilizer applied to the crop.
Water alkalinity is a measure of the carbonates and bicarbonates present that act as a pH buffer. Someone once told me to think of alkalinity as a measure of the dissolved lime in your water. If your water has low alkalinity (generally considered as ≤50 ppm or 1 meq CaCO3), it has very little buffering capacity.
A grower will easily be able to adjust pH, but will have to monitor it closely as it can fluctuate with fertilizer application. If your water has high alkalinity (generally considered as ≤150 ppm or 3 meq CaCO3), it has a large buffering capacity that needs to be neutralized before pH can be adjusted. Many growers with high alkalinity in their irrigation water will use acid injection as a management tool. The type of acid used and the injection rate will depend on the alkalinity in your irrigation water.
Lime incorporated into the substrate balances out the naturally low pH of the peat and provides buffering capacity over the life of the crop. Substrates generally have a standard lime incorporation rate, but growers can request custom blends or mix their own. If pH tends to rise over time, growers can consider lowering the lime rate. If substrate pH has a tendency to decline over time, then growers can look at incorporating more lime.
Fertilizer selection can affect substrate pH. Fertilizers containing ammonium (NH4+) forms of nitrogen are likely to lower pH, and fertilizers with nitrate (NO3-) are apt to raise pH. Fertilizer labels will indicate the potential acidity or basicity of a fertilizer.
Know Your Crops
The next step in pH management is to understand the requirements of the crop(s) you grow. Some crops prefer a lower pH (generally below 6.0). If the substrate pH is too high, these crops (e.g., calibrachoa, petunia, and sutera) will show symptoms of iron and micronutrient deficiency, most commonly expressed as interveinal chlorosis of the young leaves. Other crops prefer a higher pH (generally above 6.0), where iron and manganese are less available. If the substrate pH is too low, they will show symptoms of micronutrient toxicity (e.g., geranium), most commonly expressed as necrotic spots on the lower leaves.
To manage pH effectively, check it on a regular basis. It is generally recommended to test every one to two weeks, depending on the stage of the crop and if you have had problems in the past. It is hard to fix a problem if you don’t know where you are or know how severe the problem is. It is also helpful to track pH over time to see if a general rise or decline is occurring.
Finally, it is important to know how to adjust the substrate pH. It is much easier to correct a problem early on, before it becomes severe and you start seeing deficiency or toxicity symptoms. If your substrate pH is high and you need to lower it, some management strategies to consider include acidifying your irrigation water source if the alkalinity is high, applying acidic fertilizers, and reducing the lime rate in your growing media. If your substrate pH is too low, some management strategies to try include applying basic fertilizers, applying lime (most commonly in the form of flowable lime during the production cycle), and increasing the lime incorporation rate in your substrate.