Lowering substrate pH after planting

Crops vary in their response to substrate pH. Many crops grow well with a pH of 5.8 to 6.2, defined as the “general range.” Other crops such as marigolds and zonal geraniums grow well at elevated pH (6.0 to 6.6) and are classified as the “geranium group.” Conversely, a pH of 5.4 to 5.8 is best for plants like petunia and calibrachoa, and these plants are in the “petunia group.” Managing pH can be a challenge, especially for plants in the “petunia group.” One of the primary reasons is that it is more common for growers to have irrigation water with higher alkalinity (which raises pH) than it is for them to have water with too little alkalinity (which drops pH). This article will focus on how to lower substrate pH after your crop is planted and in the greenhouse.

The reason that pH affects plant growth is because it influences micronutrient availability. When the pH is elevated, it makes micronutrients less available to plants; when the pH is low, micronutrients are more available. That is why a synonym for the “petunia group” is “iron inefficient;” plants in this group do not take up micronutrients well, so they require a greater concentration of micronutrients for successful uptake (Fig. 1). If crops are nearly finished or have a very short production time and substrate pH is high, you may be able to simply provide more micronutrients instead of correcting the root problem. You can elevate the concentration of micronutrients by supplementing your regular fertilizer with a water-soluble blend of micronutrients. Alternatively, you can also apply water-soluble micronutrient blends as a drench as needed instead of adding them to your fertilizer mix. However, know that this is not going to fix the substrate pH. Rather, it is only going to temporarily bypass the problem.

The first step in lowering substrate pH is to re-evaluate fertilizer selection. Fertilizers can influence substrate pH, and it is the nitrogen form in fertilizer that can change substrate pH. Nitrogen is provided as nitrate (NO₃^-), ammonium (NH₄⁺), or urea (urea is converted to ammonium, so it “counts” as NH₄⁺). When NO₃^- is taken up by plants, it raises the substrate pH. Alternatively, when NH₄⁺ (and urea) is applied to plants, it lowers substrate pH as NH₄⁺ is either taken up by plants or converted to NO₃^-. Most complete, balanced, water-soluble fertilizers provide nitrogen as a blend of NO₃^- and NH₄⁺. This blend of nitrogen influences the potential effect of fertilizer on substrate pH. Fertilizers such as 15-5-15 have greater proportions of NO₃^- as the nitrogen source and will raise substrate pH. Alternatively, fertilizers with greater proportions of NH₄⁺ such as 20-10-20 will decrease substrate pH. Commercial fertilizers will be labeled with their potential basicity or acidity, which is the tendency to raise or lower pH, respectively, over time. Selecting a fertilizer with a greater proportion of NH₄⁺ and higher potential acidity will help bring down substrate pH.

If changing fertilizers is not providing you with the reduction in substrate pH that you are looking for, the next step is to acidify your water. Acid is injected to neutralize the alkalinity in water that buffers the substrate pH from dropping and raises pH. There are several different types of acid that may be used to lower pH, including nitric, phosphoric, and sulfuric acids (Fig. 2). When using these acids, it is important to remember that each of these acids will not only neutralize alkalinity and drop pH, but they will also contribute mineral nutrients to your irrigation water and fertilizer solution. For example, nitric, phosphoric, and sulfuric acids will contribute nitrogen, phosphorous, and sulfur, respectively, and you will want to keep this in mind when evaluating your fertilizer program. For instance, plug growers avoid using phosphorous so they use sulfuric or nitric acid to avoid adding any phosphorous to their plugs.

If changing fertilizers and acidifying your water is not providing you with the pH drop that you are looking for, or if you are trying to drop pH much more rapidly, you can apply an acid drench. Acid drenches are not applying straight acid directly to the substrate; rather, they entail adding acid to water and then applying that diluted solution to the substrate. This is something that should only be considered as a last resort to managing pH, as it requires experience to safely mix and apply acidic solutions.

Lowering substrate pH is one of the most common challenges growers face. Fortunately, there are a range of options for dealing with this problem. You will need to take into consideration the crop requirements, duration of the production time, and the potential severity of the problem to decide which course of action will be best for you.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu