Growing the Best Vegetables and Small Fruit: pH and Water Quality Matters

As we look around at ways to improve produce production, yield, and quality, one area that offers the greatest return in both fruit quality and reduced fertilizer inputs is in getting the pH of your irrigation solution correct. Every crop has an ideal pH range where it can best remove nutrients from the soil solution. 

First, you need to understand pH a bit to make any sense of this: pH is a logarithmic scale; therefore, even seemingly minor adjustments in the pH of a solution can have a major impact on nutrient uptake. Note: 1 point on the pH scale is a ten-fold change, so a pH of 7 is ten times more alkaline than a pH of 6. A pH of 5 is 100 times more acidic than a pH of 7 (10 x 10). A pH of 4 is 1,000 times more acidic than a pH of 7 (10x10x10) and so on. Since tomatoes are the number one dollar-per-acre crop in PA, this crop will be used as an example for this article.

Tomatoes (peppers and eggplant too) prefer a soil solution pH of 6.2–6.5. That range provides the maximum uptake of nutrients, so whatever fertilizer you provide is likely to be used by healthy plants. As the soil solution pH moves above that range, it becomes increasingly difficult for tomatoes to extract potassium, even if plenty is present. Yellow shoulders, gray wall, and internal white core are all nutrient-driven disorders that result from insufficient potassium at fruit set. If a fruit is set with insufficient potassium, no amount applied later will fix that fruit. In addition, blossom end rot and cracking, while largely Ca/Mg disorders, are part of this picture since getting the nutrients calcium, magnesium, and potassium, all in the correct proportions, into tomatoes, reduces or eliminates virtually all common nutrient disorders. A grower cannot get the proportions of these nutrients correct without first addressing the pH of the soil solution.  Growers have been able to reduce potassium applications substantially and still get improved pack outs by simply getting their irrigation water and fertigation solution pH to 6.2–6.5.

Throughout much of Pennsylvania and the Mid-Atlantic, our well, spring, and surface waters are sourced from limestone-based aquifers. These waters typically have high pH and alkalinity, which, for the purpose of growing the best tomatoes, is any pH above 6.5. pH is the relative acidity or alkalinity, where 7 is considered neutral, and anything below that is acidic and anything above that is alkaline. Alkalinity is measured as the bicarbonate content of water (typically CaCO₃) and is most easily understood as the resistance to changing the pH of a solution. For example, it takes very little acid to neutralize a solution with a pH of 7.8 (relatively alkaline) and an alkalinity of 120 ppm CaCO₃, while it will take a lot more acid to neutralize a solution with a pH of 7.5 and an alkalinity of 350 ppm CaCO₃. The higher the alkalinity value, the more acid will be needed to neutralize the solution. Alkalinity and pH are related but are not the same.

Before a prescription for acid application can be created, your water supply needs to be tested for both pH and alkalinity. Always specify to your laboratory that you need the alkalinity value in 'ppm bicarbonate', as many labs are also testing home water for the calibration of water softeners, where grains per gallon is the measurement.

If you are an organic grower, then you will probably be using powdered citric acid to reduce the pH of your irrigation water. It takes about 7–9 ounces of powdered citric acid per 100 gallons of water to reduce most well water by one point on the pH scale. Tom McCarty, Retired Extension Water Quality Educator, Penn State Extension, did extensive experiments with well water collected from sites all over Central PA and found that this formula worked most of the time. Use a two-point calibrated pH meter to refine any acid application. Concentrated vinegar is also allowed under organic standards, but that is probably substantially more expensive and harder to handle due to the weight of handling so much liquid versus powdered citric acid.

Most conventional growers use either 35% or 93–96% sulfuric acid for pH reduction. The 93–96% is readily available from greenhouse suppliers in one-gallon jugs and was available from chemical suppliers in carboys and totes. However, due to illegal drug operations using it as an ingredient, it has become hard to easily purchase large supplies of this higher percentage. 35% sulfuric acid has become the standard for most farm irrigation systems. In order to determine your dose, enter your present irrigation water source pH and alkalinity in ppm along with your target pH into the UNH AlkCalc. Pay careful attention to the units in the pull-down menus when entering values or the program will return nonsense numbers.

The acid recommendations provided by AlkCalc and the citric acid value provided earlier in this article are good starting points. In order to refine your specific acid dosage, you will need to use a digital pH meter that is temperature-compensating and uses a two-point calibration. We generally work in the lower end of the pH range, so use pH 4 and 7 calibration solutions. Wire prong meters and litmus paper are not nearly accurate enough to adjust an acid dose rate. Good pH meters use a porous glass bulb sensor that allows hydrogen ions to move through, then compares your water's hydrogen ion concentration to that of a reference electrode in the sensor.

A pH meter as described will cost between $75 and $200, with better meters on the higher end of this range. Make this a useful purchase by following all directions for use, calibration, and storage between uses. Be sure to remove the batteries at the end of the growing season and repurchase calibration solutions at least yearly. Check with your local greenhouse or vegetable supply dealer, or with catalog and online suppliers such as Ben Meadows, Forestry Suppliers, and Gempler's for meters.

If your water requires acid to maintain an irrigation water pH of 6.2–6.5, then the best practice is to run your proportional injectors (Dosatron, Dosmatic, Chemilyzer) in a series with acid as the first station that always runs. This provides a constant supply of adjusted water to the root zone. When fertigating, it is likely that the dose of acid will need to be adjusted, as every fertilizer has some degree of acidifying or alkalizing potential. Use your calibrated pH meter to adjust the concentration of acid based on the output from the injectors. Your acid injector will probably need to be rebuilt at least annually to maintain an accurate dose rate. Most regular proportional injectors designed for soluble nutrients can be used with the rates of sulfuric acid and citric acid as described in this article.

Greenhouse and high-tunnel grown produce are usually the first to receive pH and alkalinity correction due to the high crop value. However, field-grown tomatoes and peppers can also benefit from water treatment. Due to the amount of water and acid needed for large areas, acid-safe and peristaltic pumps are often better suited for field use. Work with a specialist who has a background in field irrigation to get the best system for your farm.

The greater the range between your current irrigation water pH and ideal values, the more you'll see a difference in improved production and return on your investment through these practices. You'll pick fewer cull tomatoes and have a significantly easier time sorting and packing. Sounds like a great plan.