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Growing The Best Vegetables And Small Fruit: Getting The pH Right

As we look around at ways to improve produce production, one area that offers the greatest return in both fruit quality and decreased fertilizer inputs is in getting the pH of your irrigation solution correct. Every crop has an ideal pH range where it removes nutrients from the soil solution optimally. Getting your soil and water pH right can be the difference between a profitable crop and high field / packing house losses.
As we look around at ways to improve produce production, one area that offers the greatest return in both fruit quality and decreased fertilizer inputs is in getting the pH of your irrigation solution correct. Every crop has an ideal pH range where it removes nutrients from the soil solution optimally. Getting your soil and water pH right can be the difference between a profitable crop and high field / packing house losses.


Since pH is a logarithmic scale, even seemingly minor adjustments in the pH of a solution can have a major impact on nutrient uptake. Note: 1 point on the 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 and much of my research goes into tomato nutrition, that crop will be our example for this article.

Tomatoes (peppers and eggplant too) prefer a pH in their soil solution of 6.2-6.5. That range provides the optimum uptake of nutrients, so whatever nutrients you provide are 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, Internal White Core… are all nutrient driven disorders that result from insufficient potassium at fruit set. Once fruit set with insufficient potassium, no amount applied later will fix the fruit. In addition, Blossom End Rot and Cracking, while largely Ca/Mg disorders are part of this picture as 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 packouts 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 source 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 acid and above that is alkaline. Alkalinity is measured as bicarbonate (typically CaCO3) and is most easily understood as the resistance to changing the pH of a solution. That is, it takes very little acid to neutralize a solution with a pH of 7.8 (relatively alkaline) and an alkalinity of 120 ppm CaCO3 where it would take a lot more acid to neutralize a solution with a pH of 7.5 and an alkalinity of 350 ppm CaCO3. The greater the alkalinity value, the more acid 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 approximately 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, and I 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 in handling so much liquid than powdered citric acid.

Most conventional growers use either 35% or 93% sulfuric acid for pH reduction. The 93% is readily available from greenhouse suppliers in one gallon jugs and was available from chemical suppliers in carboys and totes, but due to illegal drug operations using it as an ingredient, it has gotten hard to 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.

The acid recommendations provided by the 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 hydrogen ion concentration to that of a reference electrode also in the sensor.

A pH meter as described will cost between $60 and $200 with better meters on the higher end of this range. Make this a good 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 annually. Check with your local greenhouse or vegetable supply dealer or with catalog and online suppliers such as Ben Meadows, Forestry Suppliers, and Gemplers’ 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. That 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.

Source : psu.edu


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