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Lowering Soil pH


Several of my clients have a problem with steadily increasing pH in the soil under trickle/plastic mulch situations. Does anyone have experience with correcting pH downwards? A bit of background. Soil type is straight sand with organic matter commonly less than 0.5%. Waters around pH 7.2, hard with high magnesium and calcium. Crops (mainly ornamentals) are in for 18 months to 2 years and all fertilizer added via the drip. Soil pH (H2O) climbs from around 6.5 to 8.0 over this time, giving problems with manganese and zinc deficiencies in the plants. Main fertilizers used are Urea, Potassium nitrate, MAP, MKP, Calcium nitrate, Magnesium sulfate. (plus TE's). Perhaps I should swap to more sulfates? Is there a "quick fix" chemical I can put through from time to time? Thanks for your help.

by Julian Baker


I suspect the calcium and bicarbonate content of the irrigation water is resulting in the precipitation of calcite in the sand as it is concentrated by evapotranspiration. The pH of a soil which contains calcite can range from 7.2 to 8.5 provided the exchangeable sodium level doesn't exceed about 20 percent. If it does, the pH can be higher. Lowering the bicarbonate concentration by adding sulfuric acid to the water would be simplest way to reduce the pH. Adjusting the pH of the water to about 6.5 by adding sulfuric acid would remove most of the bicarbonate: adding about 130 lbs of sulfuric acid to an acre foot of water would reduce the bicarbonate ion concentration about 1 meq/l. Another option would be to increase the fraction of nitrogen fertilizer applied as urea. Its hydrolysis generates acid. Using N-pHuric would be another option: it is a blend of urea and sulfuric acid (the table I am reading from has N-pHuric as 10 % N and 18 % S). Whether either of these fertilizer options will work depends on the amount of bicarbonate in the irrigation water relative to the amount of N fertilizer the plants need.

by J.D.(Jim) Oster


The easiest way I know is to inject Sulfuric Acid. My recommendation would be to start at about 38ml/m^3 of water. You don't want much less than about pH 6.2 or so. Remember that if there are any carbonate deposits on the inside of your lines that these will have to be dissolved first before the pH of the water at the emitters will change. Also, on drip, that you will be creating a pH gradient in your soil. If you are having problems with a particular micronutrient because of high pH, the roots nearest the emitter will be able to take in the micronutients as soon as the pH of the soil solution allows for the material to be in an absorbable form.

by Alan S. Wicks


There is a small program available from University of Florida that calculates the injection rate required to neutralize bicarbonates as well as the cost of the treatment. Check http://hammock.ifas.ufl.edu/it/ The name of the program is ACID. A windows version has been developed and will soon be available.

by Fedro S. Zazueta