ACID INJECTION IN IRRIGATION WATER - IMPROVING pH ADJUSTMENT FOR BLUEBERRIES
 The acidification of irrigation water is a useful tool in a variety of production situations. Soil and water pH tend to run relatively high in many production areas of coastal and inland central California. It is not uncommon to encounter soil and water pH above 7 and routinely accompanied by high levels of bicarbonates. The bicarbonate ion can be toxic to plants, but more importantly, it interferes with other nutrients and makes them less available to plants. These bicarbonates also react with nutrients in the water, such as calcium or magnesium, causing precipitates to block irrigation emitters, micro-sprinklers, and fittings in lines.
Beyond adjusting the water pH and maintaining the micro-irrigation lines, acidifying the soil and water has other potential benefits that, depending upon the crop, may or may not be economical. Lowering the pH of the soil often improves availability of phosphorus, calcium, zinc, iron, and other micronutrients on calcareous (high pH) soils. Acidifying water also helps to improve water penetration and alleviates some of the stress of sodic soils, although those types of problems are rare along the coast.
Chemigation is the timely application of small amounts of fertilizers or other amendment materials through micro-irrigation systems. Compared to conventional ground fertilizer application, chemigation improves the efficiency of application of nutrients (fertigation). Comparable or improved yield and quality of many crops can be produced with 20 percent to 50 percent less fertilizer. Chemigation systems can also be used to apply acid to acidify the irrigation water. Mineral precipitates (often seen as white scale deposits), algae, and bacteria clog drip emitters, micro-sprinklers, and lines. Clogged emitters and sprinklers result in variable water distribution during irrigation and uneven fertilizer application during fertigation. Variable water or fertilizer application hinders uniform crop development, reduces yields, and jeopardizes quality. For growers to effectively use micro-irrigation technology, they must prevent clogging of drip lines and emitters.
BLUEBERRIES REQUIRE ACID SOIL
Acidifying irrigation water is a valuable technique for lowering the pH of irrigation water to aid in creating the optimum soil pH for blueberries. Blueberries require a soil pH of 4.5 to 5.0, and the accompanying low levels of soil bicarbonate. We typically attain the pH suitable for blueberries by application of sulfur or sulfuric acid to high pH soils. Sulfuric acid application acidifies the soil more quickly and more uniformly, depending upon the equipment used and the skill of the applicator, but sulfuric acid application requires specialized equipment that may not be economical on smaller fields. Soil pH can be quite variable initially in fields following application of sulfur to lower pH. It is difficult to get the sulfur uniformly distributed in newly sulfured fields, and the decomposition of the sulfur granules can be slow, depending upon the type of sulfur applied and the field conditions.
Acidifying the irrigation water improves the uniformity of pH in the field and enhances the early establishment and growth of newly planted blueberries. Repeated application of higher pH, high bicarbonate water, however, contributes to the general tendency for acidified soils to return to the original, higher pH. Acidification of the water slows the rise in pH of acidified soil and creates a more uniform growing environment for blueberries.
MATERIALS FOR ACIDIFICATION OF IRRIGATION WATER
The most commonly used materials for acidifying irrigation water are sulfuric acid, urea sulfuric acid, and phosphoric acid. These materials vary somewhat in their agronomic characteristics as well as their cost and ease of use. The phosphoric acid supplies the nutrient phosphorus (P) to the soil, while the urea sulfuric acid provides nitrogen (N). In some cases these materials may be combined with each other and with other materials to make more complete fertilizers for injection. As a rule, the phosphoric acid materials are more expensive than the urea sulfuric acid materials, and N tends to be more commonly limiting than P in production fields.
Specialized injection equipment is required for the injection of sulfuric or phosphoric acid. The urea sulfuric acid is buffered with urea, making it easier to handle, and the equipment requirements are not as restrictive. Generally speaking, the urea sulfuric acid works fine with plastic materials. These materials are not permitted by organic certification agencies, and organic growers are limited to the use of citric acid or acetic acid in the form of food grade vinegar.
DETERMINING ACIDIFICATION REQUIREMENTS
Different irrigation water sources have different requirements for acid addition, depending upon the pH and bicarbonate and carbonate load. The injection dose and rate of acid injection will depend upon the following factors:
- Characteristics of the water source
- Flow of the system
- Irrigation time
- Type of injector
The amount of acid to apply to a specific amount of water is determined through a titration procedure. Distributors of the acidifying products (see below) will often perform a titration of the water sample for prospective growers, and directions for growers to perform their own titrations are also available (Granberry, Harrison, and Kelley, 1996). Once the amount of acid required to lower the pH of the water to below pH 5 is known, the water flow in gallons per minute and the irrigation time can be used to calculate the amount of acid to inject per hour, the irrigation system is running.
INJECTION EQUIPMENT
Two types of equipment are typically used to inject acid into irrigation water:
- Positive pressure pump
- Venturi
The venturi is the most economical of the two pumping alternatives, but a venturi does not allow the precise flow rate that a pump allows. The venturi is "powered" off of the differential flow that is created when the irrigation stream is channeled by the venturi, causing a sucking pressure on the acid solution. The flow through the venturi equipment can be adjusted to enable injection of appropriate amounts of acid, but typically requires more careful calibration than the positive pump.
The positive displacement pressure pumps are the more sophisticated of the two types of injection equipment and allow the grower to more carefully meter in the injected acid material. The pumps require an external electrical source, although solar-powered equipment is available. The pump should be wired to the irrigation pump so that if it stops, the injection pump will also stop. Suppliers of acid injection materials and other agricultural supply distributors typically sell appropriate pumps or venturi equipment (see below).
There are several sources for sulfuric and phosphoric acid injection materials, whereas the most common urea sulfuric acid sources are the US10, US15, and US28 materials available from Verdegaal Brothers in Hanford, CA.
http://www.verdegaalbrothers.com
Telephone: 559/582-9205
Similar N-phuric concentrations are available from Agrium, Inc., 13131 Lake Fraser Drive SE, Calgary, AB, T2J 7E8
Telephone: 403/225-7000 or 877/700-5490
Citric acid is available from Bulkfoods.com USA Chemical, Telephone: 901/507-1400; and food grade vinegar is available from Nakano Food, 46 Walker Street, Watsonville, CA 95076. Telephone: 831/728-2061
For additional information see: Granberry, D.M., K.A. Harrison, and W.T. Kelley. 1996. Drip Chemigation: Injecting Fertilizer, Acid and Chlorine Bulletin 1130. University of Georgia, College of Agricultural & Environmental Sciences, Cooperative Extension, Athens, GA. Available at: http://www.ces.uga.edu/pubcd/b1130-w.html
Hart, J. and D. Homeck. 1997. Acidifying Soil for Crop Production. Crop and Soil News/Notes. Vol. 11. No. 6. Oregon State Univ. Extension Service. Corvallis, OR. Available at: http://www.css.orst.edu/newsnotes/9707/soils.html
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