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Winegrowing - Page 1

March /April 1999


Gypsum injection into vineyard drip systems
By Lizanne E. Wheeler, Patrick D. Brown

Vineyard Water Systems

A common component of a vineyard irrigation system in the western U.S is the "gypsum machine," which consists of a tank with agitation paddles and an injection pump. Finely ground gypsum (calcium sulfate) is mixed with water to form a slurry within the machine, and is then injected into the irrigation system. Ideally, when the slurry mixes with the irrigation water, most of the gypsum dissolves into calcium and sulfate ions, which pass through the irrigation system and into the soil with the irrigation water. That is the idea, and the idea is quite sound. It is the practice that presents the challenges.

Why inject gypsum?

There are three generally recognized agronomic reasons for injecting gypsum into an irrigation system:
  1) Gypsum contains calcium, and calcium is a secondary plant macro-nutrient. It plays an important metabolic role in cell elongation and division. Some crops require high levels of calcium during the growing season, and supplying sufficient quantities sometimes becomes a management challenge, depending on the Ca:Mg ratio in the soil. Magnesium ions may interfere with calcium uptake into plants. Calcium-magnesium ratios (optimal Ca:Mg ratio is 2:1 to 5:1) can also be favorably influenced with gypsum applications.
  2) In irrigated viticulture, calcium plays an important role in the maintenance of soil structure. The soil colloids (clay and humus) possess negatively charged sites, which attract and hold positively charged ions (cations) in an exchangeable form. The structure of the soil is determined by organic matter, microbiological processes, and the relative ratio of calcium, magnesium, and sodium cations that are bonded to the negatively charged sites. If too many "cation exchange sites" are occupied by sodium ions, the soil colloids tend to disperse and pack tightly together. This reduces the size and number of pore spaces between the aggregates, making it difficult for water and air to infiltrate the soil. Replacing most of the sodium ions with calcium ions tends to aggregate the soil colloids, thus providing larger pore spaces, better water penetration, and improved soil structure.
  3) The cation balance of the soil tends to come into equilibrium with the cation balance of the irrigation water, so irrigating with high-sodium water, Colorado River water for example, can lead to "sodic soils" (high levels of sodium in the soil causing de-flocculation of the soil colloids) and poor water infiltration. The cation balance of the irrigation water is expressed as the sodium adsorption ratio (SAR) and is a common laboratory analysis. Irrigation water high in sodium (SAR above 6.0) can be modified through the addition of calcium, either through the irrigation water or by direct application to the soil.

Irrigation water that is very low in total salts can also be a problem because its low infiltration rate produces more run-off under the emitters. This is due to the high surface tension of low-salinity water, such as Sierra snow melt. The addition of any salt will tend to reduce this surface tension and increase water infiltration rates. Gypsum is a very common choice for this application.

Dealing with the challenges:

The practical challenges of injecting gypsum relate to plugging up the drip system. The most immediate incidence of gypsum plugging occurs either when more gypsum is injected than can be dissolved into the irrigation stream or when not enough time is allowed between the injection of the slurry and its arrival at the system filter. In either case undissolved gypsum coats the filter and plugs it. This can be overcome by reducing the concentration rate of the injection and, when possible, by moving the injection point farther upstream from the filter. In some cases growers move the injection point below the filter, but if the injection stream is not adequately pre-filtered, severe problems with plugged emitters can result.

If the gypsum is ground finer than the mesh size of the irrigation filters, the insoluble (non-gypsum) fractions can pass into the system, which presents another challenge. In many cases, these particles will pass through the emitter without causing any plugging problems. But often, because of their density, they settle out in areas of the irrigation system where the velocity of the water is low. This can occur in large mainlines as well as in the emitters themselves. Over time these silt-like deposits build up and affect the system performance. The deposits become difficult to remove, especially if allowed to remain over long periods of time. Because of this, it is recommended to frequently flush all mainlines and hose laterals with sufficient velocity to prevent these deposits from building up into a chronic plugging problem.

Perhaps the most common plugging problem associated with gypsum injections is lime scale formation. Lime scale (calcium carbonate) is formed when calcium ions (either naturally present in the source water or added as calcium sulfate in gypsum) combine with naturally occurring bicarbonates in the water. As described in "Formation of Lime Scale in Drip Irrigation Systems," (PWV March/ April 1998), calcium ions are often the missing element in a source water’s ability to deposit lime scale.

When we dissolve gypsum into water that contains appreciable (100 mg/L or greater) amounts of bicarbonates and has a pH of 7.0 or greater, we are setting up a system to actively precipitate calcium carbonate (lime scale). When dealing with waters with high potential for lime scale formation (high total alkalinity), it may be more cost effective to seek methods of applying calcium to the vineyard other than through irrigation injection.

If the irrigation water has a high (>6.0) sodium adsorption ratio (SAR), you may need to supply calcium to the soil to keep sodic soils from developing. The product you use depends upon the pH of the soil, and whether or not there is any limestone (calcium carbonate) already present in the soil. If the soil is acidic, the addition of limestone will supply calcium and tend to raise the pH of the soil. If the soil is neutral or basic, the addition of gypsum (calcium sulfate) will supply the needed calcium with little effect on the soil pH.

Applying bands of gypsum to the soil beneath the emitters will be as effective as injecting the material into the water, without the plugging potential. Soil applications also have the advantage of using natural rainfall to carry calcium into the soil. If the soil contains limestone (also called "free lime"), then soil applications of elemental sulfur or sulfuric acid will react with the calcium carbonate to release calcium ions. Sulfur and sulfuric acid will lower the pH of the soil as well.

If the irrigation water is very low in total dissolved solids (TDS) or electro-conductivity (EC), making water infiltration a problem, applying a band of gypsum underneath the emitters may be a better choice than injecting gypsum into the irrigation system. Another option is the use of "water penetrants" or "surfactants," which decrease the surface tension of the water, allowing for better infiltration. There are many products on the market formulated specifically for this application. Promoting cultural practices that increase the organic content of the soils will also help improve soil texture and water infiltration.

Another common occurrence on irrigated soils in the arid western U.S. is surface crusting beneath drip emitters. In many cases, this is caused by the formation of lime scale where evaporation causes the calcium salts in the water to concentrate beyond their solubility point, and calcium carbonate is deposited as a thin layer of concrete. Increasing the calcium content through the injection of gypsum will only make this situation worse. This surface scale can be removed by adding sulfuric acid through the irrigation water. Elemental sulfur may be added to the soil. The microbial oxidation of soil sulfur by bacteria digests elemental sulfur, freeing hydrogen protons. The protons react with the calcium carbonate, forming carbon dioxide, water, and calcium ions.

How to avoid plugging

If the water source is high in bicarbonates and the pH is high (conditions which lead to lime scale precipitation), how do we avoid the potential plugging caused by chronic lime scale formation?

Gypsum may be injected without the worry of lime scale formation if the pH and the bicarbonate (often expressed as total alkalinity) levels of the water are low enough. If the natural water is too high in either of these two factors, it can be modified with the injection of sulfuric acid prior to the gypsum injection point, effectively reducing the total alkalinity and pH.

To determine how much acid you will need, obtain a valid water quality analysis from a reputable independent water laboratory. Before obtaining the sample, contact the lab and tell them that you want to know what quantity of gypsum you can add to the water before lime scale starts to precipitate. Also tell them how much gypsum you would like to add to the water. If you need to add enough gypsum to make lime scale precipitate, ask them to determine how much acid you need to add to prevent the lime scale from forming. They will be able to instruct you on how to sample and preserve the water, so their test will be valid. These analyses should be relatively inexpensive.

One alternative to acid injections is the use of "lime scale inhibitors." These compounds have been around for years in "spotless" dishwasher detergents and the steam-generating industry, and several have been registered for use in agricultural irrigation systems. Because of their cost effectiveness and inherent safety factors, lime scale inhibitors are becoming quite popular alternatives to acid injections.

Another potential plugging problem may be avoided by refraining from injecting phosphorus fertilizers into irrigation systems that have had gypsum injections until the mainlines and hose laterals have been completely flushed. The calcium remaining in the system can react with the phosphorus to form insoluble precipitates of calcium phosphate, which are very difficult to flush from the system.

Summary

Addition of soluble calcium to the vineyard has many agronomic benefits, but there is a high potential that adding calcium to the irrigation water will cause emitter-plugging lime scale to form, which will affect the uniformity of the water and nutrient applications. There are many viable alternatives to gypsum injections, and there are ways to modify the irrigation water to prevent lime scale formation during gypsum injections. Many vineyard managers are finding that soil applications every four or five years take less management time than irrigation applications, which require constantly dealing with gypsum machines and acid injectors.

Lizanne E. Wheeler and Patrick D. Brown are independent irrigation design consultants, specializing in developing bilingual training and management programs for micro-irrigation systems. They may be contacted through Vineyard Water Systems, 521 Lucerne Road, Cayucos, CA 93430, tel: 805/995-0587. Reprints of their recent publications are available at www.vineyardwater.com (will open a new window).

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