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Arthur Wallace Wallace Laboratories
365 Coral Circle, El Segundo, CA 90245

ABSTRACT: Gypsum alone has profound beneficial effects on soil because of its chemical effects. Gypsum is used to improve sodic soils, to create more favorable solute concentrations in soil, especially after leaching with heavy rainfall, and even to correct subsoil acidity. The combination of gypsum and organics can result in biological improvement of soil more than can organics alone. This is an extremely important aspect of soil quality. The combination of gypsum and water-soluble polymers, including with organics, can maximize the improvement of physical properties to soil. The chemical, biological, and physical properties of soil when improved together with gypsum, organics and water-soluble polymers constitute a triangle for major spoil improvement.

INTRODUCTION
The gypsum triangle results in better quality soil – chemically, biologically and physically. The need for gypsum and other amendments is urgent in the Intermountain West and other arid and semi-arid areas. For example, in the spring of 1993 many soils lost their structure when heavy rains removed most readily soluble solutes. On one farm involved, the fall barley died when the soil became compacted so that the field had to be replanted late in the spring. Gypsum contains both calcium and sulfur; each is an essential plant nutrient; however, calcium does much more than its role as a plant nutrient. Without it in a soluble form, soils would not be tillable. Without it in soluble or exchangeable form, other plant nutrients would not function properly (Epstein 1972). Soils usually contain considerable calcium in the soluble and exchangeable forms. Some soils also contain large quantities of calcium in the form of lime, but that form is not readily available to plants nor can it improve soil when existing as lime. When soil pH is over 8, the calcium in soil is not soluble enough to be of maximum value for either plants or soil. Large crop responses can be obtained to gypsum when soil pH is high and even under other circumstances.

GYPSUM – THE CHEMICAL EFFECT
Gypsum is the time-honored ingredient for correction of sodic soils (Aldrich and Schoonover 1951). Calcium from gypsum replaces exchangeable sodium in soil so that the sodium can be leached away. Calcium in the exchangeable form gives stability to clay particles. It keeps the pH of the clay around 7 if it is the dominant cation on the clay complex. Gypsum additionally, is a means for obtaining a favorable electrical conductivity (EC) of the soil solution so that soil is permeable to water, air and roots. A low EC makes sodic soils even less permeable (Sumner 1993b). Problems that arise from excessive rainfall can be considerably lessened with applications of modest amounts of gypsum like 400 pounds per acre. Of course, gypsum supplies calcium and sulfur nutrients. Addition also makes for better calcium to magnesium ratio when magnesium is in excess. Excess magnesium frequently happens, and it can be detected by soil analyses. Gypsum also has chemical effects on acid soils (Sumner 1993a).

GYPSUM PLUS ORGANIC MATTER – THE BIOLOGICAL EFFECT
Soil organic matter is essential to the tillability of soil. As its concentration decreases steadily with years of cultivation, soil becomes less and less fertile; they compact more and they erode more. A healthy soil needs continual addition of organic matter in the form of crop residue, green manure, organic wastes, or other forms (Parr et al 1992). But all of this can merely burn away with action of soil; microorganisms unless accompanied with calcium in the soluble form and if excessively tilled (Wallace 1994). Addition of calcium from gypsum plus organic matter can reserve the disastrous loss of soil organic matter which results in erosion, crusting, compaction and poor crop growth. Calcium is the agent which quite permanently binds components of soil organic matter to clay. The combination of gypsum and organic matter can considerably improve most soils. Soil microbes flourish with this improvement. The cycles of nature then function more smoothly. Much better crop production results.

GYPSUM PLUS WATER-SOLUBLE POLYMER SOIL CONDITIONERS – THE PHYSICAL EFFECT
Water-soluble polymer soil conditioners, even more so than gypsum alone, have the ability to dramatically improve the physical properties of soil. Appropriating water-soluble polymers add stability to soil aggregates to keep them from breaking down when it rains or when the soil is irrigated. With proper use, they prevent crusting, prevent soil erosion, prevent compaction, aid in water conservation, and greatly improve plant growth. But they cannot do this unless they are applied together with gypsum if sufficient soil calcium is lacking. A gypsum plus water-soluble polymer application program can be very cost effective. Only three pounds per acre of the water-soluble polymer applied in sprinkler irrigation when the crop is planted plus 50 to 100 pounds or more of gypsum in an inch of irrigation water per acre all off which can also go into the irrigation stream have increased potato yields by six tons per acre for a cost of only $15 per acre. Rates of water-soluble polymers to achieve this may go up to 10 pounds per acre on other crops or other soils. Other crops do equally well. Gypsum plus water-soluble polymers together in irrigation water can decrease soil erosion to almost zero. The water-soluble polymer materials used by the author’s group are food grade and leave no residues. With gypsum, the polymers can vastly improve the physical properties of soil.

WILL YOUR SOIL RESPOND TO GYPSUM? YES, WHEN –
Soils and plants will very likely respond to gypsum (Aldrich and Schoonover 1951, Handbook 1985, Traynor 1980, Wallace and Wallace 1994): When the soil pH is over about 8.2 and maybe even if it is less. When exchangeable sodium percentage (ESP) is over 3 and definitely if it is over 9. When water puddles on it. When the soil particles slake or disperse when added to water. When the subsoil pH is lower than 5. When there is waterlogging in the soil. When there is crust on the soil after a rain or irrigation. When there is excessive cracking of the soil after rain or irrigation. When the sol contains clay that is very dusty when dry. When irrigation water contains substantial amounts of bicarbonate; however, beware of caliche forming in the subsoil when gypsum reacts with bicarbonate. When intense rain falls on soil that is not acid and where nearly all solutes may be leached from the soil surface. When no-till is used. When organics are simultaneously applied to the soil with the gypsum or if water-soluble polymers are also applied with the gypsum. When crops require the developments of fruits or seeds. When sulfur may be deficient. When the soil contains clays that swell and contract. When irrigation water contains less than 200 parts per million of salts in solution (electrical conductivity of less than about 0.3 mmho/cm or dS/m).

Some of these needs may be met with as little as 100 pounds of gypsum per acre, others by a few hundred pounds per acre, while some others may require one to several tons per acre. Gypsum even deserves to be part of regular and annual fertility programs.