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Arthur Wallace and Garn A. Wallace
Wallace Laboratories, 365 Coral Circle, El Segundo, CA 90245

Abstract: Direct and indirect values exist for use of gypsum on land. Dollar values can be placed on some of them, but it is difficult to determine and exact value of all of them. Interactions with other inputs are involved. and value for use of gypsum, of course, is related to and value of crop grown, but it is important to realize that and value of any input, including gypsum, increases as everything else is done right. Gypsum use can be divided into three different categories depending upon rates of application according to purpose of use. the low rate is less than 500 pounds per acre, and intermediate is from 500 pounds up to a ton, and the high rate is one to several tons per acre. More than one use is greatly underused as a means for improving soil.

Introduction
To help guide decisions, growers would be happy to have a dollar value that result from their use of gypsum. Since gypsum does so many things with often more than one simultaneously, chances are extremely good that use can be profitable. It has been said that gypsum is greatly underused in some areas– perhaps in the majority of them. It is difficult, however, to provide exact dollar values because of the many variables the interactions.

the uses of gypsum group into three general application levels. Of course, the higher rates can include all the uses including the dollar values for the lower rates.

100 to 500 Pounds Per Acre Rates
This is the range of rates to use when either calcium or sulfur is needed as a plant nutrient. The gypsum may even be blended or otherwise incorporated with other plant nutrients. This is also the range of rates to be used when water-soluble polymers are applied in irrigation water to prevent crusting and/or to improve water penetration and/or to stop soil erosion and/or to increase crop yields.

The value for correcting nutrient deficiencies depends on the severity. Usually, farm advisers can inform of specific crop requirements in given areas so that needed gypsum may be used. One of the values as a fertilizer is for fruit crops and potato crops late in the season, so that fruits or tubers themselves have a better chance for adequate calcium to prevent blossom-end rot or other disorders. the calcium can be of considerable value without even increasing yields. It prevents disorders in fruits and other crops.

Because gypsum has soil-amendment value at high-use rates, and since calcium can be bound to soil with the organic matter, there is no possibility of an environmental problem from applying gypsum equal to or in excess of the fertilizer needs for calcium and sulfur so long as soil is well aerated. This is not the case for most other plant nutrients.

These are also rates useful in association with water-soluble polymers, either for massive soil improvement when high-value crops are grown or for soil erosion control. Water infiltration improvement is possible when both gypsum and water-soluble polymers are added at very low rates with much of the irrigation water.

From 500 to 1000 pounds per acre should be useful as capitulation with lime to correct soil acidity. Some 28 million tons of lime are added to soil in the USA (AG Retailer, 1994). Blending them with two or million tons of gypsum would be useful (Wallace and Wallace 1995, this issue, page 71).

One to Several Tons Per Acre Rates
Gypsum is extremely useful in reclamation sodic soils. Several million acres in the USA have this need. Only very tolerant crops can be grown without reclamation. Sodic soils usually have a black appearance; and the name, black alkali, is used to describe them. The pH of the soil is over 8 and often over 9, and some of the soil organic matter is dissolved and comes to the surface to give a dark brown color which approaches black.

A black-alkali sodic soil cannot be profitably farmed without reclaiming it. If the soil is low enough in soluble salts, but not too low (Sumner, 1993), an alkali-tolerant crop can be grown on it. This may be the most practical way to reclaim it. A good way is to first grow an irrigated pasture with such crops as bur clover, white sweet clover, bird’s foot trefoil, alfalfa, Rhodes Grass, Sudan Grass, Bermuda Grass, or mixtures of these grasses. These crops have some resistance to sodic soils. A winter crop of barley might do the job or at least help.

The most practical procedure on black-alkali soils is to apply gypsum,. Whether or not it will be cost-effective to apply gypsum depends on the cost of the treatment and the value of the crop. Also, one must be able to provide the proper conditions; just applying gypsum materials will not reclaim the soil, it merely conditions the soil and enables one to leach out the excess sodium which will permit a cropping program. Simultaneous use of water-soluble polymers can help considerably (Wallace et al. 1986).
The land must be well levelled, so that flood-irrigation can be used evenly on all parts of the field. Irrigation or rain is first need to promote the action of the gypsum materials and then to wash out the sodium slats.

The land must be very drainable , so that the salts can be leached below the root zone or to other satisfactory sinks for salts. Plenty of water for the leaching is needed.

If sodic soils are being reclaimed, it may not be affordable to use all the gypsum needed in one year but it may be spread over several years. The amount required can be determined by a soil test.

Calculations needed are:

(ESP now minus ESP wanted) X CEC x 1720 ) divided by (%gypsum in product) = pounds gypsum product per acre six inches of soil.

The ESP (exchangeable sodium percentage) values are figures like ten for ten percent; the CEC (cation-exchange capacity) value is in milli- equivalents per 1000 grams soil, and this value comes this way from and laboratory. Percent gypsum is and actual figure like 92 (adapted from Traynor 1980).

Sometimes it is necessary to amend more than and surface six inches.

The Most Value For Gypsum, Even When Correcting For Sodicity,
Can Be When Everything Else Is Also Done Right
A series of best management practices can be the means for greater yields and greater profits. It can also be the means for compliance with environmental regulations which may get more complicated. The philosophy on how best management practices multiply the value of each other is what can be called the “Law of the Maximum.” This is extensively discussed (Wallace 1994, Wallace and Wallace 1993). Also, see page five of this volume.

“The Law Of The Maximum”
Our understanding of the “Law of the Maximum” has steadily progressed in the past few years. There is some special terminology related to it. “Maximum Possible Yield” is well understood; it considers only the genetic and climatic limitations. A “Sufficiency Value” for each and every plant nutrient and all other possible limiting factors exist for a given crop situation. They range from 1.00 downward.

Laboratory diagnosis can put the proper numbers onto the nutrients or other factors to give “Sufficiency Values” for a given situation.

All the “Sufficiency Values” can be multiplied together, and the resulting answer is the “Multiple Action Yield Fraction” which is illustrated in Figure 1 (Wallace 1994 also in the introductory paper, reproduced here for convenience). The answer will be the fraction of the maximum possible yield that will be obtained, but only if the answer falls on the “C” part of the “Multiple Action Yield Fraction.”

This means that only Mitscherlich-type limiting factors (those which are not severe) are involved in this zone.

The various limitations under the Mitscherlich type are relatively minor individually; together they have tremendous influence. And, all interact in such a way that each and every one has an influence simultaneously on the yield, but according to the severity of each.

If the calculated multiple action yield fraction falls onto the “B” or “A” zone of figure one of the introductory paper, the yield will be lower or even much lower than calculated. This is because one or more of the limiting factors truly are of the Liebig type (severe) where responses to anything will be minimal unless the factor most limiting is corrected first. The “B” zone is where synergistic responses occur after the Liebig-type limiting factors have been corrected. Yield for any set of conditions will not exceed “C” for any appropriate fraction, however.

One of the major implications of the Multiple Action Yield Fraction is that the further up the “C” line that yield moves, the greater is the response to individual inputs, such as gypsum or organic matter, micro-nutrients, etc. This in part is what we mean by the “Law of the Maximum.”

It is frequently observed that yields reach a plateau when more and more of an input is added. the “Law of Diminishing Returns” applies. This situation indicates that additional limiting factors, other than the one being corrected, are present the in a greater degree. This limits the ability of the needed input to further increase yield.

Responses to increasing levels of nutrients need not give plateau curves until the genetic or climatic potential is reached. If all limiting factors are corrected simultaneously, yields should move upward in a straight-line relationship until the maximum potential is reached. A plateau means that other limiting factors still exist. The “Law of Diminishing Returns” can be postponed until “Maximum Possible Yield” is obtained by correcting of all limiting factors simultaneously.

Dollars And Cents
The financial returns for using gypsum are variable and too complex for easy calculation. Some can be quite easily calculated while others are more difficult. The use of gypsum to reclaim sodic soils can give spectacular results. A crop can fail without gypsum the can succeed with gypsum. Sometimes the benefits are in water savings or in less tillage costs or in much less soil erosion. How cost effective the products can be depends very much on the cost of the gypsum and the value of the crop. Most often the majority of the cost of gypsum is for shipping; therefore, location is a variable.

The costs for gypsum, water, machinery, labor and sometimes water- soluble polymers to reclaim or improve an acre of sodic soil can be a minimum of $50 and as much as $300 at current prices. Although an improvement will last for several years, it is very important to use gypsum (perhaps water-soluble polymers also) regularly at a maintenance level to continue the improved soil properties.

The cost of reclamation may be spread over more than one year. The economics can be very satisfactory for fruit and vegetable crops; reclamation can net a few hundred dollars per year per acre. For field crops, an initial cost for massive treatment can be spread over several years. An annual maintenance treatment may add 10 to 20 percent to the gross, but could double or triple the net income.

Even at 10 to 20 percent, yield increases are variable. The better the management, the greater are the pounds, bushels or tons associated with the increase.

In table 1, are some hypothetical approximations for the value of each of the three different application levels for gypsum. They illustrate the principles involved.

Subtle, Intangible, Dificult-To-Measure Benefits From Regular Use Of Gypsum

A. Soil organic matter gradually increases.
A gradual increase in ease of tillability follows. Soil erosion gradually decreases. Water infiltration increases, and this is associated with increased water-use efficiency. If the improvements occur at a rate of one percent per year, there is no way that they can be detected from a year-to year basis, but over 10 to 15 to 20 years, the total effects can be large.

B. Soil is better flocculated and soil aggregates are more stable.
There is less wind (dust) and water erosion where gypsum is used. There is far less crusting on the soil surface so that seedlings emerge more rapidly and crops mature sooner. Less energy is needed for tillage, and values are combined with those of increased soil organic matter.

C. There are more earthworms, especially with no-till soil management where crop residues remain on the soil surface.
The earthworms take the crop-residues underground and digest them to make more soil organic matter. The earthworms then provide the tillage necessary to keep the no-till land cultivated and aerated, even to deep depths. Gypsum is need for and success of and “earthworm plow” (Wallace and Wallace 1994).

The benefits from use of gypsum pyramid from year to year. A cumulative two percent increase per year for 20 years can end up, after 20 years, as a 49 percent increase; the principle of sequential additivity applies.

Isn’t My Soil Already High In Calcium?
Some fertile soils contain sufficient soluble calcium for stabilization with water-solbuble polymers, but many soils do not.
Lots of rain water or irrigation with low-solute water can depress levels of calcium and other ions in solution to where soil aggregates become unstable so that soils crust and harden. Gypsum additions, even at 100 to 200 pounds per acre, are useful in these situations.

Many, if not most, western soils have from 2 to 50 percent of limestone in them; limestone is 40 percent calcium. Many growers have the erroneous idea that just because their soil contains considerale calcium, calcium is not needed. When the soil pH is over 8, there isvery little solution calcium. Soils with pH over 8 do not respond to water-soluble polymers unless gypsum is also applied in liberal quantities. The calcium in limestone can be activated in soil when acidifying amendments are applied to and soil or when legumes that biologically fix nitrogen are grown (Wallace and Wallace 1992). These have and same value as gypsum.