Tropical soils with moderate to high rainfall are likely subject to high leaching and surface runoff. Crusting of the soil surface is obvious everywhere in the country. It has been observed in cultivated soil for several years under intensive cropping that the crops have failed to sustain high productive yields due to the unsuitable soil conditions that include chemical, physical and biological properties of soil. Farmers apply chemical fertilizers to increase crop yields with varying results depending on soil structure. The organic manure includes cattle, pig and chicken manure and industrial by-products. Farmers also apply to economical crops the supplies of these materials that are available in limited amounts in Thailand. The practice of liming is used to correct soil acidity in areas of intensive cultivation. The long -term effects of continuous application of chemical fertilizers have been evident to cause poor soil structure and increase soil acidity. This situation will result in poor crop yield and quality. Besides there are a great number of degraded soils in need of improvement and reclamation such as high acidic soils and sodic soils.
     It is interesting to introduce gypsum for soil improvement in the country. It has been used as a soil amendment and fertilizer in advanced countries for over 200 years. Gypsum is calcium sulfate. The most common form of it is dihydrate that means that each molecule of calcium sulfate has two water molecules associated with it. It is expressed as CaSO₄ 2 H₂O. The Plaster of Paris used commercially has only one half water and another form called gypsum anhydrite has no water. Much of the gypsum used in agriculture is mined gypsum some is then pulverized to small particle sizes. The smaller sizes are easier to dissolve in water than are larger sizes.

     Gypsum is also a by-product of chemical fertilizers production, The most common is the phosphate fertilizer industry. This produces phosphoric acid from rock phosphate (apatite) by wet -process acidulation:

Ca10(PO4)6F2(s)+10 H2SO4+20 H2O

10Ca SO4 2H2O(s)+6H3PO4+2HF

     The by-product gypsum accumulates as a waste and may contain fluoride and heavy metals. This fine- grained, high purity material called phospho-gypsum (PG) is produced which becomes a serious disposal problem. By-product gypsum can be obtained from chemical processing and plating industries that produce waste sulfuric acid by neutralization with hydrated lime

Ca (OH)2+ H2SO4

CaSO4.2H2O

     The industrial use of lignite coal as fuel for generating electricity will produce SO₂ gases which causes environmental pollution, this is accomplished by passing the SO₂ gases through a lime slurry, where the SO₂ gas is oxidized to SO₄^2-

2H2O+ CaCO3 (s) + SO2+ 1/2O2

CaSO4 .2H2O(s) +CO2 (g)

     This kind of gypsum is called flue gas desulfurization gypsum (FDG). Use of Flue gas gypsum in agriculture has been banned in the United States. Fortunately, Thailand has plenty of mined gypsum deposited that contains a high percentage of purity, suitable for use in agriculture.      Gypsum is slightly soluble in aqueous solutions, which approximately contains 2.5 g l^-1 or 15 mM. This level of solubility makes a substantial contribution to the ionic strength of most soil solutions, yet it is low enough to allow continued release of salt into solution over a considerable period.
     For many reasons, gypsum can be considered as one of the agricultural materials to be a farmer's sincere friend. Several benefits from its uses on land have been published. Some of the reasons are multiple and interrelated.

1. Gypsum helps reclaim sodic soils
Gypsum is used in the reclamation of sodic soils where the exchangeable sodium percentage (ESP) of sodic soils is too high. It must be decreased for soil improvement and better crop growth. The most economical way is to add gypsum, which supplies calcium to the water supply. The calcium replaces the sodium held on the clay-binding sites. The sodium can then be leached from the soil as sodium sulfate. The sulfate is the residue from the gypsum. Without gypsum, the soil would not be leachable.

2Na-Clay + CaSO4

Ca-Clay + Na2SO4

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:



Remarks:
To convert lb ac^-1 to kg ha^-1 multiply by 1.12
To convert lb ac^-1 to kg rai^-1 multiply by 0.18



Where Na⁺, Ca²⁺, Mg²⁺ refer to the concentrations of the designated soluble cat-ions expressed in me/l^-1 (US Salinity Laboratory staff 1954)

2. Gypsum as an ameliorant in acid soils
     The use of lime has proved to correct acidity in soils surface but the high costs of incorporation and the fact that it does not move readily down the profile have made it unsuitable for subsoil acidity amelioration. Following the pioneering work of Sumner (1970) and Reeve and Sumner (1972) who first demonstrated the feasibility of using gypsum to counter effects of subsoil acidity, many cases in which substantial yield response (10-100%) in a variety of crops (corn, sugarcane, coffee, wheat, alfalfa, soybean, barley) were obtained have been reported (Sumner, 1993,1994) as presented in Table 1

Table 1. Yield responses of various crops to topsoil incorporated gypsum

Source : Guy J.Levy and Malcolm E Sumner (1998)
In most cases, the responses, which only appears after the surface incorporated gypsum has been leached into the subsoil, arise as a result of increased pH and Ca availability and reduce Al toxicity as presented in Fig.1

Figure 1. Effect of gypsum (6 Mg ha^-1 ) on soil pH and Al saturation in a Red Yellow Latosol (Xanthic Halpudox) profile from Brazil, (Souza and Ritchey, 1986)
     The improved environment allows roots to penetrate to the subsoil. The surface applied gypsum has had profound effect on root distribution in all crops. In all cases, exchangeable Ca increased and exchangeable Al decreased in the subsoil following gypsum application. As a result of this, root proliferation into acid subsoil, the crops could extract water previously out of the reach of the roots which results into improved yields when water is limiting (Table2). The gypsum effect is more marked under more severe drought conditions.
     The most likely mechanism responsible for this effect is the so called "self-liming-effect" originally proposed by Reeve and Sumner (1972) as illustrated in the following reactions:

2Al3+ + 3Ca(OH)2		2Al(OH)3 + 3Ca2+

     This requires the presence of Al and Fe oxyhydroxides from which the SO₄^2- can liberate OH^- by ligand exchange that in term neutralizes exchangeable Al³⁺. Other possibility include the precipitation of aluminum hydroxysulfates (Adam and Rawajfih; 1977), ion pair (AlSO₄⁺) formation (Pavan et al., 1982, 1984) or coabsorption of SO₄ and Al³⁺ (Sumner et al., 1986)

Table 2 Effect of gypsum application on the distribution of roots of various crops down the profile of highly weathered soils

Source : Sumner,1993.

3. Gypsum can enhance in the liming program
     When lime is added of the soil, the immediate effect is an increase in the hydroxyl ions, due to hydrolysis:

CaCO3 + 2H2O

Ca(OH)2 + H2CO3

The alkaline system causes temporary dispersion of the soil. To avoid the dispersion, it is important to repress the concentration of the OH ions. Additions of gypsum 100 kg for every 500 kg of lime (CaCO₃) accomplish this task successfully (Joffe 1953) as represented by the following equations:

2H-soil + CaSO4		Ca-soil + H2SO4
Ca(OH)2 + H2SO4		CaSO4.2H2O

It should be clear that the pH will be temporarily lowered by the additions of gypsum to the soils of the humid regions. This practice facilitates deeper penetration of calcium (Ca) and more efficient replacement of H ions

4. Gypsum improves soil structure
     Gypsum provides calcium, which is needed to flocculate clays in acid and alkaline soils (Shainberg et al. 1989; Sumner 1993, Sumner and Miller 1992). It is the process in which many individual small clay particles are bound together to give much fewer but larger particles, Such flocculation is needed to give favorable soil structure for root growth and air and water movement.

5. Gypsum prevents crusting of soil and aids seed emergence
     Gypsum can decrease and prevent the crust formation on soil surfaces that result from raindrops or from sprinkler irrigation on unstable soil (Shainberg et al. 1989; Sumner and miller 1992). It can prevent crusting that results when acid soils are limed (Sumner 1993); the gypsum is co-applied with the lime. The gypsum is either surface applied or put in the irrigation system. Prevention of crust formation means more seed emergence, more rapid seed emergence, and easily a few days sooner to harvest and market. Seed emergence has been in creased often by 50 - 100 percent. The prevention of crusting in dispersive soils is a flocculation reaction.

6. Gypsum improves compacted soil
     Gypsum can help break up compacted soil (Shainberg et al. 1989) and decrease penetrometer resistance (Hall et al. 1994). The compact in many soils can be decreased with gypsum, especially when combined with deep tillage to break up the compaction.

7. Gypsum makes slightly wet soils easier to till
     Soils that have been treated with gypsum have a wider range of soil moisture levels where it is safe to till without danger or compaction or deflocculation (Shainberg et al. 1989). This is accompanied whit greater ease of tillage and more effective seedbed preparation and weed control. Less energy is needed for the tillage.

8. Gypsum stops water runoff and erosion
     Gypsum improves water infiltration rates into the soils and the hydraulic conductivity of the soil (Shainberg et al. 1989). It aids in protection against excess water runoff from especially large storms that are accompanied with erosion.

9. Gypsum prevents waterlogging of the soil
     Gypsum improves the ability of soil to drain and not become waterlogged due excess water (Aldrich and Schoonover 1951). Improvements of infiltration rate and hydraulic conductivity with use of gypsum add to the ability of soils to have adequate drainage.

10. Gypsum increase the stability of soil organic matter
     Gypsum is a source of calcium that is a major mechanism that binds soil organic matter to clay in soil which gives stability to soil aggregates (Muneer and Oades. 1989). The value of organic matter applied to soil is increased when it is applied with gypsum.

11. Gypsum improves water use efficiency
     Gypsum increases water-use efficiency of crops. In areas and times of drought, this is extremely important. Improved water infiltration rates (Wildman et al. 1988) improved hydraulic conductivity of soil, better water storage in the soil all lead to deeper rooting and better water-use efficiency (Shainberg et al. 1989, Hall et al. 1994). From 25 to 100 percent more water is available in gypsum-treated soils than in non treated soils.

12. Gypsum decreases heavy-metal toxicity
     Calcium also acts as a regulator of the balance of particularly the micro-nutrients, such as iron, zinc, manganese and copper in plants (Alva et al. 1993, and Wallace et al 1980). It also regulates non-essential trace elements. Calcium prevents excess uptake of many of them; and once they are in the plant, Calcium in liberal quantities helps to maintain a healthy balance of nutrients and non-nutrients within plants.

13. Calcium in gypsum is also a plant nutrient
     Calcium in gypsum is an important plant nutrient. It needs to be supplied to the plant continuously and is the nutrient that aids in the correction of imbalances of all the other nutrients. Reduces the effects of non-essential elements that may be present at toxic levels. Fruit and Seeds are often deficient in calcium (Wallace 1995)

14. Gypsum improves fruit qualities and prevents some plant diseases
     Calcium is nearly always only marginally sufficient and often deficient in developing fruits (Shear 1979). Good fruit quality requires an adequate amount of calcium. Calcium moves very slowly, from one plant part to another and fruits at the end of transport system become deficient. Calcium must be constantly available to the roots. In very high pH soils, calcium is not available in proper quantities. Gypsum helps prevent blossom-end rot of watermelon (Scott et al. 1993) and tomatoes. Gypsum is preferred over lime for potatoes grown in acid soil to control scab disease. Calcium may prevent fruit cracking and immature fruits falling. Root and stem rot in fruit trees are partially controlled by gypsum.

15. Gypsum as a source of sulfur
     Gypsum is a source of fertilizer sulfur. Tropical soils are low organic matter, which is a source of sulfur. Gypsum application will correct sulfur deficiency.

16. Gypsum helps earthworms to flourish
     A continuous supply of calcium with organic is essential to earthworms that improve soil aeration, improve soil aggregate and mix the soil (Sinnes 1979)

17. Gypsum can increase crop yields
Gypsum for various combinations of the above affects can substantially increase crop yields from 10 to 50 percent. (Table 1)

Use of gypsum for agriculture in Thailand
With the limited sources of information available in Thailand, the application of gypsum at farm levels has been limited. From the worldwide information it is believed that the effect of gypsum for the soil and crop improvements will sustain long-term the agricultural development in Thailand.

The quality grade and recommendation rate of gypsum used for soil and crop improvements.
Gypsum that is recommended commercially for soil and crop improvements are mined gypsum. Its specifications are in following:

Recommendation rates of gypsum, timing and application methods

Conclusion
In conclusion, tropical soils with moderate to high rainfall are subject to high leaching of plant nutrients and mismanagement practices. The arable lands are degraded largely in the country. These situations cause crusting of soil surface, water and air are not penetrated to lower depth of soils. Increasing of soil acidity and sodicity cause leaching loss of calcium ion as well as other cations from the plow depth of soils. The research carried out in foreign countries has revealed that gypsum is a product that has several properties to improve the degraded soils into productive soils. It is interesting to introduce Green Cal as a commercial product of mined gypsum in the country. It is manufactured and distributed by DKT Company Ltd. It is assured that the long-term effect of continuous uses of Green Cal will be one of the solutions to sustain soil productivity.

References