Sodium Soils in Mississippi

D.E. Pettry
Professor and Soil Scientist
Plant and Soil Sciences Department
Mississippi State University


R.E. Switzer
Senior Research Assistant
Plant and Soil Sciences Department
Mississippi State University


For more information, contact Dr. Pettry: telephone - (662) 325-2770; e-mail - dpettry@agronomy.msstate.edu. This bulletin was published by the Office of Agricultural Communications; Division of Agriculture, Forestry , and Veterinary Medicine; Mississippi State University. It was edited and designed by Robert A. Hearn, publications editor. The cover and figures were designed by George H. Taylor, graphic artist and chief illustrator.

Introduction

      The occurrence of salt-impacted soils in humid, temperate Mississippi appears to be an enigma since ample precipitation (1,200 to 1,300 mm) would be presumed to leach salts from soil profiles. No other state in the continental United States, except Louisiana, receives as much annual precipitation per square mile of land area as Mississippi (Wax and Walker, 1986). Despite the extensive precipitation, evaporation exceeds precipitation 7 months of the year in several areas of the state, resulting in a small precipitation surplus (Figure 1). Sodium (Na) soils and salt toxicities are typically associated with arid regions of restricted precipitation and high evaporation. The sodium soils in arid regions are commonly referred to as evaporites or alka li soils.

      Salt-impacted soils and their drastic effects on crops have been increasingly recognized in Mississippi during the past few decades as pastures and forests were put into row crops. Bare soil areas in agricultural fields essentially devoid of vegetation, or containing stunted and dead plants, have been referred to as slick-spots, salt-spots, and deer-licks. The areas range in size from less than an acre to entire fields. The exposed surface soil commonly has a light-colo red, bleached appearance and is vulnerable to accelerated surface erosion. Salt efflorescence may be evident in the surface crust during dry periods. Salt-impacted areas are not as evident in forests, but the impact can be seen in areas containing stunted trees and a common vegetative understory of palmetto.

      Smith (1936) documented the occurrence of sodium soils in humid regions of Illinois receiving 40 inches of annual precipitation. The soils were located on the Illinoian glacial till plain with a loess mantle less than 1 00 inches thick. The slick-spots were small inclusions in non-sodium soils that occurred at the head of small drainages and along the gentler slopes into these drainages. Smith (1937) reported that large flat areas contained irregularly spaced slick-spots with no apparent topographic difference. Salt incrustations were most common where erosion had exposed alkaline subsoil (B) horizons. Later research by Fehrenbacher et al. (1963) on the slick-spots in Illinois indicated they were loess derived, high in e xchangeable sodium, and varied from less than 1 acre to more than 100 acres in size. Wilding et al. (1963) reported the sodium in these soils originated from in situ weathering of Na-rich feldspars of the parent loess.

      Horn et al. (1964) reported that three types of sodium soils occurred in eastern Arkansas, where they developed in loess and backwater alluvial deposits. One type of sodium soil had a sodic horizon within 10 inches dept h. Another type had acidic upper horizons and moderate depth (24 inches) to a sodic horizon. The third type included sodic horizons and acid sola. Primary minerals in the parent loess were the sources of Na and Mg (Horn et al., 1964).

      Pettry et al. (1981) documented the occurrence of sodium soils (Natraqualfs) in the coastal plain regions of Mississippi. The soils were underlain by contrasting geologic formations, and they developed on poorly drained , silty, fluvial terraces. The soil textural classes were fine-silty or fine-loamy, and soil reaction was extremely acid to strongly alkaline. The acid Natraqualfs tended to occur on older geomorphic surfaces and appeared to have incurred greater weatheri ng than neutral-to-alkaline soils. Barite (BaSO₄) was identified as a common mineral in the subsurface natric horizons of acid soils.

      "Natric horizon" is a relatively new term used to designate soils containing elevated sodium levels. Such soils were formerly called solonetz, solonchalk, and alkali. The natric horizon is defined (Soil Survey Staff, 19 96) as a special kind of argillic horizon. In addition to the properties of the argillic horizon it has the following:

1. Either
   1. Columns, or less commonly prisms in some (usually the upper part), which may break to blocks; or
   2. Both blocky structure and eluvial materials, which contain uncoated silt or sand grains, and extend more than 2.5 cm into the horizon; and
2. Either
   1. An exchangeable sodium percentage (ESP) of 15% or more (or a sodium adsorption ratio [SAR] of 13 or more) in one or more subhorizons within 40 cm of its upper boundary; or
   2. More exchangeable magnesium plus sodium than calcium plus exchange acidity (at pH 8.2) in one or more subhorizons within 40 cm of its upper boundary, if the ESP is 15 or more (or the SAR is 13 or more) in one or more horizons within 200 cm of the mine ral soil surface.

      Soil surveys published before the adoption of Soil Taxonomy (U.S. Soil Classification System) in 1965 did not recognize natric horizons in Mississippi. Natric soils are recognized and delineated in soil surveys published since 1965, and they include the Bonn, Deerford, Peoria, Rosella, and Talla soil series. Natric soils have been detected in many counties that have pre-1965 soil surveys. The salt-impacted areas are commonly detected because of crop failure or drastically reduced plant stands and yields. The sodium-impacted soils may comprise entire fields or appear as small circular areas (saline-spots). The natric soils have been detected in a number of soil mapping units including: Ariel, Calhoun, Calloway, Falaya, G uyton, Henry, Mashulaville, Myatt, Rosebloom, Stough, Quitman, Trebloc, Vimville, and Wanilla.

Methods and Materials

      Study sites were selected during field mapping operations and at the request of landowners experiencing severe crop damage. Representative pedons were described and sampled in excavated pits and via soil auger using sta ndard methods (Soil Survey Staff, 1995). At selected sites, undisturbed core samples were taken for determination of saturated hydraulic conductivity, bulk density, and moisture retention properties. Soil samples were air-dried and sieved to collect the f ine-earth fraction (<2mm).

      Particle size analysis was determined by the hydrometer method (Day, 1965). Saturated hydraulic conductivity was determined by the constant head method (Klute, 1965). Moisture retention properties were determined using pressure membrane techniques (Richards, 1949). Bulk density was determined using the core technique (Blake, 1965). Soil penetration resistance was measured with a Proctor Penetrometer and a Delphi recording penetrometer.

      Extractable bases were determined by NH₄OAC (ammonium acetate) extraction and atomic absorption spectrophotometry (Chapman, 1965a). Extractable acidity was determined by the barium chloride-triethanolamine method (Peech, 1965). Exchangeable aluminum was dete rmined in KCl extractions following the procedure of Yuan (1959). Organic matter was determined by acid dichromate digestion (Peech et al., 1947). Electrical conductivity was determined on saturated soil paste extract (1:1 soil:water) after 12 hours equil ibration using a conductance meter (YSI Model 35). Soil pH was measured in 1:1 soil:water suspension.

      Sodium adsorption ratio (SAR) was determined on saturated soil:water extracts equilibrated 12 hours. Calcium (Ca), magnesium (Mg) and sodium (Na) levels were measured using atomic absorption spectrophotometry (Soil Surv ey Staff, 1995). The SAR is calculated as follows:

The exchangeable sodium percentage (ESP) is calculated as follows:

Results and Discussion

Landscape Position

      The natric soils occur in loamy sediments of high-silt content on level fluvial terraces and large interfluves of rivers, streams, and tributaries flowing from the north. They also occur on benches and terraces at the b ase of loessial bluffs (Figure 2). Sodium soils occur over a broad range of elevations throughout the state, and geomorphic position is a better indicator than elevation. The soils commonly have linear and concave slopes with gradients ranging f rom 0% to 2%. The concave basins often lack adequate surface drainage and have a circular or ellipsoidal shape.

      Soils with sodium levels detrimental to crops have been detected in fluvial sediments in the Coastal Plains, Blackland Prairie, Delta, Interior Flatwoods, and Loessial regions of the state. The natric soils overlie vari ous geologic formations ranging in age from cretaceous to holocene. Research indicated that sodium is related to the silty fluvial sediments rather than the underlying formations (Pettry et al., 1981). Acid natric soils are on older, higher geomorphic sur faces, and they may have been exposed to greater weathering than neutral and alkaline soils.

Vegetation Relationships

      Vegetation studies (Furst, 1985) of a forested Rosella soil with subsoil natric horizons (fine-loamy, siliceous, thermic Albic Glossic Natraqualf) in the coastal plain region of Lowndes County identified 115 plant speci es (Table 1). Most of the species were dicotyledoneae with considerable monocotyledoneae and bryophta. The natric soil plant community was an oak-dominated (Quercus spp.) forest with secondary red maple (A. rubrum) and bla ck gum (N. sylvatica). The understory was dominantly palmetto (Sabal minor), with substantial ground cover of spagnum sp.

      In forested soils with natric horizons deep in the subsoil, there do not appear to be indicator plants for the sodium except for palmetto in certain instances. Palmetto understory is common in the southern part of the s tate over a broad range of soils, but its presence in northern Mississippi may indicate sodium soils. The lack of plant indicators of sodium in forested areas presents a challenge to recognize and delineate natric soils during mapping operations.

Morphology of Natric Soils

      The natric soils have thin surface horizons (ochric epipedons) with silt loam, silt, or loam textures. Lighter-colored eluvial (E) or transitional EB or B and E horizons underlie the surface and grade into the natric ho rizon (Btng), which commonly has gray colors in hues of 10YR and 2.5Y. Brighter-colored mottles (masses) in hues of 10YR and 7.5YR occur in the Btng and underlying C horizons. Seams (tongues) of lighter-colored, leached E horizon commonly extend into the upper natric horizon. Ped exteriors commonly have gray silt or very fine sand coatings that have been stripped of clay. Ped interior faces have thick argillans and fine tubular pores lined with argillans. Discontinuous black and dark gray laminated bands of clay and silt may coat ped faces.

      Pockets of barite (BaSO₄) commonly occur as fine white (10YR 8/1) powder in pores and channels throughout acid natric horizons. Previous research (Pettry et al., 1981) reported authigenic barite in sodium soils of Mississippi. Black ferro- manganese concretions commonly occur in the lower part of the natric horizons.

      The natric horizons have physical properties similar to fragipans. Natric horizons typically have compound structure consisting of prismatic parting to subangular blocky structure. The Btng horizons are dense with firm consistency and brittleness similar to fragipans. The horizons have very low hydraulic conductivity and tend to perch water during wet periods. The natric horizons are often considerably drier than overlying horizons. Most natric soils are somewhat poorly to poorly drained, with slow to moderate permeability.

Physical and Chemical Properties

      A summary of selected physical and chemical properties of 30 representative soil natric horizons in 12 counties located in loessial and coastal plain regions of the state is presented in Table 2. C haracterization data are presented in the Appendix. The mean depth of the natric horizons (Btn) was 29.4 inches and ranged from 6 to 50 inches. The mean texture was silt loam with silt contents ranging from 34% to 81%. Soil pH levels ranged from 4.1 (extr emely acid) to 8.7 (strongly alkaline) with a mean value of 5.89 (medium acid). Mean organic matter content was less than 0.2%. Sodium was the dominant extractable cation, with Na>Ca>Mg>K. The natric horizons exhibited wide variation in base satu ration with a mean value of 71%. Exchangeable sodium percentage (ESP) ranged from 15% to 65%, with a mean of 32%. The mean ESP is twice as high as the minimum level (15%) necessary for designation as the natric horizon. The ESP was positively correlated w ith pH and Na and negatively correlated with acidity and base saturation (Table 3).

Salt Spots

      Investigations of representative salt spots negatively impacting cotton and soybean fields in several counties revealed high exchangeable sodium percentages (ESP) in the plant root zone (Table 4) and natric horizons. The areas ranged from 0.1 to 5 acres in size, and spots were randomly scattered across the cultivated fields. The cotton and soybean plants in the salt-impacted areas were stunted and/or dead, represent ing a terminal impact. Light-colored bare soil comprised a large proportion of the salt spots. A sharp boundary existed between the spots and surrounding normal plants. Normal plants adjacent to the impacted areas presented a sharp visual contrast to the salt-impacted plants.

      The salt spot in Noxubee County (Table 4) had a low ESP (5%) in the 0- to 6-inch surface horizon (Ap) of leached, siliceous sandy loam, which was abruptly underlain at 6 inches by an E/B horizon with very high ESP (43.7%). Surface horizons in the other impacted sit es had silt loam textures and high ESP values ranging from 20% to 44%, which are commonly toxic to crops. The sites were sampled in mid-summer when soil moisture levels were low. Apparently, wetter conditions during the planting and early season diluted s alt levels, permitting germination and stunted growth. Soil pH levels in the salt spots ranged from 5.2 (strongly acid) to 8.7 (strongly alkaline) in the surface horizons, and from 4.9 (very strongly acid) to 8.7 (strongly alkaline) at depths of 6 to 12 i nches. Extractable Ca/Mg ratios ranged from 1.7 to 9.7 in the soils with high pH levels, which is in sharp contrast to ratios of 0.7 in the very strongly acid (pH 4.9) BE horizon at Attala County.

Salt Spot Characterization

      A representative salt spot exhibiting terminal impact to cotton was investigated and characterized in September 1996 in eastern Calhoun County. Other salt spots investigated in the state have very similar properties and characteristics. The affected area was about 2 acres, and it occurred in a slightly depressed basin of a terrace landscape. The slope gradient was 0% to 1%, and the soil mapping unit was a Falaya silt loam. Soil mapping in Calhoun County was completed in 1962 before adoption of Soil Taxonomy and recognition of natric soils in the state.

      The impacted area was bare of living vegetation, and the exposed soil surface had light yellowish brown and gray colors. A pronounced surface crust contained many vesicular (terminal) pores. Examination of the cotton ro ws in the affected area revealed that about 40% to 50% of the planted seeds had germinated, and the plants grew 5 to 6 inches before dying. The landowner reported the salty spot seemed to become larger each year. Healthy cotton plants adjacent to the affe cted area were 36 inches tall and presented a stark contrast to the bare area.

      The soil in the salt spot had an ochric epipedon underlain by a weakly developed argillic (natric) horizon evidenced by increased clay and patchy clay skins on ped faces (Table 5). The pedon had co mpound structure in the subsoil with prismatic parting to subangular blocky structure. The A2 and upper natric horizons (Btn) were firm and brittle in 20% to 35% of the volume. Clay contents ranged from 4.5% in the surface to 28.5% at 46 inches. Silt cont ents exceeded 50% throughout the pedon. Sand contents ranged from 21% to 34%, with fine sand the dominant fraction (Table 6).

      Soil bulk density was highest in the natric horizons at depths of 12 to 31 inches (Table 7). Values exceeding 1.6 g/cc in the natric horizon are comparable to bulk densities in fragipan horizons. < p>       Soil penetration resistance was similar to fragipan horizons, with maximum values in the upper natric horizon (Figure 3). Saturated hydraulic conductivity was lowest (0.04 inch per hour) in the upper natric horizon, indicating it would perch water during wet periods. The natric horizons also had the lowest available water contents (0.03-1.5 megapascal [Mpa]) as shown in Table 8.

      Organic matter was dispersed, with contents ranging from 0.79% in the surface, to 0.41% in the upper natric horizon, and 0.11% at 46 inches depth (Table 9). Soil reaction ranged from moderately aci d in the surface to extremely acid in the natric horizons. Exchangeable Na exceeded Ca, Mg, and K in the pedon. Exchangeable Al increased in the subsoil to levels potentially toxic to common crops. Aluminum saturation exceeded 30% in the natric horizon. B ase saturation exceeded 35% throughout the pedon.

      Exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR), and electrical conductivity (EC) values were extremely high in the surface horizon (Table 10). The elevated salinity levels are toxic to cotton and other common crops. The ESP decreased with increased depth, but values exceeded 15% (criteria for natric horizon) to 60 inches depth. The high soluble Na levels in the surface horizon suggests concentration by evaporation and capillar y rise with no surface removal.

      Sodium levels varied across the bare salt spot but remained at toxic levels. The ESP abruptly decreased at the outer boundary to levels less than 10% in the bounding area (Figure 4). Healthy cotton plants reflected the lower ESP levels. Detailed characterization of the soil containing the dead plants indicated toxic Na levels in the upper 12-inch root zone (Figure 5). Apparently, higher soil moisture at spring planting diluted Na to levels where some seeds could germinate and plants emerge. However, few plants exceeded 6 inches in height, indicating decreased soil moisture a nd increased Na concentration to toxic levels.

Crop Impacts

      Natric soils with elevated Na levels in the root zone have a detrimental impact on common row crops. Crops have different sensitivities to elevated salt contents. Generally, cotton and bermudagrass are more tolerant tha n corn and soybeans. Varieties of a crop also differ in their tolerance of salts. Bresler et al. (1982) proposed that the adverse effects of salts can be divided into three categories: osmotic or total salt effects; specific-ion effects; and the secondary specific-ion effect of Na. Decreasing osmotic potential reduces the availability of water to plants (Bernstein, 1975). Plants impacted by high salt levels may appear drought-stricken at water contents above the permanent-wilting point. Plants affected by high salinity are generally stunted, with smaller, thicker, and darker green leaves than normal plants (Bresler et al., 1982).

      Seasonal changes in soil moisture affect the ability of crops to tolerate high salt levels. Greater soil moisture at spring planting and early summer may maintain salt levels low enough for plant growth. However, drier conditions in mid-summer may concentrate salt levels to a point at which they become toxic to plants. The seeds of a variety may successfully germinate, and plants may reach a few inches in height before succumbing to sodium toxicity. Plants stressed from high sodium levels often exhibit various symptoms of nutritional disorders due to competitive uptake of ions.

Soil Reclamation

      Natric soils commonly have poor physical conditions due to dispersive properties associated with sodium (U.S. Salinity Lab Staff, 1954) that negatively impact reclamation. The poor physical conditions reduce permeabilit y to air and water movement. These adverse properties give rise to the observable physical characteristic of the soil being sticky when wet and hard when dry (U.S. Salinity Lab Staff, 1954). Early reclamation of sodic soils used ponded water to leach salt s below the root zone in arid regions (Bresler et al., 1982). These practices often required long periods of time, and they were not always successful because of low soil permeability, which prevented flushing the salts.

      Sodium must be displaced on the soil exchange complex and leached from the soil to remediate the Na-toxicity and improve physical conditions. The critical soil toxicity level is about 10% of the cation exchange capacity (Evangelou and Marsi, 1990). When the sodium levels are high enough to exceed about 10% of the cation exchange capacity, particle dispersion results, and water infiltration and gas movement are greatly reduced. Calcium is commonly used to displace Na⁺ from the exchange complex.

      The soil cation exchange capacity (CEC) and exchangeable sodium percentage (ESP) are commonly used to determine how much Na⁺ must be displaced (exchanged) by Ca²⁺ in reclamation. A silt loam soil with a CEC of 10 cmol kg^-1 could retain about 1 cmol kg^-1 Na (10% ESP) before reaching its critical toxicity level. One cmol Na⁺ per kg of soil represents 460 pounds of Na per acre 6 inches deep (acre-furrow slice). One cmol Ca²⁺ per kg of soil equals 400 pounds per acre 6 inches deep. In contrast, a clayey soil with a CEC of 30 cmol kg-1 could retain about 3 cmol kg^-1 of Na⁺ or 1,380 pounds per acre before exceeding the critical toxicity level (10%). Clay soils have higher exchange capacities, retain more water, and require more Ca²⁺ to displace the Na⁺. Productive agricultural soils in Mississippi typically have ESP less than 2%.

      Gypsum (CaSO₄ ·2H₂O) is commonly used for reclamation of sodic soils because of its efficiency and cost. The amount of gypsum needed to displace sodium depends upon the soil texture, cation exchange capacity, and sodium content. A relative guide for the amounts of gypsum needed to replace exchangeable Na⁺ with Ca²⁺ is shown in Table 11.

      Since not all of the replacement of exchangeable sodium occurs to the soil depth on which the application is based, it is suggested the gypsum rates be multiplied by 1.25 to compensate for incomplete replacement (Soil S alinity Lab Staff, 1954). Large quantities of straw, wood chips, or other coarse organic matter incorporated in the soil at or before gypsum application increase replacement efficiency. The organic matter improves soil structure and drainage, and it incre ases cation exchange capacity and water-holding capacity. Generally, gypsum is used when soil pH values are above 6, and lime (fine-grade CaCO₃) may be used to reclaim acidic sodium soils.

      Repeated deep plowing to break up the restricting layers and permit exposure and leaching of the sodium may be effective where sodium is concentrated in the soil surface. Rasmussen et al. (1972) reported deep plowing ca lcareous saline-soils ameliorated the soil to 36 inches depth up to 4 years. In soils with greater depth to sodium-impacted layers, hipping or bedding with incorporated organic matter and gypsum or lime may provide a deeper root zone free of toxic sodium levels. Surface mulches effectively deter upward capillary water movement and salt accumulation at the soil surface.

      Proper arrangement of plant rows and surface field ditches are needed to remove surface water. Ditches and/or subsurface drains may be needed to drain depressions and swales to remove salts. The salts tend to concentrat e in low areas lacking outlets for water movement and flushing action.

      It is important not only to recognize salinity stress and toxicity, but also to determine where the sodium problem is located in the soil profile. Maintaining effective rooting depth and a buffer zone above subsurface n atric horizons are critical for optimum agricultural production. Accelerated soil erosion and land leveling can truncate the surface soil and expose the underlying natric horizons with disastrous consequences. Thick natric horizons exposed in the root zon e may have a terminal impact on many row crops.

References Cited

Bernstein, L. 1975. Effects of salinity and sodicity on plant growth. In K.F. Baker (ed.) Annual Review of phytopathology, Vol. 13:295-312.

Blake, G.R. 1965. Bulk density. In C.A. Black (ed.) Methods of soil analysis, Part 1. Agron. 9:374-390. Am. Soc. of Agron., Madison, WI.

Bresler, E., B.L. McNeal, and D.L. Carter. 1982. Saline and sodic soils. Advanced series in agricultural sciences; 10. Springer Verlag Press. pp. 166-198.

Chapman, H.D. 1965. Total exchangeable bases. In C.A. Black (ed.) Methods of soil analysis, part 2. Agron. 9:901-904. Amer. Soc. of Agron., Madison, WI.

Day, P.R. 1965. Particle fractionation and particle size analysis. In C.A. Black (ed.) Methods of soil analysis, Part I. Agron. 9:562-566. Amer. Soc. of Agron., Madison, WI.

Evangelou, V.P. and M. Marsi. 1990. The chemistry and management of oil well brines discharged in soil-water environments. Agricultural Experiment Station Bulletin 773. Univ. of Kentucky, Lexington, KY. 24 pp.

Fehrenbacher, J.B., L.P. Wilding, R.T. Odell, and S.W. Mested. 1963. Characteristics of solonetzic soils in Illinois. Soil Sci. Soc. Am. Proc. 17:432-438.

Furst, T.H. 1985. Genesis, plant growth, and floristic composition of natric soils in Mississippi. MS Thesis. Miss. State Univ. pp. 183.

Horn, M.E., E.M. Rutledge, H.C. Dean, and M. Lawson. 1964. Classification and genesis of some solonetz (sodic) soils in eastern Arkansas. Soil Sci. Soc. Am. Proc. 28:688-692.

Klute, A. 1965. Laboratory measurement of hydraulic conductivity of saturated soil. In C.A. Black (ed.). Methods of soil analysis, Part I. Agron. 9:214-215. Amer. Soc. of Agron., Madison, WI.

Peech, M. 1965. Exchange acidity:Barium chloride-triethanolamine method. In C.A. Black (ed.). Methods of soil analysis, Part 2. Agron. 9:910-911. Amer. Soc. of Agron., Madison, WI.

Peech, L.T.A., L.A. Dean, and J.F. Reed. 1947. Methods of soil analysis for soil fertility investigations. U.S. Dept. of Agric. C. 757. 25pp.

Pettry, D.E., F.V. Brent, V.E. Nash, and W.M. Koos. 1981. Properties of Natraqualfs in the Upper Coastal Plain of Mississippi. Soil. Sci. Soc. Am. J. 45:587-592.

Rasmussen, W.W., D.P. Moore, and L.A. Alban. 1972. Improvement of a solonetzic (slickspot) soil by deep plowing, subsoiling, and amendments. Soil Sci. Soc. Am. Proc. 36:137-142.

Richards, L.A. 1949. Methods of measuring soil moisture tension. Soil Sci. 68:95-112.

Smith, G.D. 1936. Intrazonal soils: a study of some Solonetz soils found under humid conditions. Soil Sci. Soc. Am. Proc. 2:461-469.

Soil Survey Staff. 1995. Soil survey laboratory information manual. U.S. Dept. of Agric. Soil Survey Investigations Report No. 45. 305pp.

Soil Survey Staff. 1996. Keys to soil taxonomy, sixth edition. U.S. Dept. of Agric. Washington, DC. 305pp.

U.S. Salinity Laboratory Staff. 1954. Diagnosis and improvements of saline and alkali soils. L.A. Richards (ed.). U.S. Dept. Agric. Handbk. No. 60. U.S. Govt. Printing Office, Washington, DC.

Wax, C.L. and J.C. Walker. 1986. Climatological patterns and probabilities of weekly participation in Mississippi. MAFES Info. Bull. 79. Mississippi State University.150pp.

Yuan, T.L. 1959. Determination of exchangeable aluminum in soils by a titration method. Soil Sci. 88:164-167.

Appendix