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II. Uses of Phosphogypsum in Agriculture

II. Uses of Phosphogypsum in Agriculture

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free, high-analysis fertilizers is seen as the most important reason for the

increasing S deficiency worldwide (Jordan, 1964; Moms, 1986).

Calcium, with concentrations ranging from 0.2 to 1.OYo in plant tissues,

is also essential to plant life. Calcium deficiency manifests in the failure of

terminal buds and apical tips of roots to develop. Also, lack of Ca results in

general breakdown of membrane structures, with resultant loss in retention

of cellular diffusible compounds. Disorders in the storage tissues of fruits

and vegetables frequently indicate Ca deficiency (Tisdale et al., 1985).

The need for Ca by plants may be readily satisfied by liming materials

such as calcitic and dolomitic limestone. However, lime application in

large amounts on certain soils can be detrimental to plant growth. Kamprath (1971), in a review of the effect of lime on Oxisols and Ultisols,

reported that lime application that raised the soil pH to 7 resulted in

reduced rates of water infiltration, reduced availability of P, B, Mn, and

Zn, and reduced growth of sudangrass(Sorghum vulgare var. Sudanese L.),

corn (Zea mays L.), and soybean. Therefore, for certain soils that require

large amounts of Ca to support commercially viable crop yields, or for

crops such as peanut that need large amounts of readily soluble Ca, a Ca

source other than lime may be necessary.

Thus, with increasing S deficiencies worldwide and the need for a Ca

source other than liming materials, phosphogypsum deserves serious consideration for agricultural applications where mined gypsum has traditionally been used. Because phosphogypsum and mined gypsum are chemically similar, studies on mined gypsum are also cited throughout this paper

when they may be of assistance in evaluating phosphogypsum for a particular agricultural application.

The present review highlights a scarcity of studies on phosphogypsum.

Amacher and Miller (1 987), in assessing the use of phosphogypsum in

agriculture, cited only eight papers on its use as a source of S and Ca for

crops, all of which were conducted outside the United States. Shainberg et

al. (1989), in their extensive review of the use of gypsum on soils, cited 10

studies on phosphogypsum published from 1983 to 1986, all of which were

conducted in Brazil, and all pertained to its use in alleviating subsoil

acidity rather than as a source of S or Ca for crops. The scarcity of studies

on phosphogypsum use in agriculture worldwide could be due to the

availability of other sources of S and Ca. In the United States, environmental concerns and government regulations hindered the use of phosphogypsum even for agricultural research (Federal Register, 1989). However, results of on-going environmental studies on phosphogypsum use in

Florida (Rechcigl et al., 1992a) and in other southeastern states (Mullins

and Mitchell, 1990) may help change existing regulations to allow for the

use of phosphogypsum in commercial agriculture.



2. Cereal Crops

The majority of Coastal Plain soils in the southeastern United States

contain relatively small amounts of extractable SO, sulfur in their sandy

surface horizons (Jordan, 1964). On the basis of the results of numerous

field tests of S, Reneau and Hawkins (1980) suggested that corn planted on

Coastal Plain soils that are moderately well drained to well drained, low in

organic matter, and fine loamy to coarser textured, with extractable soil S

of 6-7 kg ha-' in the surface horizon, would probably respond to S

application, therefore also to gypsum. In North Carolina, Rabuffetti and

Kamprath (1977) concluded that the effect of gypsum on corn was highly

dependent on the rate of N applied. At 56 or 112 kg N ha-', gypsum had

no effect on corn yield or N content of grain. At 168 and 224 kg N ha-',

gypsum at 30 and 60 kg S ha-' increased grain yield and N content of

grain. Nitrogen had a greater effect than did gypsum on total S accumulation in grain and stover. In Florida, field trials using 1.68 to 2.24 Mg ha-'

of phosphogypsum increased green corn yields by as much as 10790 in

1986 and by 5590 in 1987 (Hunter, 1989). Raw phosphogypsum was used

in 1986 and pelleted phosphogypsum was used in 1987. Friesen and Chien

(1986), citing studies camed out by the International Fertilizer Development Center ( 1985)in Togo, West Africa, reported that phosphogypsum at

10 to 50 kg S ha-' increased corn grain yields by 44 to 7790 over the

control. In the same experiment, elemental S increased yields by 45 to

5490. In Iraq, 1.28 Mg phosphogypsum ha-' applied to corn growing on a

calcareous soil increased yields by as much as 15090 over the control

(Khalil et a!., 1990). Superphosphate at SO kg P ha-' also increased yields

by as much as the phosphogypsum treatment, but P in phosphogypsum

(0.18%)was insufficient to influence the yield.

Oates and Kamprath (1985) found that gypsum was as effective as

ammonium sulfate as a source of S for winter wheat (Triticum aestivum

L.), Plants responded to gypsum at 22 to 90 kg S ha-' where nonfertilized

plants had S concentrations of 0.6 g kg-' of dry matter and a total N : S

ratio of 21 : 1. No response to gypsum was observed when there was

appreciable root growth in the B horizon that contained an accumulation

of SO, sulfur. Sulfate sulfur leached from the surface horizon tends to

accumulate in the B horizon where large amounts of hydrated oxides of A1

and Fe are present (Beaton et al., 1974). Based on studies on leaching of

SO, sulfur from various sources applied in the fall (Rhue, 1971;Rhue and

Kamprath, 1973), Baird and Kamprath (1980) suggested that improved

efficiency of S uptake by winter wheat should occur on sandy soils when

gypsum is applied as a topdressing in early spring. In Bangladesh, Mazid

(1986) reported that wheat yields from 1042 fertilization trials conducted



from 1981 to 1983 increased by an average of 21% due to gypsum applied

at 20 kg S ha-'.

Results from demonstration trials on the effect of 125 kg gypsum

(16%S) ha-' on rice (0. sativa L.) carried out from 1981 to 1983 in

Bangladesh showed that 97% of 3368 demonstration sites responded to

gypsum (Mazid, 1986).Rice yields in gypsum-treated sites increased by 19

to 4 1% over those from sites with the recommended NPK fertilizer without

gypsum. Crop responses to gypsum occurred mainly in calcareous and

continuously submerged soils and were more profitable in the monsoon

season than in the dry season. Studies in Indonesia indicated that ammonium sulfate, potassium sulfate, elemental S, and gypsum were equally

effective as a source of S for rice (Ismunadji and Zulkamaini, 1978;

Momuat et al., 1983). Chien et al. (1987), in a greenhouse study, demonstrated that response of rice to gypsum was not dependent on the method

of application. Sulfur uptake and grain yields were not different whether

gypsum was broadcast, incorporated, or placed deeply in the soil.

3. Grain Legumes

Peanuts possess a unique nutritional requirement in that supplemental

Ca must be supplied to the "peg," a modified stem that penetrates the soil

surface to form the pod or nut. Numerous experiments (Colwell and

Brady, 1945; Hallock and Garren, 1968; Cox et al., 1976; Aha et al., 1989)

have shown that supplemental Ca applied at flowering improved yield and

quality of large-seeded peanuts. The role of Ca in reducing pod rot incidence in peanuts is also well known (Garren, 1964; Hallock and Garren,

1968; Moore and Wills, 1974; Porter el al., 1975). Walker and Csinos

( 1980)demonstrated that increasing rates of gypsum from 0.56 to 1.68 Mg

ha-' resulted in corresponding reductions in pod rot in five peanut cultivars.

As early as 1945, Colwell and Brady (1945) had established the superiority of gypsum over limestone in supplying the Ca requirements of peanut.

Since then, the peanut-producing belt of the southeastern United States has

used fine-ground (anhydrite)mined gypsum as the principal Ca source for

peanut, broadcast at 0.5 to 1.0 Mg ha-' at first flowering on soils whose

Mehlich I extractable Ca is <560 kg ha-' (Mehlich, 1953). This value is

still the current critical soil test for Ca for the runner-type peanut in

Georgia (Alva et al., 1989).

Sullivan et al. ( 1974) showed that application of dolomitic limestone on

peanuts, based on soil tests, increased soil pH and soil Ca levels but did not

improve seed quality and yield. On the other hand, gypsum at 0.673 Mg

ha-' reduced soil pH and the detrimental effects of K on fruit yield and



quality, improved seed germination, seedling survival and vigor, and increased yield and improved seed quality. Daughtry and Cox ( 1974) found

that three commercial gypsum materials, namely, fine-ground and granular anhydrite gypsum and phosphogypsum, applied at 0.76 Mg CaSO, ha-'

at flowering, produced no difference in the yield of the Florigiant peanut.

Hallock and Allison ( 1 980) used similar commercially formulated fineground (Bagged LP) and granulated (420 LP Bulk) anhydrite gypsum, and

granulated phosphogypsum [Texas Gulf (Tg) Gypsum], as sources of Ca

for Virginia-type peanuts at 0.605 Mg ha-'. After 2 years of testing (1977

and 1978), the results indicated, in general, that granulated phosphogypsum and mined gypsum were as effectiveas fine-ground gypsum for supplemental Ca for peanuts. When fruit matured under very dry conditions,

granulated phosphogypsum and fine-ground mined gypsum were superior

over granulated mined gypsum. Gascho and Alva (1990) used seven gypsum materials, including phosphogypsum, as sources of Ca for Florunner

peanuts. They concluded that no other source of gypsum exceeded phosphogypsum in solubility or in its beneficial effects on peanut grade and

yield when broadcast at 224 kg Ca ha-' at first bloom.

In Brazil, Vitti et al, (1986) found that application of 0.1 Mg ha-' of

phosphogypsum to soybean on an Oxisol and on an Ultisol increased grain

yield by as much as 43 and 37%, respectively. At 0.25 Mg ha-', phosphogypsum increased grain yield of beans (Phaseolus vulgaris L.) by 13%on

an Ultisol and by 54%on an Oxisol. Phcsphogypsum rates used were very

low, so that the positive responses of the crops could be attributed more to

S or Ca as nutrients than to the ameliorative effect of phosphogypsum on

subsoil acidity.

4. Sugarcane

Gypsum applied on sugarcane (Saccharurn oficinarurn L.) as a source of

Ca on Ca-deficient soils in Hawaii increased cane yield as effectively as

limestone and ordinary superphosphates (Ayres, 1962). In Rhodesia, application of 22 or 45 kg S ha-' of rock S, gypsum, and MgSO, to sugarcane

increased yields by 44.3, 50.8, and 44.9 tons ha-', respectively (Gosnell

and Long, 1969). Buselli (1988) found that mined gypsum at 22.4 Mg ha-'

applied to sugarcane on an alligator clay in Louisiana increased stalk

population and yield.

Golden ( 1982) observed that the application of phosphogypsum at 1.12

and 2.24 Mg ha-' to sugarcane in Louisiana resulted in total increases in

cane yields over a 4-year period of 18.26 and 24.64 Mg ha-', and sugar

yields of 1.69 and 2.70 Mg ha-', respectively. Breithaupt (1989), using

both phosphogypsum and fluorogypsum on sugarcane at rates ranging



from 2.24 to 22.40 Mg ha-', reported significant increases in cane and

sugar yields in treated plots over the control in both plant cane and

first-year stubble harvests. Both gypsum by-products were equally effective

in increasing cane and sugar yields.

5 . Fruits and Vegetables

In Florida, phosphogypsum applied to citrus (Citrus sinensis) increased

fruit yields with increasing rates up to 1.12 Mg ha-' on a Myakka soil

(sandy, siliceous, hyperthermic, Aeric Haplaquods). It also increased juice

brix, brix :acid ratio, and Ca content. In Oldsmar soils, phosphogypsum

not only increased juice brix, brix:acid ratio, and Ca content but also

reduced titratable acid (Myhre et al., 1990).

In Brazil, pineapple [Ananas cornosus (L.) Merill, cv. Smooth Cayenne]

fertilized with phosphogypsum in combination with KCl as a substitute for

&SO4 gave fruit yields similar to those fertilized with K2S04. Fruits

fertilized with K2S04, however, had better fruit juice quality than those

fertilized with KC1 alone or in combination with phosphogypsum (Bianco

et al., 1990).

Use of raw phosphogypsum at 1.68 and 2.24 Mg ha-' on various vegetable crops in 1986 in Florida increased the yields of tomatoes (Lycopersicon

esculenturn Mill) by 696, potatoes (Solanurn tuberosurn L.) by 19%, and

watermelons (Citrullus vulgaris) by 49%.Residuals from phosphogypsum

applied in 1986 at 2.24 Mg ha-' also increased the yields of potatoes by

2290 and cataloupes (Cucurnis rnelo) by 42%, with a greater number of

melons weighing 1 .O kg or more each. Pelleted phosphogypsum applied to

the 1987 crop did not increase the yields of potato and bell pepper (Capsicum annuurn). The phosphogypsum pellets remained intact, albeit soft,

indicating only partial dissolution (Hunter, 1989).

6. Forage Crops

Thomas et al. (195 1) demonstrated conclusively that S deficiency limits

nonprotein N utilization in purified diets for ruminants, and that SO,

sulfur as sole source of S can correct the deficiency. Hume and Bird ( 1970)

showed that an intake of 1.9 g S day-' by sheep produced the maximum

protein production in the rumen, and that inorganic SO4sulfur was used as

efficiently as that from cystine for synthesis of protein by rumen microorganisms. Bray and Hemsley (1969) showed that S supplement to the diet

increased both crude fiber digestion and S and N retention by sheep. Moir

et al. (1967 - 1968) demonstrated that narrowing the mean dietary N :S

ratio of a basal ration for sheep from 12 : 1 to 9.5 : 1 increased the mean N



retention from 28.8 to 36.0%. Metabolic studies by Whanger ( 1968- 1969)

supported the findings of Moir et al. (1967 - 1968). Rees et al. (1982) found

that sheep ate substantially more S-fertilized digitgrass forage (Digitaria

pentzii Stent) than forage not S fertilized. Akin and Hogan (1983) indicated that S fertilization did not affect plant anatomy of digitgrass but

enhanced the fiber-digesting capability of the microbial rumen population.

Application of 86 kg S ha-' using (NH&SO, on bahiagrass (Paspalum

notatum Flugge) increased dry matter yield by 25%, crude protein by 1.2

percentage unit, digestibility by 3 to 4 percentage units 30 days after

application, and S content by 100%. (Rechcigl, 1989; Rechcigl et al.,

1989). On a larger scale, studies in Ireland (Murphy et al., 1983) showed

that cattle that grazed on S-fertilized pastures gained up to 29% more

weight than those that grazed on S-deficient fields. Also, for any given daily

liveweight gain, S-treated areas had 21 and 19% greater stock-carrying

capacity during the first year and the second year, respectively, than the

untreated pastures. These studies point not only to the need for S fertilization of forage crops for yield but also to the need to achieve a desirable

range of N :S ratios to assure better quality forage.

In plant protein, the N :S ratio is about 15 : 1 and remains fairly constant. If either S or N is limiting, protein synthesis is restricted, but the

protein already synthesized will have a N :S ratio of about 15 : 1. Excess N

relative to S supply accumulates as NO, nitrogen, amides, and amino

acids. Excess S leads to SO, sulfur accumulation (Stewart and Porter,

1969). Thus the wide variation in N :S ratios.

Sulfur fertilization of forage crops almost invariably results in a reduced

N : S ratio in plant tissue. Lancaster et al. (197 1) reported that application

of S at 40 mg kg-I of soil in the form of Na,SO, reduced the N : S ratio as

follows: from 32 to 9 for orchardgrass (Dactylis glomerutu L.); from 45 to

19 and 72 to 14 for first and second clippings, respectively, of sudangrass;

from 36 to 5 for ryegrass (Lolium multiforum L.); from 27 to 8 for alfalfa

(Medicago sativa L.); and from 33 to 16 for clover (Trifolium repens L.).

On the other hand, results from an 8-year field experiment using bermudagrass [Cynodon dactylon (L.) Pers.] showed that despite S fertilization

excessive N application could result in a forage crop with N: S ratios in

excess of 60 : 1 (Woodhouse, 1969).

In North Carolina, mined gypsum applied annually on coastal bermudagrass at the rates of 28 and 56 kg S ha-' increased forage yields in 7 of 8

years (Woodhouse, 1969). In Louisiana, Eichhorn ei al. (1990) reported

that annual application of 108 kg S ha-', using gypsum, increased bermudagrass hay yield by 16% over a 4-year period, with the highest increase

(2990)occurring in the fourth year. Digestible dry matter also increased by

14.5% over the same period. In Florida, Mitchell and Blue (1989) con-



ducted a 6-year study to evaluate the effect of gypsum applied annually on

Pensacola bahiagrass at 200 and 400 N kg ha-'. They reported that at the

low N rate, gypsum application did not increase dry matter yield until the

fourth year, with maximum yields thereafter predicted at an annual S

application between 27 and 33 kg S ha-l. At the high N rate, 10 kg S ha-I

increased dry matter yield in the second year. By the fifth and sixth years,

maximum dry matter yield was predicted at an annual rate of 40 to 5 1 kg S

ha-'. Results also showed that S fertilization enhanced N recovery. Maximum relative forage yield was obtained at a concentration of 1.6 1 g S kg-'

dry matter. In a l-year study in Oklahoma, application of gypsum at 64 kg

S ha-' reduced the N: S ratio of bermudagrass forage from 1 1.6: 1 to 7.2 : 1

but did not increase yield, N uptake, or improve N efficiency (Westerman

et al., 1983).

To date few studies have been camed out on the use of phosphogypsum

on forage crops. Paulino and Malavolta (1989) used phosphogypsum on

andropogon grass (Andropogon gayanus cv. Planaltina) grown in pots with

top soil taken from a Brazilian Cerrado site. Results showed that phosphogypsum, in the absence of lime, increased regrowth dry matter yields

linearly up to the maximum rate of 120 kg S ha-' used in the study.

Maximum protein content was attained at 63 kg S or 380 kg phosphogypsum hav1. Lime had a significant negative effect on andropogon grass.

Mullins and Mitchell (1990) used phosphogypsum at 11 to 90 kg S ha-' on

wheat cut for forage in Alabama. Average increases in forage yield over a

3-year period ranged from 5.4 to 9.3% for two soil series. Comparison

between mined gypsum and phosphogypsum showed no difference in

forage yield of wheat. Phosphogypsum applied during fall or spring had no

residual effect on yield of millet [Setaria italica (L.) Beauv] or sudangrass

planted for summer forage after the winter wheat crop. In Florida, use of

fresh phosphogypsum as a source of Ca applied at 2.24 to 4.48 ton ha-'

reduced soil pH and forage yield of ryegrass to levels below those of the

control (Rechcigl and Payne, 1989). Fresh phosphogypsum can be very

acidic, with pH only slightly over 2. Phosphogypsum was evaluated as a

source of S and Ca for bahiagrass on a Myakka soil in a 3-year study

(Rechcigl and Alcordo, 1991;Rechcigl et al., 1992a), without and with 1%

dolomite or calcium carbonate needed to bring phosphogypsum pH (1 : 1)

to 5.5. Annual rates of 0.2, 0.4, and 1.0 Mg ha-' were compared to single

phosphogypsum application rates of 2.0 and 4.0 Mg ha-'. Results showed

that phosphogypsum, with or without lime, increased the 2-year total

forage dry matter yields of bahiagrass by as much as 28% at 0.2 to 0.4 Mg

phosphogypsum ha-'. Phosphogypsum, across phosphogypsum rates, with

dolomite gave the highest increase in dry matter yield, with 12% over the









1. Acid Soils and Their Formation

Jackson ( 1963) grouped soil acidity according to the dominant protondonor constituents in the soil. The most important of these are the ( I ) free

mineral acids such as H,SO,, (2) organic acids, and (3) active A1 and Fe.

The corresponding examples of a soil dominated by each of these proton

donors are (1) the sulfate soils of Thailand (Parkpian et al., 1991), (2) the

virgin Spodosols of Florida (Carlisle and Fiskell, 1962; Pettry et al., 1969,

and (3) the Oxisols and Ultisols of the tropics and subtropics (Kamprath,

1970, 1971). It is with the last group of acid soils that gypsum has found

application as a subsoil acidity ameliorant.

Acid soils, estimated at more than 800 million ha worldwide (Orvedal

and Ackerson, 1972), constitute from 40 to 50% of potentially arable

highly weathered soils (Sanchez, 1977). Typical pH in water is < 5.0 and

pH in salt is on the order of 4.0 at the surface to a depth of 1 m (Shainberg

et al., 1989). Acid soils are located primarily in the tropics and subtropics,

where intense chemical weathering occurs.

High temperature and rainfall are two factors that promote rapid weathering of primary as well as secondary A1- Fe - silicate minerals, releasing, in

the process, basic cations as well as A1 and Fe into the soil solution. The

basic cations react with anions to form highly soluble salts, carbonates, and

hydroxides. Aluminum and Fe ions, in the presence of salts, hydrolyze into

hydrated A1 and Fe with the release of H ions (Chang and Thomas, 1963;

Jackson, 1963), acidifying the surrounding solution. In time, the hydrated

A1 and Fe, by themselves or by reacting with the various soil constituents,

resynthesize into amorphous or crystalline oxides and hydrous oxides

(Singh and Brydon, 1969; Pariitt and Smart, 1978; Adams and Hajek,

1978). The mineral thus formed supports a characteristic level of active

ionic species in equilibrium with it. The work by Hue et al. (1987) and

Walthall and Day (1988) suggests that at a given pH halloysite supports the

greatest equilibrium concentration of active A1 in soil solution, followed by

gibbsite, kaolinite, and smectite, in that order.

In areas where rainfall exceeds evapotranspiration, the highly soluble

bases are leached to greater depths than are A1 and Fe, leaving behind soil

horizons enriched in active A1 and Fe and compacted by their oxides and

hydrous oxides. As the horizon becomes more acidic, more A1 and Fe are

solubilized from primary minerals and from their secondary oxides and

hydrous oxides (Magistad, 1925). This leads to increased A1 and Fe saturation of the exchange complex of the colloidal soil constituents and to

subsoil infertility. When A1 saturation of the exchange capacity exceeds



60%, appreciable amounts of A13+start to get into the soil solution (Nye et

al., 1961). At this point A1 toxicity, caused by subsoil acidity, could set in.

Heavy fertilization, could induce A1 toxicity even in soils with a relatively

low A1 saturation (Kamprath, 1970, 1971). Excessive cropping and use of

acid-forming fertilizers, without proper liming, could only aggravate the

condition (Beverly and Anderson, 1987).

2. Al Toxicity Indices for Subsoil Acidity Amelioration

Poor root penetration and proliferation frequently observed in the highly

weathered acid soils of the southeastern United States (Pearson, 1966)

have been attributed to physical (Bowen, 1981) and chemical factors (Rios

and Pearson, 1964). The chemical factor identified as most responsible for

poor root growth is excess soluble A1 (Rios and Pearson, 1964; Adams and

Lund, 1966; Adams et al., 1967; Soileau and Engelstad, 1969).

The phytotoxicity of excess A1 has long been recognized (Ligon and

Pierre, 1932). Trivalent A1 has been reported to inhibit root growth by

binding to the PO, portion of DNA in the root cell nuclei, reducing

template activity and thus cell division (Matsumoto et al., 1976; Matsumot0 and Morimura, 1980; Horst et al., 1983). In legumes, it has been

shown to impair the growth of root hairs and rhizobia, thus root nodule

initiation and function (Munns and Franco, 1982; de Carvalho et al.,

1982). Excess A1 may also adversely affect root as well as overall plant

growth in nonphytotoxic ways by competing with Ca and Mg for uptake by

plants (Rengel and Robinson, 1989), by precipitating with anion nutrients

such as PO, (Plucknett and Sherman, 1963) and SO, (Singh and Brydon,

1969; Adams and Rawajfih, 1977) to render them less available to plants,

and by supplying the soil solution with H ions, because exchangeable A1

(pH range 4.5-5.4) and hydroxy Fe and A1 (pH range > 5.2) act as buffers

to keep the soil at a pH of < 5.4 (Coleman et al., 1964; Kamprath, 1970).

Studies on the phytotoxicity of A1 are complicated by the hydrolysis and

polymerization of trivalent A1 to form numerous mononuclear and polynuclear ionic species coexisting in the same solution. Trivalent A1 also

reacts with various ligands to form several ionic species that remain labile

in the solution (Cameron et al., 1986). These complexities have been

somewhat surmounted with the availability of computer speciation models

based on the thermodynamics of solutions (Sposito and Mattigod, 1980).

Use of these models has led to a clearer picture of the phytotoxicity of the

various A1 ionic species.

Pavan et al. (1982) demonstrated that reduction in root growth of coffee

(Cofea arabica L.) seedlings was best correlated with AP+ activity, and

shoot and root weight correlated with KC1-extractable A1 and percent A1



saturation of the soil. Cameron et al. (1986), using barley (Hordeum

vulgure L.), showed that A13+ concentration, but not total Al, correlated

best with root elongation. In soybean, reduction in tap root growth was

best correlated with the sum of the concentrations or calculated activities

of monomeric A1 [AP+ A1(OH)2+ Al(0H)fl species (Blarney et al.,

1983; Alva et al., 1986; Noble et al., 1988b). Strong correlations were also

found between root growth and activities of either A1(OH)2+or Al(0H);

with soybean, subterranean clover (Trifolium subterraneum L.), alfalfa,

~ u ~ L.) (Alva et al., 1986). Parker et a/.

and sunflower ( H e l i ~ n t annus

(1988), using wheat, confirmed previous studies on A13+as the best indicator of A1 stress on plants in the absence of toxic polymers. They also found

polynuclear hydroxy A1 to be demonstrably toxic (Bartlett and Riego,

1972; Wagatsuma and Ezoe, 1985). These species have been ignored in

most studies until now. However, they failed to confirm with wheat the

correlation between root growth of soybean and the sum of the activities of

monomeric A1 ions reported by Alva et al. (1 986) and others. Shuman et

al. (1990), in a greenhouse study using sorghum [Sorghum bicolor (L.)

Moench], found that the best predictors of plant height were soil solution

A13+ activity ( r = -0.91), A1 saturation of the exchange complex ( I =

-0.89), and 0.01 MCaC1,-extractable A1 (r = -0.78). Taking into consideration the ameliorative effect of Ca on A1 toxicity, Noble et al. (1988a)

proposed the so-called Ca-A1 balance (CAB) index: CAB = [2 l o g ( a ~ )]

[3 log(ac) 2 bg(a&,H))

log(a&,H)2)] as a predictor for potential A1

toxicity. The index was found to have a good correlation (R2= 0.88) with

the root length of soybean. Shamshuddin et al. (1991) found that the soil

solution A1 concentration, Ca- A1 ratio, activity of A13+and A1(OH)2+,and

sum of monomeric A1 activities were highly correlated with corn and

peanut yields growing on a Malaysian Ultisol soil.

Thus, it may be concluded that for most agronomic plants A13+concentration or activity in the soil solution or A1 saturation in the exchange

complex appears to be the best single measure to assess potential A1

toxicity for a given soil. For management of A1 toxicity with the use of lime

and gypsum materials, the Ca-A1 balance index of Noble et al. (1988a)

should be helpful.





3. Al Toxicity and Subsoil Acidity and Their Amelioration

For soils with serious subsoil acidity, surface application of lime may not

be the practical answer to the problem. Metzger (1934) and Brown and

Munsell (1938) reported that 10 to 14 years were required for surfaceapplied lime to increase soil pH to a depth of 15 cm. This is due primarily

to the low solubility and mobility of lime in soils. The reaction of lime with



the soil moisture supplies the soil with both OH and HCO, . These, however, are immediately neutralized by H at the surface. Any increase in pH

in the surface soil may also increase Ca adsorption due to the extra

negative charges generated in the amphoteric soil constituents (Reeve and

Sumner, 1972). Neutralization of the basic constituents of lime and the

adsorption of Ca at the surface work to keep the ameliorative effect of lime

limited to the surface (Reeve and Sumner, 1972; Pearson et al., 1973).

Thus, not only is subsoil acidity not checked by surface application of lime,

but it also fails to supply Ca to the deeper horizons to alleviate subsoil

infertility. Adams and Moore (1983), investigating root growth in subsoil

horizons of coastal Plain soils in the United States, found not only A1

toxicity in roots in the argillic horizons (Bt) but also a more prevalent Ca

deficiency in both eluvial and illuvial horizons where Ca saturation was

5 17%. Heavy application of lime to overcome low solubility and downward mobility may prove deleterious to the physicochemical properties of

these soils (Kamprath, 1971). Deep placement of lime to raise subsoil pH

even in advanced countries has not found widespread use because of the

cost involved. In developing countries, where the problem primarily exists,

it is an unacceptable solution because of the prohibitive cost and the heavy

equipment required. The use of gypsum and gypsum by-products, alone or

in combination with other chemical or mechanical treatments to enhance

their ameliorative efficiency, appears to be the most practical approach to

the worldwide problem of subsoil acidity and infertility.

To alleviate A1 toxicity to plant roots, excess A1 had to be precipitated

out from the soil solution or complexed into something less toxic than the

previous ionic forms and, ideally, leached out of the root zone. Precipitation studies of A1 and Fe from AlCl, and FeCl, solutions, respectively, had

shown that A1 and Fe were easily precipitated out of the solution onto clay

surfaces by OH supplied by addition of NaOH (Alcordo, 1968). In the

absence of clay, similar precipitation procedures produced gibbsite or

bayerite from A1 and goethite from Fe solutions (El-Swaify and Emerson,

1975). Titration of A12S04solution with NaOH, KOH, or Ca(OH), also

precipitated A12S04.On aging, the precipitate gave a chemical composition similar to basaluminite, A14(OH),oS045H,O (Singh and Brydon,

1969), and, with KOH as base, to alunite, KAl,(OH),(SO,), (Adams and

Rawajfih, 1977; Adams and Hajek, 1978; Sin& and Miles, 1978).

Aluminum in solution, in the presence of large concentrations of SO4

ions, forms a complex ion pair, Also,+ (Pavan et al., 1982; Cameron et al.,

1986; Noble et al., 1988b;Alva and Sumner, 1989). Cameron et al. (1986),

Kinraide and Parker (1987a,b), and Noble et al. (1988b) showed that roots

do not appear to be adversely affected by the Also,+complex ion.

Fluoride is also an important element in A1 toxicity amelioration. Triva-


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II. Uses of Phosphogypsum in Agriculture

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