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Soil Science Society of America Journal 67:1133-1139 (2003)
© 2003 Soil Science Society of America

DIVISION S-2—SOIL CHEMISTRY

Soil Saturation Extract Composition and Sulfate Solubility in a Tropical Semiarid Soil

John O. Agbenin*

Dep. of Soil Science, Faculty of Agriculture, Ahmadu Bello Univ., Zaria, Nigeria

* Corresponding author (joagbenin{at}yahoo.com)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil solution is the major source of plant nutrients, nutrient cycling in ecosystems, and pollutant transformation and transport in soil. I examined the composition of soil saturation extract of a cultivated and an uncultivated savanna Alfisol to determine the relations of Ca2+, Mg2+, and K+ concentrations in soil solution with exchangeable Ca, Mg, and K; and to determine the solubility relations of SO4 for which there is little information in savanna soils. The soil saturation extract had ionic strength (I) below 0.001 except for one field under intensive fertilization with NPK fertilizer and farmyard manure (FYM) whose ionic strength was between 0.001 and 0.002 at a depth below 40 cm due to leaching. The Ca2+, Mg2+, and K+ in the soil saturation extract correlated weakly with exchangeable Ca, Mg, and K respectively, but K+ intensity was fairly well predicted by percentage of K saturation of cation-exchange capacity (CEC), whereas the intensities of Ca2+, Mg2+, and SO42- in soil solution were best predicted by electrolytic conductivity of the saturation extract. The K activity ratios in solution defined as aK/(aCa + aMg)1/2 were <0.01 in the cultivated soil, suggesting a preponderance of K adsorbed to edge rather than planar sites. The Ca2+ activity ratios defined as aCa/[{sum}aCations] were <0.15 for the cultivated soil, indicating possible Ca deficiency. The SO2-4 activities in solution were in apparent equilibrium with basaluminite in the cultivated fields, whereas in the uncultivated field, SO2-4 activities were in apparent equilibrium with alunite.

Abbreviations: AR,K, activity ratio of K • CEC, cation-exchange capacity • FYM, farmyard manure • I, ionic strength • IAP, ion activity product, OM, organic matter • pIAP, negative logarithm of ion activity product, pKsp negative logarithm of solubility product constant


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE UPTAKE OF CATIONS AND ANIONS as plant nutrients is directly related to their concentration in soil solution. This has been demonstrated not only in solution culture but also in soil solution. Ion uptake rate by plants is determined by diffusive and convective movement of ions in soil solution to plant roots (Barber, 1962; Barber et al., 1963). Only a small quantity of ion uptake is accounted for by root interception—a direct exchange between plant roots and ion exchange sites in soils. The soil solution is a source of plant nutrients and a medium for all reactions, nutrient cycling in ecosystems, and pollutant transformation and transport in soils (Lawrence and David, 1996). Chemically, the soil solution can be defined as the soil water and its dissolved electrolytes, gases, and water-soluble compounds (Adams, 1974). The composition of the soil solution is greatly affected by nutrient uptake, fertilization, leaching (Nemeth et al., 1970), and other soil properties, which vary in time and space. As an example, the cations dissolved in soil solution are greatly influenced by the degree of saturation of the exchange sites, exchange capacity of the soils, the complementary ions, and the conductivity of soil solution (Nemeth et al., 1970; Benians, 1985).

Also of interest in the study is to determine the control mechanism for SO4 solubility in cultivated and uncultivated savanna soil. Sulfate adsorption-desorption, as a control mechanism for SO4 availability in weathered soils, has received considerable attention (Gebhardt and Coleman, 1974; Marsh et al., 1987; Marcano-Martinez and McBride, 1989). The retention of SO4 by soils occurs by both specific and nonspecific adsorption (Marsh et al., 1987; Marcano-Martinez and McBride, 1989). However, a preponderance of the evidence seemed to support specific adsorption of SO4 retention in weathered tropical soils (Parfitt and Smart, 1978; Rajan, 1979; Agbenin, 1997).

Some investigators have evoked precipitation-dissolution of basic aluminum hydroxyl sulfate as a control mechanism for SO4 availability and concentration in soil solution of weathered acidic soils (Adams and Rawajfih, 1977; Wolt et al., 1992). To this end, alunite and basaluminite have been suggested as the Al2(SO4)3 solids likely to control SO4 in soil solution of weathered acidic soils (Adams and Rawajfih, 1977). Agbenin (1997) found no precipitation of basic aluminum hydroxyl sulfate in a savanna soil equilibrated with SO4 solution, but did not rule out the possibility of alunite and basaluminite controlling SO4 solubility depending on the solid phase controlling Al3+ solubility in the soil. However, Marcano-Martinez and McBride (1989) did not find any evidence of precipitation of alunite or basaluminite in Brazilian Oxisols equilibrated with 0.003 M SO4 solution, which was three times higher than the SO4 concentration used by Agbenin (1997) for savanna soils.

The short time involved in adsorption studies is not enough for equilibrium to be established between SO4 in solution and SO4 solids in soils. Furthermore, even if precipitation of basic aluminum hydroxyl sulfate solids occurred in those adsorption studies, they would initially be amorphous with solubility differing considerably from crystalline aluminum hydroxyl sulfate. Therefore, I thought it worthwhile to examine SO2-4 solubility in this savanna soil, which had consistently received incidental additions of SO4 through the application of single superphosphate and FYM for 50 yr. The objectives of this study are to examine soil saturation extract composition of a typical soil under long-term cultivation, and explore the relations between soil solution cations and exchangeable cations, and to determine the solubility control mechanism for SO2-4 in soil solution.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Twenty soil samples were used for this study. The samples were obtained from five field plots at Samaru: 11°11'E lat. and 7°38'E lat. in the northern Guinea savanna of Nigeria. The climate of the area is marked by distinct wet and dry seasons with a mean annual rainfall of 1060 mm. The mean monthly maximum temperature ranges from 28 to 36°C and minimum temperature of between 14 and 18°C. The underlying geology of the area is the Precambrian Basement Complex rocks consisting mainly of granite and gneisses (Montimore, 1970). The soils are classified as Isohyperthermic Typic Haplustalf in Soil Taxomomy or Orthic Acrisol in the FAO system (Valette and Ibanga, 1984). The clay mineralogy of the soils is predominantly kaolinite, and with small amounts of illite (Ojanuga, 1979). Four of the field plots have been under continuous cultivation and fertilization with FYM and NPK fertilizers for 50 yr before sampling. An uncultivated natural site adjacent the cultivated field plots was also sampled. Four samples were taken from each field at depths ranging from 0 to 100 cm with three replicates. The properties of the soils have been described previously (Agbenin, 2001). A brief description of the fields is given in Table 1 .


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  		Table 1. Description of fields sampled for the study at Samaru, Nigeria.

 
Soil Saturation Extract Composition and Analysis
For the purpose of this study, the soil solution was assumed to be the soil saturation extract obtained for 2-mm sieved soil moistened to 1:1 soil/water and equilibrated for 24 h under laboratory condition at 25 ± 2°C. Soil saturation extract obtained at this soil/water ratio does give a good prediction of soil solution (Gillman and Bell, 1978). An equilibration time of 24 h was also considered adequate for equilibrium or steady state to be established between the solution and the solid phase of the soil (LeRoux and Sumner, 1967; Nemeth et al., 1970). One hundred grams of 2-mm sieved soil were weighed into polystyrene centrifuge tubes and moistened with 100 mL of deionized water. The centrifuge tubes were covered with screw caps, and left for 24 h after agitating in a reciprocal shaker for 30 min. After 24 h, the centrifuge tubes were loaded in a centrifuge vessel and centrifuged at 2000 rpm or at a relative centrifugal force of 900 x g as described by Gillman and Bell (1978). The soil solution obtained by centrifugation was filtered through 0.45-µm Millipore Filter Assembly under vacuum (Whatmann International Ltd., Maidstone, England). The electrical conductivity of the clear extract was determined with cell calibrated with KCl standard while the pH was determined with a glass electrode. Calcium, Mg, and Al in the extract were determined by atomic absorption spectroscopy while Na and K were determined by flame photometry. Nitrate, NH4, SO4, and Cl in the extracts were determined by ion chromatography. There was no measurable phosphate concentration in the extract suggesting that phosphate had no significant effect on ionic strength of the soil solution.

Ionic strength, I, of the soil solution was computed from either the concentration of ions in solution directly or from electrolytic conductivity of the saturation extract. Direct computation of I from soil solution was done with Eq. [1] with or without correcting for ion pairs.

[1]
where Ci is the ion concentration and Zi is the ion valence. The electrical conductivity of the soil extract was converted to ionic strength by using the Marion-Babcock equation given by Sposito (1989) as:

[2]
where I is the ionic strength in moles per cubic meter (mol m-3) and k is the electrical conductivity in decisiemen per meter (dS m-1). The ionic activity coefficients were calculated by the Davies equation given by Sposito (1989) as:

[3]
where Zi is the species valence. The activities of SO2-4, K+, and Al3+ were determined by multiplying the molar concentrations of the ions by their activity coefficients. The OH- ion activities in soil solution were estimated from soil pH using the relation: pOH- = pKw - pH where pKw is 14, which is the negative logarithm of the dissociation constant of water. Ion activity products (IAP) were calculated for three basic hydroxyl aluminum sulfate solids: jurbanite (Al)(OH)(SO4), basaluminite (Al4)(OH)10(SO4), and alunite (K)(Al3)(OH)6(SO4)2 in the soil solution.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Cation-Exchange Properties
The soils had a wide range of CEC varying from 38 to 136 mmolc kg-1 soil (Table 2) . Two fields fertilized with FYM (F-3 and F-5) had greater CEC than other fields suggesting that FYM application contributed to CEC through organic matter (OM) increases in the soils. Jones (1973) reported that 80% of CEC of surface savanna soils is derived from OM. Typically, exchangeable Ca was the dominant cation followed by Mg and K in that order (Goladi and Agbenin, 1997). Calcium accounted for between 55 and 76% of total CEC while Mg accounted for between 18 and 29%. The K saturation of the CEC was up to 15% in the F-1 field, but declined drastically to between 2 and 6% after 50 yr of continuous cultivation. Generally, exchangeable Na accounted for <1% of cationic saturation of the CEC. These results are in concord with reports that savanna soils have adequate Ca, Mg, and K saturation of CEC even though soil pH varies between mildly acidic (pH < 6.0) to strongly acidic (pH ≤ 4.5). The abundance of Ca and Mg in the exchange complex has generally led to the notion that savanna soils have adequate reserves of Ca and Mg to meet crop nutrition (Jones and Wild, 1975; Kowal and Kassam, 1978) even though this hypothesis has never been tested in the region.


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  		Table 2. The cation–exchange capacity (CEC), exchangeable cations, and cation saturation of CEC of cultivated and uncultivated savanna Alfisol.

 
Despite the relatively low pH of the soils, exchangeable Al accounted for just between 1 and 5% of CEC consistent with reports that soil acidity in the savanna is not caused by the hydrolytic reactions of Al (Jones and Wild, 1975). In fact, Kowal and Kassam (1978) reported that savanna soils have no history of crop responses to liming even at pH below 4.5 because of low concentrations of soluble Al in the soil solutions.

Soil Saturation Extract Composition
The cationic and anionic compositions of the soil saturation extracts are presented in Table 3 . The ionic strength estimated by the Marion-Babcock equation from electrolytic conductivity was higher than the ionic strength computed from soil solution data. Ionic strength corrected for ion pairs (data not shown) was slightly lower than that those computed directly from the concentrations of cations and anions in solution without correcting for ion pairs, but were highly correlated (r = 0.975***). Such correction is hardly necessary because of the low ionic strength of the soil solution. However, the higher ionic strength estimated from electrolytic conductivity than that computed directly from soil solution data is quite understandable because not all dissolved ionic species in the solution were determined. However, ionic strength computed from ionic concentration data correlated strongly with electrolytic conductivity (Fig. 1) . Thus, these results are in concord with reports that electrolytic conductivity measurements provide convenient estimates of ionic strength of soil solution (Griffin and Jurinak, 1973; Gillman and Bell, 1978; Sposito, 1989). The variability in ionic strength distribution between fields and profile depth probably reflected the intensity of leaching and fertilizer and manure inputs, consistent with the lower ionic strength in the F-2 than F-3 to F-5 fields fertilized with NPK and FYM (Table 3).


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  		Table 3. The soil pH, ionic strength, and soil saturation extract composition of cultivated and uncultivated savanna Alfisol at Samaru, Nigeria.

 


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  		Fig. 1. The relationship between ionic strength computed from ionic concentrations of soil saturation extract and electrolytic conductivity of the extract (EC).

 
The cationic and anionic compositions of the soil solution showed considerable variations between fields. In the fields, K+ and Na+ were the dominant cations in solution except for F-5 field fertilized with NPK + FYM where Ca2+ and Na+ rather than K+ and Na+ were about the dominant ions especially at 60- to 100-cm depth, probably reflecting the leaching of Ca and Na from FYM and NPK. There was a strong leaching of NO3 to lower soil depth in the natural site (F-1), and SO4 in F-3 and F-5 fields. Incidental additions of SO4 from long-term application of single superphosphate seemed to have provided leachable SO4 to these fields.

Since the soluble cations in solution are the pools from which plant roots draw nutrients, it is important to determine the relations between solution cations and exchangeable cations with a view to examining the validity of the use of exchangeable cations for soil fertility evaluation. Surprisingly, exchangeable Ca, Mg, and K had no significant linear relations with Ca2+, Mg2+, and K+ in solution. Electrolytic conductivity was the best predictor of Ca2+, Mg2+, and SO2-4, and explained between 70 and 72% of the variability in Ca2+ (Ca2+ = 1.11EC - 0.58, r2 = 0.70**), Mg2+ (Mg2+ = 0.43EC + 1.46, r2 = 0.72**), and SO2-4 in solution. However, the degree of K saturation of the exchange complex best predicted K in soil solution (Fig. 2) . The results of this study corroborated the findings of Benians (1985) who reported that the solubility of divalent cations was best predicted from the electrolytic conductivity of the solution with which the soil was in contact. The exchangeable Ca, Mg, and K determined by ammonium acetate poorly predicted the solubility of these cations probably because of the complexity of mono-divalent cation-exchange relationships. This calls to question the validity of the use of exchangeable Ca, Mg, and K extractable with neutral ammonium acetate for soil fertility evaluation in weathered tropical soils with respect to the availability of basic cations for plant nutrition. As suggested by Gillman and Bell (1978), and re-emphasized by Benians (1985), much money, time, and labor will be saved by discontinuing the use of neutral ammonium acetate as extractant for basic cations in weathered tropical soils.



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  		Fig. 2. The relationship between soluble K and K saturation of cation-exchange capacity of the soils.

 
Cation-Activity Ratios
Adams et al. (1966) reported that Ca activity ratio defined as aCa/{sum}aCations in solution, where superscript a is activity, is a significant index of Ca2+ availability in soils. When aCa/{sum}aCations < 0.15, inhibition of root growth and development of cotton because of Ca deficiency has been reported, while aCa/{sum}aCations < 0.05 caused the death of cotton plant roots (Adams et al., 1966). The aCa/{sum}aCations ratios showed widespread Ca deficiency in the soils, with ratios far below 0.15, except for the natural site (F-1) (Table 4) . For too long, it has been assumed that savanna soils are well supplied with Ca because of the dominance of Ca in the exchange complex (Table 2), even though there is no unequivocal evidence that exchangeable Ca is directly related to Ca2+ in soil solution. The notion that savanna soils are well supplied with adequate amounts of Ca has been further perpetuated by the lack of apparent responses of field crops to liming in the region even at pH far below 5.0 (Jones and Wild, 1975). My results suggested that Ca deficiency might have been masked by regular application of single superphosphate, which is a calcium-phosphate compound. The aCa/{sum}aCations ratios clearly showed apparent Ca deficiency under continuous cultivation. Consequently soil augmentation with Ca might be necessary under continuous cultivation where single superphosphate is not used as a fertilizer P source.


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  		Table 4. The K+ and Ca2+ activity ratios in soil solution extract of a savanna Alfisol under long-term cultivation at Samaru, Nigeria.

 
It is well known that K+ activity ratio in soil solution, AR,k, has a profound effect on K availability and crop uptake in savanna soils (Wild, 1971). The activity ratio of K+ can be defined with respect to Ca2+ and Mg2+ in soil solution according to Eq. [4]

[4]

The AR,K, averaged across the sampling depth for each field, was much lower than 0.01 except for the natural site (F-1) where AR,K was 0.021 in the 0- to 20-cm depth (Table 4). The AR,K, was >0.01 in the surface horizon of natural site (F-1), F-3, and F-4 fields but <0.01 in the F-2 and F-5 fields. These variations between fields appear to reflect differences in the intensity of K utilization and K adsorption sites in the soil.

Potassium can be adsorbed to planar, edge, and interlattice positions in clay particles (Schouwenburg and Schuffelen, 1963). These three adsorption sites differ in their selectivity for K, which can be indexed by AR,K or by use of Gapon coefficients (Schouwenburg and Schuffelen, 1963; Nemeth et al., 1970). When AR,K < 0.01, it suggests the predominance of K adsorbed to edge sites, whereas AR,K > 0.01 indicates a preponderance of K adsorbed to planar positions. It is likely that continuous cultivation might have depleted K adsorbed to planar sites in the silicate clay minerals, which is more available than K sorbed to edge or interlattice positions (Nemeth et al., 1970). This depletion extended to the subsoil because of deep removal of K by plant root. However, annual applications of NPK and FYM had managed to maintain AR,K slightly higher than 0.01 but not so in the F-5 field probably because of high K removal or export from the field in crop harvests (Agbenin, 2001).

It is interesting that in no field was AR,K < 0.001 which is an indication of K adsorbed to interlattice position because of the near absence of expansible 2:1 clay minerals in this soil and the preponderance of 1:1 clay, typically kaolinite. Furthermore, the variation in K adsorption sites in the soils seems to clarify why exchangeable K was a poor predictor of K intensity. Potassium adsorbed to different sites varies in availability and solubility (White and Zelazny, 1986). If K were adsorbed to sites of similar energy, exchangeable K would have been highly correlated with K intensity (Nemeth et al., 1970). Similar explanation can be extended to Ca and Mg adsorption sites in the soils. In fact, there is some evidence that Ca and Mg are mostly specifically sorbed in tropical soils (Chan et al., 1979), which might explain the weak correlation of exchangeable Ca and Mg with their intensities in soil solution.

Sulfate Solubility
The SO4 intensities ranged from 0.012 to 0.777 mmol L-1 in the soil solution. The SO4 intensities in 15 of the 20 soils were below the critical limit of 0.16 to 0.47 mmol L-1 for plant growth (Fox, 1974; Wolt et al., 1992). In soils fertilized with NPK (F-3) and FYM + NPK (F-5), SO4 intensities were higher than this critical range probably because of incidental addition of SO4 from regular single superphosphate applications for 50 yr of cultivation before sampling. The low intensity of SO4 in the natural site is an indication that weathered savanna soils have inherent SO4 deficiency.

Calculation of ion activity products (pIAP) showed that soil solutions were consistently undersaturated with respect to jurbanite (Table 5) but oversaturated with respect to alunite except for F-0 in which pIAP of alunite [p(K)(Al)3(OH)6(SO4)2] in soil solution reasonably approached the pKsp of alunite. For majority of the soils, the pIAPs of basaluminite [p(Al)4(OH)10(SO4)2] in soil solutions were somewhat in agreement with pKsp of basaluminite. It would, therefore, appear that hydroxy aluminum sulfate compounds having solubility similar to basaluminite and, to a limited extent, alunite, if present in the soils, are the solid phases likely to control the concentration of SO4-S in this savanna soil. The average pIAP of basaluminite in soil solution of the different fields ranged from 117.1 to 118.6, which is close to the range of solubility of basaluminite (pKsp = 115.7 - 118.4) reported by Adams and Rawajfih (1977).


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  		Table 5. The activities of Al3+, K+, SO2-4, and OH-, and the ion activity products (IAP) of jurbanite, basaluminite, and alunite in soil solution.

 
Basaluminite solubility in soil depends on the solid phases controlling Al3+ activities in soil solution (Wolt et al., 1992). In most of the soils, calculations of pIAP for basic hydroxyl aluminum sulfate were based on the assumption that Al3+ activity in soil solution was controlled by the solubility of cryptocrystalline gibbsite, in which case, Al3+ activity in soil solution was estimated from soil pH using the relation: pAl3+ = 3pH - 9.2 (Hegelson, 1969; Wolt et al., 1992). I considered this approach valid for the soils because in five to six of the soils where Al concentration in soil solution was measurable, calculated Al3+ activities were fairly close to Al3+ activities estimated from soil pH.

The solubility relations of basic hydroxyl aluminum sulfates in soil solution can also be expressed as [2pH + SO4] and [pAl + pOH + pSO4] (Wolt et al., 1992) using appropriate pKsp for jurbanite, alunite, and basaluminite as given by Adams and Rawajfih (1977) and van Breeman (1973), and the negative logarithm of the ion activity product of water: pKw = 14. Wolt et al. (1992) derived the following relations for the basic hydroxyl Al sulfates considered in the study:



If gibbsite is present in the system (Wolt et al., 1992), the following relation holds: [pAl + pOH + pSO4] = 4.77 + [2pH + pSO4]. The soil solution ion activities were plotted as [pAl + pOH + pSO4] versus [2pH + pSO4] for jurbanite, alunite, and basaluminite (Fig. 3) . The ion activities fell within the area circumscribed by alunite and basaluminite stability. Clearly, SO2-4 activities were undersaturated with respect to jurbanite, but appeared to be supersaturated with respect to alunite solubility. Basaluminite and, to some extent, alunite are the stable solid SO4 solids, which are likely to control SO4 solubility in soil solution especially at [2pH + SO4] ≤ 15.3. Beyond this point, gibbsite in the system becomes more stable than basaluminite and alunite (Fig. 3).



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  		Fig. 3. The equilibrium relations between SO2-4 activities and the solubility of three basic aluminum hydroxyl sulfate solids: jurbanite, alunite, and basaluminite.

 
The assumptions used in deriving the stability lines of aluminum hydroxy sulfate minerals and gibbsite shown in Fig. 3 have been criticized by Ross (1993) because of the dependent nature of the variables. However, these stability diagrams, taken together with the IAPs of the aluminum hydroxy sulfate minerals (Table 5), did provide some insight into the nature and solubility of the solid phases that were likely to control the concentration of SO2-4 concentration or activity in the soil solution (Wolt and Hue, 1993). My results are consistent with reports by Wolt et al. (1992), but in an entirely different set of soils and fertilization practices. For savanna soils under natural vegetation, alunite appeared to be more stable than basaluminite, whereas with continuous cultivation and long-term fertilization with either NPK or FYM + NPK, hydroxy aluminum sulfate compounds with solubility close to the solubility of basaluminte seemed to control SO2-4 concentration in soil solution especially at soil pH ≤ 5.6.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The solubility of Ca2+ and Mg2+ depended on electrolytic conductivity of soil saturation extract, whereas K+ in solution was best predicted by the degree of K saturation of CEC and not the exchangeable K per se. The lack of strong linear relations between exchangeable Ca, Mg, and K with the Ca2+, Mg2+, and K+ in solution indicated the cations were sorbed to edge sites rather than to planar sites in soil colloids.

Similarly, SO4 in solution had strong linear relation with electrolytic conductivity. Sulfate solubility relations indicated that basaluminite and, to some extent, alunite are the stable solid SO4 solids which are likely to control SO2-4 activities in soil solution especially at [2pH + SO4] ≤ 15.3, above which gibbsite in the system becomes more stable than basaluminite and alunite.

This study has two implications for soil fertility management in the savanna. First, exchangeable Ca, Mg, and K determined by neutral ammonium acetate were not useful for predicting the solubility of Ca2+, Mg2+, and K+ in soil solution from which plants draw to meet their nutritional requirement. Electrolytic conductivity best predicted soluble Ca, Mg, and SO4, corroborating the view of Gillman and Bell (1978) and Benians (1985) that valuable time, effort and money will be saved by discontinuing the use of neutral ammonium acetate to assess the fertility of basic cations in weathered tropical soils. Second, should another phosphate fertilizer compound other than single superphosphate be used as a source of P in savanna soils, widespread Ca deficiency is likely to appear which, hitherto, has been masked by widespread application of single superphosphate because of its high Ca content.

Received for publication April 25, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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