Soil Science Society of America Journal 63:891-895 (1999)
© 1999 Soil Science Society of America
DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION
Long-term Effects of Gypsum on Crop Yield and Subsoil Chemical Properties
M. Tomaa,
M.E. Sumnera,
G. Weeksa and
M. Saigusab
a Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA USA
b Experimental Farm, Tohoku Univ., Kawatabi, Naruko, Japan
sumnerme{at}arches.uga.edu
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ABSTRACT
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In hot humid climates with highly leached, variable charge soils, the yields of many crops are limited due to subsoil acidity. Although surface-applied gypsum has been shown to be effective in ameliorating this acidity in ferruginous and aluminous soils, the length of time that the effect is likely to last has not been satisfactorily established. A number of long-term experiments with gypsum incorporated into the topsoil at 10 and 35 Mg ha-1 were sampled to evaluate its effects on soil profile chemical properties and yields of corn (Zea mays L.) and alfalfa (Medicago sativa L.). Even after 16 yr, the gypsum effects were still clearly visible. Exchangeable Ca and SO4 were higher down the soil profile in the gypsum than in the control treatment. A complementary reduction in exchangeable Al was observed in the gypsum treatment to the 80-cm depth. However, pH was not greatly altered down the profile. This amelioration of the effects of subsoil acidity was reflected in improved crop yields of both corn (29–50%) and alfalfa (
50%) on the gypsum treatments. Because the gypsum effect is so long-lasting, its use as a subsoil acidity ameliorant becomes highly economic because the initially high cost can be amortized over an extended period of time.
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INTRODUCTION
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SUBSOIL ACIDITY is one of the major yield-limiting factors in acid soils because it restricts root growth. Because vertical movement of lime is extremely slow in variable-charge soils, specialized management strategies are required to alleviate subsoil acidity. To overcome these problems, much work (Bradford and Blanchar, 1977; Hammel et al., 1985; Sumner et al., 1986b) has been conducted in which lime has been mechanically mixed into the soil to great depth (0.5–1.0 m). In all cases, such treatments have proven to be highly effective in promoting deep rooting, which resulted in large yield increases; however, deep lime incorporation is impractical and uneconomical. On the other hand, practical methods to ameliorate subsoil acidity by using surface applications of gypsum or phosphogypsum have been developed (Reeve and Sumner, 1972; Ritchey et al., 1980; Hammel et al., 1985; Shainberg et al., 1989; Alcordo and Rechcigl, 1993; Sumner, 1993, 1995; Saigusa et al., 1996). In all these studies, increased exchangeable Ca and reduced exchangeable Al together with deep rooting in the subsoil have been observed. Surface application of gypsum is an effective technique to ameliorate the effects of subsoil acidity, but there is a lack of information on the long-term effects of surface-applied gypsum on subsoil acidity.
In view of the high initial cost of gypsum applications, the objective of this study was to evaluate the long-term effects of gypsum on soil chemical properties (exchangeable Ca, Al, Mg, pH, SO4, and electrical conductivity) and yields of corn and alfalfa, using field plots to which gypsum had been applied 16 yr earlier.
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Materials and methods
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The experimental site was on an Appling coarse sandy loam (clayey, kaolinitic, thermic Typic Kanhapludults) with a 3 to 5% south facing slope at the Plant Sciences Farm near Watkinsville, GA. At this site, there were two small plot experiments, the first begun in April 1981 and the second in October 1981. The first was cropped with 2 yr of soybean [Glycine max (L.) Merr.] followed by one year of corn, and then, continuously with alfalfa for 5 yr. The second experiment was continuously cropped to alfalfa for 8 yr, after which both experiments were abandoned and remained in weed fallow (largely grasses) until the present cropping was begun. The treatments were a control and gypsum that was surface-applied at a rate of 35 Mg ha-1 and mixed to a depth of 15 cm. The experiment was arranged in a randomized block design with three replications (Exp. 1). Full details of the experiment are described by Hammel et al. (1985). The second experiment, essentially similar in design, was established 1 yr later with a gypsum rate of 10 Mg ha-1 and used in the evaluation of yield response (Sumner et al., 1986b) (Exp. 2). Prior to planting, both experiments were fertilized (93 kg ha-1 K as KCl, 10 kg ha-1 P as triple superphosphate, 45 kg ha-1 N at planting, and two applications of 45 kg ha-1 N topdressings as urea). Corn was planted on 20 May 1997 at a population of 70000 plants ha-1 in 50-cm-wide rows on both experiments. The experiments were harvested on 16 and 18 Sept. 1997, respectively, with yield expressed on a dry matter basis.
Immediately after the corn was harvested, alfalfa (cv. Alfagraze) was planted on 28 Sept. 1997 after application of 210 kg ha-1 K as KCl, 0.5 kg ha-1 Mo as Na molybdate, and 3 kg ha-1 B as Solubor (sodium tetraborate decahydrate). No P nor lime was required according to soil tests. Three cuts of alfalfa were taken on 25 April, 4 June, and 17 Sept. 1998, subsamples were dried, and yields expressed on a dry matter basis.
Because both Exp. 1 and 2 had behaved similarly over time, soil samples (composite of 15 cores plot-1) were collected only from Exp. 1 in February 1997 to a depth of 120 cm (using a 25-mm tube sampler), and the cores were divided into 0- to 20-cm and then 10-cm increments. The soil samples were air dried and crushed to pass a 2-mm sieve. Soil pH measurements were made in water and 0.05 M CaCl2 in 1:2.5 soil/liquid suspensions. Exchangeable Al was extracted with 2 M KCl (Barnhisel and Bertsch, 1982) and the other exchangeable cations with 1 M NH4OAc prior to analysis by atomic absorption spectroscopy (Baker and Suhr, 1982). Sulfate was extracted with 0.05 M Ca(H2PO4)2 at a soil/solution ratio of 1:10 and determined turbidimetrically (Tabatabai, 1996). Electrical conductivity was measured on a 1:5 soil/water extract (Rengasamy et al., 1984).
All data sets were statistically analyzed by the least statistical difference method of mean separation (SAS Institute, 1985).
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Results and discussion
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In the gypsum treatment in 1997, exchangeable Ca increased significantly down to 1.2 m, while the increases in SO4 were less significant (Fig. 1)
, illustrating that the effects of gypsum application were still present in the profile 16 yr after application. From data of Hammel et al. (1985), Ca applied as gypsum moved down to only 0.3 m in 1 yr, and to 0.9 m 2 yr after application (Fig. 2) . These results show that Ca applied as gypsum moves from the topsoil quickly, but in the subsoil, is strongly adsorbed and retained for long periods of time. Several investigations supporting our research (Farina and Channon, 1988; Sumner, 1990) have shown that SO4 moves from top- to subsoil within the first few years after application and begins to accumulate there. Sumner et al. (1986a) reported that SO4 does not appear in leachate from soil columns until after all the Cl and NO3 have been removed. Our results show that SO4 is retained strongly for at least 16 yr in the subsoil.
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Fig. 1 Effects of surface-applied gypsum on exchangeable Ca and extractable SO4 at different depths, 16 yr after application. * at a particular depth indicates a significant difference between treatments at
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Sixteen years after gypsum application, pHH2O of the gypsum treatment was 0.1 to 0.2 units lower than the control at depths of 0.3 to 1.2 m (Fig. 3)
due to the salt effect caused by the higher electrical conductivity in the gypsum treatment (Fig. 4) . Similar results were obtained by O'Brien and Sumner (1988) and Sumner et al. (1986b) on Appling soils and by Saigusa et al. (1996) on nonallophanic Andosols. On the other hand, at the same depths (Fig. 3), the differences in pHCaCl2 between the gypsum and control were much smaller and only significantly different above 0.3 m. In this case, the self-liming effect (Reeve and Sumner, 1972) was overshadowed by the salt effect in the pHH2O measurements, whereas, in the pHCaCl2 measurements, the added salt far exceeded that present in the gypsum treatment. Above 0.3 m, pHH2O and pHCaCl2 in the gypsum treatment were higher than in the control, reflecting the low levels of exchangeable Al present (Fig. 5)
. Pavan et al. (1984) showed that, in highly weathered soils low in exchangeable Al, OH release from the self-liming effect was likely to exceed H release by hydrolysis, causing soil pH to increase, whereas in similar soils high in exchangeable Al, the reverse was likely (Ritchey et al., 1980). The amelioration of the soil acidity complex (low Ca and high Al), which gives rise to improved crop growth, is more important than changes in an index such as pH, which can change quite markedly (e.g., by salt additions) without changes in the actual level of acidity. The higher pH in the surface of the gypsum treatment may be due to the CaCO3 impurity (3%) in the gypsum, which would have added 1 Mg CaCO3 ha-1.
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Fig. 3 Effects of surface-applied gypsum on soil pH values measured in H2O and CaCl2 at different depths 16 yr after application. * at a particular depth indicates a significant difference between treatments at
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Fig. 4 Effects of surface-applied gypsum on electrical conductivity at different depths 16 yr after application. * at a particular depth indicates a significant difference between treatments at
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Fig. 5 Effects of surface-applied gypsum on exchangeable Al at different depths 16 yr after application. * at a particular depth indicates a significant difference between treatments at
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The EC in the gypsum-treated profile below 0.2 m (Fig. 4) is significantly higher than the control. In view of the solubility of gypsum, one would have expected that after 16 yr it would have been completely leached from the profile. The higher EC values in the gypsum treatment could arise from the elevated levels of sorbed SO4 and Ca, which would subtend slightly more electrolyte in the soil solution than the control. This is supported by the decreasing levels of subsoil Ca from 1983 to 1997 (Fig. 2). Further work is required to elucidate whether this is true.
Exchangeable Al was still markedly reduced with gypsum treatment down to 1.2 m (only significant to 0.7 m), 16 yr after application (Fig. 5). These reductions in exchangeable Al were mirrored by the downward movement of gypsum. The reduction in exchangeable Al is one of the major benefits of gypsum in the amelioration of subsoil acidity. The possible mechanisms responsible for the precipitation of Al have been discussed fully by Sumner (1995). These results show that the effects of surface applied gypsum can last as long as 16 yr. Comparing the data of Hammel et al. (1985) with the 1997 sampling shows that exchangeable Al 16 yr after gypsum application was lower throughout the profile than in the third year after application (Gypsum 83; Fig. 6)
, yet exchangeable Ca decreased from the third to 16th year (Fig. 2). Various proposals have been presented to explain the reduction in exchangeable Al by gypsum application (Shainberg et al., 1989; Sumner, 1993, 1995; Saigusa and Toma, 1998), but further work is needed to clarify the reactions which took place between gypsum and exchangeable Al over the long term.
The gypsum treatments have significantly reduced exchangeable Mg between 0.3 and 0.5 m and transferred Mg to lower depths (significant at 1.1 m) (Fig. 7)
. This resulted from the replacement of Mg by Ca applied in the gypsum. The effects of gypsum on Mg status have been presented previously (Ritchey et al., 1980; Pavan et al., 1984; Farina and Channon, 1988; O'Brien and Sumner, 1988), leading Sumner (1990, 1993) to suggest that Mg should be applied after gypsum application to maintain an adequate Mg level, especially in sandy soils. There is no obvious explanation for the higher Mg levels in the surface of the gypsum treatment.
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Fig. 7 Effects of surface-applied gypsum on exchangeable Mg at different depths, 16 yr after application. * at a particular depth indicates a significant difference between treatments at
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In the case of both experiments, significant yield responses to gypsum applied 16 and 15 yr previously (Table 1)
, both in terms of total biomass and grain, were obtained, indicating that the effects of gypsum were still apparent even though corn is a relatively shallow-rooted crop. Although the yield response to gypsum in Exp. 1 receiving 35 Mg ha-1 was greater than in Exp. 2 (10 Mg ha-1), the results indicate that the lower rate was sufficient to bring about amelioration of acidity. As has been shown previously (Hammel et al., 1985; Sumner et al., 1986b), yield responses arise from improved water extraction from the subsoil brought about by the reduction in Al3+ and the increase in Ca2+, which permits better root growth in gypsum-treated soil.
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Table 1 Effect of gypsum applications, made either 16 (Exp. 1) or 15 yr (Exp. 2) previously, on corn yields on an Appling coarse sandy loam soil in 1997
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Alfalfa yields (Table 2)
mirrored those of the corn crops, with the response to gypsum being greater as the season progressed. This was most likely due to the extraction of greater quantities of subsoil water in the gypsum treatment, as has been observed previously (Sumner et al., 1986b). In fact in 1998, the season grew progressively drier with time. Based on the alfalfa results of Sumner et al. (1986b) for Exp. 2, the marginal cost of the gypsum application would be recovered within 2 yr, making responses in subsequent years highly lucrative. In fact, the alfalfa response to gypsum in the 1998 season would still more than offset the cost of the 10 Mg gypsum ha-1 originally applied on Exp. 2. However, Sumner (1990) reported that in a loamy sand profile, surface-applied gypsum could be leached out in 5 yr. Therefore, the long-term effects of gypsum will be more pronounced on heavier-textured soils.
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Table 2 Effect of gypsum applications, made either 17 (Exp. 1) or 16 yr (Exp. 2) previously, on alfalfa yields on an Appling coarse sandy loam soil in 1998
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The results presented here provide clear evidence that the effects of surface-applied gypsum on subsoil acidity are long lasting, making large applications on soils with heavy ferruginous and aluminous Bt horizons highly economical.
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NOTES
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Contribution from the Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602.
Received for publication June 17, 1998.
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REFERENCES
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- Barnhisel, R., and P.M. Bertsch. 1982. Aluminum. p. 275–300. In A.L. Page et al. (ed.) Methods of soil analysis. Part 2. Agron. Monogr. 9. SSSA and ASA, Madison, WI.
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ABSTRACT
INTRODUCTION
