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DIVISION S-4-SOIL FERTILITY & PLANT NUTRITION

A Comparison of Strategies for Ameliorating Subsoil Acidity

I. Long-Term Growth Effects

^a 27 Drew Ave., Howick 3290, South Africa
^b KwaZulu-Natal Dep. of Agric., Cedara College, Private Bag X9059, Pietermaritzburg 3200, South Africa

farina{at}nitrosoft.co.za

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Subsoil acidity is an important yield-limiting factor. Mechanical procedures of deep lime incorporation and surface applications of gypsum have been shown to be beneficial, but no long-term comparisons of these strategies have been published. Without such information it is difficult to make appropriate management decisions. The work reported here was conducted toward this end. In a long-term study with maize (Zea mays L.) on a strongly acidic Plinthic Paleudult, conventional moldboard incorporation of lime (15 Mg ha^-1) was compared with (i) deeper incorporation of the same quantity of lime with plowing and subsoiling operations, and (ii) treatments where large additional quantities of lime were similarly introduced below normal plow depth. The efficacy of gypsum was tested by adding 10 Mg ha^-1 to conventionally limed plots. For 11 seasons, the average grain yield benefit ranged from 5 to 17% in the case of mechanical strategies and was 25% in the case of gypsum. Yields were increased only marginally by extra lime applications and segmental (slotted) amelioration proved inferior to deep-plowing procedures. The gypsum treatment proved profitable only in the fourth season, but by the eighth season had proved more profitable than the best mechanical procedure; and by the 11th season, the gypsum treatment had resulted in a cumulative yield advantage of 3.8 Mg ha^-1. Long-term superiority of the gypsum treatment was unquestionable in this study, but gypsum is often unavailable and acidic soils are frequently unresponsive to gypsum. In such situations, deep plowing should not, as is often the case, be discarded as impractical.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
SUBSOIL ACIDITY IS A MAJOR FACTOR limiting crop yield in vast areas of the world (Shainberg et al., 1989) and is particularly prevalent in the humid tropics and subtropics, climatic zones that encompass many of the countries struggling most to achieve self-sufficiency in food production. Since incorporating lime into topsoil has very limited effects on subsoil acidity in many dystrophic soils (Farina, 1997), special strategies have had to be developed to combat the problem. Research results obtained using both mechanical procedures of profile modification (Farina and Channon, 1988a) and surface incorporation of gypsum (Shainberg et al., 1989; Ritchey et al., 1995) have proved encouraging, but both approaches have clear limitations: Mechanical profile modification, using specialized equipment to incorporate lime into subsoil horizons, has power requirements beyond the reach of many farmers, particularly in developing countries, and gypsum use depends on a ready source of supply. Even if gypsum is available at little cost, transportation and application costs are usually considerable and, in some instances, the quantities required before meaningful benefits can be expected may exceed 5 Mg ha^-1 (Farina, 1997).

Perhaps more importantly, few long-term research findings have been published, and farmers or advisors beset with subsoil acidity problems have no way of assessing the time period over which increased production costs can reasonably be prorated or, indeed, which particular strategy is the best to adopt. Whether farmers implement costly or difficult ameliorative procedures necessarily depends on economic considerations; it is essential that they receive information regarding the longevity and the probable economic benefit of the various available mechanical and chemical strategies.

The work reported here emanates from a long-term field study with maize, designed to compare the efficacy of several mechanical procedures and surface incorporation of gypsum as methods of combating subsoil acidity. Findings obtained during the first four seasons of experimentation have been reported previously (Farina and Channon, 1988a, 1988b). At that stage, both approaches to the problem appeared promising, but it was recognized that longer-term results were needed before sufficiently reliable conclusions could be drawn. The primary aim of this report and a companion paper (Farina et al., 2000) is to examine the performance of the various strategies employed throughout an extended period. Specific objectives were to (i) compare treatment effects on yield, root distribution, and nutrient recovery, (ii) assess the relative profitability of the different strategies, and (iii) determine the effects of selected mechanical procedures and gypsum application on soil-profile chemical properties. In the interests of clarity, soil effects have been examined separately (Farina et al., 2000).

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Details regarding the trial site, selected soil chemical and physical properties, treatments, and experimental design have been reported previously (Farina and Channon, 1988a, 1988b). Not reported, but since recognized as being critically important to interpreting subsoil acidity studies, is information regarding the clay mineralogy of the soil. While dominantly kaolinitic (55%), in common with most other strongly acidic South African soils, the soil contains an appreciable quantity (30%) of mixed-layer 14/10 clays, and consists of 10% mica and 5% vermiculite. The clay mineralogy is essentially constant to a depth of 0.90 m.

To assist in interpreting results to be presented here, essential features of the treatments imposed 3 mo before initiating the 1982 trial are listed below:

1. Conventional moldboard incorporation of 10 Mg ha^-1 of dolomitic lime (Ca = 17%, Mg = 10%, median particle size = 300 µm, CaCO₃ equivalent = 94%).
   
2. Deep limer at 0.90-m intervals with 14 Mg ha^-1 of lime applied below plow depth before conventional liming.
   
3. Deep limer at 0.45-m intervals with 28 Mg ha^-1 of lime applied below plow depth before conventional liming.
   
4. Modified subsoiler at 0.90-m intervals after conventional liming.
   
5. Modified subsoiler at 0.45-m intervals after conventional liming.
   
6. Modified subsoiler at 0.90-m intervals before conventional liming.
   
7. Modified subsoiler at 0.45-m intervals before conventional liming.
   
8. Wye-double-digger before conventional liming with extra 10 Mg ha^-1 of lime incorporated below plow depth.
   
9. Wye-double-digger after conventional liming, but without extra lime incorporated below plow depth.
   
10. Deep moldboard incorporation of 10 Mg ha^-1 of lime.
    
11. Mined gypsum (86% CaSO₄·2H₂O) disked in at 5 Mg ha^-1 after conventional liming.
    

In 1983, a further 5 Mg ha^-1 of lime was applied conventionally to all plots, and gypsum-treated plots received a further 5 Mg ha^-1 of gypsum. Treatments 6 and 7 were included to separate possible physical effects of deep tillage from the chemical effects of lime and gypsum.

Fertilizer application rates used each season are presented in Table 1 . From 1986 onwards, up to 38 kg N ha^-1, 50 kg P ha^-1, and all the Zn was banded at planting. The remainder of the fertilizer was broadcast and disk-incorporated immediately before planting. Sulfur was disc-incorporated periodically in the form of single superphosphate or gypsum to satisfy plant requirements. Until 1988, an Al-sensitive maize cultivar (RO430) was used. Thereafter, less sensitive cultivars (PAN6552 and RS5232) were planted. From 1989, the row spacing was reduced from 0.90 to 0.75 m and the plant population was increased from 38000 to 40000 ha^-1. In all seasons, the trial was planted during the second half of November. The soil sampling protocol adopted in 1986 (Farina and Channon, 1988a) was followed in all subsequent seasons. Similarly, rooting density determinations were made in 1992 using the same procedures followed in 1986.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Yield Effects
Yields obtained during the study are shown in Table 3 . On average, all treatments designed to mitigate subsoil acidity and so increase use of subsoil moisture reserves, resulted in significant grain yield increases. Since Treatments 6 and 7 at no stage proved superior to conventional liming (Treatment 1), it can be reasonably assumed that soil-strength effects had not played a meaningful role and that the benefits recorded were, in fact, due primarily to acid-subsoil amelioration. Yield responses obtained during the first 4 yr of this study (Farina and Channon, 1988a, 1988b) indicated that, on average, the best mechanical treatments at that stage (Treatments 3 and 10) and gypsum treatment (Treatment 11) were not statistically different, and that the remaining mechanical procedures were only marginally inferior. Nevertheless, in the third and fourth seasons, gypsum-treated plots outperformed all other strategies, and it was considered possible that the benefits of gypsum might increase with time, as greater quantities moved down the soil profile. Results obtained over a longer term support this, and it is interesting to note that, on average, during the 11-yr period, the gypsum treatment proved significantly superior to any other treatment. From the 1984–85 season onwards, gypsum resulted in yields as good as any achieved using alternative strategies of subsoil amelioration and in five of the seasons, proved to be statistically the best.

Yield benefits derived from the mechanical strategies tested were also appreciable, but as can be seen from a careful examination of Table 3 and from trends evident in the economic comparison to be discussed below (Fig. 1) , benefits were less consistent than those obtained from gypsum. Apart from Treatment 8 (Wye-double-digger with an extra 10 Mg ha^-1 of lime below conventional plow depth), the effects of the mechanical treatments proved nonsignificant in several seasons. This decrease in effectiveness was particularly marked during the latter 5 yr of the study, but was also evident in some earlier seasons. An entirely satisfactory explanation for this variability is difficult to provide. However, it is interesting to note that short-duration drought stress at flowering during 1983–84, 1986–87, and 1987–88 appears to have enhanced the relative efficacy of all the mechanical strategies, and that yield responses have generally been lower during seasons of favorable post-anthesis rainfall or prolonged drought stress. Such seasonal dependency is perhaps not unexpected in the case of treatments that are relatively less efficient and ameliorate considerably smaller volumes of subsoil than gypsum does (Farina et al., 2000).

View larger version (35K): [in this window] [in a new window]   Fig. 1 Cumulative economic performance of the treatments, relative to conventional moldboard incorporation of lime (1991–92 silage yields excluded)

View this table: [in this window] [in a new window]   Table 4 Treatment effects on topsoil (0–0.15 m) acid saturation levels and salt pH

Root Effects
Because of the heterogeneous nature of the treatments, no attempt was made to quantify root distribution in segmentally limed plots; but from soil pits opened at silking in 1992, it was clear that even where massive applications of lime had been strip-applied (Treatments 2 and 3), there had been minimal root development beyond the original zone of lime incorporation. Rather unexpectedly, considering the visual evidence of unreacted lime residues, there had clearly been very little vertical or horizontal movement of alkalinity. Rooting density data obtained from the more homogeneous treatments at the same stage of growth (Table 5) provided further evidence of the very slow movement of lime in these soils. Even where an extra 10 Mg ha^-1 of lime was incorporated to a depth of about 0.50 m with the Wye-double-digger, root development below 0.60 m was not different from that in conventionally limed plots. This effect was substantiated by soil analytical data reported elsewhere (Farina et al., 2000).

Comparison of the rooting density data discussed here with those obtained in 1986 (Farina and Channon, 1988a, 1988b) indicates that root development had increased appreciably in all treatments with the passage of time. Soil analytical data (Farina et al., 2000) suggest that an improvement in rooting depth could reasonably have been anticipated in the gypsum-treated plots; but no further amelioration of acidity below 0.45 m in the conventionally plowed plots, and below 0.60 m in the deeply tilled plots, was evident. It would appear, therefore, that much of the apparent increase in root distribution was due to other factors. Lower sensitivity to Al of the cultivar used in 1991 is one possibility and marked seasonal difference is another. The 1985–86 season was exceptionally good, while the 1991–92 season was unusually dry up to anthesis (Table 2). Several workers have demonstrated that enhanced root development may result under conditions of moisture stress (Malik et al., 1979).

Economic Effects
Since commodity prices and input costs vary appreciably both spatially and temporally, and profitability is naturally yield dependent, an accurate economic analysis of the input–output data discussed here would have little relevance beyond the site at which these data were acquired. Nevertheless, any study comparing costly ameliorative procedures demands that some consideration be given to the relative profitability of these procedures.

In making this comparison, costs common to all treatments were not considered and no attempt was made to account for possible interest charges on extra lime and gypsum inputs. Also, since silage frequently has no economic value to crop farmers, 1991–92 yield data were excluded. Lime and gypsum were considered to have equal cost because transportation is usually the major component in the cost of these products; this is generally true for South Africa and probably also in many other areas of the world. The ratio of lime or gypsum cost to maize price was taken as 0.23. Extra diesel fuel employed in implementing the various mechanical procedures (Farina and Channon, 1988a) was costed at local prices, and the extra spreading cost incurred in the gypsum treatment was taken as 0.05 the value of 1 Mg of maize grain.

The cumulative economic gains or losses of the various treatments relative to conventional liming are shown in Fig. 1. The negative effects of extra lime and gypsum applications are strikingly evident. Clearly, the treatments that received extra applications of lime (Treatments 2, 3, and 8) were seriously disadvantaged economically, even though yields were, on average, significantly increased. The relative cost-effectiveness of the modified subsoiler treatments (Treatments 4 and 5) and the deep incorporation of the basal lime application using the Wye-double-digger or deep moldboard plow (Treatments 9 and 10) is encouraging, however. Such treatments clearly provide economically viable partial solutions to the problem of subsoil acidity and may well have proved profitable in the first season had the 1982–83 season not been exceptionally dry (Table 2).

The gypsum treatment (Treatment 11) was unquestionably the most profitable over the long term. Costs of the initial heavy applications had been redeemed by the third season; by the fifth season, only Treatments 9 and 10 were superior; and from the eighth season onward, the gypsum treatment proved appreciably more profitable than any other. Soil chemical data discussed elsewhere (Farina et al., 2000) indicate that the ameliorative effects of gypsum had not declined during the period under review. It is particularly interesting that during the 4-yr period when yield data were acquired after 1992–93 (data not shown) and another 5 Mg ha^-1 of lime was applied to all plots before the 1993–94 season, the cumulative yield benefit of gypsum over the control increased by another 3.7 Mg ha^-1. The trends evident in Fig. 1 remained essentially unchanged.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The data discussed here provide valuable long-term evidence to demonstrate the benefits of acid-subsoil amelioration using either mechanical strategies of incorporating lime to depths greater than conventional plow depth or surface applications of gypsum. The effects of incorporation depth were particularly striking in those treatments where the quantity of lime employed was the same as that conventionally incorporated in control plots. In spite of the fact that deeper incorporation and consequent dilution of the basal lime application resulted in suboptimal amelioration of topsoil acidity, grain yields were appreciably increased. Also, the direct costs of deep incorporation were rapidly recovered. This suggests that where subsoils are prohibitively acidic, the returns on lime are likely to be enhanced by incorporation to depths greater than those achieved by conventional moldboard plowing. Clearly, mechanical procedures, which do not employ extra quantities of lime or particularly specialized equipment, have a practical role to play in acid-subsoil amelioration. In soils that have been anthropogenically acidified to depth and are frequently not gypsum-responsive, or where gypsum is unavailable, such procedures become particularly relevant.

The relative inefficiency of slotting procedures that place extra quantities of lime directly into subsurface horizons (deep limer compared with modified subsoiler at 0.90-m intervals) suggests that such strategies are of questionable value. Not only was there, on average, no significant benefit in terms of yield, but very specialized equipment is required (Farina and Channon, 1988a). Similarly, the Wye-double-digger with extra lime in the plow furrow did not perform as well as might have been expected, relative to the deep moldboard plow treatment. Not only was there, on average, no benefit from the extra lime, but the Wye-double-digger is a highly specialized piece of equipment and has power requirements considerably higher than any of the other strategies compared (Farina and Channon, 1988a).

The marked superiority of the gypsum treatment, in terms of both grain yield and relative profitability, very clearly demonstrates the potentially valuable role gypsum can play in acid-subsoil amelioration. Of course, gypsum must be available at a reasonable cost and the soil involved must be identified as gypsum-responsive (Sumner, 1993), but it is encouraging to note the excellent responses recorded here on a soil with a particularly high gypsum requirement (Farina, 1997).

	ACKNOWLEDGMENTS

 
Appreciation is expressed to Margie Whitwell of the KwaZulu-Natal Department of Agriculture for statistical advice and assistance, and to numerous field and laboratory personnel who have participated in the program over the years. We are particularly grateful to Gillie Jacobtz for donating the experimental site and for logistical support.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Contribution from Agric. Research Council, South Africa.

Received for publication November 9, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

• Farina M.P.W. Management of subsoil acidity in environments outside the humid tropics. In: Moniz A.C., et al. , ed. Plant–soil interactions at low pH: Sustainable agriculture and forestry production. Campinas, Brazil: Brazilian Soil Sci. Soc, 1997:179-190.
• Farina M.P.W., Channon P. Acid-subsoil amelioration: I. A comparison of several mechanical procedures. Soil Sci. Soc. Am. J. 1988;52:169-175 a.[Abstract/Free Full Text]
• Farina M.P.W., Channon P. Acid-subsoil amelioration: II. Gypsum effects on growth and subsoil chemical properties. Soil Sci. Soc. Am. J. 1988;52:175-180 b.[Abstract/Free Full Text]
• Farina M.P.W., Channon P. A field comparison of lime requirement indices for maize. Plant Soil 1991;134:127-135.
• Farina M.P.W., Channon P., Thibaud G.R. A comparison of strategies for ameliorating subsoil acidity: II. Long-term soil effects. Soil Sci. Soc. Am. J. 2000;64:652-658 [this issue].[Abstract/Free Full Text]
• Jones J.B., Eck H.V. Plant analysis as an aid in fertilizing corn and grain sorghum. In: Walsh L.M., Beaton J.D., eds. Soil testing and plant analysis. Madison, WI: SSSA, 1973:349-364.
• Malik R.S., Dhankar J.S., Turner N.C. Influence of soil water deficits on root growth of cotton seedlings. Plant Soil 1979;53:109-115.
• Ritchey, K.D., C.M. Feldhake, R.B. Clark, and D.M.G. Sousa. 1995. Improved water and nutrient uptake from subsurface layers of gypsum-amended soils. p. 157–181. In Agricultural utilization of urban and industrial by-products. ASA, Madison, WI.
• Shainberg I., Sumner M.E., Miller W.P., Farina M.P.W., Pavan M.A., Fey M.V. Use of gypsum on soils: A review. Adv. Soil Sci. 1989;9:1-111.
• Sumner M.E. Gypsum and acid soils: The world scene. Adv. Agron. 1993;51:1-32.

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