WO1997002333A1

WO1997002333A1 - Methods for ameliorating/neutralizing acidity in soils

Info

Publication number: WO1997002333A1
Application number: PCT/AU1996/000396
Authority: WO
Inventors: Nicholas Charles Uren
Original assignee: La Trobe University
Priority date: 1995-07-05
Filing date: 1996-06-27
Publication date: 1997-01-23
Also published as: AUPN396595A0

Abstract

There are provided methods for the rapid amelioration/neutralization of acidity in acidic or sodic soil or in acid mine drainage areas, by the application of a volume of a calcium bicarbonate [Ca(HCO)3)2] solution to the surface of subsurface of the soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2cm below the surface of said soil or acid mine drainage areas.

Description

METHODS FOR AMELIORATING/NEUTRALIZING ACIDITY IN SOILS

BACKGROUND OF THE INVENTION

This invention relates to methods for ameliorating/neutralizing acidity in soils or other such porous bodies, especially those in relatively inaccessible places, including the amelioration/neutralization/prevention of acid mine drainage, the methods of the invention being particularly directed to the neutralization of acidic soils with the object of changing the acidic environment of soils so as to encourage and support plant growth in otherwise normally acidic soils.

The impact of excessive surface and sub-surface soil acidity on plant growth is well documented. Surface acidity limits the growth of some acid sensitive crops and pasture species which have shallow root systems, such as annual medics, barley and some wheat varieties (Scott and Fisher 1989). Subsoil acidity affects the growth of deep-rooted plants such as alfalfa (Rechcigl and Reneau 1984) and orchard trees (Hoyt and Drought 1990) that extend into acidic subsoils.

Surface applied forms of lime, such as calcium carbonate, dolomitic lime or limestone, hereinafter referred to simply as lime, or lime which has been incoφorated into the surface 5 cm of a plant growth site, hitherto have generally been the best methods by which surface soil acidity can be managed. However, the neutralization of subsoil acidity without damaging plant roots by mechanical incorporation remains unsolved. Also, although sub-soil acidity can be neutralized at high uneconomic rates, it is well documented that lime such as dolomitic lime or limestone, applied to surface soils at economic rates, fails to neutralize subsoil acidity because of its low solubility in soils (Messick et al. 1984; Hoyt et al. 1986; and Hoyt and Drought 1990). Although any Na-alkali is water soluble and may neutralize subsoil acidity, it is not recommended to be used as a source of base for managing soil acidity because it could lead to sodicity and associated problems such as the dispersion of clay colloids.

Summarized, acidic porous media such as soils and mine tailings often require in situ amelioration to increase the pH. The incoφoration of ground limestone by cultivation into the surface few centimetres of soil is generally regarded as the best method by which surface acidity can be managed, but the effect is restricted to the volume of incoφoration and fails to ameliorate subsurface soil acidity (Messick et al. 1984; Hoyt et al. 1986; Bromfield et al. 1987; Hoyt and Drought 1990: Conyers and Scot 1989) because ofthe low solubility of CaCO The amelioration of subsurface soil acidity without damaging plant roots by mechanical incoφoration remains unsolved.

Lime (CaO) and slaked lime (Ca(OH)₂) are more soluble than calcium carbonate (CaC0₃), but in some situations the high initial alkalinity can be harmful and they may revert to CaC0₃ prior to application. Soluble bases such as NaOH and NaHC0₃ are effective but have high initial pHs and cause sodicity. Ca(N0₃)₂ has been used with some success but there is concern that it may contribute to contamination of groundwater with nitrate. Calcium salts of organic anions are effective (Adams and Pearson 1969; van der Watt et al. 1991 ; Noble et al. 1995; Smith et al. 1995) but are likely to introduce special problems of their own such as complexing of trace metals which may facilitate losses by leaching.

We have found that solutions of calcium bicarbonate [Ca(HC0₃)₂] do not have any ofthe above-mentioned problems, except for possible reversion, and provide a soluble and mobile form of base. Also, such solutions can be considered as environmentally friendly, since soils become acidic through the leaching of bicarbonates. predominantly Ca(HC0₃)₂. Thus, there is herein provided, methods of ameliorating/neutralizing acidic or sodic soils and subsoils which avoid at least some of the disadvantages of the prior practices in the field, said methods including the amelioration/neutralization of acid mine drainage in mine tailings, especially where the source of acidity is in a relatively inaccessible sub-surface location.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method for the rapid amelioration of acidity in acidic or sodic soil or in acid mine drainage areas, which comprises the application of an effective volume of a calcium bicarbonate [Ca(HC0₃)_:] solution to the surface or subsurface of the soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2 cm below the surface of said soil or acid mine drainage areas.

More particularly, the present invention provides a method for the rapid amelioration of acidity in acidic or sodic soil or in acid mine drainage areas at depths below that at which lime is effective, which comprises the application of an effective volume of a calcium bicarbonate [Ca(HC0₃)₂] solution to the surface or subsurface ofthe soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2 cm below the surface of said soil or acid mine drainage areas. We have found that in operating the methods ofthe invention, certainly in relation to the rapid amelioration of acidity in acidic soils, the calcium bicarbonate [Ca(HC0₃)₂] solution is effective not only throughout the topsoil of normally 10-15 cm but also at depths up to about 50 cm of subsoil, the rates at which the calcium bicarbonate is applied to the soil having importance in being related to the depth ofthe soil at which the acidic pHs are ameliorated.

Specifically, the present invention provides a method for the rapid amelioration of acidity in acidic or sodic soils or in acid mine drainage areas, which comprises the application of a volume of calcium bicarbonate [Ca(HC0₃)₂] solution to the surface or subsurface of said soil or acid mine drainage areas, equivalent to and at a rate of calcium bicarbonate [Ca(HC0₃)₂] equivalent to calcium carbonate [CaC0₃] within the range of about 0.5 to about 10 tonne per hectare, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm below the surface of said soil or acid mine drainage area.

Said method may comprise the application of a volume of calcium bicarbonate 0.02196N [Ca(HC0₃)₂] solution to the surface or subsurface of said soil or acid mine drainage areas, equivalent to an amount ofthe order of about 1.048 tonne per hectare of calcium carbonate [CaC0₃], optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm, preferably to a depth at least between 2 cm and 8 cm, below the surface of said soil or acid mine drainage areas. In practice, the methods ofthe invention are conveniently based upon ascertaining the initial pH ofthe soil or acid mine drainage areas, the buffering capacity ofthe soil or acid mine drainage areas, and the localised temperature and rainfall, in leading to an amount of calcium bicarbonate being applied to the soil or to the acid mine drainage areas, on the basis of the lime requirement.

The calcium bicarbonate is conveniently applied by irrigation or by injection to the soil or acid mine drainage areas as a solution in water as the most convenient or appropriate solvent. Application of the calcium bicarbonate solution is optionally followed by the application of a leaching volume of water to effect penetration ofthe calcium bicarbonate into the subsoil and affect the pH of the subsoil.

In one practical embodiment, the calcium bicarbonate is added to the soil or mine drainage areas as a carbon dioxide-pressurized solution formed from any form of lime including calcium carbonate, water and carbon dioxide. A leaching volume of water is then added to the soil to effect penetration of the calcium bicarbonate into the subsoil and affect the pH ofthe subsoil.

DISCUSSION OF THE INVENTION

The effectiveness of Ca(HC0₃)₂ as a base, and the mobility of alkalinity associated with Ca(HC0₃)₂ in leaching columns of three different soil types in the laboratory, has been investigated in connection with the invention. A discussion of those investigations and the results are set out below.

In this connection, the invention will now be described with reference to the practical examples set out below in relation to the accompanying figures of drawings, in which: Figure 1 is a graph showing the change in pH with depth in a leaching column containing Toomuc sandy loam from near Cranboume. State of Victoria. Australia, treated with rate I CaC0₃ and rate I Ca(HC0₃)₂;

Figure 2 is a graph showing the change in pH (1 :5 soil:0.01 M CaCl₂) with depth in a leaching column containing Cranboume sand from near Cranboume, State of Victoria. Australia, treated with CaC0₃ rate I and Ca(HC0₃)₂ rate I;

Figure 3 is a graph showing the change in pH ( 1 :5 soil: 0.01 M CaCl₂) with depth in a leaching column containing Toolangi loam from near Kinglake. State of Victoria, Australia, treated with rate II CaC0₃ compared to a control treated with an equivalent volume of water;

Figure 4 is a graph showing the change in pH (1 :5 soi O.Ol M CaCl₂) with depth in a leaching column containing Toolangi loam from near Kinglake, State of Victoria, Australia, treated with rate I Ca(HC0₃)₂ compared to a control treated with an equivalent volume of water;

Figure 5 is a graph showing the change in pH (1 :5 soil:0.01 M CaCl₂) with depth in a leaching column containing Toolangi loam from near Kinglake, State of Victoria, Australia, treated with rate II Ca(HC0₃)₂ compared to a control treated with an equivalent volume of water;

Figure 6 is a graph showing change in pH (1 :5 soihO.Ol M CaCl₂) with depth in a leaching column containing Toolangi loam from near Kinglake, State of Victoria, Australia, treated with Ca(HC0₃)₂ at rate I and rate II, and with CaC0₃ rate II, compared to a control; and Figure 7 is a graph showing the pH (1 :5 soil:0.01M CaCl₂) vs. depth in Stawell Gold Mine tailings treated with 0. 2, 4. 8 and 16 pore volumes of Ca(HC0₃)₂ solution (2pv = 3.5 t ha-' of CaC0₃).

Referring to Figures 1 -6 of the drawings, in the columns of Toomuc sandy loam. CaCO, at rate I increased the pH significantly (p<0.05) in the surface 2 cm more than Ca(HC0₃)₂ at rate I. However, below the surface 2 cm, the increase in pH associated with Ca(HC0₃)₂ at rate I was constantly higher than that of CaC0₃ at rate I (Fig. 1 ) to a depth of 20 cm or more.

Both CaC0₃ at rate I and Ca(HC0₃)₂ at rate I with Cranboume sand gave similar results. Both treatments increased the pH significantly in the surface 3 cm and had no significant effect on the pH below this depth (Fig. 2).

With Toolangi loam (Figs. 3-6). each treatment was compared to the control to determine the mobility of alkalinity associated with the treatments. Both CaC0₃ at rate I (Fig. 3) and Ca(HC0₃)₂ at rate I (Fig. 4) equally increased the pH significantly (p<0.05) in the surface 4 cm but not below that depth. Ca(HC0₃)₂ at rate II (Fig. 5) increased the pH significantly (p<0.05) in the surface 8 cm. but had no significant effect below that depth.

In relation to the foregomg, we note that Adams (1984) reported that the effectiveness of added lime to soil depends on the initial pH, buffering capacity ofthe soil, the rate and the form ofthe lime and the seasonal pattem of temperature and rainfall. Since the leaching columns referred to above were under constant room temperature and were treated with equal amounts of water, the discussion ofthe results will be seen to concentrate on the soils' initial pH. their buffering capacity and the rate and form of lime used.

In these leaching columns, the movement of alkalinity may be due to both physical and chemical processes. Physical process refers to movement of CaCO, in suspension and alkalinity associated with it. and chemical process refers to the movement of alkalinity in solution by diffusion and mass flow. Physical movement is much more likely to happen in Toolangi loam (bulk density 1.00 g cm^"3) than in Cranboume sand and/or Toomuc sandy loam (bulk density 1.40 and 1.50 g cm^"3, respectively).

The mobility of alkalinity associated with the surface-applied CaC0₃ in Toomuc sandy loam was limited to the surface 2 cm. This may have been due to the low solubility and reaction rate of CaC0₃. As a result, the alkalinity produced during the dissociation of lime may have been neutralized immediately by the residual acidity in the surface 2 cm. resulting in less available alkalinity to move down to the lower depths. In contrast, Ca(HC0₃)₂ increased the pH less than did CaC0₃ in the surface 2 cm. but below this depth Ca(HC0₃)₂ increased the pH more than CaC0₃ and to a greater depth.

Neutralization of acidity and mobility of alkalinity is expected normally to be more complete and more rapid respectively in soils with low buffering capacity (Messick et al. 1984). In Cranboume sand, both CaC0₃ and Ca(HC0₃)₂ increased the pH considerably in the surface 1 cm, and although CaC0₃ gave a higher pH at this depth, Ca(HC0₃)₂ increased the pH more at 2,3 and 4 cm; below 4 cm there was no significant change in pH either with CaC0₃ or Ca(HC0₃)₂. The increase of pH in Toolangi loam associated with application of CaCO, at rate II. and Ca(HC0₃)₂ at rate 1. was restricted to the surface 4 cm in both treatments. Doubling the rate of Ca(HC0₃)₂ also increased the pH to a higher value and to a greater depth than the increase in pH achieved by the application at rate I of Ca(HC0₃)₂. These results indicate that increasing the rate of Ca(HC0₃)₂ even further could neutralize subsoil acidity. Thus, further study is being conducted using progressively higher rates of Ca(HC0₃)₂ and accompanying leaching conditions to evaluate the extent of effective neutralization ofthe subsoil acidity.

Although the amounts of Ca leached out of the columns were not significantly different for the treatments imposed, there was a positive correlation between the amount of Ca leached and the relative mobility of alkalinity in the columns. However, the amount of Ca leached may not necessarily indicate the mobility of alkalinity, because Ca can be leached as a neutral salt (Messick et al. 1984).

Ofthe comparisons made between CaC0₃ and Ca(HC0₃)₂. the following observations are the most significant: (a) the mobility of alkalinity associated with the application of CaC0₃ was restricted to the surface 2 cm. 3 cm. and 4 cm in Cranboume sand. Toomuc sandy loam, and Toolangi loam, respectively, the relatively greater mobility of CaC0₃ in Toolangi loam possibly being due to physical movement; and (b) Ca(HC0₃)₂ showed the highest mobility of alkalinity in Toomuc sandy loam (Fig. 1), but its effect in Cranboume sand and Toolangi loam was of somewhat less significance (Figures 2 and 6). We consider it is most likely that at higher rates of application and the imposition of optimal leaching conditions, that the higher mobility of Ca(HC0₃)₂ over that of CaC0₃ will be accentuated. On the basis of these results, the sources of acid in soils can be put into several classes based on their physical differences and on their activity. There are those which are readily accessible in the soil solution and which have high activity; there are those associated with readily accessible surfaces, some of which are active and others of which are less so: and there are those which are associated with poorly accessible surfaces, some of which are active and others of which are less so (Table 3). These may be subdivided on the basis of their reactivity toward base, either in the form of OH- or HCO₃-. Active acidity is H+ and most acid in the soil solution is likely to be of this form or associated with weak acids e.g. acetic acid. Exchangeable H+ are an active source of acid, whereas exchangeable Al is a slow source of acid by virtue of the requirement to desorb and hydrolyse.

MATERIALS AND METHODS Preparation of leaching columns

Clear vinyl tube. 2 cm in diameter, was cut into 24 lengths of 54 cm each. Each piece was set up vertically supported by two clamps. At the bottom of each tube a cut-off 25 cm³ syringe was fitted to facilitate drainage, and glass wool was put into the bottom ofthe syringe to support the soil.

150 g of soil (weighed out from Toomuc sandy loam (6 lots), Cranboume sand (6 lots) and Toolangi loam (12 lots)), were introduced into the columns with a small amount of 'packing'. These soils were selected because of their low pHs, and they had not been treated with lime before. In this experiment, only surface soils were used and some of their physical and chemical properties are given below. (a) Toomuc sandy loam

Sandy loam light grey surface soil. Duplex profile (Dy 3.41 , Northcote 1971 ) with 10 to 15% clay in the top 50 cm and >50% clay below 50 cm depth. The soil, developed from Tertiary sand, has 5 mmole(+)% C.E.C. and 3.9% organic matter in the surface (0 to 20 cm). The pH value of this soil is 5.00 when measured in 1 :2 soikwater suspension and 4.20 when measured in 1 :2 soiksolution (0.01 M CaCl₂) suspension, and its vertical drainage is poor due to clay subsoil (Holmes et al. 1940; Baldwin 1949).

(b) Cranboume sand

Sandy textured grey soil. Uniform coarse sand profile (Uc 2.33, Northcote 1971) developed from Tertiary alluvium. Owing to its texture, the soil has very low C.E.C. and as a result, its buffering capacity is also low. However, it is a well drained profile and the pH, when measured in 1 :5 soikwater suspension, is 4.67 (Holmes et al 1940).

(c) Toolangi loam

Loamy to clay loam dark brown (5YR 3/4) soil. Gradational non calcareous profile (Gn 4.1 1. Northcote 1960) developed from Silurian mudstone. It has 9.6% organic matter and 44.2 mmole(+)% C.E.C. in the surface (0 to 20 cm) depth, it is well drained and moderately acidic (pH 5.1 measured in 1 :5 soil: water suspension) (Sargeant and Skene 1970).

Preparation of Ca(HCO₃)₂

Ca(HC0₃)₂ was prepared by the method given by Shirlaw (1967). Using this method an excess amount of CaC0₃ was placed in a soda syphon with distilled water. A carbon dioxide bulb was discharged into the syphon and was shaken vigorously and allowed to stand for 24 hours. The suspension was then filtered and the filtrate titrated with 0.1N HCl using methyl orange indicator. the concentration of Ca(HC0₃)₂ determined, and the rates of application of Ca(HC0₃)₂ equivalent to CaC0₃ were then calculated.

Treatments

The treatments and the rates applied to the soils are given in Table 1. CaC0₃ was added into the columns as a suspension of 32.94 mg of CaCO, in 30.00 cm³ water in rate 1. and twice this amount was used in rate II, whereas Ca(HC0₃)₂ was added as a solution of 0.02196 N for rate I and twice of this amount was used for rate II. Rate I is equivalent for the amount of 1.048 t/h and rate II is twice that amount.

All the treatments were added, applied to the surface of each soil in the columns. Ca(HC0₃)₂ was added using a pipette and the suspension of CaC0₃ was added using a cylinder.

The soils were leached with 10 lots of 15 cm³ (47.8 mm) of water on various occasions during the experimental period, which is equivalent to about 500 mm of rainfall per year. During the experimental period, the columns were in the dark and in a room of constant temperature (about 27°C).

The leachates of each soil column were collected in a 250 cm³ plastic bottle placed under each column. Table 1. Soils, treatments and the number of replicates used.

Treatments Soils used and their rates Toomuc sandy loam Cranboume sand Toolangi loam

CaC0₃

rate I 3 reps 3 reps rate 11 3 reps 3 reps 3 reps

Ca(HC0₃)₂

rate I 3 reps 3 reps 3 reps rate II 3 reps

Control 3 reps

Chemical analysis

The soil columns were cut carefully at 1 cm intervals. The sections of soil were removed from the sections of plastic tube and spread out on pieces of paper for 1 week and air-dried. The pH was measured in 1 :5 soil to solution (0.01 M CaCl₂) suspension using pH meter (Metrohm Herisau E520) after shaking for one hour. CaCl₂ was used primarily to reduce the error that can emerge due to the difference in the weight of the soils used, and for the following reasons also: (a) the ionic strength of 0.01 M CaCl₂ is close to that ofthe soil solution; (b) use of 0.01 M CaCl₂ reduces the disturbance ofthe width ofthe diffuse double layer; (c) the pH measured in 0.01 M CaCl₂ is less affected by the soil to solution ratio; (d) CaCl₂ gives more constant and reproducible values; and (e) CaCl₂ also gives more realistic value for the pH ofthe soil solution. The leachates were evaporated to dryness and then treated with 50 ml of 0.1 m HCl. The solution was filtered, using 542 Whatman filter paper, into a 100 ml volumetric flask and made up to the mark. The concentration of Ca in the leachates was then measured using atomic absoφtion spectrophotometer (Varian AA-1475 series).

Leachates

The average amount of Ca in the leachates increased with the increase in the rates of both CaCO, and Ca(HC0₃)₂ used in the leaching columns. However, for the same rate, the amount of Ca leached in the columns treated with Ca(HC0₃)₂ was greater than that leached from the columns treated with CaCO, (Table 2). None of these increases is statistically significantly different (p>0.05).

Table 2. The average amount of Ca leached from the soils treated with different rates of CaC0₃ and Ca(HC0₃)₂.

Soil type Treatments Average amount of Ca leached (mg/column)

Toomuc CaC0₃ 1 4.12 sandy loam Ca(HC0₃)₂ 1 5.12

Cranboume CaC0₃ 1 0.50 sand Ca(HC0₃)₂ 1 0.75

Control 7.00

Toolangi CaC0₃ II 8.50 loam Ca(HC0₃)₂ 1 7.75

Ca(HC0₃)₂ II 9.35 Table 3. Classes of sources of acid in soils based on accessibility and reactivity.

Reactivity Accessibility rapid slow

High - soil solution HN0₃. acetic

Good - surfaces Exch H+ Exch Al, S-OH

Poor - surfaces Exch H+ Exch Al, S-OH

SUMMARY OF ABOVE EXPERIMENTS

Leaching columns were used to investigate the downward movement of alkalinity (measured as a change in pH in 0.01 M CaCl₂) associated with surface application of CaC0₃ in suspension and Ca(HC0₃)₂ in solution.

A rate of 1.048 t/ha of CaC0₃ was compared with the equivalent rate of Ca(HC0₃)₂ when applied to Toomuc sandy loam and Cranboume sand, and 0. 2.096 t/ha of CaC0₃ were compared with equivalent rates of 1.048 t/ha and 2.096 t/ha of CaC0₃ and Ca(HC0₃)₂ when applied to columns of Toolangi loam. Each treatment had 3 replicates and each column was leached with the equivalent of 500 mm of rainfall over the experimental period (one year) and the leachates were collected.

Soil pH was measured (1 :5 in 0.01 M CaCl₂) at 1 cm intervals ofthe leaching columns and the amount of Ca from each column was also measured.

In the leaching columns, CaC0₃ increased the pH of the surface 2 cm more than Ca(HC0₃)₂, but at depths greater than 2 cm the Ca(HC0₃)₂ increased the pH more than !6 CaC0₃. Application of 2.096 t/ha of CaC0₃ to Toolangi loam increased the pH in the top 4 cm but Ca(HC0₃)₂ at an equivalent rate increased the pH in the top 8 cm. The amount of Ca leached from the leaching columns increased with the increase in the lime rate, but with the same rate of application, the amount of Ca in the leachate was higher in Ca(HC0₃)₂ treated soils than CaC0₃ treated soils; these differences were not statistically different.

From the data it can be postulated that Ca(HC0₃)₂ is a more mobile form of base than CaCO, and is a form of base suited for the rapid amelioration of acidity at depths below that at which CaC0₃ is effective. These properties of Ca(HC0₃)₂ provide potential to treat not only acidic soils but also sodic soils and acid mine drainage areas.

The experiments have shown that under leaching conditions, Ca(HC0₃)₂ is a more mobile base than CaC0₃. the Ca(HC0₃)₂ affecting a larger volume of soil, the extent to which Ca(HC0₃)₂ affected greater depths depending on the rate applied, the initial pH and buffering capacity ofthe soil. Depth of effect of Ca(HC0₃)₂ on the three soils as given in a decreasing order is as follows: Toomuc sandy loam. Toolangi loam and Cranboume sand. The results obtained from these experiments indicate that application of higher rates of Ca(HC0₃)₂ and more optimal leaching than those used in the experiments could lead to better results in neutralizing subsoil acidity.

ADDITIONAL EXPERIMENTS

Additional experiments were conducted on mine tailings, some results of which are illustrated in Figure 7 ofthe accompanying drawings.

Tailings from mine batteries at Taradale and Mt. Egerton, Victoria, Australia, and from a tailings dam at Stawall Gold Mines, Victoria, Australia, with 88%, 83% and 80% sand and pH's (1 :5 0.01M CaCl₂) of 3.8, 5.2 and 2.7, respectively, were used. The tailings were introduced into columns of clear vinyl tube 30 cm long and supported in opaque PVC pipe. They were subjected to leaching with 0. 2. 4. 8 and 16 pore volumes of Ca(HC0₃)₂ solution and 16. 14, 8 and 0 pore volumes of distilled water, respectively; immediately they were drained to a suction of -10 kPa. After several days, the pHs of successive layers of moist soil were measured as above.

The results for the tailings (Fig. 7) showed that the solutions of Ca(HC0₃)₂ increased the pH to between 6 and 7 at the surface and to depth; the extent of the increases at depth depended on the rates of application, the initial pH and the buffering capacity.

REFERENCES

[1 ] Adams, F., and Pearson, R.W., (1969).Neutralizing soil acidity under Bermuda grass sod. Soil Science Society America Proceedings 33, 737-420.

[2] Adams. F. ( 1984). Crop response to lime in Southern United States. In 'Soil Acidity and Liming' (Ed. F. Adams), pp. 212-59 (American Society of Agronomy. Crop Science Society of America and Soil Science Society of America: Madison, WL, USA).

[3] Baldwin, J. G. (1949). A soil survey of the shire of Whittlesea, Victoria. Royal Society of Victoria Proceedings 62, 173-98.

[4] Bromfield, S.M., Cumming, R.W., David, D.J. and Williams, CH. (1987). Long-term effects of incoφorated lime and topdressed lime on the pH in a surface and subsurface of pasture soils. Australian Joumal Experimental Agriculture 27, 533-8. [5] Conyers. M. K.. and Scot. B. J., (1989). The influence of surface incoφorated lime on subsurface acidity. Australian Joumal Experimental Agriculture 29. 201-7.

[6] Holmes, L. C. Leeper. G. W. and Nicolls, K. D. ( 1940). Soil and land utilization: survey of the country around Berwick. Royal Society of Victoria Proceedings 52, 177-45.

[7] Hoyt, P. B. and Drought. B. G. (1990). Techniques for speeding the movement of lime into an orchard soil. Canadian Joumal of Soil Science 70. 149-56.

[8] Hoyt, P. B., Drought, B.G., Neilsen, G. H. and Hogue. E. (1986). Effect of type, rate and incoφoration of lime and of nitrogen fertilizer and gypsum on movement of lime in soil column. Canadian Joumal of Soil Science 66, 757-62.

[9] Messick. D. L., Alley, M. M. and Zelazni, L. W. (1984). Movement of calcium and magnesium in Ultisols from dolomitic limestone. Soil Science Society of America Joumal 48, 1096-101.

[10] Noble, A.D, Randall, P.J., and James, T.R. ( 1995). Evaluation of two coal- derived products in ameliorating surface and subsurface soil acidity. European Joumal of Soil Science 46, 65-75.

[11 ] Northcote, K. H. (1979). A Factual Key for the Recognition of Australian Soils. 4th Edn. Rellim Technical Publications, Adelaide. [12] Rechcigl.J.E. and Reneau, Jr.. R.B. (1984). Effect of subsurface acidity or alfalfa in Tatum clay loam. Communications in Soil Science and Plant Analysis 15, 811-18.

[13] Sargeant. I. J. and Skene, J. K. M. (1970). Some properties of krasnozems of Southern Victoria. Australian Joumal of Soil Research 8, 21 1-95.

[14] Scott, B. J. and Fisher, J. A. (1989). Selection of genotypes tolerant of aluminium and manganese. In 'Soil Acidity and Plant Growth'. (Ed. A.D. Robson.) pp. 167-96. (Academic Press: Sydney.).

[15] Shirlaw, D. W. G. (1967). "A Practical Course In Agricultural Chemistry'. (Pergamon Press: Sydney).

[16] Smith. C.J.. Goli. K.M.. Bond, W.J.. and Freney. J.R., (1995). Effects of organic and inorganic calcium compounds on soil-solution pH and aluminium concentrations. European Joumal of Soil Science 46. 53-63.

[17] van der Watt. H. v.H.. Barnard, R.O., Cronje, I.J.. Dekker, Croft, G.J.B. and van Walt, M.M. (1991). Amelioration of subsoil acidity by application of a coal- derived calcium fulvate to the soil surface. Nature 350, 146-8.

Claims

CLAIMS:

1. Method for the rapid amelioration of acidity in acidic or sodic soil or in acid mine drainage areas, which comprises the application of an effective volume of a calcium bicarbonate [Ca(HC0₃)₂] solution to the surface or subsurface ofthe soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2cm below the surface of said soil or acid mine drainage areas.

2. Method for the rapid amelioration of acidity in acidic or sodic soil or in acid mine drainage areas at depths below that at which lime is effective, which comprises the application of an effective volume of a calcium bicarbonate [Ca(HC0₃)₂] solution to the surface or subsurface ofthe soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2 cm below the surface of said soil or acid mine drainage areas.

3. Method for the rapid amelioration of acidity in acidic or sodic soils or in acid mine drainage areas, which comprises the application of a volume of calcium bicarbonate [Ca(HC0₃)₂] solution to the surface of subsurface ofthe soil or acid mine drainage areas, equivalent to and at a rate of calcium bicarbonate [Ca(HC0₃)₂] equivalent to calcium carbonate [CaC0₃] within the range of about 0.5 tonne per hectare to about 10 tonne per hectare, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm below the surface of said soil or acid mine drainage areas.

4. Method for the rapid amelioration of acidity in acidic or sodic soils or in acid mine drainage areas, which comprises the application of a volume of calcium bicarbonate 0.02196N [Ca(HC0₃)₂] solution to the surface or subsurface of the soil or acid mine drainage areas, equivalent to an amount ofthe order of about 1.048 tonne per hectare of calcium carbonate [CaC0₃]. optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm, preferably to depths at least between 2 cm and 8cm, below the surface of said soil or acid mine drainage areas.

5. Method for the rapid amelioration of acidity in acidic or sodic soils or in acid mine drainage areas, based upon ascertaining the initial pH of the soil or acid mine drainage areas, the buffering capacity ofthe soil or acid mine drainage areas, and the seasonal pattern of localised temperature and rainfall, in leading to an amount of calcium bicarbonate [Ca(HC0₃)₂] being applied to the soil or to the acid mine drainage areas on the basis ofthe lime requirement, said method comprising the application of a volume of calcium bicarbonate [Ca(HC0₃)₂] solution to the surface or subsurface of the soil or to the acid mine drainage areas, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth of at least about 2cm below the surface of said soil or acid mine drainage areas.

6. Method according to claim 5, wherein the rapid amelioration of acidity in acidic or sodic soil or in acid mine drainage areas is at a depth below that at which lime is effective.

7. Method according to claim 5 or claim 6, wherein a volume of calcium bicarbonate [Ca(HC0₃)₂] solution is applied to the surface or subsurface ofthe soil or acid mine drainage areas, equivalent to and at a rate of calcium bicarbonate [Ca(HC0₃)_:] equivalent to calcium carbonate [CaC0₃] within the range of about 0.5 tonne per hectare to about 10 tonne per hectare, optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm below the surface of said soil or acid mine drainage areas.

8. Method according to claim 5 or 6, wherein a volume of calcium bicarbonate 0.02196N [Ca(HC0₃)₂] solution is applied to the surface or subsurface of the soil or acid mine drainage areas, equivalent to an amount ofthe order of about 1.048 tonne per hectare of calcium carbonate [CaC0₃], optionally followed by the application of a leaching volume of water, the calcium bicarbonate solution penetrating the soil or subsoil or the acid mine drainage areas to a depth greater than 2 cm, preferably to a depth at least between 2 cm and 8cm. below the surface of said soil or acid mine drainage area.