Now that acid soil problems have been recognised as important in limiting plant production in New South Wales, considerable publicity has been given to the cause and correction of the problem. During the past two years many landholders have reported on their experiences with lime over the past twenty to thirty years. A number have indicated that they obtained little, if any benefit from the application of lime, with the conclusion that lime was not needed.

There are many reasons for the lack of response to lime and a few of the most important ones are examined here.

1. Particle Size of Lime Applied

Limestone rock is extremely insoluble when exposed to air. Evidence for this is seen in its use as a building stone and the existence of limestone caves in many parts of the world.

Limestone dissolves slowly in the soil by the action of carbon dioxide dissolved in water (carbonic acid) and other soil acids, organic acids. To achieve rapid benefit from lime (less than one year) in soil improvement and subsequent plant growth the individual particles of lime must be small.

Application of ground limestone with particles too large for rapid dissolution is perhaps the most common reason for failure to obtain the expected response to liming.

Research in the U.S.A. has shown that particles larger than 600 microns (40 mesh) react very slowly in soil, a large portion never reacting, with consequent little increase in pH. On the other hand, particles smaller than 150 microns (120 mesh) react rapidly with almost complete dissolution in one month and a large increase in pH. Figure 1 shows the reaction rate for three particle size ranges in relation to time and pH change (4).

Figure 1: Reaction rate of various particle sizes of limestone.

Table 1 shows the amount of lime of various particle sizes required to raise the pH of a soil from 4.5 to 7 in ten weeks, twenty weeks and one year respectively (5).

Mesh No.	Mesh size (microns)	10 weeks	20 weeks	One year
10	> 1700	135.4	104.2	79.4
10-28	700-1700	46.2	36.8	29.1
28-40	400-600	20.4	15.9	12.6
40-60	300-400	11.2	9.2	7.4
60-100	300-170	10.8	8.7	7.2
< 100	< 170	8.5	7.2	6.0

Table 1 - Amount of Lime (tonnes/ha) required to raise pH of soil from 4.5 to 7 with time (based on Motto and Melstead)

Much less lime with smaller than 300 microns was required to change pH.

As a rule of thumb, if ground limestone is rubbed between the fingers and any grittiness is felt or shiny crystals seen, the material is too coarse for the best results.

2. Failure to Mix Lime with Soil

The effectiveness of lime depends on reaction with acid components in the soil to make the lime soluble. Thus surface application of lime without subsequent cultivation and mixing with the soil will not be effective in correcting problems associated with acid soils. Considerable research has been carried out in the northern hemisphere and in New Zealand to investigate benefits from surface applied lime, and only after several years is any improvement at depths down to 15 cm found.

Figure 2 shows the effect of dolomitic limestone applied at about 10 tonnes/ha to a clay loam soil in North Carolina on pH three years after application (6).

Calder et al (2) and Bishop at al (1) have presented data (Table 2) showing the penetration of limestone five years after application to permanent pastures on three soils in Nova Scotia, Canada. The soils were a silty clay loam, loam and a loamy sand. Clearly, even at very high application rates, little improvement is evident below 7.5 cm - the major root zone. The greater depth to which pH is improved on the sandy soil is most noticeable.

	Silty Clay Loam		Loam			Loamy Sand
Depth(cm)	No Lime	5 t/ha	No Lime	5 t/ha	16 t/ha	No Lime	5.2 t/ha	16 t/ha
0-2.5	5.15	6.08	4.70	5.68	6.40	4.75	5.72	6.45
0.5-5.0	5.36	5.56		5.20	6.00		5.40	6.25
5.0-7.5	5.40	5.58		5.00	5.30		5.22	5.85
7.5-15.0	5.56	5.69		4.88	4.90		5.00	5.50

Table 2 - Change of pH at different depths, five years after application of various rates of lime (tonnes/ha) to three soils of varying texture.

Generally, unless large amounts of lime are applied, rainfall is high (> 750 mm/p.a.), soil textures are light and considerable time is allowed for neutralization of soil acidity, surface application of lime will have little benefit.

If, on the other hand, lime is thoroughly mixed with soil, the lime particles react rapidly with consequent neutralization of soluble aluminium.

Figure 2: pH with depth following lime application

3. Deficiencies of Other Elements

Soil acidity problems are frequently detected when landholders consult advisory officers concerning the failure of crops or pastures despite the use of adequate super. Soil tests usually show high levels of available phosphorus but low pH’s together with low calcium and high aluminium levels.

On the other hand, some landholders when advised to use lime, frequently omit super applications in the mistaken notion that lime will render soil phosphorus more available.

Failure to ensure that other elements, especially phosphorus, are not limiting while soil acidity is being corrected is a major reason why responses to lime are not obtained.

The typical acid soil high in soluble aluminium is usually not only low in calcium but often also very low in magnesium and potassium. Molybdenum is frequently also deficient.

Soil analyses are essential to determine whether magnesium and/or potassium should also be applied. If magnesium is low, use a 50:50 mixture of lime and dolomite or magnesium sulphate with the lime.

If molybdenum has not been used for five years or more then its use should be considered where legumes are grown.

4. Over-liming

In Europe and North America liming soils to a pH of at least 7.0 has long been recommended. While this practice has been successful on soils which were not initially very acid (pH 5.5 - 6.5) there is now evidence to suggest that liming very acid soils (pH 4.8 - 5.5) to pH’s approaching 7.0 can result in considerable reduction in yield. The decline in yield is accompanied by magnesium deficiency and increased concentration of aluminium in the plant.

In recent experiments Farina et al (3) found reductions in yield of 30-40% above maximum yield achieved at pH 6.0 when the pH was raised to levels approaching 7.0 or higher.

Figure 3 taken from their data illustrates this on a soil with an initial pH of 4.6 (H₂0) and a high level of soluble aluminium.

Figure 3: Relationship between corn yield and pH for an acid soil high in soluble aluminium.

The mechanism for this effect is not understood but, as acid tolerant species were less affected, the increased aluminium content of plant tissue points towards increased aluminium toxicity at the higher pH’s.

Liming to pH’s over 6.0 or the point where soluble aluminium is precipitated (pH 5.6) is consequently considered highly undesirable.

5. Toxicities of Other Elements

Lack of response due to toxicities of other elements is less frequent. On many soils in New South Wales Mn becomes toxic under conditions of waterlogging in winter or heat exposure in summer. Generally, applications of limo will not prevent this toxicity and it may be severe enough to prevent response of plants to lime. Liming will correct manganese toxicity due to acid soil conditions. Toxicities to chloride, sodium and salt may also mask the response to liming.

6. Failure to Properly Determine the Need for Lime

Lime applications are frequently made or recommended purely on the basis of pH. pH provides a guide to soil acidity but it is not a positive indicator of soluble aluminium in the soil. Since poor growth on acid soils is largely the result of soluble aluminium, the amount of soluble aluminium and also the levels of available calcium and magnesium must be known before the need for lime can be accurately predicted.

In other words, no benefit from lime should be expected unless soil analyses indicate the presence of soluble aluminium. Even sensitive species are able to grow at pHs as low as 5.0 when there is no soluble aluminium.

Nil response will also occur if rates of lime are too low to adequately remove aluminium from soil solution.

Failure to achieve responses to lime may thus be due to any of a number of different reasons. Before lime applications are considered the soil should be analysed, the correct fineness of lime chosen, other deficiencies corrected and the lime cultivated in. All of this can best be done by consulting with chemists and advisory officers in the N.S.W. Department of Agriculture.

REFERENCES

1. Bishop, R.D. et at, 1969. Can. J. Soil Sci. 49: 47-51.

2. Calder, F .W. et at, 1965. Can. J. Soil Sci. 45: 251-256.

3. Famma, M.P.W. at at, 1980. J. Plant Nut. 2: 683-697.

4. Limelight, 1973. No. 96. Southern Limestone, Borg, Moss Vale.

5. Motto, H.L., and Melsted, S.W. 1960. Soil Sci. Soc. Amer. Proc. 24: 488-490.

6. Shelton, J . E. 1976. J. Amer. Soc. Hort. Sci. 101: 481-485.