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Irrigation - Australian Agriculture Assessment 2001 - Soil acidification: an insidious soil degradation issue

SUMMARY

Distribution and extent

• Surface and subsoil acidity exists in all Australian States. It has an estimated total area of eight to nine times that affected by dryland salinity.
• The largest areas of acid soils are in New South Wales, Western Australia, Victoria and Queensland.
• Approximately 50% of agricultural land (approximately 50 million hectares) have surface pH values less than or equal to 5.5 (or below optimum for extremely acid-sensitive agricultural plants and below the optimal level to prevent subsoil acidification).
• 12 to 24 million hectares is extremely to highly acidic with pH values less than or equal to 4.8 (below optimum for the acid-sensitive agricultural plants).
• Subsoil with a pH at or below 5.5 affect 23 million hectares of agricultural land.

Rates of acidification

• Rates of acidification vary appreciably across Australia's agricultural systems
• In the absence of remedial lime applications, from 29 to 60 million hectares are projected to reduce to pH 4.8 or lower within 10 years, and a further 14 to 39 million hectares will reduce to pH 5.5.

Projected lime requirement

• About 2 million tonnes of lime are applied to agricultural land each year. Lime use has increased in most States over the past decade.
• Approximately 12 and 66 million tonnes of lime are required to adjust existing acidic soils to a typical agricultural production pH of 4.8 and 5.5 respectively.
• Maintaining soil pH values at 4.8 and 5.5 requires ranges of 0.6 - 3.1 million tonnes and 2.4 - 12.3 million tonnes of lime each year respectively, to counter re-acidification at rates of 50 and 250 kg lime equivalent per ha each year respectively.
• Cost is a major deterrent to the wider use of lime in all States.
• Farmer awareness of soil acidification and its insidious consequences is by no means universal. Other farming practices (e.g. use of acid-tolerant plants) are being used where the cost of liming is prohibitive.

Off-site impacts

• Information linking soil acidity to off-site consequences is negligible. Many hypotheses exist and need to researched.

INTRODUCTION

Accelerated soil acidification is recognised globally as a serious soil degradation problem that is reducing agricultural production. The Audit commissioned a series of detailed assessments (Figure 4.1) on soil acidification including:

• mapping distribution and estimating extent of acidic soils in agricultural regions of Australia;
• predicting where soil acidity is likely to occur in the future;
• determining rates of soil acidification for regional farming systems;
• determining the pH buffering capacity of Australian soils, in order to assess where soils susceptible to acidification occur and how much lime is required to neutralise acidity;
• assessing plant yield penalties, financial benefits and costs associated with growing different plant species on a variety of soils with different acidity status and following the amelioration of acidity with lime applications;
• evaluating possible off-site risks of soil acidity; and
• summarising regional information on management options.

ACIDIFICATION: THE PROCESS

Natural ecosystems

Soil acidification is a natural process. It begins when rocks are first colonised by algae and lichens. Acids (or protons) produced mainly from the carbon and nitrogen cycles begin to dissolve the rocks and soil minerals to form the parent soil. In natural ecosystems, soils gradually become more acidic with time so that older and more weathered soils are usually more acidic than younger soils.

In natural ecosystems, the amount of plant material and soil organic matter approaches equilibrium levels. Internal cycling of nutrients-especially nitrogen-is tightly controlled, and as a result, nitrogen losses as nitrate are minimal.

Nitrate leaching is considered to be a major cause of soil acidification in all ecosystems. Thus, in natural ecosystems, the inputs of acids are nearly balanced by neutralising processes, so that soils only become more acidic over many thousands of years.

Australia's soils are old and highly weathered. Some of them will be naturally more acidic than others. Natural acidity can also exist deeper down in the soil profile.

Agricultural systems

Soil acidification rates increase when land is developed for agriculture. Induced acidification (as distinct from natural acidification) involves changes to the nitrogen and carbon cycles, and the accumulation, depletion and transport of acids and bases (Helyar & Porter 1989). Induced acidification in soils arises from:

• leaching of soil nitrate-nitrate is very soluble in water and leaches below the root zone before the plant can take it up, leaving acidity in the soil; soil nitrate can come from legumes or nitrogen fertiliser. Application of ammonium fertilisers, which when converted to nitrate produce acidity;
• addition of organic acids;
• removal of alkalinity through removal (offtake) of crop and livestock products-removal of legume hay is a particularly acidifying practice (Table 4.1); and
• transfer of excreta to localised stock camps leaving surrounding land more acidic.

Soil acidification is an insidious soil process, developing slowly with subtle symptoms. If not corrected, the process can continue until irreparable damage occurs.

Table 4.1 The amount of lime needed to neutralise acidification caused by removal of alkalinity in agricultural produce.

Product	Yield	Lime requirement
Wheat	2 t/ha	18 kg/ha
Lupins	2 t/ha	40 kg/ha
Grass hay	5 t/ha	125 kg/ha
Clover hay	5 t/ha	200 kg/ha
Lucerne hay	5 t/ha	350 kg/ha
Wool*	5 kg/sheep	0.07 kg/sheep
Meat*	1 lamb	0.02 kg/lamb
Milk*	1000 litres	4 kg/1000 litres

* Additional acidification for the majority of the paddock occurs under set stocking with livestock. They consume pasture, which contains alkalinity and then deposit most of this alkalinity as dung and urine in areas where they camp, making most of the paddock more acid but the camps more alkaline.

MEASURING SOIL ACIDITY

Acidity is measured as pH: a logarithmic scale ranging from 0 to 14 (Figure 4.2). A neutral pH is seven, and each consecutive pH unit below seven (e.g. six) is ten times more acidic. Conversely, soils with pH values above seven become progressively more alkaline.

Traditionally, soil pH in Australia has been measured either in water (pH_w) or in 0.01M calcium chloride (pH_Ca). The latter test provides pH values about 0.8 - 0.9 pH unit lower than measurements in water (Figure 4.3), and has become the preferred test in most Australian States because the measurements are more stable. Unless otherwise stated, hereafter references to soil pH infers the pH_Ca test.

• Acidic soils are defined as those with a pH reading less than or equal to 5.5.
• Moderately acidic soils are those with pH values between 4.8 and 5.5.
• Highly acidic soils have pH values between pH 4.3 and 4.8.
• Extremely acidic soils have pH values less than 4.3.

ASSESSING DISTRIBUTION OF ACIDIC SOILS

Two approaches were used to report on the distribution of acidic soils. They give independent estimates of soil pH (except for South Australia), but should only be used as guides because of potential errors are associated with each method.

Commercial soil testing

• 636 000 surface soil pH values were collected from 12 private and public sector agencies operating commercial soil testing services for farmers. These data (covering 1990 to 1999) for the surface soil depth (range 0 - 15 cm, but dominated by 0 - 10cm depths and 0 - 10cm in Western Australia) were merged, allocated to the nearest town and areas (polygons) were drawn around each town. The exceptions were Western Australia, (where the data were allocated to 20 km by 20 km grids) and South Australia, (where about 50% of samples were referenced to property centroids). Samples reporting pH measured in water from Queensland and northern New South Wales were converted to pH in calcium chloride using the Ahern et al. (1995) equation.
  
  The data were allocated to pH classes (see below) and the proportion of each class determined in each polygon. From this information, statistics on areas and maps were generated showing the spatial distribution of soil pH in the agricultural area, which was not limited to the intensive land use zone. The exception was again Western Australia where there was no commercial pH data for 2.9 million hectares of agricultural land.
  
  Since samples were collected from non-random locations, errors may result from particular soil types being sampled and biasing the polygon data set. Some polygons had limited number of points (< 10 values) and therefore there is a greater risk of inaccuracy.

Australian Soil Resources Information System

• These data came from CSIRO and State agency soil survey data sets. They cover soil samples from both agricultural and uncleared lands, and for surface and subsoils.
  
  A modelling approach was used to:

  • generate models for soil pH, based on the soil point data;
  • extend these models to predict the pH for 250 m by 250 m cells, based on soil type data, topographic variables and climate surfaces; and
  • assess the precision and accuracy of the models.
  
  The models satisfied statistical tests in most States (with the exception of South Australia) and a new model was derived using South Australian commercial pH data. Although the models allowed pH to be derived at a finer scale of resolution than by using commercial data, point data from government databases is not as contemporary as the commercial pH data.
  
  From the modelled pH information, statistics on areas and maps were generated showing the spatial distribution of soil pH in catchment containing areas of intensive land use.

DISTRIBUTION AND EXTENT OF ACIDIC SOILS IN AUSTRALIA

Surface soils

Maps generated from commercial farm soil testing data collected over the past decade (Figure 4.4) and State/Territory agency land resource assessment programs (Australian Soil Resources Information System) (Figures 4.5, 4.6) were broadly consistent:

• Both indicate considerable spatial variation in surface soil pH within Australia's agricultural zone.
• Both sources of pH information show that there are major areas of acidic soils (pH less than or equal to 5.5) in all States (Table 4.2)

Ranges quoted relate to the estimates from commercial agencies and Australian Soil Resources Information System.

Acidic soils are more prevalent where annual rainfall exceeds 500 mm, and are most concentrated in a broad band extending from the central Tablelands of New South Wales through central Victoria into south-eastern South Australia. Major acidic areas also occur in the agricultural zone of Western Australia.

Within the agricultural areas of Australia (Table 4.2), it was estimated that between 11 and 21 million hectares of agricultural land had strongly acidic topsoils (pH 4.3 - 4.8), and from 1 to 3 million hectares were extremely acidic (pH less than 4.3). A much larger area of land (25 to 37 million hectares) was estimated to have moderately acidic topsoils (pH 4.8 - 5.5).

• The largest area of strongly acidic soils (pH 4.3 - 4.8) existed in New South Wales (5 to 7 million hectares), Victoria (4 to 5 million hectares) and Western Australia (1 to 7 million hectares).
• The largest area of moderately acidic soils (pH 4.8 - 5.5) occurred in Western Australia (7 to 19 million hectares) and New South Wales (11 to 13 million hectares) and to a lesser extent Victoria (2 to 3 million hectares).

Table 4.2 National and State areas (million hectares) of surface soil (0 - 10 cm) pH (measured in calcium chloride) based on information from Australian Soil Resources Information System (first number) and commercial laboratories (second number).

	< 4.3	4.3a - 4.8a	4.8 - 5.5a	5.5 - 7.0a	7.0 - 8.5a	> 8.5	Totalc
	(million hectares)
New South Wales	0.2 - 1.1	4.8 - 7.1	12.7 - 10.6	18.5 - 10.9	1.5 - 7.9	0 - 0.0b	37.6 - 37.8
Queensland	~ 0.3	0.6 - 1.0	1.5 - 1.8	7.4 - 3.7	0.8 - 4.3	0	10.5 - 11.1
South Australia	<= 0.1	0.2 - 0.5	0.8 - 1.4	4.3 - 3.6	6.7	0.0b	11.9 - 12.2
Tasmania	~ 0.1	0.6 - 0.5	1.0 - 0.9	0.0b - 0.3	0.0b	0	1.6 - 1.8
Victoria	0.4 - 1.0	4.1 - 4.5	2.0 - 3.1	4.8 - 2.0	2.9 - 3.5	0 - 0.0b	14.1 - 14.2
Western Australia	0.1 - 0.7	1.0 - 7.5	18.9 - 7.4	1.4 - 2.4	0.0b - 1.1	0	21.4 - 19.2
Australiac	1.1 - 3.3	11.3 - 21.2	36.8 - 25.2	36.3 - 22.9	11.8 - 23.5	0.0b - 0.1	97.3 - 96.2

^aInclusive

^b Numbers rounded to 0.0 vary from 0.01 - 0.03.

^c Total values may be slightly different to adding up the values in the table because of rounding errors.

Subsoils

Modelled data from the Australian Soil Resources Information System were used to map the distribution of acidic pH classes pH 4.8 and 5.5 in both the surface and subsoil layers (Figures 4.7, 4.8).

Large regions of land in all States have moderately acidic soil (pH below 5.5) in both the surface and subsoil (Figure 4.8), and particularly in New South Wales, Western Australia, Victoria and Queensland. Regions with strongly acidic surface and subsoil pH values (< pH 4.8) were far less extensive (Figure 4.7).

Nationally, the extent of subsoil acidity was estimated to be large (Table 4.3):

• about 23.1 million hectares of land had subsoil acidity grading from extreme to moderate severity (pH less than or equal to 5.5);
• 21.7 million hectares had pH less than or equal to 5.5 in both the surface and subsoil layers;
• 5.3 million hectares had subsoils with pH less than 4.8 (extreme to strongly acidic), and 0.8 million hectares had a pH less than 4.3 (extremely acidic);
• 0.2 million hectares had pH values <4.3 (extremely acidic) in both soils layers;
• 3.8 million hectares had soil pH <4.8 (strongly acidic) in both soils layers; and
• 0.7 million hectares had pH <4.3 in the subsoil and pH <4.8 in the surface

At a State level, New South Wales has the largest estimated area of subsoil acidity (3.1 million hectares below pH 4.8 and 8.6 million hectares between pH 4.8 and 5.5 (Table 4.4), followed by Western Australia (0.2 and 4.8 million hectares respectively) and Victoria (0.6 and 2.5 million hectares respectively).

Table 4.3 National area (million hectares) of surface soil (0 - 10 cm) and subsoil (30 - 40 cm) pH_Ca based on information from the Australian Soil Resources Information System.

	pH	Surface soil						Totalc
		<= 4.3	4.3 - 4.8a	4.81 - 5.5a	5.51 - 7.0a	7.0 - 8.5a	> 8.5
	(million hectares)
	<= 4.3	0.2	0.5	0.1	0.0b	0	0	0.8
	4.3 - 4.8a	0.2	2.9	1.3	0.0b	0.0b	0	4.5
Subsoil	4.81 - 5.5a	0.4	5.1	10.9	1.3	0.1	0	17.8
	5.51 - 7.0a	0.3	2.6	21.3	18.4	2.3	0	45.0
	7.0 - 8.5a	0.0b	0.2	2.9	16.2	9.2	0	28.6
	> 8.5	0.0b	0	0.2	0.3	0.3	0	0.7
	Totalc	1.1	11.3	36.8	36.3	11.8	0.0b	97.3

^a Inclusive

^b Numbers rounded to 0.0 vary from 0.01 - 0.04.

^c Total values may be slightly different to adding up the values in the table because of rounding errors.

Table 4.4 National and State areas (million hectares) of surface soil pH_Ca with a subsoil pH_Ca of < 4.8 and 4.8 - 5.5 based on information from Australian Soil Resource Information System.

	Subsoil pH = 4.8				Subsoil pH = 4.8 - 5.5a
	Surface pH				Surface pH
	<= 4.8	4.8 - 5.5a	> 5.5	Totalc	<= 4.8	4.8 - 5.5a	> 5.5	Totalc
	(million hectares)				(million hectares)
New South Wales	2.2	0.9	0.0b	3.1	2.2	5.5	0.9	8.6
Queensland	0.6	0.3	0.0b	0.9	0.3	0.5	0.1	0.9
South Australia	0.0b	0.0b	0.0b	0.0b	0.0b	0.0b	0.1	0.1
Tasmania	0.3	0.1	0	0.5	0.3	0.5	0.0b	0.8
Victoria	0.6	0.0	0.0b	0.6	2.0	0.4	0.1	2.5
Western Australia	0.1	0.1	0.0b	0.2	0.7	4.0	0.2	4.8
Australiac	3.8	1.5	0.1	5.4	5.5	10.9	1.4	17.8

a Inclusive

^b Numbers rounded to 0.0 vary from 0.01 - 0.03.

^c Total values may be slightly different to adding up the values in the table because of rounding errors.

CAPACITY OF AUSTRALIAN SOILS TO RESIST pH CHANGE

Soils have an intrinsic ability to resist pH change-either a decrease from an acid input (acidification) or an increase from the application of lime (lime requirement). This is known as the pH buffering capacity and is determined by a chemical test.

Organic matter is the major determining factor influencing pH buffering; clay content is the next important factor. A higher organic matter or clay content will result in a higher pH buffering capacity (Figure 4.9). Estimates of pH buffering capacity are important for providing advice on levels of lime application required to correct soil acidity. Units are usually expressed as tonnes of lime per hectare per pH unit change.

Estimating pH buffering capacity

Review and testing of published relationships between pH buffering capacity and soil properties to determine the most appropriate test was carried out. The best relationship (correlation or r² of 0.7 - 0.9) for a wide range of surface soils was by Aitken et al. (1990):

pH BC = [0.955 OC% + 0.011 Clay%] x 1.2

A visual representation of this relationship is shown in Figure 4.9. The relationship is expressed as tonnes of lime required to change the pH by one unit per hectare assuming a surface soil bulk density of 1200 kg/m³.

This relationship was poor for wet tropical soils with variable charge characteristics and predicted pH buffering capacity on these soil types will have greater risk of error. Approximately half the lime applied to these soils is used to generate increased cation exchange capacity rather than in raising soil pH. Data for more appropriate tests for these soils were not available.

For subsoils

Australian literature contains little evidence on pH buffering capacity relationships for subsoil. Functions that rely on soil organic carbon alone as a variable would provide overestimates. A relationship (Noble et al. 1997) that relies less on the organic carbon compared to the relationship for the surface soil was used for the subsoils (30 - 40 cm):

pHBC = [12.79 - 0.19 Clay% - 0.7 OC% - 0.03 Silt% + 0.74 Silt% x OC%] x 0.06

pH BC (measured as t CaCO₃/ha/pH) refers to tonnes of lime per hectare per pH unit; and

OC% is the organic carbon percent

Clay% is the clay percent

Silt% is the silt percent

It is anticipated that these functions for pH buffering capacity, and possibly other functions listed in the Audit Soil Acidity Report (Dolling et al. 2001), will be used by agricultural service agencies to better estimate lime requirements for acidic soils identified in Australia.

Maps of pH buffering capacity based on these relationships were generated at a nominal resolution of 250 m grid cells from the modelled organic carbon, clay and silt data in the Australian Soil Resources Information System.

pH buffering capacity variations in Australian soils

• 27 million hectares (74% of the 37 million hectares of moderately acidic surface soils) require very low to low amounts of lime (less than or equal to 1.5 tonnes lime per hectare) to increase the surface soil pH by one unit (Table 4.5, Figure 4.10). In highly acidic soils, the area of soils requiring very low to low amounts of lime to increase the surface soil pH by one unit was 4 million hectares (or 33% of the 11 million hectares of the highly acidic soils; Table 4.5, Figure 4.10).
• 91% of the 19 million hectares of moderately acidic surface soils in Western Australia require very low to low amounts of lime (less than or equal to 1.5 tonnes lime per hectare) to increase the pH by one unit (Table 4.5, Figure 4.10). In comparison, greater amounts of lime are estimated to be required to increase the pH in New South Wales with 66% of the 13 million hectares requiring very low to low amounts of lime to increase the pH by one unit. In the other States, the pH buffering was generally low to moderate (0.5 - 2.5 tonnes lime per hectare per pH unit) (Table 4.6, Figure 4.10).
• 84% of the 5 million hectares of strongly acidic soils in New South Wales require low to moderate amounts of lime (0.5 - 2.5 tonnes lime per hectare per pH unit) to increase the pH by one unit (Table 4.6). In Victoria, greater amounts of lime are required, with 89% of the 4 million hectares requiring moderate to high amounts of lime (>1.5 or >2.5 tonnes lime per hectare) to increase the pH by one unit (Table 4.5, Figure 4.10).

Table 4.5 Estimated area (million hectares) of agricultural land in Australia having topsoils with very low to high pH buffering capacity.

Estimated area (million hectares)		Tonnes lime per hectare to raise one pH unit
Topsoil pH	<= 0.5	0.5 - 1.5a	1.5 - 2.5a	> 2.5
<4.3d	0.1	0.2	0.3	0.5
4.3 - 4.8a	0.3	3.4	3.6	3.9
5.5 - 7.0a	6.8	21.2	6.3	1.8
7.0 - 8.5ae	5.4	4.8	1.8	0.4
> 8.5e	0.0b	0.0	0.0	0.0
Totalc	22.1	47.5	18.0	9.6

^aInclusive

^b Numbers rounded to 0.0 vary from 0.01 - 0.03.

c Total values may be slightly different to adding up the values in the table because of rounding errors.

d Underestimates the amount of lime required, because the breakdown of clay minerals and the increase of aluminium, which increases the amount of lime required, was not taken into account.

e Underestimates the amount of lime required, because the effect of the calcium carbonate (lime) naturally occurring in these soils on their pH buffering capacity could not be estimated.

The pH buffering capacity of soil also influences the time taken for soil to decrease to a pH value which would impair plant growth. In the context of acidification, the pH buffering capacity is expressed in terms of the amount of lime required to neutralise the acid inputs.

• Most (74 - 77%) strongly and moderately acidic surface soils in Australia have a very low to low pH buffering capacity (below 1.5 tonnes of lime per hectare per one pH unit) (Table 4.5) and are therefore at risk to acidifying more rapidly, and especially where rates of acid addition are high.

Data on pH buffering capacity were also used to estimate and map lime requirement strategies and determine the time taken to become acidic, either pH 4.8 or 5.5.

Table 4.6 Estimated area (million hectares) of agricultural land by State having topsoils with very low to high pH buffering capacity.

Surface soil	State	Tonnes lime per hectare to raise one pH unit
pH		< 0.5	0.5 - 1.5a	1.5 - 2.5a	> 2.5	Totalc
		Very low	Low	Moderate	High
		(million hectares)
< 4.3	Australiad	0.1	0.2	0.3	0.5	1.1
4.3 - 4.8a	New South Wales	0.1	2.4	1.6	0.7	4.8
	Queensland	0.1	0.3	0.2	0.1	0.6
	South Australia	0.0b	0.1	0.1	0.1	0.2
	Tasmania	0	0	0.1	0.5	0.6
	Victoria	0	0.4	1.4	2.3	4.1
	Western Australia	0.2	0.2	0.3	0.3	1.0
	Australiac	0.3	3.4	3.7	4.0	11.3
4.8 - 5.5a	New South Wales	0.9	7.5	3.0	1.3	12.7
	Queensland	0.2	0.7	0.4	0.2	1.5
	South Australia	0.0b	0.3	0.3	0.2	0.8
	Tasmania	0	0	0.3	0.7	1.0
	Victoria	0.0b	0.5	1.1	0.5	2.0
	Western Australia	8.3	8.9	1.2	0.4	18.9
	Australiac	9.5	17.9	6.2	3.2	36.8
5.5 - 7.0a	New South Wales	3.3	10.9	3.4	0.9	18.5
	Queensland	1.1	4.2	1.7	0.3	7.4
	South Australia	1.1	2.4	0.6	0.1	4.3
	Tasmania	0	0	0	0	0.0b
	Victoria	0.7	3.1	0.6	0.4	4.8
	Western Australia	0.6	0.6	0.1	0.1	1.4
	Australiac	6.8	21.2	6.4	1.7	36.3
> 7.0	Australiae	5.4	4.8	1.9	0.4	12.5

^a Inclusive

^b Numbers rounded to 0.0 vary from 0.01 - 0.03.

^c Total values may be slightly different to adding up the values in the table because of rounding errors.

^d Underestimates the amount of lime required, because the breakdown of clay minerals and the increase of aluminium, which increases the amount of lime required, was not taken into account.

^e Underestimates the amount of lime required, because the effect of the calcium carbonate (lime) naturally occurring in these soils on their pH buffering capacity could not be estimated.

RATES OF SOIL ACIDIFICATION IN FARMING SYSTEMS

Annual rates of acidification

Annual rates of acid addition vary with the type of farming system and seasonal conditions (seasonal conditions affect the extent of nitrate leaching, a major factor in soil acidification). Rates of acidification are conventionally expressed as lime needed to neutralise the acid load generated each year (kg lime/ha/year).

Annual acidification rates estimated between different farming systems in tropical, temperate and Mediterranean regions across Australia (Table 4.7) are mostly positive (i.e. the farming system requires an input of lime to maintain soil pH). Where acidification values are negative, no lime is required. The data have been summarised by agro-ecological regions, farming system and commodity classification.

• Estimates of acidification rates vary from an alkalising farming system of tobacco production (-260 kg lime/ha/yr) to strongly acidifying farming systems such as banana production (2000 kg lime/ha/yr) (see case study). Rates are more commonly in the range 50 to 250 kg lime/ha/year.
• The tobacco industry uses nitrate fertilisers, which add alkalinity to the soil. By comparison, the banana industry is located in a high leaching environment and use high rates of nitrogenous fertilisers. Consequently, the industry has a high rate of nitrate leaching, which causes acidification. However changes in fertiliser types and management can significantly alter the acid addition rate.
• Acidification rates can also be high on sandy soils that are subject to extreme leaching, particularly under wheat due to the application of ammonium fertilisers and rapid leaching of nitrate during early crop development.
• Rates of nitrate leaching are generally lower under perennial pastures than under annual pastures (e.g. Ridley et al. 2001).
• Crops are generally more acidifying than pastures, and continuous legumes and pasture cut for hay can have very high acidification rates (Tables 4.7, 4.8).

Table 4.7 Published acid addition rates (annual acidification rates, kilogram lime per hectare per year) for Australian agricultural and pastoral systems.

Agro-ecological region	Farming system	Commodity classification	Annual acidification rates(mean)	Annual acidification rates(range)	Data qualitya
NW wet/dry tropics	Stylosanthes spp. based pastures	Grazed pasture	60	25 - 90	M1
NE wet/dry tropics	Stylosanthes spp. based pastures	Grazed pasture	60	0 - 175	H
NE wet/dry tropics	Stylosanthes seed production	Seed production	530		M2
NE wet/dry tropics	Tobacco monoculture	Other non-cereal crops	(-)120b	(-)260b	- 25 H
Wet tropical coast	Grass & legume pasture; grass + Nc	Pasture cut for hay	320	50-550	H
Wet tropical coast and Wet subtropical coast	Sugar cane monoculture	Sugar cane	170	140-235	H
Wet tropical coast	Banana monoculture	Plantation fruit	1710	1400-2000	H
Subtropical slopes and plains	Leucaena	Agroforestryd	50		M2
Subtropical slopes and plains	Stylosanthes spp. based pastures	Grazed pasture	55		M2
Subtropical slopes and plains	Grape monoculture	Grapes	95	65 - 125	M1
Wet subtropical coast	White clover/paspalum/carpet grass	Grazed pasture	125	60 - 180	M1
Wet temperate coasts	Continuous grazing	Grazed pasturee	55	12.5 - 132	H
Wet temperate coasts	Continuous grazing with feed supplements (intensive dairy)	Grazed pasturee	25	(-)10.5b - 95	H
Wet temperate coasts	Regular hay cutting; med to high intensty grazing	Pasture cut for hay	85	5 - 145	H
Temperate highlands	Eucalypt forest	Agroforestryd	45		M2
Temperate highlands	Sub clover/annual grasses; sub clover/perennial grasses	Grazed pasture	120	40 - 220	H
Temperate highlands	Continuous wheat + Nc	Cereals excluding rice	105	45 - 230	H
Temperate highlands	Continuous lupin	Legumes	625		M2
Temperate slopes and plains	Continuous pasture; dryland lucerne	Grazed pasture	50	25 - 80	H
Temperate slopes and plains	Continuous wheat (fertilised with N and P)	Cereals excluding rice	80	20 - 145	H
Temperate slopes and plains	Continuous lupin	Legumes	72.5		M1
Temperate slopes and plains	Canola	Oil seeds	128		M1

^aThe data quality was assessed as either; high (H) data obtained over more than 3 sites using clearly defined methodology, medium 1 (M1) data obtained over 2 - 3 sites using clearly defined methodology, medium 2 (M2) data obtained from 1 site using clearly defined methodology.

b The negative value (-) indicates lime is applied to soil rather than lime being required to neutralise the acidity.

c +N indicates nitrogen fertiliser was applied, -N indicates no applied nitrogen.

d Data on annual acidification rates under agroforestry are very limited. Some trees (e.g.. white cedar) are known to cause net alkalisation of the surface soil. The annual acidification rates for agroforestry should therefore be treated with caution, as it will depend on the species grown.

e It is important to distinguish between grazed pasture, and grazed pasture with feed supplements, as the latter can cause net alkalisation.

Table 4.8 Published acid addition rates (annual acidification rates, kilograms of lime per hectare per year) for different cropping and pasture rotations.

Agro-ecological region	Farming system	Commodity classification	Annual acidification rates (mean )	Annual acidification rates (range)	Data qualitya
NE wet/dry tropics	Summer crop - winter fallow	Summer crop rotations	75	40 - 150	H
NE wet/dry tropics	Crop - pasture rotation	Summer crop/pasture rotation	75		M1
Subtropical slopes and plains	Summer crop - winter fallow	Summer crop rotations	125	70 - 175	M1
Wet temperate coasts	Crop - pasture rotation	Winter crop/pasture rotation (wheat, barley, canola, peas, beans)	110		H
Temperate highlands	Wheat - pasture, - N b	Wheat/pasture rotation	115	62.5 - 195	H
Temperate highlands	Wheat - lupin	Wheat/lupin rotation	140	70 - 205	H
Temperate slopes and plains	Pasture - wheat	Wheat/pasture rotation	20	10 - 40	H
Temperate slopes and plains	Wheat - lupin, pasture - wheat - lupin	Wheat/lupin rotation	20	10 - 30	H
Temperate slopes and plains	Crop - pasture rotation	Winter crop/pasture rotation. (wheat, barley, canola, peas, beans)	110	25 - 345	H
Temperate slopes and plains	Continuous crop	Continuous winter crop (wheat, barley, canola, peas, beans)	220	170 - 320	H
Temperate slopes and plains	Rice - wheat - pasture (irrigated)	Irrigated rice/wheat/pasture rotations	470	395 - 520	H

^a The data quality was assessed as either; high (H) data obtained from more than 3 sites using clearly defined methodology, medium 1 (M1) data obtained over 2 - 3 sites using clearly defined methodology.

^b -N indicates no nitrogen fertiliser was applied.

Predictions for future soil acidification risk

Estimates were made of how long it would take, in the absence of lime applications, for agricultural surface soils to decrease to pH 4.8 or 5.5.

Results from this analysis are an early warning signal to land managers that soil acidity may be a problem.

The areas of, and times for Australia's soils to reach either pH 4.8 or 5.5 primarily depend on the rate of acid addition (Figures 4.11 and 4.12). Predicted areas that will become highly acidic (pH 4.8) within 5 or 10 years are substantially greater than those predicted to become moderately acidic (pH 5.5, e.g. compare Figures 4.11a with 4.12a and Figures 4.11b with
4.12b). This is related to the large areas of agricultural land in Australia that are already moderately acidic.

Nationally, almost 29 - 60 million hectares (minimum - maximum acid addition rates) are projected to reach pH 4.8 within 10 years, with Western Australia (14 to 20 million hectares) and New South Wales (8 - 22 million hectares) being the States most at risk (Table 4.9). The model also predicted that from 18% (minimum acid addition rates) to 44% (maximum acid addition rates) of the total land affected will reach pH 4.8 within five years.

By comparison, 14 - 39 million hectares (minimum - maximum acid addition rates) were predicted to reach pH 5.5 within 10 years and from 6 - 25 million hectares within five years (Table 4.9).

Table 4.9 National and State areas of agricultural land with pH greater than 4.8 and 5.5 and the predicted years to reach pH 4.8 and 5.5 at an acid addition rate of 50 (first number) and 250 (second number) kilograms lime equivalent per hectare per year.

	Currently acid	< 5 years	5a - 10 years	10a - 20 years	> 20a years	Total b
	(million hectares)
pH 4.8
New South Wales	5.0	3.9 - 15.5	3.6 - 6.8	5.8 - 9.3	19.2 - 0.9	32.6
Queensland	0.9	0.6 - 2.5	0.7 - 2.3	0.9 - 4.5	7.5 - 0.3	9.6
South Australia	0.2	0.5 - 4.8	2.1 - 2.3	1.9 - 4.2	7.1 - 1.5	11.6
Tasmania	0.6	0.3 - 0.7	0.1 - 0.6	0.2 - 0.1	0.4 - 0.0	1.0
Victoria	4.5	0.9 - 4.2	0.6 - 2.1	2.4 - 2.7	5.8 - 0.6	9.6
Western Australia	1.2	10.2 - 19.5	4.3 - 0.6	4.5 - 0.2	1.3 - 0.0	20.3
Australiab	12	16 - 47	11 - 13	16 - 21	41 - 3	85
pH5.5
New South Wales	17.6	2.5 - 10.5	2.9 - 6.4	3.8 - 3.0	10.7 - 0.2 1	9.9
Queensland	2.4	0.7 - 3.2	0.7 - 3.9	1.0	5.7 - 0. 03	8.1
South Australia	1.0	1.3 - 5.2	2.3 - 1.8	1.3 - 3.2	6.0 - 0.7	10.9
Tasmania	1.6	0.0	0.0	0.0	0.0	0.0
Victoria	6.5	0.2 - 4.6	1.7	2.1 - 1.2	3.6 - 0.1	7.6
Western Australia	20.0	0.8 - 1.3	0.2 - 0.1	0.2 - 0.02	0.2 - 0.00	1.4
Australia b	49	6 - 25	8 - 14	8	26 - 1	48

^a Inclusive

^b Total values may be slightly different to summing values in the table because of rounding errors.

Predictions for future soil acidification risk

Predictions of soil acidification risk (time to acidify) used two acid addition rates and required several modelled outputs:

• surface soil pH values above a pH of 4.8 or 5.5 (Figure 4.5);
• surface soil pH buffering capacity;
• acid addition rates equivalent to 50 and 250 kg lime/ha/year ( Table 4.7 and Table 4.8).

The time taken to acid is calculated as follows:

Time (years) = [(pH current - pH critical) x pH buffering capacity]/acid addition rate

pH current is the modelled surface soil pH for each 250 m by 250 m cell (Figure 4.5, where pH current is greater than pH critical)

pH critical is either 4.8 or 5.5

pH buffering capacity is the modelled surface soil pH buffering capacity (Figure 4.10, tonnes of lime per hectare per pH unit) for each 250 m by 250 m cell

acid addition rate (tonnes of lime per hectare per year) is the amount of lime required to balance the acid input from agriculture.

Two acid addition rates were selected because of the large variation in acid addition rates from different farming systems and different environments, and difficulties in predicting future land use. They were equivalent to 50 and 250 kg of lime per hectare per year and cover most of the variation in the published acid addition rates. Some plantation fruit, such as bananas, can be as high as 2000 kg of lime per hectare per year, but the area affected is relatively small. Acid addition rates higher than 250 kg of lime per hectare per year can also occur for the broadacre crops, but are relatively isolated and do not occur every year.

This analysis assumes that:

• all acidification occurs in the surface soil, even though subsoil also acidifies-insufficient information exists for distributing acidification throughout the soil profile, since this varies with soil type and plant species;
• no liming materials were applied to soils-a conservative assumption for soils with pH values between 4.8 and 5.5. However, predicting which soils will or will not be limed in the future is impossible to determine.

LIME APPLICATION STRATEGIES

Application of liming materials to agricultural soils to alleviate soil acidity does not stop soil acidification. Rather acidification continues (re-acidification) at new soil pH level, and over time, surface-applied lime slowly exerts its effect at lower soil depths. Further applications of lime are an ongoing requirement (depending on rates of re-acidification).

Hypothetical examples of re-acidification (Figure 4.13) show that in the absence of lime application, soil pH continues to decline, while soil pH in the lime strategies are raised and then slowly fall, to be elevated again by more lime.

For lime application strategies to be effective we need to know:

• the amount of lime required to adjust the pH of existing acidic soils to the critical values of 4.8 and 5.5
• the amount of lime that will be required to maintain soil pH at around these values.

Consequences of liming strategies

The benefits of a high lime/higher cost strategy (maintaining pH around 5.5), compared to a zero lime/no investment strategy are:

• high plant yields;
• improved soilmicrobial activity and earthworm populations (in some soils);
• improved plant nutrition and water use;
• no loss of soil clay minerals;
• less impact from subsoil acidity; and
• greater ability to cope with subsequent re-acidification.

The low lime strategy (soil pH maintained around 4.8) has less positive impacts on these benefits. Some soil clay may be lost, yield potential of sensitive plant species may not be realised, and subsoil acidity may continue.

The zero lime strategy predicts slower rates of soil pH decline, because when acidic soils reach a pH of about 4.5, the pH buffering capacity increases because of the dissolution of soil clay minerals.

Adjusting the pH

The amounts of lime required to increase soil pH to the critical values of 4.8 and 5.5 were estimated and mapped (Figures 4.14, 4.15) for pH values in surface soils predicted to be less than 4.8 and 5.5 (low and high lime application strategies respectively). Amounts of lime were estimated from the product of pH buffering capacity and the difference between existing predicted and critical soil pH values.

• Most of Australia’s acidic soil (78% or 38 million hectares) only requires relatively small amounts of lime (less than 2 tonnes lime per hectare) to increase the pH to 5.5 (Figure 4.15, Table 4.10). For many soils, this reflects the closeness of the current pH to 5.5 (Table 4.2) and their low pH buffering capacity (Table 4.5).
• Most of Australia’s extremely or highly acidic soil (89% or 11 million hectares) require less than 2 tonnes of lime per hectare to achieve a pH of 4.8 (Figure 5.14, Table 4.5).

Table 4.10 Estimated State and national areas of existing acidic land (million hectares) and the predicted quantities of lime required to reach pH

	< 1 t lime/ha	1a - 2 t lime/ha	2a - 5 t lime/ha (million hectares)	:gt;5 t lime/ha	Totalc
Critical pH 4.8
New South Wales	4.2	0.5	0.2	0.0b	5.0
Queensland	0.5	0..2	0.1	0.0b	0.9
South Australia	0.2	0.0b	0.0b	0	0.2
Tasmania	0.3	0.2	0.2	0.0b	0.6
Victoria	2.5	1.3	0.6	0.1	4.5
Western Australia	0.8	0.2	0.1	0.0b	1.1
Australiac	8.5	2.5	1.1	0.2	12.3
Critical pH 5.5
New South Wales	8.8	5.6	3.0	0.3	17.6
Queensland	1.3	0.5	0.5	0.1	2.4
South Australia	0.4	0.3	0.3	0.0	1.0
Tasmania	0.1	0.3	0.8	0.4	1.6
Victoria	0.6	1.1	3.4	1.3	6.5
Western Australia	17.5	1.5	0.9	0.1	20.0
Australiac	28.7	9.3	8.8	2.2	49.1

^a Inclusive

^b Numbers rounded to 0.0 vary from 0.01 – 0.04.

^c Total values may be slightly different to adding up the values in the table because of rounding errors.

It is estimated that Australia needs to apply a one-off 12 and 66 million tonnes of lime to its acidic soils, to increase the pH to 4.8 and 5.5 respectively. Current agricultural lime use is nearly 2 million tonnes of lime per year (Table 4.11). If lime is applied to acidic soils at the current rate and no further acidification takes place, it would take six years to increase the pH to 4.8 and 37 years to 5.5. In the meantime, soils are continuing to acidify and this time frame is being extended.

This prediction strongly indicates there is a significant deficit in addressing soil acidification in Australian farming systems.

• Victoria and New South Wales require the highest amount of lime (Table 4.11).
• At current lime use, Western Australia and South Australia will overcome current acidity problems in a shorter time than other States (Table 4.11). Victoria will take the longest to overcome current acidity problems (Table 4.11).

Table 4.11 Total quantity of lime (million tonnes) required on a national and state level for acidic soils to reach a critical pH of 4.8 and 5.5 compared to actual lime use (million tonnes).

	Lime required pH 4.8	Lime required pH 5.5	Current lime use per year	Years to increase pH to 4.8 at current lime usea	Years to increase pH to 5.5 at current lime usea
New South Wales	3.0	22.2	0.5	7	49
Queensland	0.9	3.2	0.1	12	45
South Australia	0.1	1.5	0.1	1	15
Tasmania	1.1	6.3	0.2	7	42
Victoria	5.6	21.7	0.4	15	59
Western Australia	0.9	10.7	0.7	1	16
Australia	11.6	65.6	1.8	6	37

^a Assumes current lime use and no further acidification.

Maintaining pH

The low and high lime strategies identified in Figure 4.13 also require periodic re-application of lime to counter subsequent soil re-acidification. These lime maintenance levels were estimated annually both for minimum and maximum rates of acid addition (Table 4.12).

Table 4.12 Total quantity of lime (million tonnes per year) required on a national and state level to maintain pH values at 4.8 and 5.5 using two acid addition rates (50 and 250 kg lime equivalent per hectare per year).

	pH 4.8	pH 5.5
New South Wales	0.25-1.25	0.88-4.41
Queensland	0.04-0.22	0.12-0.60
South Australia	0.01-0.06	0.05-0.25
Tasmania	0.03-0.16	0.08-0.41
Victoria	0.22-1.13	0.32-1.63
Western Australia	0.06-0.29	0.98-5.01
Australia	0.6-3.1	2.4-12.3

• National estimates indicate that for soil pH maintenance at 4.8 (low lime strategy), annual applications from 0.6 – 3.1 million tonnes of lime will be required. To maintain soil pH at 5.5 (high lime strategy), annual totals from 2.4 – 12.3 million tonnes will be required. Again, Western Australia and New South Wales are the States requiring the highest maintenance levels of application, followed by Victoria.
• These estimates indicate that Australia needs to apply between 12 and 66 million tonnes of lime to adjust soil pH to 4.8 or 5.5, with a further 1–3 or 2–12 million tonnes required nationally for pH maintenance. At present, approximately 2 million tonnes of lime are applied to agricultural lands each year.

Allowing for the assumptions and potential errors introduced by the resolution and quality of the input data sets into the spatial analyses, these results suggest a very large lime deficit exists in Australia’s farm systems. These results merit further regional investigation into soil acidification—potentially one of the sleeping giants for on-farm productivity.

MANAGEMENT OPTIONS FOR ACIDITY

Helyar (1991) comprehensively reviewed concepts and practical issues for managing complex processes associated with soil acidification. This Australian review provided a useful framework for assessing future management options and current practices. With most of these strategies, reducing acidification is not the only consideration (e.g. ammonium-based nitrogen fertilisers may be easier to use). Targeting nitrogen fertiliser use, rather than relying on legumes to provide nitrogen can reduce acidification. Other considerations include the cost of nitrogen fertilisers, the income derived from crop legumes and the other benefits of a legume system (e.g. reduced weeds, pests and diseases of wheat).

Farm management options for dealing with soil acidity (Helyar 1991)

Principal issues

1. Controlling acid additions through better managing the nitrogen and carbon cycles

2. Use of plant tolerance to reduce the effect of acidic soil conditions

3. Use of liming materials to ameliorate acidic surface and subsoils

4. Applying fertiliser nutrients (e.g. molybdenum) to correct nutrient constraints caused by soil acidity

Management strategies

Nitrogen fertiliser

• Change nitrogenous fertiliser from ammonium-based sources to urea or anhydrous ammonia or nitrate-based products
• Reducing nitrate leaching and improving nitrogen use efficiency
• Improve timing of nitrogen fertiliser application to match demand by plants
• Ensure nitrogen inputs do not exceed crop demands
• Ensure irrigation does not result in deep drainage
• Sow crops early (favouring early crop root growth) to improve efficiencies in use of fertiliser nitrogen and mineralised soil organic nitrogen reserves
• Grow perennial rather than annual species. In some regions, the existence of live plants during summer and autumn will increase nitrate uptake and thereby reduce leaching losses.
• Reduce legume dominance in mixed pastures
• Use reduced tillage to minimise build-up of soil nitrate (nitrate is more likely to leach when crop demand is low in early winter)
• Avoid long fallows
• Incorporate high carbon/nitrogen plant residues (stubbles) into the soil so that microbes use soil nitrogen to break down residues, thus preventing nitrate from being leached
• Carbon cycle
• Retain plant residues on-site rather than burning or removing
• Return plant residues to areas of active root growth (e.g. in row or plantation crops, return residues to the crop row)
• Manage stock to disperse campsites (e.g. by reducing the size of the paddocks or increasing the intensity of grazing)
• Graze pastures rather than cut for hay or silage
• If hay is required, feed hay in the paddock where it was cut

Acid tolerant plants

• Use acid-tolerant perennials in favour of annuals
• Adopt acid-tolerant strategies where lime amelioration is too costly

Lime applications (preferred where economicallyfeasible)

• Lime will also stimulate nutrient availability in acidic < soils p>
• Use of earthworms and dung beetles speeds incorporation and downward penetration of surface applied lime to help ameliorate subsoil acidity
• Mechanical mixing and injections of lime have also been used in some areas

Lime

Although lime use is lower than required, it is increasing at a rate of 10 – 15% per year (Table 4.14). Even where soil acidity and the benefits of lime applications are recognised by farmers, liming may not be tenable because:

• lime is required in large amounts (1–5 t/ha) and it may have to be transported a long way from where it is mined (Figure 4.16, Table 4.13) and hence is expensive;
• profitability in some farming sectors is declining (e.g. in Queensland low sugar prices have dramatically reduced lime use in recent years); and
• surface applications of lime to correct subsoil acidity may take many years before improvements are realised

High value industries (e.g. horticulture), where fertiliser is a small proportion of the total operating cost, are more likely to use lime compared to broadacre dryland farming industries. Broadacre industries are often located a long way from where the lime is mined and have lower profit margins, increasing the relative costs of lime use. At the same time, their farming practices induce acidification over larger areas of land.

• Including cartage and spreading costs, an application of 2.5 t/ha, costs will vary from $145 to $200 per hectare in Queensland to $52 to $110 per hectare in Western Australia and $45 to $100 per hectare in South Australia.

Table 4.13 Estimated agricultural lime/dolomite production and use in thousands of tonnes.

	Production Use
	1989/90b	1995/96c	1998/99a	1995/96c
New South Wales	144	335	453	257
Queensland	150	246	70	144
South Australia	40	88	100	31
Tasmania	100	99	150	146
Victoria	147	281	370	197
Western Australia	117	178	653	178
Australia	708	1242	1811	958

^a Data are from surveys of lime producers (Victoria, South Australia, New South Wales), State Department of Mines (Tasmania, Queensland) or survey of farmers on lime and dolomite use (Western Australia, Australian Bureau of Statistics data). Alkaline by-products were not considered. There is also some transport of lime across state borders.

_b Adapted from Porter & McLaughlin (1992).

_c Adapted from CRC Soil & Land Management (1999).

Growing acid-tolerant species

Some plants are able to tolerate more acid soils. Species with acid tolerance (see also Table 4.14) include: triticale, oats, yellow lupins, clover, perennial veldt grass, subterranean clover, perennial ryegrass, cocksfoot and tall fescue (in temperate regions); and sugar cane, macadamias and bananas (in subtropical regions).

Clay spreading

Spreading sodic clays to alleviate water repellent soils in South Australia also increases soil pH in the zone of incorporation.

Use of alkaline irrigation water

Soils irrigated from bores in South Australia are known to significantly increase soil pH.

CASE STUDY

Acidification case study: bananas in tropical north Queensland

Industry scenario

• A survey of ‘current practice’ at three commercial banana plantations indicated growers applied an average of about 600 kg N/ha/year predominantly as urea, but with some ammonium nitrate also being used.
• Banana bunches removed from plantations averaged about 8100 kg dry weight/ha/year with an alkalinity equivalent to 200 kg agricultural lime/ha.
• Nitrate leaching losses in excess of 50 kg N/ha/year have been measured under bananas in the wet tropics. This area receives over 3000 mm annual rainfall.
• During de-suckering, crop residues (suckers and dead leaves) are disposed into inter-row areas.

Estimates of acidification rates

• Contributions to soil acidification in the banana production system include nitrate leaching from applied fertiliser, organic matter accumulation (acidifying), removal of alkalinity in harvested product, removal of alkalinity in crop residues such as suckers and dead leaves during de-suckering, and application of ammonium-N (acidifying) which is balanced by the application of the equivalent amount of nitrate-N (alkalising) where ammonium nitrate is used as the nitrogen fertiliser (Figure 4.17).
• The net acidification under these current practices is managed by regular surface applications of agricultural lime or dolomite at rates of 1–2 t/ha/year.

Practices for reducing acidification

Identification of the sources of acidification and their magnitude under current practices allowed a best management practice scenario to be developed. This scenario involved:

• reducing nitrogen input to 250 kg N/ha/year;
• applying nitrogen fertiliser in the form of ammonium nitrate rather than as urea;
• returning crop residues to the plant row rather than discarding into the inter-row; and
• reduced nitrate leaching as a consequence of reduced nitrogen fertiliser inputs.

Has best practice management worked?

Confirmatory evidence that adoption of best management practices would have a positive impact on reducing acidification was obtained by comparing ‘paired site’ data from a commercial plantation using current practices and another plantation that has implemented some of the best management practices detailed above. In the ‘paired site’ approach, soil pH was measured to a depth of 1 m under bananas and under rainforest (i.e. the undeveloped situation) in close proximity on the same soil type (see Figure 4.18).

• Best practice management has resulted in a net increase in soil pH to a depth of 40 cm (Figure 4.18a).
• Subsoil acidification has occurred under current practice (Figure 4.18b) despite the regular surface application of agricultural lime. Lime applications have caused pH in the surface 20 cm to increase, but had no effect below 20 cm. The correction of subsoil acidification is expensive and prevention is the best option.

Industry implications

The banana industry is moving towards adoption of these best management practices as research removes some of the uncertainties associated with current fertiliser and plant residue management (e.g. it has been shown that nitrogen fertiliser inputs can be reduced to around 250 kg N/ha/year where fertigation is used; a major improvement in fertiliser nitrogen efficiency; fertigation also allows flexibility in the form of fertiliser applied).

The use of nitrate fertilisers in the industry is expected to increase as growers become aware that reduced acidification rates occur where nitrate rather than ammonium fertilisers are used. In addition, reduced nitrate leaching under best management practices will reduce nitrate contamination of surface and groundwaters.

Further information

Moody P.W. & Aitken R.L. 1997, ‘Soil acidification under some tropical systems. 1. Rates of acidification and contributing factors’, Australian Journal of Soil Research vol. 35, pp. 163–173.

IMPACTS OF ACIDIC SOIL CONDITIONS ON PLANT YIELD

Soil acidity:

• reduces the availability of essential plant nutrients to plants;
• detrimentally affects microbial soil processes, legume nodulation and nitrogen fixation; and
• induces the dissolution and accumulation of toxic levels of aluminium and sometimes manganese in soils.

Moderately acidic soils reduce growth of some legume species; while highly and extremely acidic soils are suboptimal for the growth of most agricultural plant species. Under these conditions, plant growth can be impaired by:

• toxic concentrations of aluminium and manganese that accumulate in the acidic environment and seriously impede root growth (occurs below pH 4.8 and leads to reduced shoot growth, and inefficient use of soil water and nutrients; manganese toxicity is also observed more frequently on less weathered soils);
• reduction in nodulation in legumes;
• unavailability of essential plant nutrients (e.g. phosphorus and molybdenum); and
• depression of nutrient cycling through adverse effects on soil biological processes.

Different plants have different tolerances to soil acidity (Table 4.15). Between a pH of 4.8 and 5.5, growth of very highly to highly acid sensitive species, mainly legumes, will be reduced. Below pH 4.8, plant growth starts to be progressively depressed by aluminium and manganese toxicity and other acidity-related disorders. At pH values below 4.3, the yields of most plant species are markedly reduced. Soil pH is the most common test used in determining whether acid soils are restricting plant growth, because soil tests for aluminium and manganese are not yet available to assist farmers.

Plant yield declines with decreasing surface soil pH (Figure 4.19). The extent of the decline depends on the concentration of aluminium and manganese in each soil. Subsoil acidity also reduces the growth of plant species.

The yield relationships were used to estimate economic penalties associated with farming acidic soils in Australia’s agricultural regions, using plant species differing in tolerance to acidic soil conditions. These assessments were also contrasted where lime had been applied to alleviate soil acidity. These findings are reported in the Audit’s socioeconomic report (Australians and Natural Resource Management 2001).

Table 4.14 Allocation of plants to each yield tolerance class indicates the tolerance to aluminium, manganese and pH (cultivars do vary in their tolerance so the table can only be taken as a guide).

Tolerance class	Examples
Extremely tolerant	Italian and perennial rye-grasses, lovegrass, oats, native pasturea, pineapple, sugar cane, yellow serradella
Highly tolerant	Bananas, cereal rye, cocksfoot, lupins, macadamia, peanutsb, potatoes, rice, triticale, turf
Moderately tolerant	Avocados, cotton, maize, mangoes, Rhodes grass, soybeans, subterranean clover, wheat
Slightly tolerant	Crimson clover, grain sorghum, Medicago murex, millet, mustard, phalaris
Slightly sensitive	Buffel grass, Faba beans, field peas, vetches, white clover
Moderately sensitive	Almonds, apricots, barley, canola, cherries, fennel, grapes, lavender, mandarins, nectarines, oranges, peaches, pears, plums, red clover, sunflower, tobacco
Highly sensitive	Apples2, balansa clover, chick peas, coriander, lentils, lucerne, mung beans, oil poppies, pyrethrum
Extremely sensitive	Persian clover, strand medic, strawberry clover, tall wheatgrass

^a Tolerance depends on soils at origin.

^b Very sensitive to calcium deficiency, which can occur on acid soils.

IMPLICATIONS FOR AUSTRALIAN AGRICULTURE

Acidification looms as a major soil degradation issue in all Australian States, and farmer awareness of its insidious nature has been heightened in recent years by research and extension programs in some, but not all States. At present, only three States (Western Australia, New South Wales and South Australia) have active extension programs on soil acidity.

Problems arising from induced acidification are reversible (mostly), but costly. This means that farmers only treat small areas of their farms with lime at any one time, where liming is an economically viable option.

• Efficiency in lime use could be achieved by basing application rates on soil types, as variations exist in the amounts of toxic aluminium and manganese in the soil solution at a given pH.
• Innovative ways for treating subsoil acidity and subsoil acidification processes remains a priority for future research.

Farmers responses to treating soil acidity are partly attributed to preserving or enhancing the capital value of their farm. Regional resource managers are also concerned with wider implications of soil acidification on the resource base. Reduced plant growth due to soil salinity can lead to increased erosion, less water use (increasing recharge to groundwater) and downstream effects of sedimentation and possibly salinity.

Understanding off-site impacts from soil acidification on-farm are rudimentary and need to be assessed with scientific rigor. To date, some postulations have been assembled, but not tested. Significant off-site implications would move soil acidification into the ‘public good’ arena. By far the best option would be to treat soil acidification on farm before it becomes a downstream issue.

REFERENCES

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