Acid soilaction investment for your soil now for the future

Farmers who: • are not aware of the impact of soil acidity • know they have a problem, but are not sure of the implications • are ready to act, but unsure of what to do • have made some

How to use the kit 5

What’s in it for me? 5

Support material 5

Why worry about acid soils? 7 Nature of the problem 8

The acid attack 10

The cost 11

Benefits of action 13

Understanding acidity 17 What can you do to stop soils becoming more acid? 17

What needs to be done? 17

Soil pH - What does the figure mean? 22

The strategies 23

Interpreting soil pH 24

What do I need to do? 29 Acid soil management decision 29

Action Guide for soil acidity 39 Check the signs 39

Indicators of Acid Soil 41

Taking soil samples 44

Measuring soil pH 46

Possible causes 47

Monitoring checklist 48

Field texture 50

How much lime? 53 To maintain production level 53

To increase soil pH to grow sensitive plants 53

Lime Guide 63 Which liming material is best? 63

Types of lime 65

Comparing limes 67

I N V E S T M E N T F O R Y O U R S O I L

N O W A N D F O R T H E F U T U R E

This publication is intended as a guide for farmers,

advisers and industry field representatives to help

assess acid soil problems and decide on

appropriate management strategies The kit has

been designed and compiled by Carole Hollier,

Agriculture Victoria, Rutherglen with the assistance

of numerous colleagues in research, extension and

industry The technical information has been

supplied by many sources and is gratefully

acknowledged Special thanks to Anna Ridley, Bill

Slattery, Eloise Rich, Agriculture Victoria; Greg

Fenton, NSW Agriculture; Brian Hughes, Primary

Industries, South Australia; Mike Frost, Limestone

Association of Australia; and Trevor Tovey, David

Mitchell Limited

Special thanks also to the farmers in south-eastern

Australia who provided crucial input in determining

what problems to address and how to present

information – especially the 12 farmer collaborators in

north-eastern Victoria for their valuable contribution.

Suggestions for improving the manual are most

welcome.

The kit has been funded by the Land and Water

Resources Research and Development Corporation,

as part of the National Soil Acidification Program.

© Department of Natural Resources and Environment 1999.

DISCLAIMER

This publication may be of assistance to you but the State

of Victoria and its employees do not guarantee that the

publication is without flaw of any kind or is wholly

appropriate for your particular purposes and therefore

disclaims all liability for any error, loss of other consequence

which may arise from you relying on any information in this

A C I D S O I L A C T I O N • A B O U T T H I S K I T 3

This kit is for

Farmers who:

• are not aware of the impact of soil acidity

• know they have a problem, but are not sure of the implications

• are ready to act, but unsure of what to do

• have made some attempts at soil diagnosis/management, but unsystematically

• are uncomfortable with the technical level of data in available materials

• are uncertain about management options, costs, benefits, time scales

• need clear, practical information

• are ready to act further, but need clear options

and industry/farm advisers who:

• know about acid soils but want to update their skills

• are busy people and want a quick reference guide

• talk to farmers to help solve their problems

• require industry information (lime sources/quality)

• want more detail

It features:

• action-oriented information: what to do next

• clear steps, essential minimum information only

• clear recommendations to assist decision making

• organised around tasks, not topics

• easy to use planning and recording tools

• basic guide to cost-benefits of alternative approaches

The kit will be successful if you:

• are convinced to test for soil acidity

• implement monitoring or action strategies in the current year

• seek further support and information

• establish new farm management practices to minimise

soil acidification

• move to a whole farm planning approach

• obtain increases in yield and profitability by

implementing a liming strategy

Follow the tractors:

Go to page 5 to get started

Soil acidification - Is your farm at risk?

If acid is attacking your soil and burning a hole in the farm account Acid Soil Action

- a practical decision support system to encourage step-wise adoption of management

options - is for you

It aims to increase awareness of the impact of soil acidification and knowledge of

management options to reduce soil acidity

How to use the kit

You can read single sheets or work through the whole package

A series of colour coded tractors makes your information search and

decision making easier This “go to” approach is designed to guide

your decision making process You decide the level of involvement

and your technical information needs

The tractor symbols will drive you through the awareness and assessment of soil

acidification into management options to improve farm management decisions

It provides direction for further information and action

What’s in it for me?

The material is divided into colour coded sections The first section introduces acid soils

and describes the nature of the problem It helps you answer the question; Is acidification

an issue for my farm?

The second part is a step-wise farm management decision support system followed by

a series of technical information updates For many land managers the information sheets

and work plans will not go into enough depth They are designed to provide a starting

point to build individual knowledge and improve discussions with your farm adviser

Support material

This resource kit can be adapted for discussion groups

or group activities (such as Landcare) to update

skill base on recognising acid soil problems

and appropriate farm management options

Photographs and illustrations help to

reinforce key elements for your

For example,

Go to page x

management decisions Technical terms are explained on the most relevant

sheet and a glossary of terms is provided

However, the package is not complete It is designed as an initial building block to add

specific information and relevant material from other sources for your region and

industry.

Your local Department of Agriculture will be able to supply you with a range of resource

material and can advise on commercial soil testing packages Research results from local

or regional lime evaluation or acidification studies may also be available

With the help of this guide, you can make an initial assessment of an acid soil problem

and determine, in consultation with your farm adviser and Australian Fertiliser Services

Association local lime suppliers, management actions to minimise and treat acidification

and boost productivity

6 A C I D S O I L A C T I O N • G E T T I N G S T A R T E D

The process of soil acidification or change in the amount of acid in the soil is a serious

land degradation issue Without treatment soil acidification will have a major impact on

agricultural productivity and sustainable farming systems

Ninety million hectares of agricultural land in Australia is acid and at risk of further soil

degradation Nearly half of this land is already strongly acid (pHCa<4.8) and requires

urgent action for sustainable production

The area damaged by acidification is increasing and many soils are acidifying with current

agricultural practices Without treatment, acidification is extending into subsoil layers

posing serious problems for plant root development and remedial action

For most acid soils, the most practical management option is to add lime to increase

For more information:

Go to map section on page 109

8 A C I D S O I L A C T I O N • W H Y W O R R Y A B O U T A C I D S O I L S ?

Nature of the problem

Soil acidification is a natural process, accelerated by some agricultural practices

In some regions, there has been a drop of one pH unit over the last 20-30 years

Already, some farming areas have lost the ability to grow preferred agricultural species

such as phalaris and canola simply because, without lime, the soil is too acid

Many clover-based annual pastures

are becoming progressively more

acid over time In cropping areas,

where subterranean clover pastures

are an important part of cereal

growing, soil pH decline trends are

emerging

If soils are allowed to become

extremely acid there is a risk of

irreversible soil texture change

If subsoils are allowed to acidify, treatment is extremely difficult and costly

Acid soil problems can cause poor establishment, growth and persistence of pastures and

crops These soils are characterised by plant nutrient imbalances, toxicities and

deficiencies that reduce plant growth

Aluminium toxicity is the major problem associated with acid soils As soil becomes more

acid, aluminium becomes more available to the plant and stunts root growth

Acidification has been described as a ‘sleeping giant’ Unlike dryland salinity, it is insidious

with few visual symptoms

Symptoms of reduced growth in acid soils are often subtle and can be explained

away by other factors, such as poor season or inadequate fertiliser It is only over a

period of time (usually decades) that major problems become apparent

Problems are likely to develop if you farm:

• in high rainfall areas (greater than 500mm/rainfall) with dry summers

• annual pasture systems

• lightly textured soil

• with legume rotations

• and have a history of nitrogenous fertiliser use

• with high product removal, particularly lucerne

In most grazing systems acidification processes work unnoticed - until pasture production

declines The rate of acidification in pastures depends on the species grown In cropping

systems, lack of plant vigour in sensitive species and reduced yields are more obvious

Soil acidification is linked to other degradation issues The water using capacity of

agricultural systems is reduced as soils become more acid Valuable perennial species such

as lucerne and phalaris, recommended to reduce recharge and the impact of salinisation,

are sensitive to aluminium toxicity and will not establish and grow

Other economically important species, such as canola, barley and most varieties of wheat

are also sensitive to acidity

Accelerated acidification occurs in

agricultural soils as a result of:

• removal of plant and animal products

• leaching of excess nitrate

• addition of some nitrogen based

fertilisers

• build up in organic matter, largely of

plant origin

The more acid the soil, the fewer the

choices of crops and pastures that will

grow productively Even acid tolerant

plants show a reduction in growth

Some acid soils support productive systems but acidification processes can be masked.

Soil alkalinity removed by some farm products (kilograms)

Lucerne hay 3-7 tonnes per hectare 360 to 840

Clover/grass hay 3-7 tonnes per hectare 270 to 630

Lupins 0.6 to 1.3 tonnes per hectare 12 to 26

Wheat 1.2 to 2.1 tonnes per hectare 11 to 19

10 lambs per hectare 6

6kg of wool per sheep 0.8

Product Yield Lime equivalent per year (kilograms)

The acid attack

Acidity itself is not responsible for restricting plant growth The associated chemical

changes in the soil can restrict the availability of essential plant nutrients

(for example, phosphorus, molybdenum) and increase the availability of toxic

elements (for example, aluminium, manganese) Essential plant nutrients can also be

leached below the rooting zone Biological processes favourable to plant growth may

be affected adversely by acidity

Acid soils have a major effect on plant productivity once the soil pHCafalls below 5

pHCa6.5 Optimum for most plant growth Neutral soil Trace elements may become

unavailable

pHCa5.5 Balance of major nutrients and trace elements available

pHCa5 Below pH5 aluminium may become soluble in the soil depending on soil type

Phosphorus combines with aluminium and may be less available to plants

pHCa4.5 Manganese becomes soluble and toxic to plants in some soils

Molybdenum is less available Soil bacterial activity slows down

Aluminium becomes soluble in toxic quantities

pHCa4 Soil structural damage

1 0 A C I D S O I L A C T I O N • W H Y W O R R Y A B O U T A C I D S O I L S ?

The causes of soil acidity

Source: NSW Agriculture Agfact 19 Soil acidity and liming

The Cost

Acidification is costing the national agricultural economy around $300 million each year

Beyond farm-gate impacts triple this cost to the community

The impacts of soil acidification are potentially enormous and include:

• increasing dryland salinity

• increasing nitrate pollution of groundwater and reduced water quality

• reduced plant yields, farm income and domestic/export earnings

• reduced options for agriculture

• reduced vegetative cover and accelerated water runoff and erosion

• irreversible degradation of the clay content of soil, hence reduced fertility

• declining pH of waterways and aquatic environment

• increased infrastructure cost

• decreased land values

The effects of soil pH on aluminium.

1 2 A C I D S O I L A C T I O N • W H Y W O R R Y A B O U T A C I D S O I L S ?

Off Site Impact

Acidification

Reduced plant growth

Death of acid

sensitive species

Increased runoff

Increasederosion

Reduced wateruptake

Reduced nitrateuptake

Water table rise Increased nitrate

Benefits of action

If preventative or remedial strategies are put in place, acidification can be slowed down or

soil pH (the measure of acidity) adjusted so that a wide range of plant species can be

• increase molybdenum (Mo),

calcium (Ca) and phosphorus (P)

availability

• provide a more favourable

environment for soil microbes

• improve nutrient cycling by earthworms and soil bacteria

• help reduce other forms of land degradation (for example salinity, erosion)

• increase cropping/pasture options so more sensitive or valuable species can be grown

Plants respond to lime in different ways Altering soil pH affects the availability of

nutrients and also influences soil microbial activity Some crop and pasture species are

more tolerant or sensitive to acidity than others Plant production responses also vary

depending upon soil type and environmental factors

There is more dry matter production where lime responses are seen Because soil growing

long term perennial pastures are higher in organic matter than cropping soils, they often

require larger amounts of lime to obtain a similar response

Responses to lime have been inconsistent on some acid soils Subject to soil pH

and plant species, visual responses are sometimes seen in autumn/winter but

more commonly in spring Dry matter responses to lime with pastures based

on tolerant species have been generally small and inconsistent

Research data has shown that responses to lime can be profitable where soil pH is

strongly acid Most economic advantage is achieved by liming highly productive

perennial pastures or sensitive crops and pastures such as canola and lucerne

1 4 A C I D S O I L A C T I O N • W H Y W O R R Y A B O U T A C I D S O I L S ?

Yield increase

Research has shown that applying lime to an acid soil can make the difference as to

whether barley will or will not grow and will significantly increase yields of canola, wheat

and triticale Benefits have also been seen with faba beans, chick peas and the more tolerant

pasture species such as clovers and cocksfoot Sensitive species such as phalaris and lucerne

respond dramatically to lime application on acid soils Trials have shown subterranean

clover production responses ranging from 10 to 50% on acid soils in cropping areas

Research at Agriculture Victoria – Rutherglen has shown improved canola (right)

and wheat (left) yields with 2.5 t/ha lime application to raise soil pH.

Understanding soil pH

Soil pH is one of the most routinely measured soil parameters It is used as a benchmark to

interpret soil chemical processes and governs the availability of many essential or toxic

elements for plant growth

Soil pH is a common measure of the soil’s acidity or alkalinity because:

• testing is relatively easy

• laboratory and field equipment to measure pH is not expensive

Sending soil samples to a laboratory is recommended to ensure the most accurate results

Test kits are available that use colour to indicate pH levels The kits are cheap (around

$20), easy to use and will test a lot of samples but cannot be relied on for decisions such as

rates of lime application Test kits will only tell you whether your soil is acid or alkaline

A number of compact testing meters that can be used out in the paddock are available, most

of which are capable of giving accurate results if used correctly They range from $130 to $500

Topsoil and subsoil pH testing is recommended When interpreting plant responses based

on soil pH, the surface (A horizon) and sub-surface (B horizon) need to be considered

There are two common measurements of soil acidity or alkalinity - pH as measured in water

(pHw) and pH as measured in 0.01M CaCl2(pHCa), both at a soil to solution ratio of 1:5

Soil test reports usually provide soil pH results measured in both calcium chloride and water

The soil pH as measured in water is considered to be closer to the pH that the plant roots

are exposed to in the soil But it is subject to large variation within the paddock because of

seasonal changes in soil moisture and the ionic concentration of the soil solution which is

related to the amount of total salts in the soil Research has shown that seasonal variation

of pH measured in water can vary up to 0.6 of a pH unit in any one year

In comparison, the measurements of soil pHCais less affected by seasons Farmers can take soil

samples at different times during the year without affecting the final diagnosis or interpretation

Soil pHCameasurements in Australia vary from pH 3.6 to pH 8 for a range of

different soil textures (sandy loams to heavy clays) Soil pHwvalues lie

between 4 and 9

Higher pHwvalues to around 10 may be associated with alkali mineral

soils containing sodium carbonates and bicarbonates Soil pHw

less than 4 may arise from oxidation of sulfides in acid

sulphate soil (confined to North Coast NSW and

sub-coastal mangrove soils) or from highly

organic peat conditions

1 6 A C I D S O I L A C T I O N • W H Y W O R R Y A B O U T A C I D S O I L S ?

Some tips

• Soil testing will tell you the current acidity status of your paddock If your soil

pH measured in calcium chloride (pHCa) is above 5.5 then there is little immediate

risk of acidity

• Lime can restore productivity in acid soils and should be considered once the pH drops

below pHCa5 if sensitive species are to be grown successfully

• You are unlikely to get responses to lime if other nutrients are lacking This should show

up in a soil test or plant tissue analysis and should be corrected

• Lime responses are generally seen in the first and second year for cropping systems, but

can take up to five years depending on soil type, rainfall and lime quality for permanent

pasture systems

• It is necessary to re-lime your paddock (about every 10 years), depending on the rate of

re-acidification

• If paddocks with an acidity problem are not limed, the soil pH will continue to fall

• The amount of lime you need to apply varies according to soil type - field experiments

have shown that up to 5 tonnes a hectare on clay loams and down to 1.5 tonnes a

hectare on sandy soils is needed to increase pH by one unit

• Lime moves slowly (0.5 to 1cm yr) down the soil profile, so incorporation is

recommended where possible

• In grazing situations, spreading the lime on the surface and allowing it to work its way

into the soil is recommended Surface application is better than no application

• Soil pH is measured in either water or in calcium chloride When measured in calcium

chloride, the result is lower than pH measured in water

• The pH using the water method may be higher by 0.6 to 1.2 in low salinity soils and

higher by 0.1 to 0.5 in high salinity soils Research has shown a difference of 0.8 for a

large range of soils

pH measured in calcium chloride

What can you do to stop soils becoming more acid?

• Recognise soil acidity

• Monitor soil pH

• Know crop and pasture requirements

• Keep accurate records for each paddock

The aim of monitoring soil acidity (knowing your current soil pH status) is to identify

problems at an early stage and not at the crisis management stage when production loss is

severe and remedial treatment expensive

Do not assume that every paddock has the same soil pH or acidification rate across

your farm

Your management options:

• Use acid tolerant plant species (Remember this is only a short-term strategy as the soil

will continue to acidify over time.)

• Apply lime at a rate based on soil pH, soil type, rainfall and land use

• Reduce the rate of acidification

- sow perennial pasture to reduce nitrate leaching

- match fertilisers to plant requirements, monitor plant and soil nutrient levels,

use least acidifying nitrogen fertilisers

- recycle nutrients and alkalinity by feeding hay back onto paddocks

What needs to be done?

• Soil test

• When should lime

be applied?

Lime neutralises surface soil acidity

and helps prevent future subsoil acidity

If your soil pHCais below 4.5 you will

need to apply lime as soon as possible

to regain productive yields On less acid

soils (pHCa4.5 to 5) liming within the

next few years will help prevent future

acidification

Go to page 39 Action Guide for Soil Acidity

Go to pages 46-47 for more details

1 8 A C I D S O I L A C T I O N • U N D E R S T A N D I N G A C I D I T Y

If you intend to grow sensitive species an urgent lime requirement

may be needed Soil pH needs to be kept above pH Ca 5.5.

Lime requirement will depend on:

• current soil pH (topsoil and subsoil)

Horticulture (most crops) 5.5 to 6.5 6.0 to 7.0

Application rates vary according to

your soil type Heavier textured soils

require more lime to raise the soil

pH Most acid soils need at least 1.5

to 2.5t/ha to raise pH in the top

10cm by half a unit Use 1.5t/ha

for sandy soil; 2t/ha for a clay

loam; and 2.5t/ha for a clay

soil Lime works best when it is

finely ground and incorporated

into the soil

Subsoil acidity:

• Target pH after liming needs to be pHCa

greater than 5.5 for lime to effect the subsoil

• When surface pHCafalls below 5 acidity starts to move into

the subsoil

Go to page 53 How much lime?

• Replace nutrients

Soils become more acid when crop and pasture products leave the farm gate and lime

equivalents and soil nutrients are not replaced; for example lucerne hay removal is

very acidifying to the soil Soil test or plant tissue analysis will tell you what

nutrients your soil needs - see your local agronomist or farm adviser to help

interpret your test results and nutrients requirements

• Select the right nitrogen fertiliser

Choose nitrogen fertiliser carefully and aim to use the least

acidifying fertilisers The most acidifying nitrogen fertilisers

are ammonium sulphate and mono-ammonium phosphate

(MAP) and the least acidifying nitrogen fertiliser is potassium

nitrate

For more detail:

Go to page 93 Acidifying fertilisers Slightly-acidifying

Highly acidifying Nitrogen fertilisers

• Mono-ammonium phosphate (MAP)

Amount of lime needed to neutralise

acidification caused by product removal

Your decision to manage acid soils should be viewed as a:

• Preventative strategy - to ensure continuous, profitable farming

• Remedial strategy - to fix it now

If your soil pHCais above 5.5 Your only decision is to MONITOR acidity with regular soil testing

Target pHCa5.5

2 0 A C I D S O I L A C T I O N • U N D E R S T A N D I N G A C I D I T Y

• Monitor and keep records

Implement a regular soil testing program Remember you may not see an immediate

response to any farm practice you introduce to reduce acidity Keeping good records

helps you track your progress and compare performance

Soil testing is the first step to identify an acid soil problem and decide on future

management actions

Soil testing is the only way to determine the

severity of acidity as decline in production

associated with acidification is a gradual and

often subtle process

Your soil test report alone may be insufficient

because factors related to soil acidity may not

be the major limitation to production Seek a

specialist with local knowledge to help interpret your

soil test results

Go to page 22 Soil pH - What does the figure mean?

pH (water) values pH of common substances

Optimum growth range

Pure Water

Caustic soda Ammonia

Milk of magnesia Soap solution Sea water

Blood Fresh milk

Sour milk Wine Vinegar Lemon juice Battery acid Sulphuric acid

pH of common substances

coastal and sub

coastal low lying

areas

pHCa4 to 4.5 natural or induced process

acidification associated with nitrate leaching from improved pasture, fertiliser effects, removal of alkali in produce

process of mineral weathering dominant

exchangeable cations such as aluminium (Al) and manganese (Mn) exhibit toxicity to plant roots toxicity increasing in concentrations with decreasing soil pH

deficiencies of nutrients such as molybdenum (Mo) can occur

populations and activity of some microorganisms affected soil type and organic matter controlling soil buffering capacity (ability to resist pH change) determines the rate

a soil is likely to decrease in pH

pHCa4.5 to 5 critical level with optimum growth

of acid tolerant plants providing adequate fertiliser supplied

effects of toxicities

of exchangeable manganese (Mn)

in some soils may still limit

production of sensitive plants below pHCa 4.8 aluminium (Al) toxicity will begin

to limit production

Amelioration economically viable

Liming strategy needs to be determined according to acidification rate

of farming system.

pHCa 5 to 6.5 optimal growth

of most plants

no soil acidity affect

on production other than possibly manganese below

pHCa 5.8

soils likely to be productive providing no nutrient deficiencies

or degradation effects (eg salinity)

pHCa6.5 to 7.5 neutral

no soil acidity affect on production

stage, minimum 3t/ha high

grade lime to increase soil

rethink land value and use

What type of lime?

breakdown.

Acid soil infertility Nutrient toxicities and deficiency likely Check aluminium levels

in topsoil and if greater than 5 mg kg -1 LIME Values for exchangeable

Al generally greater than

70 mg kg -1 in this soil range

Soils characterised by low CEC levels (less than

5 milli equivalents/100g) are likely to develop acid problems.

Cation Exchange Capacity (CEC) If less than 5 me % can only be increased by organic carbon CEC 5 to 8 me %

or higher and Ca + Mg%

CEC high, further cation unlikely or process very slow.

acidifi-Check subsoil pH

If acid need lime sooner than later because lime moves slowly (0.5 to 1cm yr) down the soil profile.

Minimum 2 to 3.5 t/ha high grade lime to increase topsoil pH 0.5

to 1 unit.

ONLY option acid tolerant species plus lime.

Values for exchangeable

Al generally vary from 5 to

70 mg kg -1

What species do you grow?

If ACID TOLERANT you don’t need to lime but monitor so that soil pH doesn’t fall below pHCa 4.5.

If your species selection is moderate

or sensitive to acid soils you will need to LIME TO ABOVE

pHCa5 At pHCa4.8 and above Al toxicity

is unlikely Only toxicities of Mn, if present will affect plant yield.

Check subsoil pH Which management strategy?

Remedial strategy to increase soil pH and the choice of species you can grow Preventative strategy

to maintain current production level and prevent gradual pH decline.

generally less than

5 mg kg -1

Can grow all species providing nutrient requirements are met.

MONITOR to maintain pHCaabove 5 Regular soil testing (at least every five years).

Application

•Incorporation: If practical, fastest result.

•Surface application: Prior sowing,

autumn break

•Timing: Ideally season before sowing

sensitive species or prior sowing Lime

can be applied any time of the year.

Go to page 53 How much lime?

Go to page 63 Which liming material

is best

Go to page 79 for more information

on aluminium interpretation

or revisit page 18

2 4 A C I D S O I L A C T I O N • U N D E R S T A N D I N G A C I D I T Y

Interpreting soil pH

Soil pH governs the availability of many elements that are

essential for, or toxic to plant growth It is the measure of

the hydrogen activity in solution indicating the intensity of

acidity or alkalinity

It is the imbalance of the nutrients and other chemicals, caused by the acidity, that

affects plant growth Interpretation of soil pH is greatly improved knowing other

soil measurements such as electrical conductivity, effective cation exchange

capacity and the concentration of each exchangeable cation

Knowing your soil pH and estimating your target soil pH to grow sensitive species is

essential in determining lime requirement However, interpretation of pH for plant

production relies on the ability to match the results with studies that have

determined responses for the same soil type and climatic conditions If there were

no other limiting factors, optimum soil pH for a plant species would be the same for

all soil types This is rarely the case because in strongly acidic soils other limitations as

well as low pH are usually present

With each unit drop in

soil pH the number of

hydrogen ions multiplies

by 10 At pH 7 the

hydrogen and hydroxyl

ions are in balance At

pH 6 there are 10 times

more hydrogen ions; at

pH 5 there are 100 times

more hydrogen ions, and

at pH 4 there are 1000

times more hydrogen

ions As this graph

shows, the concentration

Go to page 15 Understanding soil pH

Soil clay minerals contain aluminium but it has no impact on plant productivity unless the

soil pHCafalls below 4.8 (pHwless than 5.5) Below pH 4.8Ca aluminium (Al) becomes toxic

and root growth is severely reduced in sensitive and moderately sensitive plant species

A damaged root system reduces the plant’s ability to obtain nutrients and water from the

soil As a result there is a significant decrease in plant yield

Plants vary in their tolerance to Al Tolerant species, for example

lupins, some wheat cultivars, triticale and cocksfoot may achieve

maximum yield potential without significant effects on plant

growth due to Al toxicity

Growing tolerant species needs to be considered as a short-term

option to manage acid soils Soils will continue to acidify over time

Soil pHCabelow 4.3 Soil pHCa4.3 to 4.8 Soil pHCaabove 4.8

Al concentrations increase Al concentrations increase Al concentration extremely

to levels that affect even the to levels that will affect the low and not likely to be

most tolerant plants most sensitive species toxic to plant growth.

Significant decrease in Significant decrease in Values for exchangeable

plant yield plant yield Al generally less than

Exchangeable Al greater Exchangeable Al varies 5 mg kg -1

than 70 mg kg -1 from 5 to 70 mg kg -1

(0.78 cmol (+) kg -1 ) (0.06 to 0.78 cmol (+) kg -1 ) (less than 0.06 cmol (+) kg -1 )

Manganese (Mn)

At low pH and in high concentrations, manganese can

become toxic to plant growth but not in all soil types

Some soils do not contain high levels of Mn even at low

pH Aluminium has a dominant effect at low pH

Seasonal fluctuations in Mn may vary in wet or dry soil

Concentrations increase in waterlogged soils or poorly drained soils in

spring, when warm and wet conditions coincide Elevated Mn

concentrations may also occur in hot, dry summer but the

likely period of toxicity in species other than lucerne

is in autumn after the first rains Above soil pHCa

5.5 Mn is not toxic to plant growth

Revisit page 30 for interpreting aluminium level from a soil test.

What do I need to do?

For more information:

Go to page 78 Measuring aluminium

2 6 A C I D S O I L A C T I O N • U N D E R S T A N D I N G A C I D I T Y

What do the results mean?

There are a number of aspects to consider when interpreting soil test figures for pH

• Which crops or pastures will grow best in a soil of a particular level of acidity or alkalinity?

• What is the availability or non availability of soil nutrients due to the soil pH level?

• What are the trends in soil pH levels over time?

• Is lime required?

1 Plant Responses

Most plants prefer a pHwrange between 6.0 and 7.5, (pH measured in calcium chloride 5.5 to 7)

but will grow outside this range although yield may be affected How far outside this range

plants will grow varies between the species

Preferred pHwranges for plants

Soil biological activity is also affected by soil pH This becomes important whenapproaching the extremes of acidity or alkalinity, when for example variousspecies of earthworms, and nitrifying bacteria, disappear Rhizobia strains vary

in their sensitivity to soil pH, and have preferred ranges in which they areeffective Most soil organisms function best between pH 6.0 to 7.0

Barley

Red clover Canola

Sub clover Linseed

White clover Lupins

Cocksfoot Maize

Cowpeas Millet

Medics Oats

Phalaris Peas

Ryegrass Rice

Serradella Ryecorn

range

Acidity

Alkalinity

2 Nutrient AvailabilitySoil pH will influence both the availability of soil nutrients toplants and how the nutrients react with each other At a low pHmany elements become less available to plants, while otherssuch as iron, aluminium and manganese become toxic to plantsand in addition, aluminium, iron and phosphorus combine toform insoluble compounds In contrast, at high pH levelscalcium ties up phosphorus, making it unavailable to plants, andmolybdenum becomes toxic in some soils Boron may also betoxic at high pH levels in some soils.

Influence of pH on soil organisms

3 4 5 6 7 8 9 10 11

FUNGI BACTERIA ALKALI TOLERANT WORMS ACID TOLERANT WORMS

NUMBERS

pH

2 8 A C I D S O I L A C T I O N • U N D E R S T A N D I N G A C I D I T Y

Effect of pHCaon availability of plant elements

The relative availability of 12 essential plantnutrients in well-drained mineral soils intemperate regions in relation to soil pH A pHCarange between 5.0 and 6.0 (between heavy lines)

is considered ideal for most plants

NITRATES POTASSIUM

CALCUIM PHOSPHATE

IRON ALUMINIUM

MAGNESIUM SULPHUR MANGANESE

MOLYBDENUM ZINC

COPPER BORON 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

pH

To maximise profitability of your farming system an essential

part of your whole farm plan is to:

• Find out the current level of soil acidity across your farm

• Assess whether soil acidity is limiting production

• Estimate the rate at which the soil is becoming more acid

• Consider the future risk of doing nothing

If you suspect you have an acid soil problem and this is confirmed by your soil test

results, the next step is to decide your management strategy.

You have three main acid soil management options:

• Grow acid tolerant species to maintain cash flow

• Apply and incorporate lime when sowing a crop or pasture or topdress with lime

• Adopt less acidifying farming systems and practices to slow down acidification

A combination of the above strategies is usually needed to manage acid soils and maintain

economic production

Increase soil pH

Applying lime to increase surface soil pH is the most practical option to manage acid

soils After the initial application, soil pH will increase, plateau, then gradually the soil

will reacidify and pH will fall Liming frequency depends on the intensity of production,

the environment and soil type Reapplication will be necessary after 4 to 10 years If your

soil pH is just above the range for optimal plant growth of most species and there is no

affect of soil acidity, preventative liming should be considered The cost of liming should

at least be covered by the value of production which would have been lost without the

liming operation

Tolerance

Growing acid tolerant plants is one way to deal with declining yields on

acid soils but it needs to be viewed as a short term option Eventually,

even the most acid tolerant species will be affected as soils continue to

acidify However, this strategy can maintain short term

profitability on acid soils and maybe useful “to buy

some time” before you implement your liming

strategy It may also be necessary if subsoil

acidity is a problem

Interpreting aluminium level from a soil test:

Aluminium is measured in different ways by different laboratories

The result on your soil test is determined by using one of the following

methods:

• Aluminium (mg/kg) using a potassium

chloride extract (Al mg/kg KCl)

• Aluminium (mg/kg) using a calcium

chloride extractant (Al mg/kg CaCl2)

• Aluminium as a percentage of cation

exchange capacity (Al ex%) This

percentage varies according to the salt

content of the soil

KCl Aluminium: This method uses potassium chloride to extract the

aluminium from the soil particles It is a measure of exchangeable

aluminium This method tells you the amount of aluminium that is

potentially toxic to the plant during the growing season, it doesn’t

indicate total available toxicity

CaCl 2 Aluminium: The calcium chloride method is used to measure

extractable aluminium This method estimates the amount of aluminium

that the plant root may encounter in the soil

3 0 A C I D S O I L A C T I O N • W H A T D O I N E E D T O K N O W ?

Go to page 79 for more detail on measuring Al

Aluminium becomes available in acid soils Some plants are more

affected than others

lucerne, barley highly sensitive

phalaris, canola sensitive

Aluminium Aluminium Aluminium Aluminium Aluminium Aluminium

tolerance of (mg/kg) KCl (mg/kg) CaCl2 ex% ex% ex%

plants Low salinity Medium High salinity

Aluminium as % CEC: This method determines the exchangeable aluminium as a

percentage of effective cation exchange capacity This means that a number of important

soil elements are measured such as calcium, magnesium, sodium, potassium and

aluminium The result is a percentage of aluminium compared to all the other cations

measured

When the soil becomes more saline the proportions of different cations changes so the

aluminium result varies according to the salt content of the soil Aluminium percentages

must be interpreted with the known salinity level (expressed as ds/m)

The following table summarises the critical concentrations of aluminium that will cause a

reduction in plant yield according to the three aluminium

methods:-Comparing soil aluminium concentrations with the species sensitivity to aluminium gives

a good indication as to the types of plants you can expect to grow in your soil For more

detailed test interpretation, please contact your local extension officer

Note: Within species, varieties can vary significantly in their reaction to aluminium toxicity.

Generally lucerne should not be sown when subsoil exchangeable aluminium is greater than

50 milligrams per kilogram, although this may depend on the cation exchange capacity (CEC)

of the soil There are interactions between calcium and magnesium in the soil which may allow

lucerne to be grown, even if the exchangeable aluminium levels would seem to preclude the

option High levels of organic carbon can also reduce the toxic effect of aluminium on plants.

Toxicity levels of aluminium.

The tolerance of crop and pasture plants to aluminium

Level of exchangeable aluminium above which yields are reduced.

(Aluminium percentage of CEC)

Aluminium as % of CEC Toxicity

Slow down the rate

Changing farm

management practice

can slow the rate of

acidification It can also

influence the amount of

lime needed to neutralise

acidity For example,

replacing annual plants with

perennial species to reduce

nitrate leaching or sowing

early to enhance root

development prior to the

autumn rains Feeding hay

back onto paddocks that have

been previously cut for hay

will help reduce the rate of

acidification as you are

replacing alkali material

Plants vary in tolerance to soil acidity Lupins (very tolerant) and barley (very sensitive) growing

on strongly acid soils high in aluminium.

Plant product removal such as hay has

an acidifying effect on your soil.

Acidifying effect of various farm enterprises in the

>500mm rainfall zone of South Eastern Australia.

Lime application is the most practical

management option.

Some farm management systems are more acidifying than others

Acidity of farm management systems

Lower acidifying Higher acidifying

Perennial pasture Annual pasture or crop

Rotational grazing Set stocking

Early sowing crops Late crop sowing

Cereals with high nitrogen fertiliser producing 80%

of district average

More nutrients

As acidity increases some soil nutrients become less available Increasing fertiliser rates

may overcome some short term deficiencies However, if you continue down this pathway,

you risk increasing the soil bank of nutrients that remain unavailable to plants and

subsequent productivity loss

Do nothing

If current soil pH status represents minimal risk to profits in the coming years, the do

nothing scenario may be appropriate But doing nothing over the long term will lead to

reduced soil productivity Your soil will continue to slowly acidify Eventually a liming

strategy will need to be put in place

Factor associated with high risk acidification

Topsoil has low clay content Nitrogen leaches easily

Low pH buffer capacity

Topsoil has low organic matter Low pH buffer capacity

Rainfall over 500mm Nitrogen easily leaches

More plant production so greater product removal More nitrogen input to support production Productive legumes More nitrogen input, increase risk of N leaching

Annual plant system No roots to absorb N at beginnning of season

Cultivated regularly Higher rate of N mineralisation and leaching

Semi-arable and non-arable land:

On acid non-arable grazing land based on improved pastures, lime application

will reduce acidification, although the lime spread on the surface moves slowly

into the soil profile The alternative is to rely on acid tolerant species,

increase fertiliser rates or take the land out of production, stop applying

fertilisers and discourage clover growth

It may take several years before the benefits of surface applied lime are

demonstrated, particularly on clay soils with medium rainfall A rapid

response to surface applied lime is most likely due to the release of

molybdenum, release of nitrogen through mineralisation or an improvement

in nodulation

The effectiveness of the surface applied lime can

be improved by using high quality fine grade lime and

spreading when there is a good ground cover to ensure

it does not blow away Spreading lime prior to a direct

drill operation will give some incorporation

8.4 or higher Highly alkaline

Go to page 63 for Lime Guide

Use of super-lime, a mixture of lime and superphosphate, banded with seed

when direct seeding will provide a channel and allow roots of acid sensitive

species to initially establish However, once the plant has germinated, it will

need to send roots out into less favourable surrounding soil

Super-lime will supply some neutralising value to the soil but as an alternative to

lime it is uneconomic Its use will also have no impact on the management of

subsoil acidification

Frequent applications of small amounts of lime to the surface, instead of larger

amounts of lime applied once every 5 to 10 years will not increase soil pH

enough to allow leaching of alkalinity into the subsurface

Applying lime will not correct a phosphorus deficiency Normal

phosphorus applications are still required when lime is used

The activity of soil fauna, particularly earthworms, can increase

movement of lime into the soil

In cropping rotations acidification rates can be reduced by liming on a regular

basis usually before a responsive crop such as canola or barley Acidification

rates will also be reduced using less acidifying fertilisers, avoiding fallows in

cropping areas over summer, retaining stubble and sowing crops as soon as

possible after the first autumn rain

3 8 A C I D S O I L A C T I O N • W H A T D O I N E E D T O K N O W ?

Acid Soil Management – What do I need to know?

Annual legume

based pasture

Processes that acidify soil

• unused nitrate leached below root zone

• product removal

• build up of organic matter

• use of acidifying fertilisers

Topsoil pH 4 to 4.5 (pH water 4.8 to 5.3)

• caused bysignificantacidification

• natural or longterm use ofintensiveagriculturalpractices

• exchangeablecations such asaluminium andmanganesebecome toxic toroots

• deficiencies ofnutrients(molybdenum andcalcium)

• reduced soilmicrobialpopulations

Crop/pasturerotation

Perennial basedpasture system Horticulture

Soil test for pH (CaC12)Topsoil 0 - 10cmSubsoil 10 - 20cm

Topsoil pH < 4

(pH water 4.8)

• acid sulphate or

well weathered low

fertility soils in high

• critical pH level

• optimum growthfor acid-tolerantcultivars if fertiliserrequirement met

• effects of toxicitiesmay limit optimumyield potential ofsensitive species

• below pH 4.8aluminium toxicitylimits production ofsensitive species

• ameliorationeconomically viable

• strategy depends

on land use, soiltype

Topsoil pH 5 to 6.5 (pH water 5.8 to 7)

• no effect of soilacidity

• optimum forgrowth of mostplant species

• no nutrienttoxicities ordeficienciesassociated with pH

Urgent lime

requirement

Lime to above pH 5.5Grow acid tolerantplant species

Lime to above pH 5.5Grow acid tolerantplant species

Monitor to keep pHabove 5.5Soil test every 5 years

Check subsoil prior to sowing sensitive

perennial species

Step 1 Check the signs

You probably already suspect an acid soil problem If you are not sure or need more

information, use the action guides and inspect your soils, pastures and crops

Acidity checklist

❏ Yields are fallingOverall production is down and hasbeen declining over time

Poor persistence of perennial plants

❏ Sensitive crops are failing

Poor establishment and growth of plants which don’t

like acid soil including any of these:

❏ Toxicity problems are showing up

Acid soils can be toxic, killing or injuring plants Look for:

• stunted root growth

• poor establishment and persistence of pasture species such as

lucerne and phalaris where previously they grew well

• delayed maturity in wheat

• uneven pasture and crop growth (especially sensitive

species)

• poor nodulation of legumes

• abnormal leaf color, pale green, yellow (or even red) early in the

season

4 0 A C I D S O I L A C T I O N • A C T I O N G U I D E F O R S O I L A C I D I T Y

❏ Indicator plants are

appearing

More grasses and acid-tolerant weeds

(for example, sorrel)

See page 44 for help

in taking soil samples

Tolerant weeds such as sorrel thrive on acid soils