soil acidity liming

7/24/2019 Soil Acidity Liming

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AGFACTS

AGFACTSAGFACTSwww.dpi.nsw.gov.au

Soil acidity andliming

Agfact AC.19, 3rd edi tio n 20 05Brett Upjohn1, Greg Fenton2, Mark Conyers3

ORDER NO. AC.19 AGDEX 534

1District Agronomist, Tumut, NSW Department of Primary Industries.2Former Program Coordinator, Acid Soils Action, NSW Department of Primary Industries.3Senior Research Scientist, Wagga Wagga Agricultural Institute, NSW Department of Primary Industries.


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DISCLAIMERThe information contained in this publication is basedon knowledge and understanding at the time of writingin March 2005. However, because of advances inknowledge, users are reminded of the need to ensure

that information upon which they rely is up-to-dateand to check the currency of the information with theappropriate officer of NSW Department of PrimaryIndustries or the users independent adviser.

ACKNOWLEDGEMENTS

Much of the content of this Agfact has beenderived from Agfact AC.19, editions 1 & 2.The authors of Edition 3 acknowledge thecontribution of previous authors including:Keith Helyar, Peter Orchard, Terry Abbott, PeterCregan and Brendan Scott.

Acknowledgement is also given to Ross Ballard(SARDI) for rhizobia nodule photos, and MichelDignand for diagram design. Also to David and

Jamie Elworthy and Jade Francis for permissionto take photos of liming related activities.

ACIDIC SOILS IN NSW 3THE IMPACTS OF SOIL ACIDITY 5RECOGNISING SOIL ACIDITY 5 Soil tests 6 Measuring soil pH 6CAUSES OF ACIDIFICATION OF THE SOIL 7 Leaching of nitrate nitrogen 7 Use of nitrogenous fertilisers and

legume pastures 8 Removal of produce 9 Build-up of soil organic matter 9HOW SOIL ACIDITY REDUCES CROP ANDPASTURE PRODUCTION 10 The effect of aluminium (Al) toxicity 10 Soil testing for aluminium 10 The effect of manganese (Mn) toxicity 11 Plant analysis 12 Seasonal changes in availability of

manganese 13 Soil testing for available manganese 13 Managing toxic levels of soil manganese 14 The effects of molybdenum (Mo) deficiency 14 Correcting a molybdenum deficiency 14 The effects of calcium (Ca) deficiency 14 The effects of magnesium (Mg) deficiency 15 Magnesium and soil structure 16 Magnesium and grass tetany in cattle 16 Using the Ca:Mg ratio to predict plant

growth 16 Effect of soil acidity on the micro-

organisms that affect plant growth 16

Reduced fixation of nitrogen 16

The effect of pHCa

on the activity of

Take-all root disease in cereals 16

REDUCING THE IMPACT OF SOIL ACIDITY

ON AGRICULTURE 17 Growing acid tolerant crops and pastures 17

Protecting soils at risk of becoming acidic 17

Reducing the leaching of nitrate nitrogen 17

Using less acidifying fertilisers 17

Minimising product removal effects 18

Preventing erosion of top soil 18

MANAGING SOIL ACIDITY WITH LIMESTONE 18

Limestone quality 19

Fineness 19

Neutralising value 19

Calcium and magnesium contents 19 Comparing liming materials 20

The quantity and timing of limestone

application 20

Timing of limestone application 20

Check list before applying lime 21

Applying and incorporating lime 21

Spreading machinery 21

Dust problems 21

Incorporation 22

Surface applied limestone 22

ESTIMATING RATES OF SOIL ACIDIFICATION 22FURTHER INFORMATION 23

ACKNOWLEDGEMENTS 24

GLOSSARY 24

CONTENTS

NOTES ON PASTURE IMPROVEMENT

1. Pasture improvement may be associated with anincrease in the incidence of certain l ivestock healthdisorders. Livestock and production losses from somedisorders are possible. Management may need to be

modified to minimise risk. Consult your veterinarian oradviser when planning pasture improvement.2. Legislation covering conservation of native vegetation

may regulate some pasture improvement practiceswhere existing pasture contains native species. Inquirethrough your office of the Department of Infrastructure,Planning and Natural Resources for further details.


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1pH in 1:5 soil:0.01 M CaCl2 see Measuring soil pH page 6.

Figure 1. Soils with a low pHca

and soils at risk of developing a low pHca

.

Tweed Heads/Coolangatta Murwillumbah

Byron Bay

Kyogle

Lismore Ballina

Casino

Grafton

Stanthorpe

Tenterfield

Texas Yetman

Warialda

Coffs Harbour

Inverell Glen Innes

Dorrigo

Guyra

Armidale

Walcha

Nambucca Heads

Port Macquarie

Kempsey Tamworth

Wauchope

Scone

Muswellbrook

NewcastleMaitland

TareeGloucester

SYDNEY

Nowra

Wollongong

Goulburn Yass

Cooma Jindabyne

Bateman s Bay

Eden

CANBERRA

Bombala

Corryong

AlburyWodonga

Beechworth Wangarattaa

Benalla

Shepparton Bendigo

EchucaYarrawonga

Swan Hill

Balranald Hay

DeniliquinJerilderie

Leeton

Narrandera

Lockhart

Wagga Wagga

Junee Cootamundra

Young

Tumut

Batlow

Mildura

Wentworth

Ivanhoe

Roto

Hillston

Booligal

Griffith

Lake Cargelligo

Condobolin

Forbes

Parkes

Wellington

Braidwood

Lithgow

Cowra Oberon

Bathurst

Orange

Kandos

Mudgee

Narromine Dubbo

Merriwa

Coolah

Binnaway

Coonabarabran

Gilgandra

Manilla

Barraba

Bingara

Narrabri

Moree Collarenebri

Coonamble

Walgett

Lightning Ridge Tibooburra

Wanaaring

Tilba White Cliffs

Wilcannia

Broken Hill

Menindee

Enngonia

Bourke

Cobar

Byrock

Brewarrina

Mungindi

pHCarange

< 4.25 (acid soils)

4.254.5

4.65.0

5.15.5 (soils at risk from acidity

(slightly acid soils)}

ACIDIC SOILS IN NSW

Acidic soils are an impediment to agriculturalproduction. More than half of the intensivelyused agricultural land in NSW is affected bysoil acidity. The gross value of agriculturalproduction lost in NSW due to soil acidity has

been estimated in the Land and Water Audit(2002) at $378 million per year. Ongoingacidification caused largely by normalagricultural practice increases limitations onfuture production and in some areas results inpermanent degradation of soils in NSW.

The average pHCa

1of surface soil throughout

eastern and central NSW based on data collectedin 1989 is shown in Figure 1. It can be seen thatlarge areas of the northern, central and southerntablelands, the central and northern coast, and

the central and southern slopes have soil with apHCa

of 5.0 or less.

At that time it was estimated that 13.7 millionhectares of agricultural land had a surface soil(0 to 10 cm) pH

Caof less than 5.0. It was also

estimated that a further 5.7 million hectareswas at risk of developing acidic soil problemsbecause they had a topsoil pH of 5.1 to 5.5.

A soil pHCa

between 5.5 and 8.0 provides thebest conditions for most agricultural plants. Ifthe pH

Cadrops below 5.0, plants that are highly

sensitive to acidity, such as lucerne and barley,are adversely affected. Plants that are moretolerant of acidity continue to grow normallyuntil the pH

Cafalls below 4.5. Below pH

Ca4.4

most plants, except the very highly acid tolerantplants like oats, narrow leaf lupins and thenative pasture grassMicrolaena spp, show asignificant reduction in production.

If only the top 10 cm of the soil profile isacidic it can be readily corrected by applyingand incorporating finely ground limestone.However, if acidification of the soil continuesand the surface pH

Cadrops below 5.0 the acidity

will leach into the subsurface soil (Figure 2a).

The further the acidity has moved down theprofile the greater the effect on plant growthand the more difficult it is to correct. This iscalled subsurface soil acidity and is a long termdegradation of the soil. Not all areas of NSW

with subsurface soil acidity have been identified,but it can be assumed that most soils with pH

Ca

values below 4.8 in the surface soil and an


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Figure 2a. The development of subsoil acidity and the implications for acid sensitive plants.

a) No acidic soil problems.b) Acidification starts at the surface restricting surface root development.c) Acidity is leached to depth when the pH

Caof the surface soils drops below 5.0 and all root growth is restricted.

d) Subsurface soil acidity is permanent as surface applied lime only corrects acidity in the surface soil.

Figure 2b: Sampling of the 1020 cm (subsurface) soil layer, as well as the surface soil, indicateswhether acidity is a problem in subsurface layers.

pH of 010 cm sample issimilar for both situations

pH of 1020 cm sample will show if acidity

has moved into the sub-surface layers

acid soil

MichelDignand

(a)

(b)

(c)

(d)

Acid Soi l


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Photo 1: Subterranean clover root stunting dueto increasing aluminium.

Photo2: Acid tolerant triticale on right grew much better than acid sensitive barley on left.

average annual rainfall of at least 500 mm wouldhave developed acidic subsurface soils.

The pHCa

of a soil rarely falls below 3.8. Iffurther acid is added to a soil, it causes abreakdown of the clay minerals. This is apermanent change to the soil and cannot be

reversed. Soils that are very acidic, with pH lessthan 3, are usually associated with acid sulphateconditions and these soils are not discussed inthis Agfact.

THE IMPACTS OF SOIL ACIDITY

Farmers see the direct impacts of soil acidity aslost productivity and reduced income through:

reduced yields from acid sensitive crops andpastures

poor establishment of perennial pastures failure of perennial pastures to persist.

Acidic soils also impact on the community.

There is permanent degradation of the soilwhen the acidity leaches to a depth whereit cannot be practically or economicallycorrected (Figure 2b.). This is a slow processand will most likely affect future generationsmore than it affects the present landmanagers.

Recharge of aquifers is due to less wateruse by plants affected by soil acidity. Thiscan lead to dryland salinity and damage toinfrastructure such as the break up of roads.

There is an increase in soil erosion andaddition of silt and organic matter to

waterways as annual vegetation predominateson acidic soils, leaving the soils exposed toerosion for a significant part of the year.

The details of the impact of soil acidity onspecific crops and pastures are explained inthe section How soil acidity reduces crop andpasture production.

RECOGNISING SOIL ACIDITY

The signs of soil acidity are more subtle thanthe clearly visible symptoms of salinity and soilerosion.

Cereal growers may predict that their soil isacidic when acid sensitive crops fail to establish,or crop production is lower than expected,particularly in dry years. In pasture paddockspoor establishment or lack of persistence ofacid sensitive pastures such as lucerne, and to alesser degree phalaris, is an indication that thesoil may be acidic.

More definitive indications of acidic soil are:

Stunted or shallow root growth in crops andpastures (see photos 1,2, 3 and 4)

poor nodulation in legumes or ineffectivenodules (see photos 5 and 6)


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Photo 4: The effect of aluminium toxicity onthe roots of wheat plants. From left to right theplants were grown in solutions containing 0, 5and 10 ppm aluminium.

Photo 3: Berseem clover grown in a highaluminium (pH

Ca4.0) soil. The small purple leaves

are characteristic of aluminium toxicity in clover.

manganese toxicity symptoms in susceptibleplants (see photos 6 and 7).

Soil tests

A soil test is the most reliable way to assessif soils are acidic. A comprehensive chemicalanalysis of the surface soil (010 cm depth)gives information that will assist farmers indetermining if a crop or pasture will be affectedby acidity. In order to assess whether acid

sensitive crops will be affected by subsurfaceacidity, it is recommended that subsurface soil(10 cm to 20 or 30 cm) be tested for pH.

For soil tests to be meaningful, paddocksneed to be divided into management units forsampling. For example, dividing the paddock on

the basis of slope and aspect. To be confidentthat the soil sample which goes to the laboratoryrepresents the variability of the paddock, itis necessary to collect and thoroughly mix 30cores (or greater than 0.5 kg of soil) from eachmanagement unit.

Common soil analysis includes the followingtests:

soil pHCa

or soil pHw

. (See Measuring soilpHbelow for an explanation of these terms.)

exchangeable cations and their percentage ofthe ECEC (see the Glossary for explanation ofterms)

available phosphorus

electrical conductivity (EC)

soil texture.

Measuring soil pH

Acidity and alkalinity in any solution ismeasured as pH. Soil pH is an estimate of theacidity/alkalinity of the soil solution, which is

the water that is held in the soil (Figure 3). Mostsoil pH measurements in Australia are made byshaking soil samples for an hour in either a 1:5soil to water suspension (pH

w) or a 1:5 soil to

0.01M calcium chloride suspension (pHCa

) andusing an electrode to measure the pH of theresultant mixture.

The pH measured in calcium chloride is onaverage 0.5 to 0.8 less than pH measured in

water, although the difference can vary from

nil to 2.0 for different soils. In this Agfact, pH isnominated as pHCa

.

The timing for collection of the soil sampleand the sampling depth are important as theacidity of soil varies throughout the year, anddown the profile. The pH in summer is in mostcircumstances higher than that in winter by upto 0.5 of a unit. This is important when makingrecommendations for winter crops based onanalysis of samples taken over summer.

The pH varies down the soil profile, thesurface soil usually being more acidic thanthe subsurface soil layers. In this Agfact,recommendations are made on the basis that


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Figure 3. Plant growth in relation to pH, and examples of some of commonly used terms to describeeffect of acidity/alkalinity on plant growth.

the samples for analysis are taken to a depth of10 cm, in late summer or early autumn.

If the pHCa

of the 010 cm layer is less than 4.8then the subsurface soil may be acidic. In higherrainfall areas, subsurface soil layers are morelikely to be acidic. The failure of lucerne to

persist over summer after lime has been appliedis a potential indicator of an acid subsurface soil(Figure 2b). The only reliable way to determineif a paddock has an acidic subsurface soil is tosample and analyse the 1020 cm, and possibly2030 cm layers.

CAUSES OF ACIDIFICATION OFTHE SOIL

Acidification of the soil is a slow naturalprocess and part of normal weathering. Many

farming activities cause an increase in the rateof acidification of the soil. Changes in soil pH

Ca

under agricultural use are measured in tens orhundreds of years rather than thousands of yearsas in the natural environment.

There are four ways that agriculture contributesto the accelerated acidification of the soil andthese are shown in Figure 4.

They are:

use of fertilisers containing ammonium or urea

leaching of nitrate nitrogen sourced fromlegume fixation or from ammonium fertilisers

removal of produce

build-up of soil organic matter.

In some older texts the removal of base cations,that is calcium, magnesium, potassium andsodium, is given as a cause of soil acidity. This ismisleading, and in a similar vein acidity cannotbe corrected by applying calcium.

Leaching of nitrate nitrogen

Leaching of the nitrate form of nitrogen is amajor contributor to agriculturally induced soilacidification. Nitrate nitrogen is produced in thesoil by the breakdown of organic matter, or ofammonium forms of nitrogen.

The chemical processes that produce nitratenitrogen from fertiliser and organic matterleave the soil slightly more acidic. This acidityis neutralised by plants discharging an alkalinesubstance as they take up nitrate nitrogen andto a smaller extent by conversion of nitratenitrogen to nitrogen gas.

While the plants continue to take up all thenitrate nitrogen, the acid/alkali balance of thesoil surrounding the roots remains in balance.Nitrate nitrogen is very soluble and easily


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Figure 4. Agricultural practices that increase the rate of soil acidification.

leached. Leaching breaks the balance of theacid/alkali processes and results in increasedsoil acidity. Deep rooted perennial plants reducethe risk of leaching which is most prevalent inautumn/early winter. Perennial plants are moresuited to control leaching as they are able to

grow quickly after the autumn break rains andcapture soil water before leaching can occur.Where farming systems are based only on

Table 1. Acidifying effect of nitrogenous fertilisers and legume-fixed nitrogen in terms of lime requiredto neutralise the acid added.

Source of nitrogen

Lime required to balance acidification(kg lime/kg N)

0% nitrateleached

50% nitrateleached

100% nitrateleached

High acidificationSulphate of ammonium,Mono-ammonium phosphate (MAP)

3.7 5.4 7.1

Medium acidificationDi-ammonium phosphate (DAP)

1.8 3.6 5.3

Low acidificationUrea

Am monium ni tr at eAqua am moni aAn hydr ou s am moni aLegume fixed N

0 1.8 3.6

Alka li sa ti onSodium and calcium nitrate

3.61 1.8 0

1Equivalent to applying 3.6 kg lime/kg N.

annual plants, leaching can occur before newroot development has occurred.

Use of nitrogenous fertilisers and legumepastures

The amount of acidification that results fromusing nitrogenous fertilisers depends on thefertiliser type (Table 1). Fertilisers that containnitrogen as ammonium, for example ammonium


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sulphate, acidify the soil within weeks afterapplication. Calcium nitrate and sodium nitratehave a neutralising effect on soil acidity, unlessall the nitrate is leached (Table 1) but they areexpensive and use is restricted to horticulture.

Using superphosphate fertiliser on crops and

pastures does not directly acidify the soil.However, its use stimulates growth of clover andother legumes, resulting in a build-up in organicmatter which in turn increases soil acidity. Alsothere is an increase in nitrate nitrogen in thesoil that comes with the higher levels of organicmatter. This increases the likelihood of soilacidification from leaching of nitrate nitrogen.

Applying pure sulphur or flowers of sulphurwill acidify the soil. An application of 3 kg oflimestone for each kg of sulphur is required to

neutralise this effect.

Removal of produce

Grain, pasture and animal products are slightlyalkaline and continued removal will lower thesoil pH over time. This contribution to acidity ispart of the carbon cycle (see the Glossary). If

very little produce is removed, such as in woolproduction, then the system remains almostbalanced. Where a large quantity of produceis removed as in the case of hay making

(particularly clover or lucerne hay), the soil isleft significantly more acidic. For details on thequantity of lime needed to neutralise acidityrelating to common agriculture products seeTable 2.

Removal of produce by burning, for exampleburning of stubble, does not change theacid/alkali balance of the soil, but gives aredistribution, leaving alkali at the soil surfaceas ash. If the ash is then washed away, asmight occur after a fire, this would leave the

soil more acidic.Build-up of soil organic matter

Over the last 50 years the regular use of fertiliserand improved pastures, particularly subterranean

Table 2. The amount of lime needed to neutralise

the acidification caused by removal of produce.

Produce removed Lime requirement kg/t of produce

Milk 4

Wh ea t 9

Woo l* 14

Meat* 17

Lupins 20

Grass hay 25

Clover hay 40

Maize silage 40

Lucerne hay 70

* Further acidification occurs with set stocking of sheep due to theuneven deposition of animal excreta in stock ca mps.

Table 3. Estimates of the relative importance of factors causing agriculturally induced soil acidity fortwo farming systems.

Cause of acidification Annual pasture SouthernTablelands NS W(%)

Cropping/pasture rotation,Wagga Wagga (%)

Leaching of nitrate nitrogen 5070 5070

Build-up of soil organic matter 1030 Nil or reverseRemoval of product 1030 2030

Use of nitrogenous fertilisers n/a 510

clover, has generally led to increasedorganic matter in the soil. While increasingorganic matter has many benefits, includingimprovement of soil structure, it also increasessoil acidity. The build up of organic matter willnot continue indefinitely, and there is no further

acidification due to this cause once the organicmatter stabilises at a new level.

The acidification caused by a build up in organic

matter is not permanent and can be reversed if

the organic matter breaks down. However, there

will be a permanent change in the acid status

of the soil if the topsoil containing the organic

matter is eroded or removed.

The relative importance of these causes of soil

acidification for two farming systems is given in

Table 3.A simple demonstration of the extent to which

agricultural land use has acidified a paddock

is to compare the soil test of the paddock with

a similar test of a sample collected from an

area close to the paddock that has not been

cultivated or fertilised, such as an adjacent

roadside.


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Photo 5: Yellow leaf margins of clover indicatemanganese toxicity in the soil.

HOW SOIL ACIDITY REDUCES CROPAND PASTURE PRODUCTION

Nutrient solubility, and thus availability to plants,varies with soil pH. Some nutrients may reachtoxic levels, while others become unavailableleading to deficiencies. The increased availability

of aluminium in the soil solution associated withdeclining pH is an example of this, aluminiumtoxicity being a major problem for crop andpasture production in acid soils.

Other production losses may occur where acidityreduces the activity of beneficial soil micro-organisms. It is recognised that the nitrogenfixation byRhizobia spp. on legume roots isretarded in acid soils, resulting in lower nitrogenavailability and reduced production.

In general the changes in the availability of plant

nutrients associated with increasing soil acidity are: Increased available aluminium (Al3+) causing

stunted root development in crops andpastures (see photos 1, 2, 3 and 4). Stuntedroots result in reduced capability to accesssoil moisture, and reduced nutrient uptake.

Increased available manganese (Mn2+)causing reduced growth in some plants insome soils (see photos 3, 4 and 5).

Reduced solubility and availability ofmolybdenum, phosphorus, magnesium andcalcium.

The actual amount of aluminium and manganesein solution in a soil at a given pH varies betweendifferent soil types. Weakly weathered soilsthat are acidic tend to release toxic amountsof soluble manganese, but lesser amounts ofaluminium. Alternatively, highly weathered soils(other than a group of soils high in iron andaluminium oxides) tend to release large amountsof aluminium and lesser amounts of manganese.

Different soils release different amounts ofaluminium and manganese at the same pH

Ca.

As the tolerance to aluminium and manganesevaries between plant species, it is not possible torecommend a single pH at which liming shouldbegin for all situations.

The effect of aluminium (Al) toxicity

Soluble aluminium in the soil solution causesmost of the problems associated with acidicsoils. The principal effects on plant growth from

soluble aluminium in the soil solution are:

Reduced root mass and function. Theprincipal effect of aluminium toxicity is

to reduce the mass and function of roots.Generally this is seen in the field as stunted,club shaped roots. This reduces their abilityto extract moisture from deep in the soil. (Seephotos 1, 2, 3 and 4.)

Tying up phosphorus. Soluble aluminium

immobilises phosphorus in the soil and theplant, causing symptoms of phosphorusdeficiency, that is, small and dark-green oroccasionally purple leaves. The symptomsbecome more pronounced as the aluminiumlevel increases.

Notethat applying lime to strongly acidicsoils slightly increases plant access tosoil phosphorus that is normally of lowavailability (such as residues of previouslyapplied fertiliser). This effect is usually small,and normal phosphorus applications are stillrequired when lime is used.

Reduced availability of calcium andmagnesium. Very high levels of aluminiumin the soil also reduce the uptake andutilisation of calcium and magnesium.

Aluminium toxicity does not usually occur insoils where the pH

Cais above 5.2. Applying

sufficient lime to lift the pHCa

above 5.5 will

remove aluminium from the soil solution.Alternatively the impact of aluminium can bereduced by growing plants that can toleratealuminium. Different plants show different levelsof tolerance to soluble aluminium levels in soil.See Table 4.

Soil testing for aluminium

Two methods are commonly used to measureavailable aluminium in the soil. The first methodmeasures the aluminium in the 0.01M CaCl

2


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extract used to determine pHCa

and is called thecalcium chloride extractable aluminium, Al

Ca.

This is the best estimate of the aluminium thatwill be encountered by the plant root. It givesthe best prediction of the effect of aluminium onplant growth.

The alternative method measures theexchangeable aluminium as part of thedetermination of the effective cation exchangecapacity (ECEC see the explanation in theGlossary). The proportion of aluminium inthe ECEC expressed as a percentage (Al

ex),

reflects the aluminium in the soil solution.This measurement is determined routinely bycommercial laboratories. When interpretingthese results the electrical conductivity (EC)of the soil is needed to accurately predict the

effect of the aluminium on plant growth. Table5 shows critical concentrations of calciumchloride extractable aluminium (Al

Ca) and the

exchangeable aluminium percentage (Alex

)for different electrical conductivities for themajor groups of plant tolerance to aluminium.

A critical concentration is one that will reduceplant growth by 10%.

Note when the organic matter is over 5% it mayadsorb soluble aluminium resulting in sensitiveplants being able to grow in soils with a lowpHCa. These plants will often have shallowroots as the subsurface soil contains little

Table 4. Aluminium sensitivity (tolerance) of some crop and pasture plants.

Highly sensitive Durum wheat, most barley cultivars, faba beans, lentils, chickpeas, lucerne, medics,Strawberry, Berseem and Persian clovers, Buffel grass, tall wheatgrass

Sensitive Cunningham & Janz wheat, Canola, Yambla barley, albus lupins, red grass (Wagga),wa ll aby gr as s (D. linkii), phalaris, red clover, Balansa clover, Caucasian and Kenyawh it e cl ov er s.

Tolerant Wh is tl er, Suns ta te & Diam ondb ir d wh ea ts , an nual & pe re nn ia l rye -g ra ss , ta ll fe scue ,Haifa white and subterranean clovers, chicory.

Highly tolerant Narrow leaf lupins, oats, triticale, cereal rye, cocksfoot, kikuyu, paspalum, yellow &slender serradella, Maku lotus, common couch, Consol love grass

These are only examples. For the current information on the tolerance of current varieties of winter crops to soil acidity see the Winter crop variety sowing guidepublished annually by NSW Department of Primary Industries.

Table 5. Critical concentrations of calcium chloride extractable aluminium

Al umin ium to le ranceof plants (Table 4)

Critical levelsAl Ca mg/L

Equivalent % Alex for soils at different electrical conductivities(EC 1:5 dS/m)

Low EC < 0.07 Med. EC 0.070.23 high EC > 0.23

Highly sensitive 0.10.4 916 28 0.52

Sensitive 0.50.8 1720 912 36Tolerant 0.91.6 2132 1321 710

Highly tolerant 1.72.7 3343 2230 1116

organic matter and is therefore high in solublealuminium. Both soil tests, calcium chlorideextractable and the exchangeable aluminium,

will indicate a reduction in the availablealuminium caused by organic matter taking thealuminium out of solution.

The effect of manganese (Mn) toxicity

Toxicity from excessive amounts of availablemanganese can affect the growth of crops,pasture and horticultural crops in soils wherepH

Cais less than 5.5, but only in some soils

and then only at certain times of the year.Plants require manganese in small amountsfor photosynthesis and for several enzymesincluding those controlling the plant hormones.Toxic amounts of manganese disruptphotosynthesis and the function of planthormones.

Toxic levels of manganese do not affect theproductivity of crops and pastures to the sameextent as aluminium toxicity. Manganese toxicityeffects are sometimes complicated by relatedproblems. The anaerobic conditions associated

with waterlogged soils induce manganesetoxicity but, other problems such as loss ofgaseous nitrogen may result in more yield lossesthan the toxic manganese.

While both toxicities and deficiencies ofmanganese can occur in NSW, the main problem


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Photo 6: The effect of manganese toxicity onTeal wheat. From left to right plants were grownin solutions ranging from 0, 90 to 180 ppmmanganese.

Photo 7: Canola leaves that are cup shaped withyellow margins indicating manganese toxicity.

is toxicity. However, excessive applications oflime may result in manganese deficiency insome light textured soils.

The visual symptoms of manganese toxicity insome common agricultural plants are:

Canola. The effect of manganese toxicity

is reduced vigour with yellowing of theleaf margins (see photo 7). Higher levels ofmanganese result in yellowing of the wholeleaf, necrosis of the leaf margins and greatlyreduced vigour or death. In most seasonscanola grows despite the toxic effects ofmanganese as the solubility of the manganesedrops in late autumn.

Lucerne, medics, serradella and subclover. The effect of excess manganese isreduced seedling vigour and red or yellowleaf margins. Normal growth rates return

with a decrease in available manganeseas the season progresses. If warm to hotconditions return after the first germinationthe level of available manganese increases,renewing the effect and possibly causing

death of the seedlings. The seedlings oflegume pasture species germinating in theautumn can be affected by high levels ofavailable manganese. Lucerne and mostmedics are highly sensitive, while sub cloverand serradella seedlings are only affected by

higher levels of manganese. Grasses. Lack of seedling vigour, yellowingat the tips and margins, and some flecking ofthe older leaves are indicators of manganesetoxicity (see photo 6). However, othernutritional problems can have similar effectsin grasses and tissue analysis is required toconfirm manganese toxicity.

Plant analysis

Because of the complexity of identifying

toxicity using visual symptoms, analysis of planttissue can help to determine if there is a toxicmanganese problem.

Some plants are affected by only a small amountof manganese in the tissue, while others aretolerant of high levels. The concentration of


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Table 6. Critical levels of manganese for various plants.

Manganese

tolerance category Plant

Critical leaf

manganese level(mg/kg)

Highly sensitive Lucerne, pigeon pea, barrel and burr medics 200400

Sensitive Wh it e cl ov er, st rawb er ry cl ov er, ch ic kpea , ca no la 40 0 700

Tolerant Su b c lo ve r, co tt on , c owp ea, so ybe an , w he at , bar le y, tri ti ca le , o at s 7 00 1 00 0

Highly tolerant Rice, sugar cane, tobacco, sunflower, most pasture grasses, oats,triticale, Tiga, Currency, cereal rye

> 1000

See NSW DPI Winter crop variety sowing guidefor more information.

Figure 5. Variation of manganese availability with season.

unlimed soil

limed soil

manganese in plants at which a small declinein growth will occur can vary from 200 to over1000 mg/kg of plant dry matter, depending onthe tolerance of the plant. Critical manganeselevels sufficient to cause a 10% decline ingrowth for a number of species are given in

Table 6. These levels are determined on theyoungest fully developed leaf.

Some plants are more sensitive to aluminiumthan to manganese and vice versa. For example,

white clover is tolerant of aluminium butsensitive to manganese.

Seasonal changes in availability of manganese

The availability of manganese can vary up to

four-fold through the year as shown in Figure 5.Manganese is most available in summer when

hot and dry conditions stimulate the chemicalchanges from an unavailable form (oxidised) to

an available form (reduced). This effect is less ina wet summer.

Waterlogged conditions in spring can producehigh levels of available manganese. However,

low levels of oxygen and loss of nitrogenassociated with waterlogging are likely to affectthe plants more than the high manganese levels.

Rain in autumn creates a soil environment thatfavours microflora which convert the manganeseback to an unavailable (oxidised) form. It

follows that the availability of manganese is at itslowest in winter, although potentially toxic levelsmay remain if the conditions are too cold inautumn (soil temperatures less than 10C) for themicroflora to change the available manganese tothe unavailable form.

Soil testing for available manganese

Because the levels of available manganesecan vary up to four-fold through the year inresponse to temperature and rainfall, a soil

test which measures available manganese atone time in the year cannot be relied on todetermine if manganese toxicity is likely to bea problem at another time. For example, it isnot possible to predict the level of availablemanganese in April, when sowing canola orlucerne, from a soil sample taken in February.


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This problem can be overcome by using soiltests that determine the potentially availablemanganese (the maximum that could becomeavailable under the most harmful conditions)rather than that which is available at the timeof testing and then to estimate the available

manganese based on the time of the year andsoil conditions at sowing.

To assist in the interpretation of soil test results,Table 7 lists three tests that are commerciallyavailable and a research test that givespotentially available manganese, the form ofmanganese that is extracted by each test, andthe range of results that might be expected. Notethat the values obtained from different tests cannotbe compared in absolute amounts.

Managing toxic levels of soil manganeseApplying lime sufficient to lift the pH

Cato

above 5.5 will decrease available manganese.Alternatively, the effects of manganese toxicityon autumn sown crops and pastures which aresensitive to manganese can be decreased bysowing on the second autumn rain.

The effects of molybdenum (Mo) deficiency

Many Australian soils are naturally low inmolybdenum and deficiencies are more likely to

occur in soils with pHCabelow 5.5.

All plants need molybdenum in trace amountsto facilitate the use of nitrate nitrogen.Molybdenum deficiency causes an accumulationof unused nitrate nitrogen, resulting in irregulartwisting of the leaves. The whole plant may bepale green and the older leaves can also showchlorotic striping and burnt leaf margins.

Legumes have higher molybdenum requirementsthan grasses and cereals as molybdenum plays

a part in the nitrogen fixing process. Where

Table 7: Tests for soil manganese that are commercially available.

Extraction method Form of Mn extracted Likely range of values(mg/kg soil)

1:10 soil:solution DTPATriethanolamine CaCl2

soluble + exchangeable 450

1:5 soil:solution0.05 M EDTA

soluble + exchangeable + portionof potentially reducible

50600

1:5 soil:solution

0.01 M CaCl2

soluble + exchangeable 0175

1:10 hydroquinone in1 M ammonium acetate

potentially available 01200

molybdenum is deficient, nitrogen fixingnodules occur more frequently than usual and

when opened the normal pink colour is replacedwith a pale green (indicating that they are notfunctional). The symptoms of a molybdenumdeficiency in a legume are those of a nitrogen

deficiency, that is, pale green to yellow leaves.Correcting a molybdenum deficiency

Because molybdenum is required in such smallamounts a soil test is not a reliable methodfor assessing if there is a deficiency. The onlyrecommendation is that where the pH

Caof a

soil is below 5.5 a response in legumes andsome cruciferous vegetables may occur to anapplication of 50100 g of sodium molybdateper hectare. This response varies and localinformation from your district agronomistor fertiliser outlet should be sought beforeproceeding. An application every 35 years(depending on soil type) as a spray of sodiummolybdate or molybdenum trioxide, or as acomponent of a fertiliser, is sufficient for allplants. A lime application that increases pH

Caby

one unit, for example from 4.5 to 5.5, increasesthe availability of both applied and naturallyoccurring molybdenum.

The effects of calcium (Ca) deficiency

Calcium is required to form new plant cells, so itis essential for the growing points of shoots androots, for root hair and for leaf development. Lowlevels of soil calcium also adversely affect thenodulation of legumes. Short term deficiency cancause petiole collapse of young expanding leaves.

Most soils in NSW have an adequate supply ofavailable calcium for field crops, pastures andhorticultural crops. Vigorously growing plants inmarginal calcium soils will show symptoms on

the parts of the plant that are furthest from the


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Photo 8: Lucerne plants showing leaf yellowing characteristic of molybdenum deficiency. Similarsymptoms occur where nitrogen is deficient.

Photo 9: Rhizobia nodules from a medic. Pinknodules on the left are active, white/greennodules on the right are non-functioning.

main flow of water. Blossom end rot in tomatoes(Photo 10) and watermelons and poor seed setin peanuts and subterranean clover are examplesof the effect of moderate calcium deficiency.More severe calcium deficiency causes death ofgrowing points, for example November leaf in

bananas.Very severe calcium deficiency is very rare andwill only occur in acidic soils that are sandy andlow in organic matter, or where there has beenexcessive use of highly acidifying fertilisers.Under these circumstances the percentage ofexchangeable calcium of the ECEC can drop tolow levels (< 40%), leaving the exchangeablealuminium as the dominant exchangeablecation (see the Glossary for an explanationof these terms). The symptoms may appear

as stubby, unbranched and discoloured rootsor as dead growing points in the shoots. Theroot symptoms are difficult to distinguish fromsymptoms of aluminium toxicity.

The effects of magnesium (Mg) deficiency

Magnesium deficiency has been recordedin seedling crops and pastures in light soilsPhoto 10: Tomato blossom end rot.


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in southern NSW where there is less than0.2 meq/100 g exchangeable magnesium in the010 cm soil layer, but such reports are rare.Usually these crops or pastures recover byspring, as nearly all soils in NSW have an amplesupply of magnesium in the subsoil that plants

access as their roots extend down the profile.The signs of magnesium deficiency in cereals are

yellowing of the oldest leaves and this can beconfused with nitrogen deficiency. In clovers thesymptoms can include reddening of the oldestleaves.

Magnesium and soil structure

There have been some reports that high levels ofmagnesium (exchangeable Mg > 50% of ECEC)may cause loss of structure in soil. Research has

shown that magnesium itself has no effect onspontaneous dispersion of undisturbed soil. Itcan, however, affect dispersion of a soil that hasbeen cultivated.

Where the exchangeable magnesium is greaterthan 30% of the ECEC it may enhance theeffect of sodium in causing dispersion of a soil,provided that the Ca:Mg ratio is less than 1 and:

the ESP > 4% and/or

the sum of ESP plus (Ex.Mg% divided by 10)

is greater than 6.(See glossary for explanation of terms.)

In practice this means that when the ESP is morethan 6%, the contribution of sodium (Na) is muchmore important than that of magnesium. ESP isthe principal measurement used to predict soildispersion. Note that the critical values of Mgand Na for dispersion given above are for a soil

with a low electrical conductivity (EC < 0.2 dS/min 1:5 soil:water). As the electrical conductivityincreases, so will the critical value of (ESP plus

Mg/10) need to increase to cause dispersion.

Magnesium and grass tetany in cattle

The onset of grass tetany in cattle is sometimesattributed to a low soil magnesium but a numberof factors might influence the onset of thiscondition. Check with a veterinarian or livestockofficer before applying dolomite or magnesiteto correct a grass tetany problem. There maybe more effective and less expensive ways toreduce the occurrence of grass tetany in cattle.

Using the Ca:Mg ratio to predict plant growth

Since 1901 there have been claims that theratio of exchangeable calcium to exchangeable

magnesium needs to be of the order of 4 to 6 toachieve a healthy soil and therefore optimumagricultural production. This claim has notbeen proven. Furthermore, there have beenseveral scientific reports that show that the Ca:Mg ratio can vary significantly with no effect on

agricultural production.Effect of soil acidity on the micro-organismsthat affect plant growth

Sometimes the effect of acidic soils on thegrowth and production of crops and pastures isnot direct but rather through the effect on soilmicro-organisms that in turn affect plant growth.

Photo 11: Rhizobia on lucerne roots.

Reduced fixation of nitrogen

Acidity reduces the survival of Rhizobia andthe effective infection of legume roots. Thesensitivity to acidity varies greatly betweenspecies. When a Rhizobia sp is affected by soilacidity it shows as poor nodulation and resultsin reduced nitrogen fixation. Often Rhizobiumbacteria are more sensitive to soil acidity thanthe host plant, for example lucerne and medics.

Lime pelleting of inoculated legume seed is usedto protect the inoculum against drying out andcontact with fertiliser. Sowing into bands of lime-super also creates an environment suitable forsurvival of the inoculum in an acidic soil.

For further information on the effect of soilacidity on legume nodulation and how tomanage it, see Agfact P2.2.7,Inoculating and

pelleting pasture legume seeds.

The effect of pHCa

on the activity ofTake-all root disease in cereals

The fungus that causes the root disease Take-all in cereals, Gaeumannomycesgraminisvar.


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tritici, is most active in soils with a pHCa

greaterthan 4.8, and has a low level of activity in soils

with a pH less than 4.6. Liming greatly increasesthe activity of Take-all.

Wheat and triticale should not be grown afterliming a paddock unless the population of the

fungus has been greatly reduced with a breakcrop or early fallow. Take-all can build uprapidly in wet seasons on roots of wheat, barley,triticale and many grasses and pose a threat to

wheat grown in the paddock the next year. Abreak crop of a broadleaf crop (canola, lupins,peas, etc.) or winter cleaning of grasses fromclover or lucerne pastures reduces the threat fromtake-all and should be done routinely after liming.

REDUCING THE IMPACT OF SOIL

ACIDITY ON AGRICULTUREGrowing acid tolerant crops and pastures

If a paddock is already acidic, particularly whereboth the surface and subsurface soils are acidic,the economic and downstream impacts and therate of acidification can be reduced by growingacid tolerant crops and pastures.

Acid tolerant species can help farmers to reducethe impact of soil acidity by:

maintaining cash flow if limestone cannot be

applied when required maintaining or increasing production on soils

with acidic subsurface layers that are toodeep to be limed economically, or are onnon-arable land

slowing the rate of acidification with moreefficient use of nitrate nitrogen and soilmoisture; particularly by replacing annual

winter grasses with vigorous, perennialgrasses that have some summer growth

allowing crop and pasture rotation sequencesto match the typical decline of soil pH

Ca

during a 1015 year liming cycle

increasing water use and ground cover andthus reducing the downstream impacts.

Acid tolerant plants may slow the acidificationof the soil but will not prevent it. Eventuallythe soil will become so acidic that only themost tolerant species can grow, and then withreduced production.

Where the soil is very acidic and sowing acidtolerant species is not practical then retiring landfrom agriculture may well be the best option forthe farm and the environment.

Refer to the list of aluminium tolerant crops andpastures in Table 4 when selecting crops andpastures for acidic soils. Tolerance of plants toaluminium closely reflects acid soil tolerance inall soils except the weakly weathered soils.

Protecting soils at risk of becoming acidic

Where soils are at risk of becoming acidic thefuture impact of soil acidity can be reduced,but not eliminated, by slowing the rate ofacidification. This can be achieved by:

minimising leaching of nitrate nitrogen

using less acidifying fertilisers

reducing the effect of removal of product

preventing erosion of the surface soil.

Reducing the leaching of nitrate nitrogen

Nitrate nitrogen (NO3-N) is easily leached as itis highly soluble. When NO

3-N is leached away

it leaves that part of the soil more acidic. If theNO

3-N is taken up further down the profile

there can be an increase in pHCa

at the point ofuptake. However when the NO

3-N is leached

below the root zone it leaves the soil profilemore acidic (Figure 4).

Table 8 lists the factors that affect NO3-N

leaching in order of importance. It qualifies theeffect of each factor and indicates how the effect

can be influenced. The absolute effect of eachfactor will increase with higher rainfall.

Using less acidifying fertilisers

Acidification of the soil can be reduced byavoiding the use of highly acidifying fertiliserssuch as sulphate of ammonium and mono-ammonium phosphate (MAP) (see table 1).Nitrogen fertiliser (including urea) that is pre-sown should be drilled into narrow bands toslow nitrification and subsequent leaching.

Surface application of nitrogenous fertiliser forcrops before sowing, even if harrowed, canresult in acidification due to nitrate leaching.Nitrate nitrogen will be better utilised byapplying top-dressed nitrogen to activelygrowing rather than dormant crops, resulting inless acidification risk.

The acidification caused by applying elementalsulphur can be eliminated by using productsthat contain sulphur in the sulphate formsuch as gypsum, potassium sulphate andsuperphosphate. Correcting acidification causedby using elemental sulphur requires 3 kg oflimestone for each 1 kg of sulphur applied.


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Table 8. Factors that affect NO 3N leaching in order of importance.

Factor affectingNO 3N leaching

Nature of effect Reducing the effect

Poor plant growth Inefficient water use increases leakageof water containing NO 3N into the subsoil and into the water table.

Efficient water use by healthy wellmanaged crops and pastures reducesleakage into the water table.

Nitrogen fixed by cloversin pastures

NO 3N will leach with autumn rainbefore annual pastures establish.

Perennial pastures will utilise the NO 3Nas it comes available soon after rain.

High clover to grass ratio

in pastures

High N producing pastures increase

NO 3N available to be leached.

Ai m fo r maximu m of 30 % cl ov er,

minimise annual weeds and maximiseperennial grass component.

Annual cr ops Delay in sowing annual crops will allowthe NO3to move down the profile onthe first water front ahead of the roots.

Sow as early as possible after weedcontrol.

N Fertil iser Soluble forms of N are leached awaybefore the plant can use it

Appl y top dr es se d N fe r ti li se r ac co rd in gto N budget guidelines.

Soil pH Higher pH increases nitrif ication thusincreasing NO3N available.

Maintain high productivity (andmaximise NO3N use) with perennialpastures and efficient crop management.

Lack of adsorption ontoclay

NO 3is weakly adsorbed by clays, soleaching occurs.

Utilise the nitrogen before it is leached.e.g. early sowing of crops, perennialpastures.

Minimising product removal effects

Grain, pasture and animal products are slightlyalkaline and their removal from a paddockleaves the soil more acidic. When a largequantity of produce is removed, particularlyclover or lucerne hay, the soil becomes

significantly more acidic. If very little produce isremoved, such as in meat or wool production,then the effect on the soil acidity is far less. Therates of limestone required to neutralise theacidification caused by removal of produce aregiven in Table 2.

If the produce is sold off-farm, regular liming isthe only way to maintain pH

Ca. The effect on soil

acidity of removing hay will be greatly reducedif the hay is fed back in the paddock whereit was cut. On dairies, effluent from the shed

should ideally be spread back on the paddocksand more than one night paddock should beused.

Preventing erosion of topsoil

A build-up of soil organic matter and/orincreasing the biomass, both dead and alive,has the same effect on soil acidity as removingproduce from the paddock. The increase insoil organic matter or biomass will stabilise ata new level over time and can be reversed if

the biomass is reduced and the organic mattermineralised.

However, if the topsoil is removed by erosionthe increase in the acidity of the soil ispermanent. Similarly if the biomass is burnedand the ash is washed or blown away then soil

is left a little more acidic.

Higher organic matter levels often mean higherlevels of available nitrate nitrogen and thepotential for further acidification due to leaching.

MANAGING SOIL ACIDITY WITHLIMESTONE

Application of finely crushed limestone, orother liming material, is the only practical wayto neutralise soil acidity. Limestone is most

effective if sufficient is applied to raise thepHCa

to 5.5 and it is well incorporated intothe soil. Where acidity occurs deeper than theplough layer, the limestone will only neutralisesubsurface soil acidity if the pH

Caof the surface

soil is maintained above 5.5.

The liming materials most commonly used areagricultural limestone and dolomite, but othermaterials are available. A list of the principalliming materials, together with some of theirproperties, is given in Table 9.


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Table9.

Chemicalanalysesao

fpureandc

ommercialgradesoftheprincipa

llimingmaterials.

Limingmaterial

Neutralisingvalue

Calcium(

%C

a)

Magnesium

(%M

g)

Pu

reform

Commercialgradesa

Pureform

Commercialgrades

Pureform

Com

mercialgrades

Good

Poortofa

ir

Good

Poor

Good

Poor

Agricu

ltura

llime(ca

lciumcarbonate)

100

9598

6075

40

3639

2832

0

Usua

lly

soil acidity liming

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