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PART B. COMMON PROBLEMS ANDSTRATEGIES

Chapter B1 Common problems

Chapter B2 Weed control

Chapter B3 Harvesting on wet soil

Chapter B4 Poor seedling emergence

Chapter B5 Does my soil need lime?

Chapter B6 Does my soil need gypsum?

Chapter B7 Managing saline soils

Chapter B8 Dispersion

Chapter B9 How do I control erosion?

Chapter B10 Does my soil need fertiliser?

Chapter B11 Case study 1

B1. Common problems

B1.1Vegetable SOILpak

Chapter B1. Common problemsPURPOSE OF THIS CHAPTER

To summarise the problems commonly encountered in horticulture.

CHAPTER CONTENTS

• summary of common problems

ASSOCIATED CHAPTERS

• Parts D and E

COMMON PROBLEMS

Erosion control (keeping the soil in place) is the most importantpractice. After that, other problems may need solving, as they reduceproductivity and often contribute to erosion risk.

The ideal soil for farming should supply plants with adequate water,oxygen, nutrients and support. When the soil does not supply theseneeds there is a soil problem. An example of a soil problem is a crustedsurface that reduces infiltration and increases run-off. Less water isstored in the soil for plants to use.

A soil problem may be due to:

• recent management techniques (for example, tillage when the soil istoo wet compacts, remoulds and smears the soil)

• a long history of a certain management (for example, continuouscropping for many years may deplete soil organic matter to the pointwhere the surface sets hard when dry)

• a property of the soil itself; the problem may always have been there(for example, if a soil is sodic, it has probably been sodic for a verylong time).

Consider the needs of plants, examine the soil, and then deduce theproblem. You will then be able to choose a management strategy to dealwith the problem. Economics will decide whether the strategy isfeasible.

Some common problems

Common soil problems in horticulture are:

• soil salinity

• loss of soil and plant nutrients by erosion

• soil acidity

• declining chemical fertility, particularly nitrogen

• damaged topsoil structure caused by traffic, wet tillage or stocktrampling

• plough pan caused by wet tillage

• poor surface structure causing crusting or hard-setting

• compacted subsoil caused by traffic.

B1. Common problems

B1.2 Vegetable SOILpak

B2. Weed control


Chapter B2. Weed controlPURPOSE OF THIS CHAPTER

To provide an overview of weed control

CHAPTER CONTENTS

• weed control

ASSOCIATED CHAPTERS

• D7 ‘Cultivation and soil structure’

WEED CONTROL STRATEGY

In controlling weeds between crops, consider the effect on soilerosion and soil compaction. Use herbicides rather than tillage whenthe soil is wet. Tillage on dry soil at or above the plastic limit ispermissible, but avoid creating finely tilled, bare, dry soil. Such asurface is very prone to erosion.

Wet soil

Herbicides are more effective when weeds are growing vigorously,as will happen in wet soil. The use of herbicides minimises thedisturbance of wet soil and any consequent compaction and smearing.

However, when you are spraying on wet soil, you are likely todamage the soil structure due to wheel pressure.

Drive in wheel tracks you have already made. This limits soilcompaction to a minimum area of the paddock.

Dry soil

Herbicides are not very effective when the soil is dry, because theweeds are not growing vigorously.

The timing of herbicide application is critical. Working the soilwhen it is dry is less likely to cause smearing or compaction.

Before tilling, check the moisture content of the soil well below theplough layer (say, at 30 cm, even though the tillage may not be thatdeep). Soil can appear dry on the surface, while remaining wetunderneath. If so, tillage can cause compaction at depth.

B2. Weed control


B3. Harvesting on wet soil


Chapter B3. Harvesting on wet soilPURPOSE OF THIS CHAPTER

To summarise the problems encountered when harvesting on wet soil

CHAPTER CONTENTS

• harvesting on wet soil

ASSOCIATED CHAPTERS

• D6 ‘Improving soil structure by crop rotation’


• D8 ‘Landforming and soil management’

THE DILEMMA

A mature crop loses yield and quality in wet weather, and there isurgency in the harvest. Vegetable crops destined for processing areoften harvested to suit factory requirements, and potentially harmfuleffects on the soil may be ignored. Harvesting on wet soils poses adilemma: while it is important to harvest the current crop, theharvesting operation will damage the soil for the next crop. Soilpreparation for the next crop includes what you do during this harvest.

Harvesting on wet soil is costly for three reasons:

• it takes more energy to drive on soft soil than on hard soil (creatingcompaction costs you money through higher fuel use)

• repairing damaged soil is costly

• the lower yield in the following crop is a cost.

STRATEGIES

• Where possible, avoid traffic on wet soils, especially clay soils.

• Make allowance for harvesting aids with permanent access roads.

• Consider the use of precision farming techniques, for example,permanent tracking and global positioning systems.

• Rejuvenate damaged soils with organic matter or selectivecultivation at the right soil moisture level. (See Chapter D7.)

B3. Harvesting on wet soil


B4. Poor seedling emergence


Chapter B4. Poor seedlingemergencePURPOSE OF THIS CHAPTER

To discuss the problem of poor seedling emergence and strategiesfor its control

CHAPTER CONTENTS

• poor seedling emergence

• compaction

• biological repair

• mechanical repair

• importance of moisture

ASSOCIATED CHAPTERS

• B6 ‘Does my soil need gypsum?’

• B8 ‘Dispersion’

• D4 ‘Slaking and dispersion’

• D5 ‘Sodic soil management’


POOR SEEDLING EMERGENCE

Patchy crops reduce yield and profit. Poor seedling emergence ismore of a problem under minimum tillage, because there is less soildisturbance and more stubble retention. It is less of a problem underconventional tillage, because the seedbeds are finely worked and theseeds are planted into bare ground. However, rather than rejectminimum tillage because of its problems, it is preferable to addressthose problems.

POSSIBLE CAUSES

Poor seedling emergence may be due to:

• poor seed–soil contact

• inaccurate seed placement

• a soil temperature that is too low or too high

• soil insects or soil-borne disease

• surface crusting after sowing

• poor quality seed.

Poor seedling vigour and establishment after emergence may causedby the presence of compacted soil beneath the seedling. The soil mayhave already compacted before sowing, or the planting tine may havesmeared the bottom and sides of the seed trench.



Planting machinery

If poor seedling emergence is due to poor seed–soil contact orinaccurate seed placement, then adjusting your sowing machinerycorrectly will help to improve emergence.

Surface crusting

Rain after sowing and before seedling emergence may form asurface crust that reduces emergence.

Once seedlings have emerged, the problem of surface crusting isless. However, until the crop reaches full ground cover (and protectsthe surface from raindrop impact) surface crusting will continue toreduce infiltration.

All soils seal under raindrop impact—even self-mulching soils sealif they are bare. On some soils, the seal is weak or it cracks as it dries;it offers little resistance to seedling emergence. However, on othersoils, the seal forms a crust on drying, and this crust prevents seedlingemergence.

Further light rain will help emergence by softening the surface;further heavy rain will hinder emergence by strengthening the surfacecrust.

Clay soils can crust if the surface is sodic. The treatment for suchsoils is gypsum.

COMPACTION MANAGEMENT

A small degree of compaction can be beneficial. Soil that is tooloose may not be good for plant growth: a loose seedbed dries out tooquickly and gives poor contact between the seeds and the soil; seedsmay not germinate. The optimum level of compaction depends on thesoil types, the plant, and the irrigation management during the growingseason. Too much compaction can cause yield reduction.

To reduce soil compaction:

• time mechanical operations carefully

• reduce axle loads

• keep livestock off wet cropping country

• confine traffic to laneways

• use low ground pressure (wide) tyres (with caution).

The method of compaction repair will depend on the soil’scapabilities. A cracking clay that swells and shrinks behaves quitedifferently to a hardsetting, fine, sandy or silty soil. If a soil swells andshrinks on wetting and drying you can use that natural action to breakup a compacted layer. If a hardpan has developed in a soil with littleclay, you will need to consider other options, for example:

• biological repair—that is, using growing plants to break up acompacted layer (such repair includes ‘biological ripping’ and‘biological drilling’—see below) or increasing the soil organicmatter content

• mechanical repair—that is, deep ripping or mouldboard ploughing.



BIOLOGICAL REPAIR

Biological repair means using growing plants to restore soilstructure. It usually involves a yield penalty. The plants will suffer inrestoring the soil structure, and will produce a lower yield. Repair ofcompaction should therefore work in with the season, the marketprices, and your long-term plans.

Biological repair includes:

• biological ripping (using plants to dry and crack the soil)

• biological drilling (using tap-rooted plants to ‘drill’ through acompacted layer)

• increasing soil organic matter content.

Biological ripping

The easiest way to restore soil structure in compacted cracking claysis to dry the soil, using any plants with a vigorous root system. Dryingcauses the soil to shrink and crack, allowing water, air and roots topenetrate through the cracks. The process is called biological ripping.

Biological drilling of non-cracking soils

In soils that don’t crack, such as sandy, silty or loamy soils,‘biological ripping’ is not an option. However, ‘biological drilling’ ispossible.

A strong tap-rooted plant such as lucerne can force its way through acompacted layer and leave root channels for subsequent plant roots touse. This is ‘biological drilling’. The compacted layer must be moist:no root will enter dry soil. Depending upon how badly compacted thesubsoil is, some roots will not be able to penetrate the compacted layerand the crop will suffer.

Increasing soil organic matter

Some soils, such as fine sandy and silty soils, depend heavily onorganic matter for the maintenance of their structures. Even crackingsoils benefit from increased organic matter content. Increasing theorganic matter of a soil content is a slow process. A pasture phaseincorporated into your vegetable crop rotation is perhaps the most cost-effective way to achieve this.

MECHANICAL REPAIR

Deep ripping and mouldboard ploughing are sometimes useful forquick results, but they are expensive operations. The initial success ofthe operation depends on the moisture content of the soil and the depthof tillage. Continuing success depends on managing traffic to avoidhaving machinery on the soil when it is too wet.

The importance of moisture content

Soil moisture is important when you are working soil. A soil(particularly a clay) that is worked too wet will compact, while a siltyor fine sandy soil that is worked too dry may powder. In both instancesthe soil will then structurally degrade.

Before deep ripping, dig a hole to see if there is a compacted layer.If there is, check the layer’s depth, thickness, and moisture content.



The severity of compaction will also influence your decision on deepripping.

If there is no compacted layer, there is no need to deep rip. Deepripping is expensive, and it could do more harm than good if the soil istoo wet—a costly way of creating soil structural damage where noneexisted before!

A compacted layer must be at the right moisture content to benefitfrom the ripping operation (Figure B4–1). When you are cultivatingclays (or when the compacted layer is clay) the layer must be dryenough to shatter, rather than smear. When the compacted layer is otherthan clay, some moisture is desirable so that the layer does not powder.

The depth and thickness of the compacted layer indicates how deepto set the ripper tine. To be effective, the tine should work just belowthe compacted layer. Dig a hole after a short run and check theeffectiveness of the ripping operation.

Figure B4–1.

Solar powered data logger as used in Environscans® for measuring soilmoisture at Yanco Research Station in Asian vegetable trials. (GrahamJohnson)

B5. Does my soil need lime?


Chapter B5. Does my soilneed lime?PURPOSE OF THIS CHAPTER

To describe how to work out whether your soil needs lime

CHAPTER CONTENTS

• acidity

• pH testing

• liming materials

• lime quality

ASSOCIATED CHAPTERS

• A3 ‘Features of soil’

• B1 ‘Common problems’

• D3 ‘Chemical tests’

WHAT DOES LIME DO?

Lime has two effects on soil. First, it neutralises acidity. Second, itmay be a slow-acting alternative or a complement to gypsum in thetreatment of sodic, dispersive clay soil.

OCCURRENCE OF ACIDITY

Acidity occurs naturally in many coastal, tablelands and slopesvegetable districts, as well as in parts of the Riverina. Localised acidityalso occurs in micro-irrigation systems using ammonium-basednitrogen fertilisers.

The major problems in acid soils are aluminium and manganesetoxicity. Some elements—particularly phosphorus, calcium andmolybdenum—become less available to plants.

PLANT SYMPTOMS

Soil acidity results in poor establishment, yield, and persistence ofplants, including barrel medic and lucerne, which are often used invegetable rotations. Figure B5–1 shows the effect of pH on nutrientavailability.

Plants affected by acidity also become more prone to disease.Plants affected by aluminium toxicity have smaller, dark green,

sometimes purplish leaves. Plant growth is slow and lateral root growthis stunted.

Acid soil also limits nodulation and nitrogen fixation by legumes.Potatoes, sweet potatoes and watermelons tolerate some acidity.

PH TESTING

You can do a soil pH test yourself to determine whether acidity is aproblem. Soil pH kits are available from some garden shops and plantnurseries, but these kits are only accurate to half a pH unit. They



measure pH in water, whereas laboratories measure pH in calciumchloride (CaCl

2). The pH in calcium chloride gives values about 0.5 to

0.8 lower than pH in water.More expensive kits, costing about $500, measure pH in calcium

chloride to an accuracy of about 0.2 pH units. These kits are availablefrom the Centre for Conservation Farming, Charles Sturt University,Wagga Wagga.

Test the subsoil as well as the topsoil to determine the extent of anyproblem. If the pH as shown by the pH kit is well above 6.0, thenacidity is not a problem. If the pH is below 6, get laboratory tests donefor soil pH and exchangeable cations, and seek advice on what to do.

Soil test results for topsoil pH

Interpret the pH of topsoil as follows (these pH values are for pH incalcium chloride):

• pH above 5.6: soil acidity is not a problem yet.

• pH between 5.1 and 5.5: there is a risk of soil acidity problems.Think about a maintenance liming program and methods that willreduce the rate of acidification.

• pH between 4.6 and 5.0: you need to check the exchangeablealuminium percentage as a percentage of the total cation exchangecomplex. If aluminium is present, even at low levels (1%–5%), thensensitive species will be affected. Subsoil pH should not be aproblem, but test to make sure.

Strongly acid Mediumacid

Slightlyacid

Veryslightlyacid

Veryslightlyalkaline

Slightlyalkaline

Mediumalkaline

Strongly alkaline

NITROGEN

PHOSPHORUS

POTASSIUM

SULFUR

CALCIUM

MAGNESIUM

IRON

MANGANESE

BORON

COPPER and ZINC

MOLYBDENUM

4.0 4.5 5.0 5.5 6.0 7.0 8.06.5 7.5 8.5 9.0 9.5 10.0

Figure B5–1. Influence of pH on nutrient availability



• pH below 4.5: you need to check the exchangeable aluminiumpercentage and the subsoil pH. With this level of acidity, the subsoilis probably acid, and a liming program needs to be implemented.

LIMING

The aim is to apply sufficient lime to reduce the soil’s exchangeablealuminium percentage to zero. Ask your soil testing laboratory or yourhorticultural adviser for advice on how much lime to apply. Thereafter,make maintenance applications of lime at regular intervals, dependingon the rate of acidification. Don’t wait until the problem is seriouslyeffecting yields. When a soil becomes strongly acid, you are alreadylosing production.

Acidity is a potential problem on any farm, and its preventionshould be part of your soil management plan.

Caution: Strongly acid soils (pH in calcium chloride less than 5.0,or pH in water less than 5.5) require special care. Adding large amountsof lime can result in potassium and magnesium deficiencies.

In sandy soils, over-liming can cause deficiencies in trace elementssuch as zinc, manganese and iron. Over-liming causes problems! Getlaboratory testing for pH and exchangeable cations before applyinglarge amounts of lime.

If finely ground agricultural lime (100% passing a 0.25-mm sieve) isincorporated into the soil, it reacts with the soil that it contacts as soonas moisture is available. However, the lime moves very slowly throughsoil and may take many years to reach an acid subsoil. On pasturesused in rotation where the lime is not incorporated, it may take a yearor two to have an effect on the topsoil. Coarse lime is slow to act, andits use is inadvisable.

MINIMISING SOIL ACIDIFICATION

As well as liming, better management of nitrogen and soil water willreduce the rate of soil acidification.

A major cause of soil acidification is the leaching of nitrate from thesoil. Use cropping rotations that include deep-rooted plants to use upnitrogen before it is leached. Also, use less-acidifying nitrogenfertilisers, such as urea, rather than ammonium sulfate. Apply nitrogenin small amounts more frequently to minimise nitrate leaching.

LIME QUALITY

Agricultural lime comes from naturally occurring limestone that ismined and crushed. The quality, and therefore the effectiveness, ofdifferent lime products varies. The sale of lime in New South Wales iscovered by the Fertilisers Act 1985. All liming material must belabelled before it is recognised under the Act.

Neutralising value

The capacity of a liming material to neutralise soil acidity is calledits neutralising value (NV). The higher the NV, the greater the ability ofthe product to correct the acidity. Pure lime—that is pure calciumcarbonate—is taken as the standard; it has an NV of 100.

Hydrated (slaked) lime and burnt (quick) lime have NVs of 120 and160 respectively. They can neutralise acidity rapidly, but must be usedwith caution. When mixed with water, the pH rises to over 11 and candamage plants.



Calcium and magnesium content

Pure calcium carbonate contains 40% calcium and no magnesium.Good commercial agricultural lime contains 35%–38% calcium and

very little magnesium.Dolomite contains 8%–11% magnesium and 12%–20% calcium.Magnesite (pure magnesium carbonate) contains 25% magnesium.If a soil test shows that your soil is deficient in magnesium, use

dolomite or magnesite as your liming material. However, beware ofoverdoing magnesium applications: too much magnesium can interferewith the potassium uptake and can aggravate clay dispersion problems.Have the soil tested for exchangeable cations. Keep the magnesium andcalcium in balance by the sparing use of liming materials containingmagnesium.

Fineness

The finer the lime, the more quickly it will react with the soil.A lime with fine particles has more surface area exposed to acid soiland more particles distributed through the soil than an equal weight ofcoarse material. The percentage of particles passing through a 0.25 mmsieve is the measure of fineness.

TYPES OF LIMING MATERIALS

Agricultural lime (calcium carbonate) is the most commonly usedliming material. It consists of limestone crushed to a fine powder, and isusually the most cost-effective material for correcting soil acidity.

Dolomite (also known as maglime) is a naturally occurring rockcontaining calcium carbonate and magnesium carbonate. It is useful foracid soils where supplies of magnesium are low. On most occasions useagricultural lime.

Magnesite (magnesium carbonate). Made from crushed magnesiumcarbonate rock, magnesite has an NV of 95 to 105. It may be usedwhere there is a magnesium deficiency.

Wet lime. Wet liming materials are sometimes available at lowprices. Their usefulness is determined by their NV and water content. Ifthe water content is 10%, then the lime will only be 90% as effective asdry lime. You need to consider the extra costs of handling, freight andspreading.

Crushed shells of oysters and other shellfish are mainly calciumcarbonate, but the shells tend to be contaminated with sand and organicmaterial and are usually too coarse to be effective quickly in soil.

Gypsum (calcium sulfate) is classified by the Fertilisers Act as aliming material, but is not considered one in agriculture because is doesnot reduce soil acidity. Although it is used mainly to improve thestructure of sodic clay soils, it also reduces aluminium levels whenapplied to some acid soils.

B6. Does my soil need gypsum?


Chapter B6. Does my soilneed gypsum?PURPOSE OF THIS CHAPTER

To describe how to determine whether your soil needs gypsum

CHAPTER CONTENTS

• behaviour of sodic soil

ASSOCIATED CHAPTERS

• B8 ‘Dispersion’


DOES MY SOIL NEED GYPSUM?

Gypsum is often promoted as a ‘clay breaker’. It does, indeed,improve the structure of sodic clays (clays with more than 5%exchangeable sodium, and low salinity). However, it does little toimprove the structure of clays that are not sodic, soils where there islittle or no clay dispersion, or the structure of soils containing onlysmall amounts of clay.

BEHAVIOUR OF SODIC CLAYS

Sodic clay surface soils disperse in water (Figure B6–1). Dispersionof surface soil causes crusting. Sodicity also causes excessive swellingwith water. The excessive swelling of a sodic subsoil closes large poresand reduces infiltration and drainage. Waterlogging may result.

Sodicity is most obvious in the soil surface, when clay dispersionleads to crusting. If your soil is prone to crusting, it could be dispersive,and could respond to gypsum. Subsoil sodicity is harder to detect byeye, but sodic subsoil exposed by erosion or earthworks will showdispersion. Such exposed subsoil is very prone to erosion, and thegypsum application should complement other erosion-controlmeasures.

Deep tillage may bring sodic subsoil up to the surface, where it willdisperse on wetting by rain. Gypsum is needed to treat the newlycreated crusting surface.

Subsoil that is not exposed is very difficult to treat with gypsum: theproblem lies in getting gypsum down to the sodic layer. Suchtreatments may not be economic, but new technology is promising.

RATES OF GYPSUM APPLICATION

Broadcasting fine grade gypsum at a rate as low as 2.5 t/ha usuallyprevents clay dispersion in the short term in marginally sodic to sodicclay soils, assuming a water application rate of up to 10 mm/h(equivalent to moderately intense rain). Higher application rates areneeded to prevent clay dispersion under the following circumstances:

• coarse grade gypsum is being used, or

• a longer term effect is required, or

B6. Does my soil need gypsum?


• the soil is highly sodic, or

• the water application rate is greater than 10 mm/h (increasing thewater application rate decreases the time available for dissolvinggypsum).

Broadcast rates of 2.5–5 t/ha usually give successful results,although higher rates can be economic, particularly for high valuecrops or where cheap gypsum is available from a local source.

If gypsum is applied in the irrigation water, a practical rate is850 kg/ML. At this rate an irrigation of 100 mm of water applies0.85 t/ha. A gypsum concentration of 850 kg/ML is approximatelyequivalent to that obtained from broadcasting fine-grade gypsum at2.5 t/ha, and applying water at a rate of 10 mm/h. Therefore, it shouldbe sufficient to prevent clay dispersion in marginally sodic to sodicsoils. For highly sodic soils a gypsum concentration higher than850 kg/ML is needed to prevent clay dispersion.

Although it is possible to calculate the theoretical amount of gypsumrequired to reduce the sodicity (as measured by the ESP) of a givendepth of soil from its present value to about zero, this is usually of littlepractical value. The calculated rate is frequently very high (exceeding10 t/ha) and therefore unlikely to be economically viable. Also, thisapproach ignores the value of the electrolyte effect in reducing swellingand preventing clay dispersion.Figure B6–1. Dispersionin a Petri dish. (M. Hill)

B7. Managing saline soils


Chapter B7. Managing salinesoilsPURPOSE OF THIS CHAPTER

To outline the management of saline soils

CHAPTER CONTENTS

• causes and signs of salinity

• management strategies

ASSOCIATED CHAPTERS

• B10 ‘Does my soil need fertiliser?’

COST AND EXTENT OF SALINITY

About 33% of all irrigated lands world-wide are affected by varyingdegrees of salinity.

In New South Wales, waterlogging and salinity cost irrigatedagriculture an estimated $40 million a year.

The key horticultural areas affected in New South Wales are theMIA, Murray Valley, Sunraysia Irrigation Area, Hunter Valley andsmall areas in the Sydney Basin and southern slopes around Young.Salinity is also an emerging problem in the Gwydir and Namoi Valleys

WHAT IS SALINITY?

Salinity is the amount of salt in the soil or water. The dominant saltin most saline soil is common salt—sodium chloride (NaCl). Varyingamounts of calcium, magnesium and potassium chlorides and sodiumsulfates can also occur.

It is important to know the level of salinity. This determines:

• the types of plants that will grow in the soil, and their yield potential

• the characteristics of a soil

• the quality of water for irrigation, domestic, industrial and stock use

• the extent of the problem.

HOW IS SALINITY CAUSED?

Natural salinity

Salt is a constituent of nearly everything in nature. Sodium is foundin nearly all rocks and is abundant in soil minerals. Coastal breezescarrying salt can deposit significant amounts, while natural rainfallcontains small but measurable quantities of sodium chloride. Naturalrainfall is, however, usually considered beneficial in flushing salt fromthe root zone.

Natural salt deposits occur where there are groundwaterfluctuations, where salty water is discharged, or where topsoil isremoved to reveal saline scalds.



Groundwater fluctuations

When the watertable (the top of the ground water) rises due tofluctuations in rainfall, salt is dissolved from the deeper layers and canbe deposited in the root zone when the water evaporates from thesurface. This tends to be a periodic occurrence, but over time it causessalt accumulation.

Salt lakes

Saline ground water has the potential to build up enormous amountsof pressure, particularly if it is confined within a layer of coarsesediment or porous rock, called an aquifer. This pressure is released atan aquifer outlet or discharge area (where the aquifer meets the soilsurface), resulting in the formation of a salt lake.

Scalds

Saline scalds develop when topsoil is removed to expose a subsoilthat is high in salt. The saline subsoil is often a result of salt blownfrom the dry beds of salt lakes. The saline topsoil is also blown fromadjacent areas and deposited over long periods of time.

Natural rainfall flushes the salt through the sandy topsoil to the lesspermeable subsoil, where it accumulates in large amounts. When winderosion exposes the subsoil a scald is formed. This frequently occurs inwestern areas of the State.

Induced salinity

In many cases human intervention is responsible for salinityproblems, in both irrigation and dryland areas.

Irrigation

Farming methods that result in excessive amounts of irrigation waterpercolating through the soil profile have been responsible forincreasing the height of the watertable.

When large quantities of water are applied to a soil that is quitepermeable, or water is left on less permeable soils for long periods oftime, the watertable will rise rapidly.

It may bring with it large amounts of dissolved salts. When thewater is evaporated from the surface, the dissolved salt will remainbehind in the root zone.

Dryland salinity

Farming practices throughout New South Wales have involved theremoval of large numbers of trees that previously pumped water fromthe soil to the atmosphere by transpiration.

Pastures and crops that have replaced tree cover pump much lessground water because of their shallow roots and smaller leaf area.Watertables therefore rise with rainfall, bringing dissolved salt nearerthe surface.

In some situations, this ground water moves through aquifers andincreases the height of watertables in nearby irrigation areas.Alternatively, watertable rises under irrigation areas may affect nearbydryland areas. These are difficult to reclaim, as irrigation cannot beused to wash salt down from the soil surface.



How a rising watertable affects salinity

The watertable will rise when the amount of water entering the soilprofile exceeds that leaving the soil profile.

If rainfall or irrigation percolates through the soil in excessiveamounts and there is either an impermeable layer or an existing highgroundwater level, the watertable will rise. When the watertable isabout two metres from the soil surface, ground water can be brought tothe surface by capillary rise. Water is then evaporated from the soilsurface, leaving dissolved salts behind in the root zone.

Capillary rise is the movement of water upwards from thewatertable (the top of the ground water) into the unsaturated soil above.It can be likened to a dry sponge (the unsaturated soil) being placed ontop of a wet surface (the watertable). The sponge sucking up water issimilar to capillary rise in soil.

Capillary rise occurs regardless of the depth of a watertable. If thewatertable is below two metres, however, capillary rise does not reachthe root zone of most agricultural plants. Therefore, one aim ofirrigation, drainage and land management should be to keep thewatertable well below two metres depth from the surface and so keepsalty ground water well away from plant roots.

The relationship between salinity and waterlogging

Salinity has been defined as the amount of salt in soil. Waterloggingis the saturation of soil with water for a period of at least one day ormore.

Waterlogging should not be confused with salinity. Waterloggingproblems can exist on their own without soil salinity. When a soil issaturated with water, for instance, plant roots are unable to breathebecause oxygen becomes unavailable. However, waterlogging can be acontributing factor to rising watertables, thus increasing the threat ofsalinity.

RECOGNISING PROBLEM AREAS

Sometimes it is quite difficult to recognise salinity problemsbecause:

• plants often respond to excess salt the same way they would to othersoil problems such as water stress

• salinity is often associated with waterlogging

• the yield of the plant may decline by 30% before signs becomeevident

• where there is no evidence of salt on the soil surface and plants arenot showing obvious characteristic signs of salt damage, landowners must rely on maps of the region or specific soil tests.

General signs

• Leaves appear smaller and darker than normal.

• Marginal and tip burning of leaves occurs, followed by yellowingand bronzing.

• Germination is slow.



• Plants grow poorly.

• Salt-tolerant species predominate.

• Vegetation dies.

• A white crust forms over bare ground.

In crops and pastures

• Legumes are more susceptible to high salinity than grasses, sograss-dominant pastures may be an indication of soil salinity.

• Overall yield will decline in saline areas.

• Establishment is often slow, leaving plants more susceptible todamage from disease and water stress.

MANAGEMENT STRATEGIES

Leaching

Leaching excess salts and maintaining a favourable salt balanceremains the best strategy to prevent detrimental salt accumulation in thesoil profile. This is achieved by supplying enough water to leach saltsbelow the root zone but not into ground water reserves.

Drainage

A prerequisite to using leaching as a management tool is goodinternal and external drainage. Poor internal soil drainage caused bysurface crusting, hardpans and sodic conditions is often managed bytillage and soil amendments.

Regular deep ripping is recommended in these situations. Whensodic conditions exist an aggressive soil amendment program isrequired, for example, using gypsum.

Surface drainage is important, particularly with furrow irrigation.Laser levelling is a standard recommendation, and growers should beaware of crop water requirements to avoid over-irrigation.

The increasing use of tile drains in horticultural plantings (and moledrains in vegetable production) is helping to improve internal drainage.

A thorough soil survey before planting new areas is stronglyrecommended.

The irrigation method

The irrigation method and volume of water applied have apronounced influence on salt accumulation and distribution. Floodirrigation and an appropriate leaching fraction generally move saltsbelow the root zone. Similar results can be obtained with a properlymanaged sprinkler irrigation system.

With furrow and pressurised irrigation, soluble salts in the soil movewith the wetting front, concentrating at its termination or at itsconvergence with another wetting front. In drip-irrigated plots, watermoves away from the emitter and salts concentrate where the waterevaporates. In furrow-irrigated plots, water movement is from thefurrow into the bed via capillary flow. When adjacent furrows areirrigated, salts concentrate in the centre of the intervening bed.Manipulating bed shape and planting arrangement are strategies oftenused to avoid salt damage in furrow-irrigated row crops. Because drip



irrigation maintains more constant favourable conditions of soilmoisture, plants tolerate higher levels of salinity than with furrowirrigation.

Fertiliser management

Many fertilisers contain soluble salts in high concentrations.Therefore, the nutrient source, rate, timing and placement are importantconsiderations in the production of horticultural crops. Salt indices formost commercial fertiliser products have been reported. For example,KCl has a salt index 205 times that of K

2SO

4. Generally, band

application of fertilisers with high salt indices near seedlings should beavoided.

The salt content of other things added to the soil, such as gypsumand manures, also should be considered. Applying gypsum is a usefulmanagement practice for precluding sodium accumulation on the soil’sexchange complex, maintaining soil structure and improving waterinfiltration.

For salt-sensitive crops such as lettuce, apply gypsum well beforesowing so that soluble salts released during dissolution do notnegatively affect production.

Soil amendments and water treatments

Soil amendments and water treatments often offer a practical andeconomical means for managing many problems common to saline andsodic soils. Soil applications of amendments are used for initialreclamation and long-term maintenance of soil quality. In general,water applications are intended to alter the chemistry of irrigation watersuch that no further degradation in soil quality will occur. Rates ofamendments used for soil application are typically large and primarilybased on economics. For water treatments, rates of amendments aretypically much smaller and are nearly always based on solubilities.

Amendments such as gypsum and elemental S have been used foryears. Gypsum is primarily used on Na-affected soils as a source ofCa++ ions to displace Na+ ions, which tend to disperse soil particles andrestrict water infiltration. The resulting displaced Na+ ions are leachedreadily from the soil profile. Gypsum is a neutral salt that does notdirectly reduce pH. However, it can indirectly lower the pH of sodicsoils by reducing the hydrolysis reactions associated with Na+ ions onthe exchange complex.

CONCLUSION

Many economically important horticultural crops are sensitive tosoil and water salinity (see Table B7–1) and to the deterioration of soilphysical properties associated with Na in soil and irrigation water.Therefore, soil chemical and physical properties, crop tolerance, waterquality, fertilisation and irrigation methods are important considerationsfor the production of horticultural crops if we are to avoid the extremesalinity seen in some parts of the world (Figure B7–1).

+

See Chapter D5 for moreinformation on sodic soilmanagement.



Table B7–1. The relative salt tolerance of a range of vegetables, expressedin terms of soil salinity and in order of sensitivity at 90% yield.

Crop Salinity threshold Soil salinity ECseECse (dS/m) at 90% yield (dS/m)

Pea 1.0 –Bean 1.0 1.5Carrot 1.0 1.7Onion 1.2 1.8Lettuce 1.3 2.1Pepper 1.5 2.2Corn (grain sweet) 1.7 2.5Potato 1.7 2.5Tomato 2.3 2.8Cabbage 1.8 2.8Spinach 2.0 –Watermelon – 2.0Cantaloupe (rockmelon) 2.2 –Cucumber 2.5 3.3Broad bean 2.3 –Celery 1.8 3.1Broccoli 2.8 3.9Squash 4.7 5.8Zucchini 4.7 5.8Garden beet 4.0 5.1

References:Landscape, Soil and Water Salinity, Brisbane Workshop May 1987.Journal of Irrigation and Drainage Division, Proceedings of American Society of CivilEngineers 103: 115–130.

Notes:1. Salinity threshold EC

se (dS/m) is a measure of the electrical conductivity of a soil

saturation extract at the point which soil salinity begins to decrease crop yield.2. Soil salinity EC

se (dS/m) at 90% yield is a measure of the electrical conductivity of

a soil saturation extract at which salinity reduces maximum crop yield by 10%.

Figure B7–1.

Extreme salinity in the Aral Valley irrigation district, Russia. This occurred asa result of poor irrigation practices and the farmers’ blasé attitude to theimpending degradation. (Don Blackmore)

B8. Dispersion


Chapter B8. DispersionPURPOSE OF THIS CHAPTER

To outline the principles of dispersion

CHAPTER CONTENTS

• testing for dispersion

• applying gypsum and lime

ASSOCIATED CHAPTERS


• B6 ‘Does my soil need gypsum’?


• D5 ‘Sodic soil management’

TESTING FOR CLAY DISPERSION

It is easy to test a soil for dispersion. Drop small, dry crumbs of soil(3 to 5 mm diameter) into rainwater or distilled water and leaveundisturbed. If a milky halo of dispersed clay develops around thecrumb, it is likely that gypsum would improve the soil structure. In verydispersive soil, the halo will develop within 10 minutes. A moderatelydispersive soil will show a halo within two hours. A non-dispersive soilwill not show a halo at all, even by the next day.

If a test shows your soil to be dispersive, have the soil analysed forexchangeable cations and electrical conductivity in a laboratory.Sample the subsoil as well as the topsoil to determine the extent of theproblem. Analysis results showing high exchangeable sodium and lowelectrical conductivity indicate a soil prone to dispersion. Generally, asoil with an exchangeable sodium percentage above 5 is prone todisperse on wetting. However, soil with an exchangeable sodiumpercentage lower than 5 may disperse if the electrical conductivity isexceptionally low (very low salinity).

You may also try some test strips of gypsum at various rates (forexample, try 2.5 t/ha and 5 t/ha). If you decide to treat a whole paddockwith gypsum, leave a strip untreated to show the benefits. If the treatedsoil responds to gypsum, you will notice increased soil friability, lesspower needed for tillage, improved infiltration of rain, less surfacewaterlogging (the soil surface dries out sooner after rain) and betterseedling emergence. Measure yields on the treated and untreated strips,even if the response is not visible.

If gypsum improves soil condition but there is little yieldimprovement, something else may be limiting plant growth—possiblyplant nutrients. Improvement in water infiltration can, in a wet year,result in greater leaching of nitrogen. Improved nitrogen nutrition maybe needed. On some soils sulfur is deficient. Crop response to gypsummay then be a response to the sulfur (gypsum is calcium sulfate) ratherthan a response to improved soil structure.

B8. Dispersion


A test strip of gypsum is more informative if combined with teststrips of other likely remedies, such as nitrogen, phosphorus and sulfurfertilisers.

GYPSUM AND LIME

Applications of gypsum (calcium sulfate) and lime (calciumcarbonate) supply calcium to soil. Lime is preferable for strongly acidsoils (pH (CaCl

2) less than 5). Lime is not recommended for alkaline

soils. Gypsum is more soluble than lime and is more commonly used; itacts quicker but leaches sooner. Lime has a slower acting but longerlasting effect. A combination of gypsum and lime may be a goodcompromise on soils with pH (CaCl

2) between 5.0 and 6.5. See Chapter

B6 for gypsum application rates.

FURTHER READING

Agfact AC.10 Abbott, T. S., McKenzie, D. C. 1996. Improving soilstructure with gypsum and lime.

B9. How do I control erosion?


Chapter B9. How do I controlerosion?PURPOSE OF THIS CHAPTER

To describe strategies for controlling erosion

CHAPTER CONTENTS

• why control erosion?

• strategies for controlling erosion

ASSOCIATED CHAPTERS


• D6 ‘Improving soil structure by crop rotation’


• D8 ‘Landforming and soil management’

WHY CONTROL EROSION?

On-farm benefits of controlling erosion are:

• reduced loss of soil

• reduced loss of nutrients attached to soil particles

• reduced damage to plants by burial and sandblasting

• less gullying to interfere with paddock operations

• reduced damage to fences and roads

• less silting of dams and channels.

Off-farm benefits of controlling erosion are:

• less pollution of watercourses with sediment and nutrients

• less silting of off-farm roads, dams and drains.

Soil lost from erosion gullies can never be completely replaced(Figure B9–1).

STRATEGIES

The adoption of an integrated package of soil management strategieswill reduce the risk and severity of soil erosion from vegetable-growinglands. The strategies are complementary and will not be effective ifused in isolation.

Assess paddock for erosion risk

Take time to assess the paddock for soil erosion. Ridge-tops andupper-side slopes will be less likely to erode than lower-side slopessubject to run-off waters from above.

Pinpoint steep slope segments and watercourses as areas of highrisk. Give some thought to the location of headlands and direction ofplanting. Care in these areas can reduce the steepness and length ofrows and associated erosion risk.



Kikuyu paddocks cultivated for potatoes, for example, have highorganic matter levels and are less prone to erosion. Native pasturecountry quickly works to a fine tilth, and is lower in organic matter andmore susceptible to erosion.

Avoid steep slopes

The erosion risk increases as the slope gradients increase.Guidelines are:

• 0%–10%: preferred slope range, lower erosion risk

• 15%–20%: very high erosion risk

• more than 20%: should not be cropped.

Manage water flow properly

• Don’t disturb natural watercourses. Keep on-site watercourses in anatural grassed condition for the safe disposal of stormwater run-off. Exclude them from all tillage operations.

• Divert external run-off. Use grassed diversion banks, waterways ordrains to divert run-off coming from up-slope paddocks, roadsurfaces and culverts away from cropped country. Use them todivert run-off from higher unworked country within a paddock toprevent run-on and erosion in lower cropped sections.

• Control and safely dispose of crop surface water. Consider usingshorter rows. Control and dispose of surface water safely within thevegetable crop by using temporary banks and strategic diversionbanks and silt traps to intercept stormwater run-off. Use natural

Figure B9–1.

Soil lost from erosion gullies like this can never be completely replaced. (Ben Rose)

☞☞☞☞☞Strategies for wind erosioncontrol on sandy soils (see E3.1)



watercourses, constructed waterways and dual purpose irrigationruns/waterways to dispose of water (Figures B9–2 and B9–3).

Figure B9–2.

A grassed waterway with temporary grade furrows leading into it. (Ben Rose)

Figure B9–3.

A three-point-linkage disc plough is ideal for constructing grade furrows. (Ben Rose)



Reduce tillage operations

Reduce the number and type of tillage operations. Excessivecultivation will break down soil structure, leading to compaction andsoil erosion.

Soil organic matter not only contains nutrients required by plants,but also binds the soil particles together to form larger aggregates thatgive a soil its structure. Each cultivation encourages decomposition oforganic matter and breaks up the bonding responsible for soilaggregates, so each cultivation further breaks down soil structure.

After rainfall and run-off, the soil settles and is less susceptible toerosion. However, each subsequent cultivation disturbs the soil anddetaches soil particles; with rain further soil erosion results.

Reducing the number of cultivation operations will benefit the soilstructure and reduce soil erosion.

Use herbicides for weed control in the growing crop in preference tocultivation.

Machines that hill and plant in the one operation will reduce thenumber of soil workings and allow the soil to settle and develop aground cover of grass and weeds. When sprayed off with herbicide, thiscover will provide mulch on the ground that will protect against rainand stormwater run-off.

Less pulverising and deeper working machinery such as mouldboardploughs can increase the seedbed depth and reduce the number ofcultivations and thence the physical damage to the soil structure.

Consider using herbicides to reduce tillage operations in groundpreparation.

Avoid the unnecessarily fine seedbeds caused by overworking andharrowing when pastures follow vegetables.

Consider minimum tillage.

B10. Does my soil need fertiliser?


Chapter B10. Does my soil needfertiliser?PURPOSE OF THIS CHAPTER

To describe how to work out whether your soil needs fertiliser

CHAPTER CONTENTS

• five ways to determine nutrient requirements

• soil sampling

• soil testing

ASSOCIATED CHAPTERS

• D3 ‘Chemical tests’

• E1 ‘Key checks for productive irrigated soils’

DOES MY SOIL NEED FERTILISER?

Paddock records of crop and pasture rotations, fertiliser use andplant yields are useful to help estimate plant nutrient requirements.

Fertiliser can supply some of the plant nutrients needed by a crop orpasture. Nutrients can also come from other sources: from thedecomposition of soil organic matter, from the weathering of soilmineral matter, and from nitrogen fixed from the air by legumes and byfree-living organisms.

FIVE WAYS TO DETERMINE NUTRIENT REQUIREMENTS

Use a combination of these five ways to estimate the nutrientrequirements for a paddock:

• for a cropping paddock, review the yield and quality of previouscrops

• set a yield goal and increase plant nutrients to achieve that goal

• be alert to signs of poor productivity

• observe plant symptoms for possible nutrient deficiencies

• send soil samples or plant tissue to a laboratory for chemicaltesting, and use test strips of fertiliser in the paddock to confirm theresults of these tests.

Regular soil testing combined with paddock history and theinformation you gain from test strips is the best way to predict plantnutrient requirements. Observing plant symptoms and plant tissue aregenerally of no use for the current crop, but can help to predict nutrientrequirements for the following crop.

SOIL TESTING

Soil testing (chemical analysis of soil samples) helps to identifynutrient deficiencies and toxicities, and to estimate soil nutrientrequirements. Interpret the results in conjunction with other methods ofestimating fertiliser requirements, for example test strips of fertiliser(useful when you are considering not to apply fertiliser because youhave high nutrient levels).

B10. Does my soil need fertiliser?


Soil testing is useful for:

• showing the availability of the major plant nutrients (nitrogen,phosphorus, potassium, sulfur, calcium and magnesium)

• problems related to sodicity

• problems related to acidity

• problems related to salinity.

Soil testing is not a good indicator of trace elements—plant tissuetesting is better.

SOIL SAMPLING

Soil testing is only as good as the samples of soil that the laboratoryreceives. Errors introduced by sampling, and by the way that you treatthe samples, are usually bigger than any laboratory error. One cause oferrors in sampling is the variability in the soil across a paddock.

Do not take samples from areas that are obviously different frommost of your land. Divide your property into sampling areas withsimilar soil types, landscapes and paddock histories. Avoid obviouslyunusual areas such as stock camps, around trees, wet areas, gatewaysand fence lines.

Another cause of errors in sampling is the way in which soil canvary with depth. Sample all soil horizons separately, or, if horizons arenot obvious, sample by arbitrarily fixed depths.

On cultivated ground, sample to the depth of cultivation. In no-tillcropping paddocks, sample at 0 to 15 cm.

When you are sampling to estimate available soil nitrogen you willneed deeper samples. Various farm advisers and laboratories havedifferent opinions on the necessary depth. A minimum depth of 30 cmis advisable.

Take 20 to 30 small cores from each sampling area, at pointsscattered over the area. Mix the samples together, keeping the depthsseparate if appropriate.

If you are sampling to help solve a problem, identify ‘good’ and‘poor’ areas and sample separately. The comparison between ‘good’and ‘poor’ greatly helps to determine whether or not soil is causing theproblem.

For further advice contact your soil testing laboratory or farmadviser.

Caution: Leaving soil samples in a warm, damp conditionstimulates the growth of soil micro-organisms that convert soilnitrogen from one form to another, with grave consequences for thesoil test result. Chilling the samples below 4°C, or freezing them, stopsmicrobial activity. They must be kept cool until reaching thelaboratory. Alternatively some people prefer to put soil samples intopaper (not plastic) bags so that they can begin drying immediately.Whatever bags you use, air-dry the samples as soon as possible. Crumbthe samples and spread them out in trays or on newspaper. Leave themin a well-ventilated place to dry before sending them to a laboratory.

Also, do not dry soil samples above 40C. Excessive heat changesthe solubility of certain nutrients, affecting the measurement of nutrientavailability.

B11. Case study 2


Chapter B11. Case study 1PURPOSE OF THIS CHAPTER

To give an example of a well-designed soil-management strategy

CHAPTER CONTENTS

• case study

ASSOCIATED CHAPTERS

• C6 ‘Case study 2’

• E3 ‘Case study 3’

CASE STUDY 1

Andrew Rix grows 28 ha of rockmelons on red undulating sandhillsoutside Wentworth in south-western New South Wales. This study is ofinterest because the land is continually cropped with no rotation.

The soil is naturally neutral pH. A cypress pine – casuarina –eucalypt windbreak is planted on the western boundary to protect fromprevailing winds.

Ryecorn is sown with MAP at 50 kg/ha in May/June. The plantingbed (180 cm) is then disced and hoed in July before the melons aresown. This mulch holds the sand together.

The remaining 90-cm strips of ryecorn are used as windbreaks.These strips are slashed at 60 cm to prevent shading. The windbreakstrips are then sprayed with glyphosate before crop emergence, or, inthe case of the early crops, while the melons are in plastic tunnelhouses (Figure B11–1).

Strip fumigation with methyl bromide is carried out in the seedbedin July using clear plastic mulch. This practice overcomes the soil-borne pests and diseases; it is currently under review.

Fertiliser and soil ameliorant or amendment practice includes theuse of humic acid on the ridges at 20 L/ha to build up the cationexchange capacity and improve soil structure.

Gypsum is applied as a calcium source every second year at 2 t/ha.Soil tests are undertaken annually, and regular leaf analysis tests are

done during the growing period.Base fertiliser is 50 kg/ha of phosphorus plus 55 kg/ha of potassium

with calcium and sulfur.Side-dressing is in the form of fertigation at 160 kg of nitrogen

spread over the growing season, starting at the two-leaf stage.Potassium at 50 kg/ha is added through the irrigation system fromflowering.

Depending on the leaf analyses, micronutrients such as zinc andmagnesium may be added.

Irrigation is by in-line drippers spaced at 0.5 m and emitting 2 L/h.Black polythene drains are installed in the low-lying gully areas of thepaddock.

The bulk stubble is disced in after the crop and holds the soiltogether before sowing of the new ryecorn.

Yields are consistently sustained at 30 t/ha.

B11. Case study 2


Figure B11–1.

Andrew Rix and NSW Agriculture District Horticulturist Gerard Kelly inspecting rockmelon seedlingson sandy soil at Wentworth under plastic mulch (top) and plastic tunnel houses for early production(below). Note the oats crop that is slashed on top and then sprayed out. The oats act as a windbreakand a soil stabiliser. (Bernie McMullen)

soilpak - vegetable growersarchive.dpi.nsw.gov.au/.../soilpak-vegetable-part-b.pdfand the crop will...

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