GET TO KNOW YOUR SOILS

Sue Briggs – DEDJTR Rutherglen

Today

• What soil conditions required for healthy plants.

• Physical characteristic of soil.

• Soil testing procedure.

• Plant tissue test.

Characteristics of the soil

Chemistry • Get water into

the soil

• Store plant

available water

• Maintain

structure for root

growth and

water extraction

Biology

Soil

Health

Physical

• Store and supply

nutrients for plant

growth

• Maintain pH

• Minimise toxicities

• Support biological

capacity for

nutrient cycling

Soil Components

Mineral Particles

45%

Air 25%

Water 25%

Organic Matter

5%

Soil Components – Mineral particles

Mineral Particles

45%

Air 25%

Water 25%

Organic Matter

5%

Soil Components – Organic Matter

Mineral Particles

45% Air

25%

Water 25%

Organic Matter

5%

Soil Components - Air

Mineral Particles

45%

Air 25%

Water 25%

Organic Matter

5%

Australian soils

• Poor water storage capacity

• Salinity and sodicity

• Low nutrient availability

• Susceptibility to wind and water erosion

• Poor physical status of surface or sub soil

horizons

• Class I Arable Land (Agricultural Land) • Land able to support a wide range of uses with minimal risk of

degradation

Soil condition required for garlic

production

• Well drained soils

• Fertile soils (good levels of N,P,K,S)

• Soils high organic matter

• Prefer lighter soils (Sandy loam – loam) with OM

• Likes friable soil bed

• Prefers pH (CaCl2) between 5.5 – 7.5

• Any others!!!! (a lot of experts in the room)

Is this possible for my soil?

What should I look out for?

Looking below the soil surface

A1

A2

B21

B22

C

We influence this zone

through management

In the subsoil must

adapt our management

to match soil capability

•Proportions of clay, silt, fine sand and coarse sand are the main determinants of field texture (as well as organic matter)

•Gravel (> 2 mm)

•Coarse sand (0.2 mm – 2 mm)

•Fine sand (0.02 – 0.2 mm)

•Silt (0.002 – 0.2 mm) - ‘silky’ smoothness

•Clay (<0.002 mm diameter) - plastic (like plasticine) - cohesive and sticky

Soil texture – what is it?

Source: Better Soils SA website

Soil texture

The Proportions of sand, silt & clay in soil texture classes

85

65

45

20 1528 25

10

25

40

60 55

3730

5 10 15 2030 35

45

0

20

40

60

80

100

Loam

Sand

Sandy

Loam

Loam Silty loam Silty clay

loam

Clay loam Clay

Soil Texture Classes

Perc

en

tag

e

Sand

Silt

Clay

Source: Adapted from Buckman & Brady (1960)

Soil structure – Why is it important?

Importance of soil structure

• Allows aeration/oxygen in the soil

• Increases availability of water & nutrients

• Increases infiltration rates and reduces erosion

risk

• Encourages plant root penetration

• Encourages microbial activity

Soil aggregate • Soil particles can be cemented together to form

aggregates

• The electrical attraction properties found in clay and

organic matter cement all soil material together.

• Fungal hyphae & bacterial glues also important

Soil structure • Soil structure relates to the way soil particles

are arranged and bound together

• The size, shape, nature of the soil aggregates

play a major role in determining water movement

and ease of root penetration.

Soil porosity Refers to the spaces within the soil which allow air, water and root movement

• Connectivity of pores is important

Macropores

between

aggregates

Water

transmission:

•Drainage

•Aeration

•Root growth

Micropores

within

aggregate

Water storage:

•Plant available &

residual soil water

Dispersion –

sodic soil

Slaking

Aggregate stability

Slaking

• Breakdown of larger aggregates into smaller ones.

• Occurs when dry soils become wet – clay swells

compressing air causing an explosion.

• Organic matter reduces slaking by binding mineral

particles and slowing rate of wetting

Slaking

Source: MAF Information Services, New Zealand

Effect of crusting on seedling emergence

The roots were

unable to

penetrate the dry

sub soil.

Resulting in

reduced nutrient

and water uptake

Slaking and dispersion

Slaking

Slaking

Stable aggregate

Dispersion

Dispersion

Aggregates broken

down

Clay particles in suspension

Breakdown of

larger

aggregates into

smaller ones

• Slaking is related to soil structure

• Dispersion is a problem with soil chemistry

Dispersion

• Separation of clay particles from aggregates when wet.

• If sodium trying to hold clay together, bond is weak and

easily broken.

• Dispersed particles fill up pore spaces.

0 1 2 3 4

No sodicity Highly sodic/low salt

Source: Dang Y. 2004

Dispersive topsoils when wet

Dispersive topsoils when dry

Sodic soil

• Occurs when the clay particles are dominated by

sodium.

• Soils > 6% ESP (exchangeable sodium

percentage) usually disperse with rainfall

• Can occur naturally but irrigating with water high

in sodium and spreading of some manures can

increase the sodium levels in the soil.

• To improve these soil – use gypsum (Calcium

Sulphate) to flocculate the clay

Poor soil structure leads to…

• Increased waterlogging

• Increased run-off

• Increased soil erosion

• Increased nutrient loss to

the environment

• Decreased plant water

availability

• Impeded root growth

How does agricultural practices impact

soil structure

• Cultivation destroys the natural

aggregation of soil structure and organic

matter levels

• Driving heavy vehicles on wet soils

causes compaction

• Grazing wet soils – pugging

Repairing soil structure - Mechanical

•Deep ripping

– success depends on moisture content, severity

of compaction, plough-pan,

– no need to rip if no compacted layer.

– Gypsum should be used for sodic and

dispersive soil.

Repairing soil structure – ‘Primer’ crops

• Biological ‘drilling’ using ‘primer’

crops

– tap-rooted plants to ‘drill’ through

compacted layers (eg lucerne,

lupins, chicory etc)

• Increasing soil organic matter

Repairing soil structure –

Green Manure Crop

• Cost effect way to increase organic matter levels

• The stage the crop is turned in, can determine

the amount of organic matter or nutrient returned

to the soil.

• Legume crop such as vetch can supply between

50 – 140 kg N/ha at the time of cultivation.

• If left to mature – will contribute to OM and less

available N for the plant

Green Manuring crop selection

Different crops have differing benefits for the soil.

Rough rule of thumb

• Rye grass – best for soil structure and building

OM (due to fibrous root system

• Oats for quick ground cover

• Lupins (legumes) for soil fertility

• Brassicas for biofumigation (to prevent

nematode problem developing)

Repairing soil structure – Applying

organic amendment

• Which organic amendment – a number to

choose from.

• Animal manures, composts, biosolids, biochar

• Do you want to supply nutrients or improve soil

structure – this will help determine which product

• Beware of the risks – heavy metals, pathogens

• Accredited product

Characteristics of the soil

Chemistry

Biology

Soil

Health

Physical

• Store and supply

nutrients for plant

growth

• Maintain pH

• Minimise toxicities

Why soil test?

• Check on the fertility of the soil and better

determine or predict nutrient requirements;

– Fertiliser type and rate of nutrient required

– Lime requirement

– Gypsum requirement

Why soil test?

• With regular soil testing – may able to see trends

in the results

Why soil test?

Investigate by

comparing

Good

Poor

Problem solving

What is a representative soil sample?

A representative sample consists of a large

number of soil cores taken from within a uniform

area of a soil type or paddock.

Topsoil 25 – 30 cores (mix in bucket and take a

subsampe of 500 grams.

Subsoil 8 – 10 cores (less variability in the

subsoils).

Vegetable crops

Sampling depth – topsoil sample 0 – 15 cm

subsoil sample 15 – 30 cm

When to sample – at least 3 to 4 months (or

longer) prior to planting to provide enough time for

effective soil amelioration.

Sampling tools

Other tools

• GPS to mark soil testing area.

• Screw driver and bucket to remove and collect

soil form the sampler.

• Collection bag and soil analysis form.

Selecting areas for sampling

In selecting areas for the soil sample consider;

• Sample high and low yield areas separately.

• Don’t mix soil samples from different production

areas or have been farmed differently.

• Where different soil types occur within the same

paddock.

MONITOR Fertility

Soil Sampling

Method

Transect

MONITOR Fertility

Soil Sampling

Methods

Cluster

MONITOR Fertility

Soil Sampling

Method

Zig Zag

MONITOR Fertility

Soil Sampling

Method

Grid

Prepared seed beds

• Care must be taken when sampling in

permanent seed beds as it can potentially give

misleading results

• The main issue is knowing where fertiliser is

banded.

Where and when not to sample!

• Unusual areas e.g. stock

camps, fence lines,

headlands, table drains,

poorly drained areas.

• Areas limed within 3

months or fertilized within 2

months

• Unusual soil conditions e.g.

waterlogged

Sampling Protocol

• Remove leaves/thatch and other debris from soil

surface.

• Using corer take samples and place in clean

bucket.

• Mix thoroughly and take a sub sample (500g).

• Send to laboratory

Selecting a soil test laboratory

• NATA accredited - National Association of Testing Authorities

• ASPAC accredited – Australasian Soil & Plant Analysis Council - check website for which test

• Interpretations field calibrated

• Cost

• Turnaround time

• Quality of service - eg. back up?

Some rules for soil testing

• Make it REPEATABLE! – Same time of the year

• Do it regularly

• USE the SAME TEST (Colwell P or Olsen P?)

• PREFERABLY Use the same Lab or if changing transition by sending samples to the ‘old’ & ‘new’ Lab

What can we learn from soil testing?

• pH and buffer capacity

• Nutrient holding capacity, nutrient levels, and

nutrient distribution over the farm

• Organic matter levels

• Soil structure stability

• Levels of toxic elements

Major nutrients

Needed in large

amounts by the

plant

pH – used to

check plant

suitability and to

determine lime

rates

OC / OM / Total Carbon

OC and Total carbon

are portion of OM

(x 1.72)

Soil texture –

Field assessment

method

Cation Exchange

Capacity – indicate soil

fertilitypotential

Indicate soil structure

Ca:Mg ratio

Na %

pH

(CaCl2) Effect on plant productivity

> 6.5 Neutral soil. Trace elements may become unavailable

5.0 - 5.5 Balance of major and trace nutrients are available

4.5 - 5.0 Al may become soluble in the soil (soil type

dependent).

Fe and Al become more available and can tie up P

4.0 - 4.5 Mn becomes soluble and toxic to plants in some soils.

Mo is less available.

Soil bacteria activity slows down affecting the

mineralisation of OM.

4.0 Soil structural damage

Reducing plant growth

How much lime?

• Applying lime is an inexact science!

Traditional recommendations - 2.5 t/ha

• Buffering capacity of the soil is important

It takes more lime to raise pH in a clay soil

compared to a sandy soil.

• Monitor with soil test

Addressing soil acidity

Lime (CaCO3)

• Liming materials consist of Ca and Mg

carbonates

• When applied CO3 replace the H ions with Ca &

Mg ions.

2H+ + CaCO3 → Ca

2+ + H2CO3 → H2O + CO2

Determining lime requirement – one option

CEC From 4.0 to

5.2

From 4.3 to

5.2

From 4.7 to

5.2

From 5.2 to

5.5

3 3.5 1.7 0.7 0.5

4 3.9 2.1 0.9 0.6

5 4.7 2.5 1.1 0.7

6 5.5 3.0 1.2 0.8

7 6.3 3.3 1.4 1.0

8 7.1 3.8 1.6 1.1

9 7.9 4.2 1.8 1.2

10 8.7 4.6 1.9 1.3

15 12.5 6.7 2.8 1.9

Source: FertSmart

Lime required (t/ha to lift pH (CaCl2) of the soil

Determining lime requirement – option 2

If your soil test OM level is

above 2% add an extra 0.4t/ha

Conversion factors

Clay 0.26

Clay loam 0.37

Sand clay loam 0.47

Sandy loam 0.57

t/ha of lime Target pH (CaCl2)

Soil test pH (CaCl2) = -

Conversion factor

Source: Acid Soil Action

From the soil test

Soil test pH of 4.4 (CaCl2), Target pH 5.5 (CaCl2),

Texture: Loam OC: 3.2%

Lime required = 5.5 – 4.4 / 0.37

= 2.9 t/ha lime

= 2.2 + 0.4

= 3.3 t/ha lime

But have to consider lime quality

Two factors determining quality of lime

Source: Gazey, 2011

Neutralising value Fineness

If the neutralising value of lime is not

100%

For example:

Lime to spread = 3.3 t/ha x 100 / 85%

= 3.9 t/ha

Lime to spread (t/ha)

Lime required (t/ha)

100

Effective neutralising value (%)

x

Plant tissue testing

• Research has found that using soil test to

determine trace nutrient deficiencies is very

inaccurate, especially on acid soils

• Diagnosing trace nutrients toxicities &

deficiencies

Soil testing v Plant testing

• Similarities

– Coping with

variability

– Laboratory selection

– Interpretation for

monitoring or

diagnosis

• Differences

– Variability within the

plant and growth

stage

– Sampling

– Sample handling

– Interpretation of

fertiliser optimisation

Trace nutrients Trace

nutrient

Conditions

conductive to

deficiency

Leaves

showing

symptoms

first

Visual symptom

Copper Acid organic

soils, high soil

Zn

Youngest Pale areas on leaf, stunted

growth

Zinc Sands, high

pH, cool

weather

Middle Pale areas between leaf veins

Iron High pH Youngest Pale areas between leaf

veins, veins dark green

Manganese High pH, cool

weather, dry

conditions

Oldest to

youngest

Yellowing between leaf veins,

veins pale green

Boron Sands, low OM,

dry weather

Youngest

Reddish colouration, lack of

legume persistence in pasture

due to poor seed set

Guidelines for plant tissue testing

• Take the sample from a representative area

• Use transects

• Do not take the sample until 8 weeks after the

last fertiliser application

• Ensure hands are clean when sampling

• Wash off (or brush off) any soil (labs do not

wash the sample

• Send the same day of sampling & sample early

in the week

Traps for the unwary

• Some nutrients are mobile within the plant, some

are not

• Nutrient contents very with growth stage and

plant part

• Contamination by soil and/or ointments

79

Growth Stages

Feekes Scale

Zadocks Decimal Scale

1 2 3 4 5 6 7 8 9

Industry Standard Textbooks

Reuter D & Robinson JB

Plant Analysis: An

Interpretation Manual 2nd Ed.

CSIRO Land and Water

How healthy is my soil? - In the field

observations

• Ground cover

• Root development and depth

• Soil colour and drainage

• Earthworm numbers can indicate biological activity

• Soil pH

• Soil structure and structural stability

• Water infiltration