- Maximising water use efficiency is all about increasing the amount of available water in
your soil in order to increase cropping yields
- The capacity to store this water depends on the soil�s physical characteristics such as
structure and depth
- Sandy soils tend to have low water storage capacity
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- Sub-soil constraints (acidity, hardpans etc.) can prevent crops accessing water in the
subsoil
- Structure and depth of crop roots affects access to available water
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Understanding the problem
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| Why is it important to me as a farmer?
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- Water use efficiency (WUE) is defined as the amount of dry matter or yield produced per millimetre of rainfall
- The key to optimising yields is to make full use of all the soil moisture available. This is
particularly apparent for large grain-growing areas of southern Australia where rainfall use
efficiency is restricted by subsoil constraints
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- Large areas of land in the south west and the south east of Australia have dense clay
subsoils that are hostile to plant growth. The prevailing attitude has been that it is too
costly to ameliorate these hostile subsoils
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| How and why we need to maximise water use efficiency
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Both the amount of water that can flow into the soil in a given time (its infiltration rate)
and the amount that the soil can hold (its storage capacity) are important
1. Storage Capacity:
A growing crop relies heavily on water that has been stored in the soil during periods of
fallow or reduced cropping activity; the capacity to store this water depends on the soil�s
physical characteristics such as structure and depth. The maximum water available for
plant growth is known as the Plant Available Water Capacity (PAWC); it varies with soil
type and crop
- Factors affecting PAWC:
- Soil texture - Soil texture is a measure of the relative proportions of the
three particle size categories: sand, silt and clay
- Sub-soil constraints - are soil chemical or physical conditions in the root
zone that restrict root penetration and thereby limit utilisation of subsoil
moisture by the crop. Subsoil constraints that can have impact on water
storage and productive use of the water by vegetation through
evapotranspiration include:
- Sodicity
- Salinity
- pH
- Exchangeable Magnesium
- Compaction
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- Why is PAWC Important?
- It is an indicator of storage capacity of individual soils
- It allows comparison between soils of potential productivity and helps to
explain variations in yield between soils
- It allows a farmer to place in context the amount of water stored prior to a
particular crop (through soil probing or coring) ie 100mm of a possible
250mm plant available water capacity
- This then enables more realistic setting of yield targets and fertiliser
strategies
- In recent years in the south-west regions of Victoria there has either been
too much or too little preseason rain, complicating the decision on when to
sow crops
- The timing of nitrogen fertiliser top dressing and fungicide
application may also depend on the current soil water status. If crops do
not have sufficient soil water for active growth and are �water-stressed�
such applications of fertiliser or fungicide can tend to have adverse
consequences
- In the heavy clay basalt plains of south-west Victoria, the
active root zone can often be confined to a depth of about 30-40cm (this
would be greater under raised beds) and farmers would benefit from a
simple technique for the determination of PAW at that depth in their day to-
day decision making
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- How do you measure PAWC?
- PAWC is the difference between the upper water storage limit of the soil
(DUL) and the lower extraction limit of a crop (CLL) over the depth of
rooting.
- Drained upper limit (DUL) is the amount of water that a soil is able
to hold after drainage has ceased. It is water held against gravity,
and may be removed only by plants - crops or weeds - or through
direct evaporation
- Crop Lower Limit (CLL) measures the extent to which a particular
crop can extract water from a particular soil type. Crops differ in
their ability to extract water, determined by the root length and
density and osmotic potential, as well as duration of crop growth
- In many seasons, the maximum water storage capacity is not reached
due to insufficient rainfall, fallow weeds, run-off and evaporation
- In these cases, the actual water present is described in terms of the
PAWC
- Tools are available for measuring PAWC. See:
- Tools for estimating plant available water in the paddock � Peries
R. DPI, Victoria. Published by SFS (See link in Resources section)
- Estimating Plant Available Water Capacity � CSIRO (See link in
Resources section)
Figure 1 � Plant Available Water Capacity. � Source: Measuring and Managing
Soil Water - GRDC Eastern Farming Systems - Soil Water Training Manual
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2. Infiltration Rate:
- The efficiency of water entry is of critical importance in determining the amount of
water stored in a soil and the amount available for crop use
- After rain saturates the soil surface, water flows into the soil through large holes such
as wormholes and the holes left by decayed plant roots. These large holes are
known as macropores and they are important in moving water away from the soil
surface
Figure 2 � Micropores and Macropores within soil. � Source: SOILpak - southern dryland farmers DPI, NSW
- The bulk of the soil absorbs water through small holes from these large pores by
capillary suction. These small holes, called micropores, are less than 0.05 mm
across, but there are huge numbers of them in a healthy soil and they spread the
water throughout the soil
- From this you can see that the ideal soil needs a wide range of pore sizes to conduct
water efficiently
- Water entry is impacted on by a number of factors including:
- Surface sealing
- Cracks
- Compaction
- Tillage System
- Surface roughness
- Stubble cover
- Rate of infiltration
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| What is the best practice?
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- Estimation of plant available water capacity in the field (see previous section)
- Improving soil structure and removing barriers to plant growth can improve both the
storage capacity of the soil itself through increasing soil organic matter levels, reducing
tillage, increasing surface cover, use of raised beds
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- Increase infiltration through use of controlled traffic and by increasing surface cover and
increasing organic matter
- Increase the area/depth of soil which plant roots may utilise for exploration through
growing deep-rooted perennials
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| How can you achieve this?
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Storage capacity improvement by structural improvement
- A good surface structure with no surface crust will ensure the rain gets in. The
topsoil structure is improved by:
- increasing soil organic matter levels
- Organic matter binds the soil together, and reduced tillage
encourages earthworms to burrow up to the surface. Soils with low
organic matter levels rapidly crust over when rain falls. Sub soil
manuring is an example of this practice
- reducing tillage
- Much of this crusting is due to raindrops battering the soil surface.
Crop and pasture residues shelter the soil, thus maintaining the
initial high infiltration rate. Retaining stubble (see - How do I manage stubble?)will not help
much if the topsoil is compacted and degraded to depth. To
ensure that water soaks through to wet all of the root zone, you
need plenty of old root channels and worm holes. You also need
to avoid subsoil compaction by not cultivating when the soil is too
wet
- Appropriate once-off deep tillage may be required to breakup hardpans
(see - How do I manage the impact of compaction?)
- increase amount of surface cover to reduce the amount of surface sealing
on the soil � this also assists in reduction of water loss through
evaporation
- use of raised beds
- Trials near Geelong have concluded that both black vertisol and
grey sodic vertisols showed continuous improvement in its microporosity
over a five years period thus significantly improving its
water retention capacity
- adding gypsum
- If your subsoil is dispersive you may need to add gypsum.
Gypsum stops dispersion, thereby stopping clay from blocking the
small crevices in the soil
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Increased infiltration
- Introduction of controlled traffic
- Conservation tillage, reduced soil disturbance, and reducing the number
of trips across a field necessary to produce a crop help leave continuous
pore spaces intact and minimize the opportunity for soil compaction
- Improving surface cover
- Stubble slows overland flow and allows more time for infiltration to occur
as well as protecting the soil surface from erosion
- Increased organic matter results in increased aggregation and improved soil
structure leading to improved infiltration rates
Rooting depth improvement
- Annual pastures and crops often have very shallow roots. This limits the amount
of soil from which they can extract water. Dense subsoils can be too hard for
roots to penetrate; air entry is slow so roots have trouble breathing, making the
soil a hostile place for roots
- This situation can be improved by:
- growing deep-rooted perennial grasses and lucerne in rotations. These
plants are better able to grow deep roots, and their roots create channels
that later crops can exploit. These roots dry out and crack the subsoil,
letting in more air and leaving organic matter to stabilise the soil when
they die
- growing healthy, disease-free crops and pastures with strong root
systems that can use the soil water. Break crops such as canola can
greatly improve wheat root growth
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Other related questions in the Brown Book
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Brown Book content has been based on published information listed in the Resources and References sections below
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- Improving crop water use efficiency in high rainfall zone (HRZ) Victoria, through
appropriate practice change to overcome subsoil limitations - Peries R, Gill J
(2010) - Australian Society of Agronomy
- Raised beds in South West Victoria: Pore structure dynamics deliver increased
plant available water in sub-optimal rainfall years - Peries R, Rab A, Bluett C -
International Union of Soil Sciences
- Increasing rainfall-use efficiency for dryland crops on duplex soils - Sale P, Gill J,
Peries R, Tang C � Australian Centre for International Agricultural Research
- The synergy of raised beds, controlled traffic, minimum tillage and stubble
retention deliver higher water use efficiency in South West Victoria, Australia - Peries R, Gill J. - Australian Centre for International Agricultural Research
- Increasing rainfall-use efficiency for dryland crops on duplex soils - Gill J, Sale P,
Peries R, Tang C Department of Agricultural Sciences, La Trobe University, Bundoora,
Victoria 3086, Australia
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- Sale P, Gill J, Peries R, Tang C Increasing rainfall-use efficiency for dryland crops on
duplex soils. Department of Agricultural Sciences, La Trobe University, Bundoora,
Victoria 3086, Australia.
- Peries R. Tools for estimating plant available water in the paddock. Published by SFS.
- Water Availability. � Factsheet � Soil Knowledge Bank.
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- Wockner G, Dalgliesh N, Dang Y, Price L and Voller J (2004) Measuring and
Managing Soil Water. GRDC Eastern Farming Systems - Soil Water Training Manual.
- Managing soil for efficient water use. (Part D12 in SOILpak - southern dryland farmers) �
Department of Primary Industries, NSW.
- Infiltration. � Soil Quality for Environmental Health.
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