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China Gibson correct
about our sodic
subsoils
Leigh Vial
Head, Experiment Station, International Rice Research Institute &
absent rice grower, Moulamein
A good decade ago, Ian ‘China’ Gibson, the resident scribe of
Moulamein, told me of his tactic for pre-irrigating his Moulamein
clay soils. He kept the water on for at least a week, “to get the
soil properly wet”. This sounded a bit stone-age in an era of quick
application and drainage rates, and the odd centre-pivot dotting the
landscape, but a decade later I see his point.
Most of our Riverina clay soils have a common trait: a compact, sodic
subsoil. Indeed, Australia is a world leader in having sodic subsoils.
There is not much pore space in these subsoils and the pores are small
and poorly connected. Getting water in or out of the pores is generally
a slow process. Roots find it hard work to penetrate and explore these
subsoils, and water only flows slowly to the root once it does get in.
Getting water in
QUICK TAKE
›› Compact, sodic subsoils, typical of many Riverina clay soils,
may not be as a big a liability as immediately thought to be.
Our rice subsoils only let water in at a rate of less than 1.0 mm/day.
Admittedly, water enters the soil more quickly when it is dry, but these
soils still take a week or three to saturate, as you can see by greater
water use in the early weeks of a rice crop. It takes a long time for
water to work its way into all the hard-to-get-to nooks and crannies of
compact clay. It is like trying to fill a dam with a half-inch hose.
Getting water out
›› With careful and deliberate water management, many
compact sodic subsoils could in fact be an asset—providing
a steady supply of end of growing season moisture to finish
cereal crops.
›› Leigh Vial uses some science to back up China Gibson’s tried
and tested tactic for pre-irrigation.
Compact clays don’t make it easy for roots. They have a high
mechanical resistance, which requires the plant to spend more of its
limited energy pushing roots through it. Similarly, for any root that does
succeed getting in, water does not flow to it quickly. Water will only
flow to a root according to its water potential compared to the soil
(how hard it sucks) and how much resistance there is to water flowing
through the soil.
Any soil scientist will tell you that the small and poorly-connected pores
of a compact clay soil make it difficult for water to flow through it, so
water does not reach the roots quickly, no matter how hard it sucks,
and that the resistance gets bigger as the soil gets drier. This is a little
like drinking beer through a narrow straw.
This all sounds like bad news...but is it?
In a collaborative experiment between The University of Basilicata in
Italy and Michigan State University in the USA, scientists grew maize
in pots, in a sandy-loam soil, with and without compacted clay. They
wetted-up the soil, drained it, planted the maize and did not water it
again.
The maize without the compacted clay grew well, then died of thirst.
The maize with the compacted clay never really grew that well and its
roots only slowly penetrated the compacted clay, but it managed to
finish and yield grain. The narrow straw limited the plant’s rate of beer
consumption, but that ensured that it made it to the end of the party!
A new take on water in compact subsoils
Up until now, compact clay subsoils have been regarded as a liability.
They don’t supply enough water to the roots to successfully finish crops
if things get dry at the end. Even in pre-watered crops, this seems to be
the case, but this may be too much of a generalisation.
Part of the problem may be that the ‘wet’ sodic subsoil was not full
of water to start with. Only some of the pores—and generally the
smaller pores that are harder to get water out of—are full of water.
Prolonged submergence gets the pores completely full and ensures the
more accessible larger pores are full.
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IREC Farmers’ Newsletter – Large Area No. 188: Autumn 2013
crop management
sodic soils
Rice submerges the soil for 2–4 months, depending on how you grow
it; I have never seen a winter cereal following rice finish poorly, and
only rarely seen the following crop finish poorly either. True, I have
seen quite a few grow poorly during the season due to poor nitrogen
nutrition or occasionally waterlogging, but never finish poorly.
CSIRO research agronomists showed that the last inch or two of
water in the season for a wheat crop can have a water productivity
of 33 kg/ha/mm, as opposed to the commonly-accepted maximum of
20 kg/ha/mm for the season, so water stored deep in a full, compact
subsoil can be highly productive water when helping to finish a crop.
If soil with sodic clay subsoil can see rice every three or four years, to
truly fill the subsoil, that narrow straw of those small, poorly-connected
pores, can let the plant draw small, strategic amounts of moisture from
the subsoil, especially late in the season for at least the following two
years. This can ensure that high-yielding irrigated crops finish well and
make spring irrigation a little less tricky, particularly if we entertain using
overhead irrigation on a conventional rice layout.
Back up with overhead irrigation
area, rather than have to supply all the water, it need only supply 1–2
ML/ha rather than 4 ML/ha or more. This means there will be less of
a supply pinch in the spring, so there is less variability in yield, and a
single machine could command a larger area and hence reduce capital
cost per hectare. Perhaps overhead irrigation does have a place in a
rice system?
Current overhead irrigation cropping programs do not succeed in
wetting the sodic subsoil, so it contributes little water in the spring
and requires the spring irrigation program to be faultless to get top
yields. The narrow straw from the subsoil can be an asset, provided
we appreciate the size of the half-inch hose filling it up. The odd chance
to thoroughly fill up our compact sodic subsoil with moisture can make
high-yielding irrigated cropping more reliable, especially with the feast
or famine year-to-year irrigation supplies we face now.
Thanks China, your thinking may be revolutionary!
Further information
Leigh Vial
E: l.vial@irri.org
A rice soil is not ideal for overhead irrigation, but if a lateral-move
irrigator can complement the subsoil reserve in spring over a large
For a plant, getting water from a sodic subsoil can be like drinking beer through a straw. However, if the subsoil is filled, e.g. during the rice phase, the slow and steady supply
of water to a subsequent crop can be the key to a successful finish.
IREC Farmers’ Newsletter – Large Area No. 188: Autumn 2013
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