[ Note I've attempted to rationalise the distribution of this
duologue by deleting aus.media-watch and aus.culture.true-blue from
the followups, and adding sci.agriculture ]
Yes. If you want to drag them into the discussion too.
pesticides.
Pollution, yes. *Salinity*? Your call.
The vast bulk of pesticides, herbicides and fertilisers
contain mineral salts and many contain chemicals that act as
catalysts to breaking mineral salts out of more complex
compounds that exist in and around the soil, both natural
and man made.
I agreed with your comment re fertilisers -- they are well known
causes of salinisation. But I'm still not sure about your
"pesticides". They are used at *very* low rates compared with
fertilisers so the _prima facie_ case is not obvious.
However, if you are talking catalytic reactions with natural soil
complexes, then that's another issue. I must ask my resident soil
chemist about this as it's something I haven't previously considered.
Thank you for raising it.
*If* the soil is deep draining, you may be able to overcome this
problem by flushing out the salt (i.e. driving it down the profile)
with some heavy irrigation. Drainage lines may help in this too.
The problem with driving salt down the profile is that the
salt ends up somewhere in unnaturally large quantities.If
its below ground - it becomes a future danger, if it is into
the catchment, it creates river salinity.
Very similar to the "pumping" technology you mention below.
However, where I've seen pumping discussed as a possible treatment,
the idea seems to be to do it and discharge the saline water at times
of strong stream flow to minimise the effect. (Whether "minimise" can
be considered synonymous with "negligible" is arguable.)
It still builds up the salt content though, and in more
complex river systems like, say, a Darling river tributary -
if the stream flow strength was due to localised weather
conditions, by the time the salt reaches further down the
basin, that strong flow has ceased, but you still have the
increased salt content.
If the discharge happens at a large number of places along
the river, the increase in salt content of the water becomes
significant.
Which then adds serious problems regarding salinity further
downstream as the water that is used to irrigate contains a
higher salt content, since the salt upstream has been pushed
downstream.Then the problem starts anew, in a worse way.
Yep. In that sort of situation, I agree. But don't forget you will
also have other "localised weather conditions" which may provide
further dilution downstream. So it still may be possible to
eventually flush a given slug of salt out of the system. (On the
other hand, I'm told by local environmentalists that "Dilution is not
a solution." :-)
Some years (decades? :-) ago I recall reading that even sea water may
be used for the flushing process -- but that was a research result and
I don't know if it has been put into practice anywhere. (Perhaps
Israel?)
On the other hand, overwatering on some soil types will also cause
salinity. Typically, this will happen if there is saline water at
depth
The water doesnt even have to be saline - fresh water can do
it when it rises and unlocks salts and minerals from the
soil and rock profile.
Obviously. ;-)
and the excess irrigation gradually builds up the water table
until it is too near the surface. At some point (maybe a metre or so)
you start to get "wicking" where the naturally saline water is
continously drawn to the surface by evaporation
In irrigation salinity - which is this particular scenario
you've outlined, it doesnt even require evaporation for
mineral salts to rise to the surface (although that does
happen), all it requires is for the water table to rise
enough to meet with the moisture footprint of the irrigation
regime.That creates large amounts of mineral salts becomming
concentrated in the root zones of the crop, and creates that
bloody awful waterlogging problem.
Mineral salts can't "become concentrated" unless water is removed. So
can we settle on "evapotranspiration" as covering the issue?
We can.
(And I think waterlogging can be a separate issue and can be damaging
to susceptible species without necessarily involving salination.)
True - its just that irrigation salinity often goes hand in
hand with waterlogging.
and you end up with salting.
As I understand it, this process is pretty similar to what happens
when trees are cleared inappropriately. The water table rises because
trees are no longer extracting water at depth, and you get "wicking"
In dryland salinty, that "wicking" is called capilliary
rise.It works a little differently to the similar phenomena
that occurs WRT irrigiation salinity.
if not actual outbreaks of surface water at times. Whether you get
salinity as a result will depend on the rock types from which the soil
is derived -- or perhaps it is more a matter of "how long" rather than
"whether".
In a country like Australia, with very old soils, most of
which were underneath oceans at some stage of their life,
and with high concentrations of natural salt in the rock
profile, its always a case of "how long".
Irrigation is used on pawpaws, cane, and bananas, on parts of the wet
tropical coastal lowlands (you're free to wonder why and in this
case I think it would be a *very* long time, if ever, before there
were significant salinity problems from irrigation _per se_.
That's mostly true, *for irrigation salinity* - yet there
are cases of salinity outbreaks (dryland salinity) in cane
country.Shitloads of them actually, I could probably find a
map of them for you if you're interested.
I would like to see a copy -- do you have a web address for such a map
(or a traditional library reference will do 8-). I can certainly
imagine there are/will be problems in the Burdekin, for example. But
that's hardly the wet tropics and they have to rely on irrigation
there (not to mention their *very* Mahathir soils in many cases ;-).
Indeed it's the dry tropics around here too, and irrigated cane
farming is new in this area; but already there are problems emerging
(almost literally) as the water table rises. In this case I'm told we
have both perched water tables, which are already fairly close to the
surface by nature, and (IIRC) lower aquifers which may cause problems
if the impermeable layer above is breeched. All this combined with
some rather nasty metamorphic parent material.
With the crops you've mentioned (and the geography that they
grow in) - salinity outbreaks around those crops would be
dryland salinity outbreaks because of the nature of the
irrigation demands (relatively low level) of those crops.
Because
(a) it's the wet tropics after all, so irrigation is small potatoes in
terms of total water input; and (b) a couple of metres of rain each
year has a pretty good flushing effect. (But, that said, I know of
cases where small near-ocean aquifers are becoming brackish as a
result of domestic demand only, combined with a couple of years now of
well below average rainfall.)
We cant just move salt around to solve the problem of
salinity, we have to lower the water tables, keep them low,
reduce the salt inputs and in some cases extract the salts.
If you extract them, aren't you going to move them around?
Not if you pump them into evaporation ponds and ship the
salt out into salt dumps (or even better, sell it to salt
refiners, or even better still - like a couple of smart
*******s are researching, generate electricity out of the
temperature differential of the water between the top and
bottom of the evaporation pond).
I don't know that the market would bear the output; but the idea of a
sort of reverse Siberia tickles the imagination. ("You are hereby
sentenced to 20 years in the salt unmines." :-)
There's no one way to do that, it requires combinations of
ecological and environmental engineering, mechanical
pumping, agricultural and cropping reform in terms of water
utilisation, catchment protection initiatives - the whole
grab bag, and usually in different ways since every
catchment is unique in its salinity profile.
Looking at irrigation for broad acre agriculture, I doubt that there
*is* a solution in the long term. Sure, we may do better than many
past civilisations that died out when their cropping systems failed;
but, if we continue to grow crops on these "agricultural soils",
whether irrigated or dryland, it's almost inevitable that they will be
buggered up eventually. [I always like to end on an optimistic note.]
I'm a bit more optimistic.
Salinity prevention is pretty simple - in a salinity neutral
environment, water and salt inputs must equal water and salt
outputs via crop outputs.As long as catchment wide, good
vegetation management is used to bring about and sustain the
initial salinity neutral environment, nutrient and water
micromanagement will do the rest.
I still see problems given (1) soils prone to salinisation and often
naturally poor in essential nutrients such that fertilisers must be
used for decent levels of production; (2) huge variation in annual
rainfall, and often very intense rainfall events; and (3) the fact
that, if you're going to grow significant areas of crops, you're going
to have significant areas cleared of trees.
There's an interesting case in central Queensland where arable farmers
were switching their land over to grazing based on the perennial
forage legume tree _Leucaena leucocephala_. I'm a bit out of touch
with current conditions, but a few years ago this was certainly a very
desirable change economically (which is why it was happening of
course) and it was/is also very environmentally friendly in that it
was largely putting back a leguminous forest with direct economic
benefit to replace the leguminous forest (brigalow) of no direct
economic benefit which was cleared in the 1960/70s. But of course
Brown's Greens still don't like it.
The difiiculty is bringing land that already suffers
salinity back to being salinity nuetral.
That and teaching those whinging Queensland farmers about
their temrinal stupidity when it comes to land clearing.
Like I've said before, I reckon we should demand southern States
*replant* an acre of trees for every acre left uncleared in
Queensland. ;-)
Salinity [Was: Environmental Hypocrisy]
and in this
Linear Mode
