Our Indispensable Soil - Soil: Reflections on the Basis of our Existence - page 13

No plants can tolerate high levels of sodium ions in their cell cytoplasms.
Special ion pumps remove the sodium from the cells, and some plants transport
them to other parts where they can do no damage. The grassleaf orache, which
grows on beaches in southernmost Sweden, secretes salt from special glands in
its leaves. I sometimes chew them on my walks near Laröd, the seaside village
near Helsingborg where I grew up. They taste nice enough in small doses but you
realise they are unsuitable fodder for animals like sheep and goats, the commonest
ruminants in Tunisia.
It is not easy for a plant to find enough potassium ions in soil rich in sodium
ions, which is why plants growing in saline conditions often have a potassium
deficiency. High salinity also makes water diffuse from plant roots via osmosis—
the same process that occurs in human kidneys when we reabsorb water from
urine prior to excretion. Thus, plants dry out in saline soil even if they have an
adequate water supply.
Salt-tolerant plants sometimes counteract this by forming substances with a
high osmotic propensity to help them retain water in their cells. This energy-inten-
sive response enables them to survive in saline conditions. A great deal of funding
is going into conventional plant breeding and genetic modification research in a
bid to add these properties to modern crops.
In southern Tunisia my colleague Hafedh is carrying out extensive test farm-
ing of both domestic plants and imported varieties from salinity-affected areas
of North America and Australia, in an effort to create favourable conditions for
symbiotic fungi. The main crop we are experimenting on is salt-tolerant bar-
ley used for animal fodder. Knowledge about new crops is constantly growing
and there is an increasing willingness to experiment. Declining fish stocks in the
Mediterranean have caused some fishermen to abandon their traditional liveli-
hoods in favour of farming—with promising results. Successful introduction of
foreign plants for high-salinity cultivation contrasts with the failure to combat
erosion through the introduction of fungi. But we should not be surprised at the
patchiness of our results: many organisms compete to use the soil and we lack
knowledge about how to manage the land to ensure that introduced organisms
will thrive. We are also largely in the dark when it comes to knowing how spe-
cies differ in their ability to stabilise soil aggregates. By contrast, we have well-
established growing techniques for specific crops and know how best to cultivate
the soil and fight the weeds to promote crop growth. Adding compost and other
organic matter can be effective in saline soil because a higher humus content helps
plants to tolerate salt. We have found in our experiments that mycorrhizal fungi
grow best, and hence are better able to absorb sufficient nutrients, when we add
compost.
Farming in high salinity environments is a constant balancing act. A type of
grass from Pakistan grows well in saline soil but needs intensive irrigation. If you
take too much of this water from wells you create a risk of sea water entering the
water table and ruining the entire area for agriculture.