Salinization

Why can irrigating a field for years eventually leave it barren?

Irrigation looks like pure gain. You take a dry field, add the one thing it lacks, and it yields. Yet some of the oldest irrigated land on earth is now white-crusted and grows almost nothing, and the same story has repeated in Pakistan, in Australia's Murray-Darling, in California's San Joaquin Valley. If the water is fresh and the field is fertile, what is being added that eventually ruins it?

Begin with the water. Is river water pure? It is not — it has run over rock and through soil, and it carries dissolved minerals: calcium, sodium, magnesium, chloride, sulphate. Even water we happily call fresh typically holds a few hundred milligrams of dissolved solids per litre. Groundwater is usually saltier still. So every irrigation adds salt as well as water, in a quantity too small to notice on any single occasion.

Now ask what leaves. The crop transpires — water is pulled up through the roots and evaporates from the leaves — and more evaporates straight off the soil surface. Here is the crucial asymmetry: what leaves is water vapour, essentially pure. The plant takes up some nutrients, but the bulk of the dissolved salt is not carried away with the vapour. It stays.

So follow the arithmetic. Water in, carrying salt. Water out, carrying none. What happens to the difference? It accumulates in the root zone. A field can receive a tonne of salt per hectare per year and nothing appears to happen for a decade — and then crops start showing the symptoms of drought in wet soil, because dissolved salt lowers the water potential of the soil solution and roots must work harder to draw water out of it. That is the first mechanism, and it is nothing more exotic than an unbalanced ledger.

Which immediately suggests the remedy. If the problem is that salt comes in and does not go out, give it an exit: apply somewhat more water than the crop needs, so the surplus percolates down through the root zone and carries the accumulated salt with it. Farmers call the surplus the leaching fraction. But notice what leaching requires — somewhere for that water to go. If the subsoil is impermeable, or the drains are absent or blocked, the water does not leave the field. It sits.

And now the second mechanism arrives, which is the one that turns a manageable problem into a landscape-scale one. Water accumulating below a field raises the water table. As the water table climbs to within a metre or two of the surface, capillary action draws that groundwater upward through the soil pores — and groundwater in old sedimentary basins is frequently very salty, because salts have been sitting in those sediments for a long time. The water reaches the surface, evaporates, and leaves its salt behind at exactly the depth where roots live. Irrigation has, in effect, begun mining the salt beneath the field and depositing it on top. This is why salinization so often appears not in the irrigated fields themselves but in the low ground downslope of them.

Is this genuinely ancient? The record from southern Mesopotamia is the standard case: cuneiform administrative tablets record complaints of "white patches" on fields, a marked shift in cropping from wheat toward barley — which tolerates roughly twice the salinity — and falling grain yields over several centuries in the third and early second millennia BCE. Whether salinization caused the political decline of the southern cities, as was argued in the mid-twentieth century, is genuinely disputed among archaeologists; that the fields were salinizing is much better supported than any claim about what it brought down.

A few refinements connect these pieces.

The first is that salinity and sodicity are different problems, often confused. Salinity is total dissolved salt, and it harms crops osmotically. Sodicity is a high proportion of sodium among those salts, and it harms the soil physically: sodium ions cause clay particles to disperse rather than cluster, collapsing the pore structure. A sodic soil loses its permeability, which cripples the very drainage that would have flushed it — so the two conditions reinforce each other.

The second is that leaching is not a fix in itself; it is a relocation. The salt goes somewhere — usually a drain, an aquifer, or a river — so irrigation districts routinely export their salinity downstream to whoever is next along the valley, and disposing of drainage water has its own failure modes.

The third is that none of this is a fate. Salinized land can often be reclaimed, given enough good-quality water, working drainage, and for sodic soils an amendment such as gypsum that supplies calcium to displace sodium from the clay. What makes salinization so persistent in practice is rarely that the agronomy is unknown — it is that drainage is expensive, its benefits are shared across a whole district rather than one farm, and the damage arrives slowly enough that nobody has to act this season.

• Why some crops tolerate salinity so much better than others, and what breeding for tolerance can and cannot achieve.
• How dryland salinity arises with no irrigation at all, simply by replacing deep-rooted native vegetation with shallow-rooted crops.

Every term the collection defines is gathered in the glossary.