Salt in the root zone: An emerging test for global potato production

New research suggests that zinc and phosphorus may help potatoes withstand salt stress. The more urgent task, however, is preventing salinity from becoming a permanent production problem.

By Lukie Pieterse, Editor/Publisher of Potato News Today

A potato crop can be surrounded by water and still behave as though it is thirsty.

The canopy may lose vigour, roots may remain weak and tubers may fail to size as expected. A grower responds with more irrigation or fertiliser, yet the crop continues to struggle.

The hidden problem may be salt.

When salts accumulate in the root zone, plants require more energy to extract water from the soil. The water is physically present, but it becomes more difficult for the roots to absorb. Certain ions can also interfere with nutrient uptake, damage plant tissues and alter soil structure.

The result can resemble drought, nutrient deficiency, compaction or poor seed performance. That makes salinity particularly dangerous: growers may spend money treating the symptoms while the underlying constraint continues to intensify.

A study published on 28 September in Scientific Reports offers new evidence that carefully managed plant nutrition may help some potato genotypes cope with salt stress. The findings are promising, but they also carry an important warning. Nutrients can support a plant under stress. They cannot remove salt from a field.

A problem extending far beyond the potato row

The first major global assessment of salt-affected soils in approximately 50 years estimated that 1.381 billion hectares—10.7% of the world’s land area—are affected by salinity.

The assessment estimated that about 10% of irrigated cropland and a similar proportion of rainfed cropland are affected, although it acknowledged substantial uncertainty because soil information remains limited in many regions.

The problem is expected to worsen as rising temperatures increase evaporation, drought places greater pressure on groundwater and lower-quality water is brought into irrigation systems. Coastal areas face the additional threat of saltwater intrusion associated with groundwater extraction, storm surges and rising sea levels.

Natural salts have always existed in many soils and water sources. Agriculture can accelerate their concentration when irrigation water evaporates or is used by the crop, leaving dissolved salts behind.

Without sufficient drainage and controlled leaching, those salts remain in the root zone. Repeated irrigation then adds another small deposit. The change may be almost invisible from one season to the next, but significant over several years.

Why potatoes are exposed

Potato is generally classified as moderately sensitive to salinity. Its precise response varies with cultivar, growth stage, soil, weather, irrigation method and the particular salts involved.

Widely used international guidance places the average root-zone salinity threshold for potato at approximately 1.7 deciSiemens per metre, measured as the electrical conductivity of a saturated soil extract. Beyond that guideline threshold, potential yield is estimated to decline by roughly 12% for each additional deciSiemens per metre.

Those figures are useful for identifying risk, but they should not be treated as universal predictions. Field response can differ substantially, and a single soil sample may miss important variation across the field or through the soil profile.

Potato’s relatively shallow and concentrated root system leaves the crop with limited ability to escape a saline layer. Tuber initiation and early development can be particularly sensitive, while prolonged or severe stress can reduce tuber number, size and marketable yield.

The crop’s need for relatively consistent soil moisture creates an additional difficulty. Irrigation may be essential for protecting yield and quality, yet every application also introduces whatever dissolved salts the water contains.

What the new study found

Researchers Abbas Saidi, Zahra Hajibarat and Ahmad Mousapour Gorji examined how zinc and phosphorus, applied individually and together, influenced four potato genotypes exposed to salinity.

The genotypes were D6, KG911-91, D220 and KG913. The researchers examined morphological, physiological, biochemical and gene-expression responses rather than relying on a single visual measure of stress.

Salinity consistently reduced root performance, biomass production and photosynthetic pigment levels. Zinc and phosphorus treatments moderated several of those effects, with the combined treatment generally producing the strongest response.

The genotypes did not react equally.

KG911-91 showed the strongest overall tolerance, followed by D220. Both maintained better osmotic regulation and photosynthetic pigment levels under salt stress. KG913 was the most sensitive of the four.

D6 produced an especially instructive result. It strongly increased the expression of certain nutrient-transporter genes, but its growth remained weaker than that of KG911-91 and D220. The researchers interpreted this as a stress reaction rather than proof of effective adaptation.

That distinction matters. A plant can show intense molecular activity because it is struggling, not because it is successfully overcoming the problem. More gene expression does not automatically mean better field performance.

Promising evidence, not a fertiliser prescription

The study helps explain how interactions between genotype and nutrition may influence salt tolerance. It does not establish a universal zinc-and-phosphorus programme for commercial potato fields.

The work must now be tested through field experiments across different soils, climates, irrigation systems and commercial cultivars. Trials will need to measure tuber yield, size distribution, dry matter, processing quality, defects, storage behaviour and economic return—not simply vegetative growth or laboratory indicators.

Application rate and method will also matter. A treatment that assists plants in an experiment may be ineffective, uneconomic or environmentally undesirable under commercial conditions.

Phosphorus can accumulate in soil and contribute to water-quality problems if it is applied beyond crop and soil requirements. Zinc is an essential micronutrient, but excessive amounts can become toxic. Fertiliser products themselves contain soluble salts and can increase osmotic stress when overapplied or concentrated near developing roots.

The practical message is therefore not “apply more zinc and phosphorus.” It is that nutrient balance, genotype and salt stress interact—and that management should begin with measurement.

Diagnosing the right problem

Electrical conductivity, usually abbreviated as EC, is the starting point for assessing salinity. It measures the ability of water or a soil extract to conduct electricity, which increases as the concentration of dissolved salts rises.

But EC does not identify which salts are present.

Water and soil analyses may also need to assess sodium, chloride, calcium, magnesium, bicarbonate, boron and the sodium adsorption ratio, or SAR. The distinction between saline and sodic conditions is important.

A saline soil contains enough soluble salt to restrict crop water uptake. A sodic soil contains excessive exchangeable sodium, which can disperse clay particles, damage soil structure and reduce infiltration. A field may be saline, sodic or affected by both conditions.

The remedy is not identical in every case. Leaching may remove soluble salts when drainage is adequate, while sodic soils may require a calcium amendment such as gypsum—but only when testing shows that it is appropriate.

Applying amendments without understanding the chemistry can waste money and, in some cases, make management more difficult.

Averages can hide the damaging zones

Salinity is seldom distributed evenly.

Water movement, soil texture, elevation, drainage, irrigation uniformity and the depth of the water table can create sharp differences within a field. Salts may accumulate on bed shoulders, along wetting-front boundaries, in poorly drained depressions or above a shallow saline water table.

A composite sample from an entire field can dilute the evidence from those problem areas.

Sampling should therefore compare visibly weak and apparently healthy zones. Soil should be assessed at relevant depths, while irrigation water should be tested at different times if its quality changes through the season.

Yield maps, electromagnetic soil surveys, satellite imagery and canopy-temperature data can help locate recurring stress patterns. None of them identifies salinity conclusively on its own. They are most useful when combined with soil and water measurements.

The purpose is not to create another colourful map. It is to discover where salts are accumulating and why.

Water management comes before rescue products

The first defence against salinity is control of water movement.

Adequate drainage allows salts to move below the active root zone. Where drainage is poor or the water table is shallow, adding more water may simply redistribute salts or raise saline groundwater closer to the roots.

Leaching can be effective, but only when suitable-quality water is available and the dissolved salts have somewhere to go. It also has environmental consequences. Saline drainage water cannot be allowed to create a problem for the next field, aquifer or river.

Irrigation uniformity is equally important. Under-irrigated areas may accumulate salt because insufficient water passes through the soil. Over-irrigated areas can increase nutrient loss, waterlogging and groundwater recharge.

Drip and low-volume irrigation can improve control, but they do not eliminate salinity. These systems often move salts towards the edge of the wetted zone. Placement of seed pieces, roots and developing tubers in relation to that zone therefore matters.

Bed shape, irrigation frequency, soil texture and rainfall all influence where salts finish.

Variety choice will become more important

The differences among the four genotypes in the new study support a broader conclusion: salt tolerance is not a single characteristic shared equally by all potatoes.

Breeding and variety evaluation should give greater attention to saline environments, especially where potato production is expanding under irrigation or increasingly dependent on marginal water.

Useful tolerance must extend beyond survival. A commercially valuable variety still has to deliver yield, marketable size, shape, dry matter, processing colour, dormancy, disease resistance and storage performance.

A plant that remains green under salt stress but produces an unacceptable crop is not a solution.

Local testing will remain essential. Material selected under one type of salt, soil or climate may perform differently elsewhere. Breeders also need access to accurate environmental data so that apparent genetic tolerance is not confused with differences in irrigation, fertility or drainage.

The burden is not shared equally

Salinity is often described as a technical soil problem, but it is also a question of resources and power.

A well-capitalised business may be able to drill another well, install drainage, blend water supplies, map the field and change irrigation equipment. A smallholder using water delivered at the end of a canal may have none of those options.

Upstream water use can affect the quality available downstream. Poorly managed drainage can transfer salt from one farm to another. Coastal intrusion and declining aquifers can damage land belonging to growers who played little role in creating the original problem.

Tenants may be reluctant to invest in drainage or long-term soil rehabilitation on land they do not own. Growers under short contracts may also be pushed towards maximising the present crop rather than protecting the field for the next decade.

Salinity therefore cannot always be solved farm by farm. Irrigation districts, processors, landowners, governments and water authorities may all be involved.

A practical first-season response

A grower does not need to wait for severe symptoms before acting.

Begin by testing the irrigation source and representative soil zones. Establish EC and, where relevant, SAR and concentrations of specific ions. Record where weak growth appears and compare those areas with elevation, drainage, irrigation coverage and yield history.

Review fertiliser applications for unnecessary salt loading. More nutrition will not compensate for roots that cannot extract water effectively.

Inspect the soil profile. A shallow water table, compacted layer or restricted drainage route can explain why salts remain in the rooting zone. Correcting the water pathway may be more valuable than another crop input.

If leaching, gypsum, water blending or a change in irrigation method is being considered, base the decision on soil chemistry, water quality and professional advice. Each intervention solves a different part of the problem.

Finally, preserve field identity in the harvest data. Grade, size, dry matter and storage results may reveal losses that total yield alone conceals.

Do not wait for a white crust

Visible salt on the soil surface is a late and obvious warning. Much of the damage begins earlier, below the row and beyond casual observation.

The new zinc-and-phosphorus study adds a useful piece to the scientific picture. It shows that potato genotypes differ meaningfully and that nutrient interactions may strengthen the plant’s response.

It does not offer a product-based escape from poor-quality water or inadequate drainage.

That is the harder message, but also the more useful one. Salinity must be managed as a connected system involving water source, soil structure, drainage, variety, nutrition and time.

A potato crop under salt stress may still look green. It may still receive every scheduled irrigation. The grower may not realise how much potential is being lost until the harvester enters the field.

By then, the salt beneath the rows has already collected its share.