Soil and nutrition
Soil alkalinity: sodic soils under cotton and wheat
Alkaline, sodic soil holds too much sodium relative to calcium and magnesium. Sodium breaks down soil structure: a crust forms, water soaks in poorly and roots lack air. At pH 8.5–10 iron, zinc and phosphorus turn into forms plants cannot use, so the crop is chronically underfed and yields fall.
Crops: Cotton, Wheat, Maize, Orchards
When it is dangerous
The most sensitive stage is emergence: the crust physically blocks seedlings and the stand stays thin. During vegetative growth the lock-up of iron, zinc and phosphorus hurts most. The risk grows when irrigation water is high in sodium; sodic areas occur in Jizzakh, Syrdarya and parts of the irrigated zone.
Causes
- Irrigation water high in sodium
- Naturally sodic and calcareous soil
- Lack of calcium in the soil
- High pH of the soil solution
Signs on the field
- A dense crust on the surface
- Puddles that stay long after irrigation
- Thin emergence
- Young leaves yellowing between green veins
- Patches of stunted crop
How ClusterOS notices it
This is the hardest risk to see from space: sodic soil has no bright salt crust, and in the visible range salinity and sodicity look almost the same. From Sentinel-2 images ClusterOS finds thin stands and patches that stay weaker than the rest of the field season after season, with no drought and no visible salt. That is a flag for soil sampling, not a diagnosis. Alkalinity can only be confirmed by a lab test of exchangeable sodium, pH and electrical conductivity.
What to do
- Inspect flagged patches: crust, puddles after irrigation, yellowing young leaves
- Take soil samples for exchangeable sodium, pH and electrical conductivity
- Test irrigation water for sodium
- If confirmed, discuss gypsum application and leaching with an agronomist – these are capital costs
Questions
How is soil alkalinity different from salinity?
Saline soil holds a lot of soluble salts and has high electrical conductivity. Sodic soil holds a lot of exchangeable sodium with moderate conductivity and no bright salt crust. Only a lab test can reliably tell them apart.
Why do young leaves turn yellow on alkaline soil?
At high pH iron, zinc and phosphorus turn into forms plants cannot take up. Iron shortage makes young leaves yellow between the veins while the veins stay green. The deficiency comes from the soil itself.
Learn about this risk on your fields before it becomes a loss
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Other risks in «Soil and nutrition»
Scientific basis
- Abrol I.P. et al. 1988, FAO Soils Bulletin 39
- Stavi I. et al. 2021, Salinity and Sodicity in Drylands: A Review
- Marschner's Mineral Nutrition of Higher Plants, 3-е изд., 2012