Nutrient deficiencies
Potassium deficiency in wheat: symptoms, soils and fixes
By the Fert Directory editorial team · Published 24 October 2026 · Last updated 24 October 2026 · Not yet reviewed by a qualified agronomist
Direct answer
Look for smaller, paler wheat with dead leaf tips on the sandier parts of a paddock. Older leaves yellow and die back from the tip and edges, often leaving a green arrow down the middle. Sandy and deep grey sandy duplex soils are most at risk. Confirm with a tissue or soil test, then top-dress potash.
Key numbers
| What | Figure | Source |
|---|---|---|
| Colwell K critical level (0–10 cm), deeper sands | about 45–50 mg/kg | Department of Primary Industries and Regional Development, Western Australia (MyCrop) 2015 |
| Whole-shoot K critical concentration | about 4.5% in young plants, falling to about 1.5% at boot | Department of Primary Industries and Regional Development, Western Australia (MyCrop) 2015 |
| Wheat yield response to K on WA loamsBest predicted by soil sampling 0–40 cm | 0.69–1.37 t/ha, in 4 of 8 trials | CSIRO Publishing (Crop & Pasture Science 75(11), CP24195) 2024 |
| Potassium removed in wheat grain | about 4 kg K per tonne | Soil Quality Pty Ltd (soilquality.org.au) 2026 |
| Potassium content | MOP 50% K; SOP 41% K | Incitec Pivot Fertilisers (manufacturer) 2026 (manufacturer) |
What potassium deficiency looks like
DPIRD (WA) describes these signs in wheat:
- In the paddock: smaller, lighter green plants with dead (necrotic) leaf tips, usually on the sandier parts of the paddock or between header or swathe rows.
- Plants look unusually water-stressed even when conditions are good, and are more prone to leaf disease.
- On the plant: older leaves go first. Tips die, and yellowing works down from the tip and edges.
- The yellow edges contrast sharply with the green centre, sometimes leaving a green 'arrow' pointing to the leaf tip.
Short potassium (K) also means poorer use of water and other nutrients, so crops cope worse with drought, waterlogging, frost and leaf disease. NSW guidance adds weak stems, lodging and high screenings.
What else could it be?
| Condition | How to tell it apart |
|---|---|
| Molybdenum deficiency | K-deficient plants have no white or rat-tail heads, and a sharper yellow-green contrast on leaves. |
| Nitrogen deficiency | K shows more leaf tip death and sharper contrast; tillering is less affected. |
| Spring drought | K deficiency is more marked in high-growth plants in good seasons. |
| Root lesion nematode | Nematode-affected plants have 'spaghetti' roots with few feeder roots. |
NSW guidance also warns the leaf symptoms can be mistaken for yellow spot and Septoria nodorum blotch.
Which soils are at risk
- Sandy soils and deep grey sandy duplex soils (sand over clay).
- Paddocks with heavy hay or grain removal. Wheat grain removes about 4 kg K per tonne; oaten hay about 25 kg per tonne.
- Sands in higher rainfall areas, which hold K poorly so it washes down (leaches).
- Some WA loams too: long-term cropping has run soil K down, and wheat on loams has shown low K even where the topsoil test looked fine.
Testing: tissue and soil
- Tissue: DPIRD recommends a whole-top plant test, ideally paired samples from good and poor patches. The critical K level falls from about 4.5% in young plants to about 1.5% at boot.
- Soil: the critical Colwell K level for 0–10 cm is about 45–50 mg/kg on deeper sands.
- On duplex soils the clay can hold K reserves, so test deeper in the profile too.
- On WA loams, a 2024 study of eight wheat trials found K responses of 0.69–1.37 t/ha in four. Sampling 0–40 cm predicted the response best.
- Tissue tests often come too late for this crop. Use them with soil tests to plan next season.
New to soil reports? Read how to read a soil test.
Fixing it: top-dress MOP or SOP
- Top-dressing potassium will generally correct the deficiency (DPIRD).
- Foliar sprays generally cannot supply enough K for a severe deficiency and can scorch the crop.
- Where a paddock is badly deficient, apply K early: at seeding or up to about four weeks after.
- Keep muriate of potash (MOP) away from the seed: the WA Soil Quality fact sheet reports rates above 30 kg/ha (22 cm rows) with the seed hurt germination significantly.
| Muriate of potash (MOP) | Sulphate of potash (SOP) | |
|---|---|---|
| Potassium | 50% K (potassium chloride) | 41% K (potassium sulphate) |
| Chloride | Yes | None |
| Cost per unit of K | Cheapest form of K | Significantly more expensive |
| With the seed | Can cut germination and establishment | Less damaging; can be drilled with seed |
- Examples: Muriate of Potash, Summit Muriate of Potash, Impact Muriate of Potash and Sulfate of Potash. Compare MOP and SOP side by side.
- N and K blends such as NKS21 are another option. Browse all potassium fertilisers.
- Full comparison: sulphate of potash vs muriate of potash. How K fits with N and P: NPK fertiliser guide.
Always check with your agronomist before acting.
What we could not verify
- DPIRD's MyCrop page is no longer live; we used an Internet Archive copy (last updated April 2015).
- GRDC potassium Update papers blocked our fetch, so newer GRDC critical values by soil type are not included.
- We found no verified potash price, so no cost per kg of K is shown.
- We could not confirm how late in the season a K top-dressing still pays in wheat beyond the 'up to four weeks after seeding' guidance.
References
- Diagnosing potassium deficiency in wheat (DPIRD (WA); Department of Primary Industries and Regional Development, Western Australia (MyCrop), 2015; Government diagnostic guide (WA). Checked 2026-10-24.)
- Potassium (fact sheet) (Department of Agriculture and Food, Western Australia; Planfarm Agricultural Consultancy; Soil Quality Pty Ltd (soilquality.org.au), 2026; Government-backed fact sheet (WA). Checked 2026-10-24.)
- Potassium—New South Wales (fact sheet) (NSW Department of Primary Industries; Department of Agriculture and Food, Western Australia; Planfarm Agricultural Consultancy; Soil Quality Pty Ltd (soilquality.org.au), 2013; Government-backed fact sheet (NSW). Checked 2026-10-24.)
- Subsoil testing required to detect the rundown of soil potassium to deficient levels for wheat production on loam-textured soils (Crop & Pasture Science paper (record held in the DPIRD WA Digital Library); CSIRO Publishing (Crop & Pasture Science 75(11), CP24195), 2024; Peer-reviewed field research (WA). Checked 2026-10-24.)
- Potassium: Sulphate of Potash (SOP) and Muriate of Potash (MOP) (Incitec Pivot Fertilisers; Incitec Pivot Fertilisers (manufacturer), 2026; Manufacturer product page. Checked 2026-10-19.)
More on how we check sources: methodology. Spot an error? Tell us.
General information only. Always check with your agronomist before acting. Full disclaimer