The Mobile Versus Immobile Rule
This is the golden rule of plant diagnostics. It narrows your search by 50% immediately. Firstly, you need to know which nutrients are mobile and which are immobile.
- Mobile nutrients (N, P, K, Mg) can move from one part of the plant to another. In effect, the plant can ‘steal’ these nutrients from old leaves to feed new growth. If the bottom (older) leaves look sick first, it’s likely a mobile nutrient deficiency.
- Immobile nutrients (Ca, Fe, B, Cu) are ‘locked in’ once they arrive at their destination, so the plant can’t move these once they are in place. If the top (new) growth looks stunted or yellow, it’s likely to be an immobile nutrient deficiency.
Now you know the basics in terms of nutrient mobility you can start to diagnose your plants’ health issues.
Why does this matter? Well, let’s say your plants’ leaves are yellowing, this could be an iron (Fe) or magnesium (Mg) deficiency as both can cause yellowing between leaf veins. However, if the yellowing is at the top, it’s likely to be deficient in iron (immobile). If it’s at the bottom, it’s likely to be magnesium deficiency (mobile).
Common Deficiency Cheat Sheet
Here is how the most common deficiencies tend to manifest themselves:
| Nutrient Deficiency | Primary Symptom | Location |
| Nitrogen (N) | General paling/yellowing (chlorosis) of the whole leaf. | Older/bottom leaves |
| Phosphorus (P) | Dark green or purple/reddish tint; stunted growth. | Older/bottom leaves |
| Potassium (K) | ‘Burnt’ or brown edges (necrosis); looks like the leaf was singed. | Leaf edges/older leaves |
| Magnesium (Mg) | Interveinal chlorosis (veins stay green, rest turns yellow). | Older/bottom leaves |
| Iron (Fe) | Sharp yellowing while veins stay bright green. | New/top leaves |
| Calcium (Ca) | Distorted hooked shapes; blossom end rot in fruit. | New/top leaves |
Correcting Nutrient Deficiency
To correct a mobile macronutrient deficiency, your aim is to supply that nutrient into the root zone for better long-term health (since these nutrients will move through the plant easily). Soil applications rather than foliar applications are usually best for introducing macronutrients, so look for a fertiliser with the correct nutrient ratio for your issue.
For immobile nutrient deficiencies, adding fertiliser to the soil might not be the best approach since these do not move well within the plant. In these cases, it is best to look for a fertiliser with the correct nutrient ratio that can be applied to the leaves – a foliar application.
The Small Print!
Calcium
A note of caution regarding calcium deficiencies (particularly those like blossom end rot in tomatoes). These can often be caused by inconsistent watering or humidity. This is because calcium is moved around by water (transpiration), so aim to get your watering as consistent as possible relevant to the weather and air humidity.
pH
If your soil is too alkaline (high pH), iron can become ‘locked’ in the soil. In this instance, the plant cannot access it, so it won’t matter how much you add to the soil – it will not benefit the plant. To overcome this, use a foliar application or if needed, a soil amendment like sulphur to change its pH. It always pays to conduct a soil test before selecting fertiliser, so you know exactly what you’re dealing with.
Temperature
Phosphorous (mobile) is difficult for plants to absorb when the ground is cold (under 12°C). Wait until the soil has warmed before making a diagnosis, as plants often fix this issue without intervention once the weather warms.
Recognising Toxicity (Over-fertilising)
Toxicity is often harder to spot because it can mimic deficiencies or cause nutrient ‘lockout’ (where too much of one thing prevents the plant from absorbing another).
- Nutrient Burn: The most common sign. Leaf tips turn brown and crispy suddenly, usually after a heavy feeding. This can be caused by any nutrient if the concentration is too high.
- Nitrogen Toxicity: Leaves turn an unnaturally dark, waxy green and may "claw" (curve downward) like a bird's talon. In this case, plants can be more vulnerable to pests.
- Salt Buildup: A white, crusty layer on the soil surface or the rim of the pot is often a precursor to nutrient burn. It means the salt concentration in the soil is becoming higher than that inside the plant.
- Check Watering: Overwatering often looks exactly like a nitrogen deficiency (yellowing) because the roots are drowning and can't breathe.
To fix nutrient overdoses, you will need to flush the soil to physically wash away the excess salts and nutrients from the root zone.
If you are flushing plants that are in the ground, slowly soak the area with a hose for 30-60 minutes to push excess nutrients away from the roots.
If you are flushing potted plants, pour a volume of water over the soil until the water runs clear from the bottom of the pot. This usually requires water in volumes 2 to 3 times the size of the pot.
Leave flushed plants free of any additional fertilisers for 14 days and when you do start feeding again, use only 50% of the recommended fertiliser dose.
Questions about nutrient deficiencies? Get in touch with our technical team for free advice.
This post was written in collaboration with Ollie Wright.
Ollie Wright MBPR, FQA
Ollie Wright serves as the Technical Manager at Agrigem, where he oversees the integrity and accuracy of the company’s technical output. A recognised figure within the UK turf-growing sector, Ollie leads Agrigem’s technical response, ensuring that all guidance provided to retail and trade clients remains rigorous, compliant, and at the forefront of industry standards.
With a career built on a foundation of practical experience in farming and landscaping - including the successful management of his own firm - Ollie bridges the gap between theoretical science and field application. His extensive hands-on background is reinforced by industry credentials, including RHS Horticulture, amenity turf and hard surface BASIS, and FACTS qualifications. This unique combination of expertise allows him to deliver high-level strategic advice across diverse sectors regarding plant protection products and complex legislative requirements.
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Article Notes:
- Use plant protection products safely. Always read the label and product information before use. For label and safety information, refer to the manufacturer’s website.
- Only certified users may apply professional plant protection products. Find out more about safe use of pesticides here.
