GenetixBayEST. 2009
Nutrients · Beginner · 2 min read

Mobile vs immobile nutrients: reading old vs new leaves

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

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Short answer. Some nutrients can be pulled out of old leaves and sent to new growth through the phloem; others stay where they were first laid down [1]. Shortages of mobile nutrients (N, P, K, Mg) show first on the oldest, lowest leaves. Shortages of immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) show first on new growth [1][4]. Where the symptom starts is the first fork in diagnosis. Cannabis has a few exceptions, below.

What's happening

Water and dissolved nutrients rise from the roots in the xylem, pulled by transpiration. Everything reaches leaves that way first. What differs is whether the plant can reload a nutrient into the phloem (the sugar-transport system) and send it to growing tips [1].

  • Mobile: N, P, K, Mg, Cl, Mo (moderate), Ni [1][5]. When supply falls, the plant strips them from old leaves. Old leaves yellow; new leaves look normal for a while.
  • Immobile: Ca and B (barely enter phloem in most species), Fe, Mn, Cu, Zn (low mobility) [1][4]. New growth depends on what the roots deliver today. New leaves suffer; old leaves stay green.
  • Intermediate: S. Mobility depends on N status; symptoms tend to appear mid-canopy and on newer leaves [1][2].

How it shows in cannabis

Nutrient Mobility Where symptoms started in cannabis trials
N Mobile Lower leaves, climbing up [2][3]
P Mobile Lower, older leaves (olive or brown spots) [2][3]
K Mobile Margins of lower leaves [2]; sugar leaves in DWC flower [3]
Mg Mobile Lower leaves, interveinal [2][3]
S Intermediate Middle of plant, then new leaves [2]; upper sugar leaves [3]
Ca Immobile Growing tips and new leaves [2]; but lower fan leaves in DWC flower [3]
Fe Immobile New, expanding leaves [2]
Mn Low Upper and central leaves [2]
Zn Low Newer leaves, margins [2]
Cu Low New leaves, distortion and limpness [2]
B Immobile Growing tips, new leaves, root tips [2]
Mo Moderate No visible symptoms in 9 weeks [2]

Decision steps

  1. Where did it start? Oldest leaves → think N, Mg, K, P. New leaves → think Fe, Mn, Zn, Ca, B, Cu, S.
  2. What pattern? Whole leaf (N, S), between veins (Mg old, Fe/Mn new), margins (K old, Zn new), spots (P, Ca), distortion (Ca, B, Cu).
  3. Then rule out the root zone and pH before any nutrient: most immobile-nutrient symptoms in a properly fed plant are pH or root failures [4]. See how to diagnose any problem.

Common mistakes

  • Treating mobility as absolute. The Ca result in DWC [3] shows spotting on lower leaves can still be Ca.
  • Forgetting that pests (broad mites, thrips) and light stress also hit new growth first.
  • Diagnosing late flower: plants remobilise N, P, K and Mg into flowers by design, so old-leaf fading near harvest is normal.

Evidence

  • Phloem mobility classes: Established [1].
  • Cannabis symptom locations matching the classic pattern: Established for most elements [2][3], with the Ca exception noted [3].

Questions this article answers

Which nutrients are mobile in cannabis?

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

Why do deficiencies show on bottom leaves or top leaves?

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

Yellow new growth vs yellow old leaves, what does it mean?

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

Is calcium mobile or immobile?

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

How do I tell nitrogen from iron deficiency?

Mobile nutrients (N, P, K, Mg, and also Cl, Mo, Ni) can be moved from old leaves to new ones through the phloem, so their shortages show on the oldest, lowest leaves first. Immobile nutrients (Ca, B, Fe, Mn, Cu, Zn, and partly S) stay where they were first deposited, so shortages show on new growth. It is the first fork in any deficiency diagnosis, with a few cannabis-specific exceptions.

Sources

  1. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. doi.org/10.1016/C2009-0-63043-9
  2. Cockson P, et al. 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. doi.org/10.3390/app9204432
  3. Llewellyn D, et al. 2023. Plants 12(3):422. doi.org/10.3390/plants12030422
  4. Lopez RG, Mickelbart MV, Pasian C. Purdue Extension HO-242-W. extension.purdue.edu/extmedia/HO/HO-242-W.pdf
  5. White PJ, Broadley MR. 2001. Annals of Botany 88(6):967-988. doi.org/10.1006/anbo.2001.1540

Related

SupportedIntermediate · Draft

Nitrogen (N) in cannabis: nitrate vs ammonium, deficiency, toxicity and targets

Nitrogen drives leaf growth and photosynthesis. Cannabis does best on mostly nitrate N (about 150–200 mg/L in soilless veg and early flower), with ammonium kept at or below about 10–30% of total N; above 30% ammonium, cannabis trials show toxicity and lost yield. Deficiency yellows the oldest leaves first; excess gives dark, clawed leaves and soft, leafy growth.

SupportedIntermediate · Draft

Calcium (Ca) in cannabis: deficiency, transport, targets and sources

Calcium builds cell walls and membranes and moves only in the transpiration stream, so it cannot be moved from old leaves to new ones. Feed about 150–200 mg/L Ca in soilless systems, and remember that most 'calcium deficiency' is a delivery problem (low transpiration, high humidity, root damage, competing K/Mg/NH4) rather than an empty bottle. Classic signs are distorted, stunted new growth and brown spots.

SupportedIntermediate · Draft

Iron (Fe) in cannabis: yellow new growth, chelates and pH

Iron is needed to make chlorophyll and run electron transport. Deficiency shows as interveinal yellowing of the newest leaves and is almost always caused by high root-zone pH or alkaline water, not a lack of iron in the feed. Supply about 1.5–3 mg/L Fe as a chelate matched to your pH: EDTA up to about 6.5, DTPA to about 7.5, EDDHA to about 9.

SupportedIntermediate · Draft

Sulfur (S) in cannabis: role, deficiency, targets and sulfate sources

Sulfur is part of the amino acids cysteine and methionine and of glutathione; cannabis also makes sulfur-containing aroma compounds. Feeds usually supply 50–120 mg/L S as sulfate without trying, via magnesium and potassium sulfate. Deficiency yellows the middle and newer leaves evenly, strongest at the leaflet base, and is rare unless a feed is built entirely from nitrates and chlorides.

DiagnosisSupportedBeginner · Draft

Yellowing lower leaves on cannabis: a differential

Lower-leaf yellowing is usually nitrogen shortage or, far more often, roots that cannot take nitrogen up (overwatering, cold or rotting roots, pH out of range). Uniform yellow climbing from the bottom suggests N; green veins with yellow between them suggests Mg; yellow-then-brown margins suggest K. In late flower, gradual fade is normal.

DiagnosisSupportedIntermediate · Draft

Yellow new growth on cannabis: a differential

Yellowing at the top of the plant points to an immobile nutrient (Fe, S, Mn, Zn, Cu, B, Ca) that the roots are failing to deliver, and the usual cause is high root-zone pH or a wet, cold or damaged root zone rather than a lack of fertiliser. Rule out light bleaching, broad/russet mites and viroid stunting before adding micronutrients.

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