Iron (Fe) in Cannabis – Deficiency and Requirements

The youngest leaves turn pale yellow, their veins stay green, and lower down the plant is dark green. That is what iron deficiency looks like. Iron is a micronutrient, one the plant needs only in small amounts. Chlorophyll contains no iron but needs it to be made. A checklist from NC State University looks for the cause in uptake first. On that list, pH and root zone come before any iron dose.

How do I recognise iron deficiency?

Iron deficiency shows on the youngest leaves as yellow between green veins. It starts at the leaf base and runs from light green through light yellow to almost white. Lower and middle leaves stay deep green. Brown spots only come after a long deficiency.

In flowering iron deficiency was barely visible. Without iron the sugar leaves and buds turned paler. The leaflet centres lightened for a while, and late on the margins of lower fan leaves browned. Sugar leaves are the small leaves inside the buds. Bud mass did not fall, and the researchers consider it possible that reserves from vegetative growth were enough.

  • Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
  • In vegetative growth, check the top: iron deficiency hits new growth, magnesium deficiency the lower leaves.
  • Rule out manganese deficiency. In geraniums it forms a fine green vein net, iron deficiency broader yellow bands.
  • Rule out too much ammonium. In cannabis it curls the leaves at the shoot tips and kills their margins.
  • Compare several plants. Root rot usually hits single plants, a pH problem many at once.

Why is iron short although I feed it?

The checklist starts with a pH that is too high and with roots that are not working. The pH value describes how acidic a solution is. In soilless substrates the NC State group saw iron deficiency above about pH 6.5. In its preliminary trials, however, cannabis showed no yellowing of young leaves at pH 7.0 but did at 7.8. The threshold is therefore not sharp.

Irrigation water with more than 120 ppm bicarbonate drives the pH up. Wet, cold or diseased roots take up little iron, and constantly wet substrate caused iron deficiency in greenhouse trials despite feeding. Too much manganese, zinc or copper crowds out iron, and in cannabis too much zinc lowered the iron in the shoot.

  1. Measure the pH in the substrate. Plant nutrition specialists recommend 5.8 to 6.2 for cannabis.
  2. Check the bicarbonate on the water analysis. It is the part of the water that absorbs acid.
  3. Check the root zone temperature. In ornamentals a root zone below 13 °C counts as a cause.
  4. Check watering intervals and fix waterlogging. Look at the roots for brown, mushy patches.
  5. Check the phosphorus in the fertiliser. In ornamentals iron problems start from about 30 ppm phosphorus, in sensitive species from 15 ppm. In cannabis no iron deficiency appeared at 75 mg/L phosphorus.
  6. Only after all of this, give more iron.

Which iron chelate do I need?

That depends on the pH in the root zone. A chelate is a molecule that wraps around iron and keeps it in solution. Unbound iron precipitates above about pH 6 and is then lost to the root.

According to one fact sheet, iron EDTA suits pH 5.8 to 6.2, iron DTPA 6.3 to 6.8 and iron EDDHA from 6.8. According to a greenhouse handbook, iron DTPA stays stable up to pH 7 and iron EDDHA up to pH 9. Chelates do not lower pH. What exactly a chelate is, is explained in the article on chelates.

  • Choose the chelate by the pH in the root zone.
  • Do not mix manganese, zinc or copper sulphate with iron chelate in the same concentrate. Depending on pH, they push 20 to 50 % of the iron out of the chelate.
  • Store concentrates in the dark. Some chelates break down under UV light.
  • Yellow leaves usually do not turn green again. New growth shows whether the dose works.

How much iron does cannabis need?

Little: for flowering the NC State group gives 3 to 3.5 ppm iron in the feed solution. In feed solution 1 ppm equals one milligram per litre. No trial with graded iron rates on cannabis was found.

Across 6119 hemp samples, the middle half of leaf values lay at 70 to 150 mg iron per kilogram of dry matter. Without iron in the solution, one cultivar showed signs in vegetative growth only after nine weeks. The leaf then held 60 mg/kg.

  • In flowering, dose iron by the guide value of 3 to 3.5 mg/L.
  • Compare a lab leaf sample with 70 to 150 mg/kg, together with an analysis of substrate or solution.
  • If too much iron is suspected, also sample the lower leaves.

What happens with too much iron?

Too much iron is mainly a consequence of a pH that is too low. Below pH 5, according to the NC State group, cannabis can take up too much iron and manganese. The lower leaves then turn bronze from the tip.

In a trial with peat and perlite, however, cannabis reacted to very low pH mainly with weaker growth. At pH 3 the shoot weighed 23.9 g, at pH 5 to 7 it was 36.6 to 39.2 g. Bronzing only appeared at four times the micronutrient rate and pH 3.8, and only in one of two cultivars.

  1. Measure the pH in the substrate and keep it above 5.5.
  2. If pH is too low, raise it with lime or feed with calcium nitrate.
  3. Irrigate with clear water for a while.
  4. Damaged leaves do not recover. What has built up in the leaf cannot be removed.

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