Nutrient Lockout in Cannabis – Deficiency Despite Feeding

Sometimes a plant shows magnesium deficiency even though the fertiliser supplies plenty of magnesium, and growers call this nutrient lockout. The term comes from growing practice and simply means a deficiency despite supply. Behind it lies either an unsuitable pH or an antagonist, that is a nutrient in excess that crowds out another one. Precipitates in the tank, too much salt and a disturbed root zone add to the list of causes. Simply giving more of the missing nutrient fixes none of them.

At which pH do which nutrients run short?

Too high a pH mainly makes micronutrients scarce, the nutrients a plant needs only in small amounts, such as iron. Too low a pH holds back calcium, magnesium and molybdenum instead. The pH value measures the acidity of a solution, and it decides whether a nutrient stays dissolved and reaches the root. The thresholds for soil in the table below come from soil science. Soilless substrates are potting materials without soil, such as peat, coco coir or rockwool, and in hydroponics the roots grow directly in the nutrient solution, the ready-mixed feed. For coco, rockwool and hydroponics, no thresholds per element have been measured on cannabis.

Medium and pH	What happens
Soil below pH 6.0	Phosphorus, potassium and sulphur become scarcer
Soil below pH 6.5	Calcium becomes scarcer
Soil between pH 6.5 and 8.5	Magnesium is most available
Soil above pH 7.0	Molybdenum becomes more available
Soil above pH 7.5	Boron, iron and manganese become scarcer
Soil above pH 8.0	Copper and zinc become scarcer
Soil between pH 7.6 and 8.4	Potassium becomes scarcer again
Soil strongly acidic or strongly alkaline	Nitrogen becomes scarcer
Soilless substrate above pH 6.5	Boron and copper as well as iron, manganese and zinc are taken up poorly
Nutrient solution above about pH 6	Unchelated iron, manganese, zinc and copper precipitate; a chelate keeps them dissolved
Hydroponics below pH 5.5	Iron and manganese become overly soluble; calcium and magnesium get bound and less available
Peat substrate below pH 5.3	Calcium and magnesium deficiency threaten, and manganese can become toxic
Cannabis in peat-perlite, very low pH	Growth is stunted at normal rates; bronzed lower leaves appear only at four times the micronutrient rate
Plants in general at low pH	Molybdenum deficiency is the only micronutrient deficiency tied to low pH

Measure pH where the roots are, in the runoff or in the tank. Runoff is the solution that drains from the bottom of the pot.
Look up the target ranges for each medium in the article on nutrient diagnosis.
If several micronutrients are short at once, check the pH first and then the injector.
With chlorosis, a yellowing of the young leaves, measure the substrate pH before feeding iron.
With stunted growth and a very low pH, raise the pH first and keep feeding micronutrients.
Judge success on the new growth, not on old yellow leaves.

What pushes the pH at the root off target?

Irrigation water, the form of nitrogen and the phosphorus supply all shift the pH in the root zone. Bicarbonate, which chemists call hydrogen carbonate, is the part of the irrigation water that neutralises acid. Water with more than 120 ppm bicarbonate, that is 120 mg per litre, drives the pH up over time. The NC State group, a team of plant nutrition specialists at North Carolina State University, then observes iron deficiency in soilless substrates. The plant takes up nitrogen either as ammonium or as nitrate, and ammonium acidifies the root zone. In the cannabis trial, pure nitrate raised the runoff pH above that of the feed solution.

If phosphorus is short, the root acidifies its own surroundings. The runoff pH then fell to 4.5, compared with 5.8 under good supply, and calcium and magnesium in the tissue were lower. That the acidification causes this drop has been proposed but not measured. Raising the pH with lime also adds calcium; in the hemp trial, leaf calcium rose from 1.03 % to 2.86 %. Potassium bicarbonate, a potassium salt of carbonic acid, raises the pH as well. It adds a lot of potassium, though, and makes the EC jump; EC, short for electrical conductivity, measures all dissolved salts together.

Read your water supplier’s analysis or have the irrigation water tested.
Above 120 ppm bicarbonate, acidify the irrigation water.
With a pure nitrate fertiliser, measure the pH in the runoff.
If the runoff pH drops without any added acid, check the phosphorus supply.
When liming, count the calcium that comes with the lime.
After a dose of potassium bicarbonate, flush with clear water the next day.

Which nutrients crowd each other out?

In cannabis it was mainly potassium, magnesium and nitrogen in excess that pushed down calcium and magnesium. This crowding out by an antagonist is called ion antagonism. The cannabis values come from perlite, a heat-expanded volcanic rock that binds hardly any nutrients, with one or two cultivars per trial. They have not been rechecked for coco, soil and rockwool. What was measured are lower levels, not a visible symptom in every case. The researchers name the drop as a cause of damage in only two series, under high potassium and under high ammonium.

Across many crops, 17 of 117 interactions affecting yield were inhibiting. Such crowding out affects yield only when the displaced nutrient is already scarce. The Mulder chart, a diagram of these interactions, is spread mainly by fertiliser makers without any literature reference. It goes back to a conference contribution on fruit growing from 1953.

In excess	Lowers	Shown in
Potassium	Calcium and magnesium in leaf, stem and root; in the root also zinc, iron and manganese	Cannabis
Magnesium	Calcium and potassium as well as manganese and zinc	Cannabis
Nitrogen above need	Phosphorus, calcium and magnesium in the root; manganese in leaf and stem	Cannabis
Ammonium	Calcium and magnesium	Cannabis
Nitrate	Phosphorus	Cannabis
Zinc	Iron in the shoot	Cannabis
Phosphorus	Zinc in the root; in a second trial leaf zinc rose slightly	Cannabis, not settled
Potassium	Magnesium; magnesium does not lower potassium there	Forage crops
Calcium	Magnesium	Soil, horticultural practice
Calcium and magnesium together	Potassium	Greenhouse cucumber
Phosphorus	Iron and manganese as well as zinc	Field crops and ornamentals
Phosphorus from about 30 ppm	Iron	Ornamentals, sensitive species from 15 ppm
Manganese or zinc or copper	Iron	Field and greenhouse crops
Ammonium	Potassium; in cannabis leaf potassium was higher under high ammonium	Other plants
Sulphur in excess	Boron and molybdenum	Trade article without primary source

Add up fertiliser and irrigation water, and check which nutrient lies far above need.
Check fertilisers high in potassium or phosphorus specifically.
Lower the antagonist and keep the amount of the missing nutrient the same.
With calcium or magnesium deficiency, do not raise nitrogen above need.
With zinc deficiency, check the phosphorus supply.
Have a lab analyse the solution or the substrate to find the nutrient in excess.

Why does something precipitate in the tank or stock solution?

Calcium forms gypsum with sulphate and calcium phosphate with phosphate, and both dissolve poorly. That is why calcium fertilisers go into one stock solution, the concentrate the nutrient solution is diluted from, and sulphate and phosphate fertilisers into another. Epsom salt, which is magnesium sulphate, is not mixed with calcium fertiliser either. Stock solutions should sit below pH 5 so that everything dissolves, but below pH 3.5 the chelates break down. A chelate is a molecule that binds a metal and keeps it in solution, and the article on chelates explains how it is built.

Manganese, zinc and copper supplied as sulphates displace iron from the chelate, and depending on pH, 20 to 50 % of the iron is lost this way. UV light and ozone used for disinfection also partly destroy chelates. In a closed hydroponic system the solution is topped up rather than replaced. Cannabis grown this way formed a precipitate at double fertiliser strength and high phosphorus. Phosphorus in the solution was then lower than with the same rate at single strength. The researchers classify the precipitate as calcium phosphate, but they did not analyse it.

Make up calcium fertilisers separately from sulphate and phosphate fertilisers.
Set stock solutions to a pH between 3.5 and 5.
With iron deficiency despite chelate, check whether manganese, zinc or copper sulphate is in the same mix.
With sediment in the tank, do not raise phosphorus above need.
Choose the iron chelate by pH, and look up the matching one in the article on iron.

When does the root zone slow uptake?

Too much salt, wet substrate, cold and diseased roots slow uptake even when the supply is complete. If the EC climbs too high, the root takes up less water. Specialists call this osmotic stress, and for nitrogen above the optimal rate the researchers name this load as one of several reasons. EC says nothing about the composition of the solution, and micronutrients contribute less than 1 % to it.

Constantly wet substrate limits iron uptake, and for cannabis the NC State group names a saturated root zone as a cause of iron deficiency. In ornamentals, iron runs short once the root zone falls below 13 °C; measurements for cannabis are lacking. Pythium and Fusarium are pathogens that cause root rot, and from practice the NC State group knows cold and Pythium as brakes on iron uptake. In deep water culture the roots hang in a container of aerated solution. There, even gentle bubbling from the aeration sharply lowered iron uptake in sunflower and maize, while tomatoes hardly reacted and cannabis was not tested.

Measure EC in the runoff or in the tank, and compare it with your own records.
If it is high, lower the fertiliser rate and flush with clear water.
Look up limits for fertiliser salt and table salt and for acidifying in the article on calculating a nutrient solution.
Let constantly wet substrate dry down between waterings.
Measure the temperature in the substrate or tank, not just the room air.
Look at the roots before feeding more.
In deep water culture with iron deficiency, lower the pH and keep phosphorus low.
With calcium deficiency in new growth and humid, still air, read the article on calcium.

How do you proceed with deficiency despite feeding?

Fix the cause first, and feed only after that. A simple deficiency from too low a rate is not lockout, and root rot, pests and light stress produce similar patterns. Answering a deficiency caused by excess with more fertiliser only makes it worse.

Take magnesium deficiency from too much potassium: here less potassium helps, while according to the cannabis data extra Epsom salt would push calcium down further. How fast new growth recovers after such a correction has not been measured in cannabis.

Measure pH and EC at the root.
Put fertiliser and irrigation water side by side and look for the nutrient in excess.
Check the roots, the wetness and the temperature in the root zone.
With yellowed young leaves, feed iron only after ruling out too high a pH and too much phosphorus.
Rule out a cold root zone and waterlogging in the same way.
Also rule out root rot, fungus gnat larvae and damage from chemicals such as pesticides.
Evaluate a lab analysis of the solution or substrate together with a leaf sample.
Only then give more of the missing nutrient.
Watch the new growth.
Read leaf patterns and salt damage with the article on identifying nutrient deficiency.
Match the location of the first sign with the article on nutrient function and mobility.