Checking Nutrient Problems by Growing Medium – What to Measure in Soil, Coco, Rockwool, and Hydroponics

Before answering a yellow or spotted leaf with more fertiliser, check what is actually reaching the root. This nutrient diagnosis by growing medium needs only two meters, one for pH and one for EC. You measure the solution you water with and the solution the roots are standing in, the root zone solution. Where you get that second sample depends on the medium: in soil and coco it is the water running out of the bottom of the pot, in rockwool the runoff from the slab, in hydroponics the tank.

Two measurements anyone can make

pH tells you how acidic the solution is. If it is off, the root cannot take up nutrients that are present. EC, the electrical conductivity, tells you how much fertiliser salt is dissolved, but not which salt. You measure both twice: in the feed solution and in the sample from the root zone.

The trend matters more than any single reading. Always measure the same way, write the values down, and compare them over the weeks. If EC in the root zone climbs from one reading to the next, salt is building up in the pot. If pH there keeps falling, the solution is turning acidic.

The pH at the root should sit in these ranges, depending on the medium:

Medium	Where to take the sample	pH at the root
Peat and peat-based potting soil	Leachate from the bottom of the pot	5.5 to 6.5, not below 5.0
Coco	Leachate or runoff	5.4 to 6.2
Rockwool	Runoff from the slab	5.5 to 6.5, feed slightly lower
Hydroponics	Tank	5.5 to 6.5
Garden soil	Soil sample	6.0 to 7.5

Only the value for peat comes from a trial with cannabis, in peat with perlite. The value for coco is the general range for soilless substrates, and the others come from greenhouse horticulture; they are a starting point. For EC no target values for cannabis are established. As a rough frame, a fact sheet for ornamental crops treats 2.7 to 4.6 mS/cm in PourThru leachate as normal.

Peat and potting soil in pots

The sample is taken with the PourThru: water as usual, wait half an hour to an hour, then pour some distilled water on top and collect what runs out at the bottom. For a pot of just under 4 litres that is about 150 ml, a little under a cup; about 50 ml should come out, enough to immerse the probe. The plant stays in its pot. For about 11 litres it is 350 ml. The fact sheet gives no amount for larger pots; settle on one and stick to it, otherwise the readings cannot be compared.

If pH falls below 5.0, cannabis grows noticeably worse; in a trial with four hemp cultivars, plants at pH 3 lacked almost two fifths of their mass. Anyone raising pH with lime is adding calcium at the same time. An EC below the frame of 2.7 mS/cm points to too little fertiliser. If it is above and still rising, salt is building up and the plant takes up less water; then let more water run through or flush with plain water.

Coco

Measure as with soil, using the PourThru or the runoff. The difference lies in the material: coco holds nutrients on its fibre and swaps them for others. Unwashed, unbuffered coco can carry a lot of potassium and sodium and takes up calcium and magnesium from the nutrient solution in exchange. On the plant this can show as calcium deficiency, even though the fertiliser contains enough.

Buffering helps: soak the coco in a calcium nitrate solution before use and rinse it afterwards. In the trial that was up to 150 g of calcium nitrate per 1.5 kg of coco in 15 litres of water, for 12 to 72 hours. In one trial this raised the bound calcium sixteenfold, while potassium fell to just over a third. Soft water makes the problem worse: reverse osmosis water or rainwater brings no calcium or magnesium of its own, so the coco takes them from the fertiliser. In that case, blend in tap water. In one comparison cannabis grew more weakly in coco than in peat, and the authors saw the cause in water: coco holds a lot of air and little readily available water. They advise not carrying the watering routine over from soil but tuning it to the coco.

Rockwool

Rockwool holds practically no nutrients. Whatever is in the feed either reaches the root or drains away. You therefore measure pH and EC daily at the dripper and in the runoff. The greenhouse handbook sets the feed slightly below the target for most crops, around pH 5.3, so that the root zone settles at 5.5 to 6.5. New rockwool starts at a pH of around 7.5. No measurements exist for pre-treating it for cannabis; the runoff shows whether the pH is arriving in the target range.

The handbook steers root zone pH through the ammonium in the feed, one of the two forms of nitrogen: if the pH falls too low, use less ammonium; if it rises too high, a little more, at most 5 to 15 % of the nitrogen in total. If a laboratory analysis of the runoff shows that a nutrient deviates from its target, change the feed in small steps, by 10 to 15 % for a deviation of a quarter. If the runoff is collected and reused, sodium builds up; only irrigation water with less than 1.5 mmol/l of sodium, by calculation 34.5 mg/l, is suitable, which you can read from your water supplier’s analysis.

Hydroponics

In hydroponics the tank is the solution at the root, so you measure there directly. If you only top up spent solution with water, the substances the plant takes up more slowly accumulate, such as calcium, boron, copper, and sulphur. In one cannabis trial concentrations could rise to twice the starting level between two changes this way. That is why the solution is replaced completely at regular intervals instead of just being topped up.

If the pH in the tank rises above 6, iron, manganese, zinc, and copper precipitate out of the solution unless they are supplied as a chelate, that is, bound to a molecule that keeps them dissolved. Iron DTPA holds reliably up to about pH 6.5. Anyone running higher uses EDDHA.

When the measurement explains nothing

If pH and EC are right and the plant still shows a problem, the cause may lie outside the root zone. Towards the end of flowering the lower leaves yellow even with full supply, because the plant breaks them down. Feeding more at that point changes nothing. How to tell the individual deficiency patterns apart is described in the article on nutrient deficiency. If the cause remains unclear, a leaf analysis in a laboratory is the next step. It shows what the plant has actually taken up.