When does pre-fertilised soil run out of nutrients?

Short answer

A typical pre-fertilised bagged potting mix feeds cannabis for about 2–4 weeks, so most plants run short in mid-veg. Richly amended or super-soil-style mixes and bigger pots last longer. When the lowest leaves start to pale and the fading climbs, begin feeding at half the label rate. Grower practice.

Why

Bagged mixes carry a modest starter charge of fertiliser. A seedling barely touches it; a plant in fast veg strips it quickly, and each watering leaches a little more.

Nitrogen usually runs out first. It is mobile, so the plant moves it from old leaves to new growth, and the oldest, lowest leaves yellow first while the top stays green ^[4]. Cannabis trials show exactly that pattern: nitrogen deficiency starts on lower leaves and climbs ^[1]^[2].

Organic mixes release nutrients through microbes, so their supply depends on warmth, moisture and how much was mixed in. Organic feeds need more nominal nitrogen to match mineral ones, around 389 mg/L N in one vegetative coir trial ^[3], which is why lightly amended mixes fade early.

The numbers

Mix	Typical feeding time before you add more
Light starter mix (seedling or general potting)	1–3 weeks
Standard pre-fertilised potting mix	2–4 weeks
Heavily amended organic / super-soil style	4–8+ weeks, often longer
Living soil with top-dressing	Topped up by top-dress, not bottles

When you start: half label rate, EC ~0.8–1.6 mS/cm, every second or third watering.

What to do

Water seedlings with plain, pH-adjusted water (pH 6.2–6.8).
Check the lowest leaves every few days from week 2.
When they pale evenly, check runoff pH to rule out lockout.
Start feeding at half the label rate.
Check runoff EC monthly; back off if it climbs.

Common mistakes

Feeding from day one on top of a fertilised mix.
Waiting until half the plant is yellow.
Reading normal late-flower leaf fade as hunger.
Treating a pH problem as a feed problem.

Full article: Growing in soil
Answers: feed every watering? · super soil
Articles: mobile vs immobile nutrients · yellowing lower leaves
Terms: top-dressing · nutrient deficiency

Potting mix is a manufactured container medium, usually peat or coco with perlite, lime and starter fertiliser, designed to drain and hold air better than garden soil.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Running a large volume of low-EC water or mild solution through the medium to wash out accumulated salts; it treats the medium, not the plant, and suits any stage.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.The programmed ageing and breakdown of leaves and other tissues, in which the plant recycles nutrients (especially nitrogen) from old leaves to developing flowers and seeds.Living soil is an organic growing medium managed to sustain an active soil food web, so microbes and invertebrates release nutrients and the soil is reused across cycles.Top dressing is spreading dry fertiliser or amendments on the soil surface, usually under mulch, so nutrients release into the root zone as the plant is watered.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.

Follow-up questions

How do I know when my soil is running out of nutrients?

Soil is running out of nutrients when the oldest, lowest leaves of a growing plant turn evenly pale, then yellow, and the fading climbs up the plant while new growth stays green. That is nitrogen, a mobile nutrient, being moved to new leaves. Check runoff pH first; if it is in range, begin feeding at half the label rate.

Should I feed seedlings in fertilised soil?

No, do not feed seedlings planted in fertilised potting mix. The mix's starter charge usually covers the first 2-4 weeks, and seedlings need very little. Extra feed on top risks burnt tips and stalled roots. Water with plain, pH-adjusted water until the lower leaves start to pale, then start feeding gently.

Do bigger pots hold nutrients longer?

Yes, bigger pots of the same mix hold more total fertiliser, so a plant in a larger pot of fertilised mix usually runs longer before needing feed. The concentration is the same, though, so a seedling isn't overfed by a big pot. Watering habits matter more there: big pots of wet mix stay soggy and slow a young plant.

Sources

Cockson P, Landis H, Smith T, Hicks K, Whipker BE, 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. doi.org/10.3390/app9204432

Llewellyn D, Golem S, Jones AMP, Zheng Y, 2023. Foliar symptomology, nutrient content, yield, and secondary metabolite variability of cannabis grown hydroponically with different single-element nutrient deficiencies. Plants 12(3):422. doi.org/10.3390/plants12030422

Caplan D, Dixon M, Zheng Y, 2017. Optimal rate of organic fertilizer during the vegetative-stage for cannabis grown in two coir-based substrates. HortScience 52(9):1307-1312. doi.org/10.21273/HORTSCI11903-17

Marschner P (ed.), 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. doi.org/10.1016/C2009-0-63043-9