Cannabis nutrition has a reputation for being complicated. Some people keep a notebook with daily measurements, others follow nutrient charts with milligram dosages, and others buy supplements for every conceivable micronutrient. And then there\u2019s the reality: most nutritional problems don\u2019t stem from giving too little, but from giving too much, at the wrong time or without monitoring the pH.<\/p>\n\n\n\n
This guide gets straight to the heart of the matter. What the plant needs at each stage, in what proportions, how to spot when something is going wrong, and how to rectify it without destroying the growing medium in the process. No miracle products. Just the NPK principle, clearly explained.<\/p>\n\n\n\n
\u2192 This guide is part of our Complete Guide to Growing Cannabis from Seed<\/a>.<\/p>\n\n\n\n Table of contents<\/strong><\/p>\n\n\n\n NPK stands for the three main macronutrients that any plant needs to grow:<\/p>\n\n\n\n Nutrition products always show the NPK ratio in that order. A \u00ab20-10-10\u00bb fertiliser contains twice as much nitrogen as phosphorus and potassium. A \u00ab0-50-30\u00bb fertiliser contains no nitrogen and is designed for peak flowering or a flush.<\/p>\n\n\n\n As well as NPK, cannabis needs secondary macronutrients<\/strong> (calcium, magnesium, sulphur) and micronutrients<\/strong> (iron, zinc, manganese, boron, amongst others). A good starting growing medium and a high-quality base fertiliser cover most of these without you having to think about them individually.<\/p>\n\n\n\n Before discussing which nutrients to use, it is important to understand why many growers use nutrients but see no results: the pH of the irrigation water is outside the correct range, and the plant cannot absorb what you are providing, even if it is present in the growing medium.<\/p>\n\n\n\n Each nutrient has a pH range within which it is available to the plant. Outside that range, it remains \u00ablocked\u00bb, even though it is physically present in the growing medium. The result is a deficiency that cannot be corrected by adding more nutrients \u2014 it can only be corrected by adjusting the pH.<\/p>\n\n\n\n Measures the pH of the water afterwards<\/em> Add the nutrients then, not before. Nutrients alter the pH of the water. Always adjust the pH at the end, using pH up or pH down, before watering.<\/p>\n\n\n\n A basic digital pH meter costs between \u20ac15 and \u20ac30 and is the most important purchase you can make after your seeds. Calibrating it every 2\u20133 weeks using a buffer solution ensures reliable readings.<\/p>\n\n\n\n NPK requirements: minimal. Ideally: no added nutrients.<\/strong><\/p>\n\n\n\n The seed contains all the energy and nutrients it needs to germinate and develop its first roots. A light starter compost (low in pre-formulated nutrients) is sufficient and preferable during the first two weeks.<\/p>\n\n\n\n Adding fertiliser at this stage is one of the most common mistakes. Seedling roots are very sensitive and are easily burnt by salt concentrations that a mature plant would tolerate without any problem.<\/p>\n\n\n\n If the growing medium already contains pre-formulated nutrients (as is the case with most bagged cannabis growing media), do not add anything until the plant is at least 2\u20133 weeks old and begins to show the first signs of a lighter green colour on its new leaves (a sign that the growing medium is running out).<\/p>\n\n\n\n NPK profile: High in nitrogen, moderate in phosphorus and potassium. Approximate ratio: 3-1-2.<\/strong><\/p>\n\n\n\n During the vegetative stage, the plant is building its structure: stem, branches, leaves and roots. Nitrogen is the key nutrient because it is the central component of chlorophyll and of all the amino acids that make up this new tissue.<\/p>\n\n\n\n The length of the vegetative phase depends on you (for feminised strains) or on the genetics (for autoflowering strains). A feminised strain can remain in the vegetative phase for 4 to 12 weeks, depending on the size you want to achieve. An autoflowering plant has between 3 and 5 weeks of actual vegetative growth before pre-flowering begins.<\/p>\n\n\n\n Rule of thumb:<\/strong> Always start with half the dose recommended by the manufacturer. Cannabis responds well to gradual feeding but poorly to sudden overdoses. It is much easier to add more than to flush out accumulated salts from a saturated growing medium.<\/p>\n\n\n\n If the leaves are a deep, rich green and the internodes are short: nitrogen levels are sufficient. If the leaves are pale green and growth is slowing down: you can increase the nitrogen dose. If the edges of the leaves turn yellow or brown: excess salts \u2014 reduce the dose and rinse the growing medium.<\/p>\n\n\n\n NPK profile: Transition. Gradually reduce N, and increase P and K. Recommended ratio: 1-3-2.<\/strong><\/p>\n\n\n\n The first few weeks of flowering are a critical transition period. The plant continues to grow \u2014 it can increase in size by between 30 and 100 % during the initial \u00abstretch\u00bb of flowering \u2014 and still needs nitrogen, but its metabolic focus begins to shift towards flower production.<\/p>\n\n\n\n The most common mistake here is to cut off the nitrogen supply abruptly. The result is premature yellowing of the leaves (the plant cannibalises its own leaves to extract the stored nitrogen), which weakens the plant before it reaches peak flowering.<\/p>\n\n\n\n Calcium and magnesium become more important during the flowering stage. Chlorophyll synthesis (magnesium) and the formation of bud cell walls (calcium) consume larger amounts of these elements. If your tap water is soft or you use reverse osmosis, add a CalMag supplement from the start of the flowering stage.<\/p>\n\n\n\n NPK profile: Minimum or zero nitrogen, maximum phosphorus and potassium. Approximate ratio: 0-3-3.<\/strong><\/p>\n\n\n\n This is the period when the plant\u2019s energy requirements are at their peak. The buds are actively plumping up and need phosphorus to build the floral structure and potassium to transport the sugars that fuel this process. Nitrogen is no longer welcome: in excess, it produces \u00abspongy\u00bb buds that are less dense and have a lower concentration of cannabinoids.<\/p>\n\n\n\n The lower leaves will naturally start to turn yellow during this period. This is normal and to be expected: the plant is redistributing the nitrogen it has stored from the leaves to the buds. Do not mistake this for a deficiency that needs to be corrected with more nitrogen.<\/p>\n\n\n\n A real cause for concern:<\/strong> If the yellowing spreads rapidly from the lower leaves to the upper ones and the buds are not yet mature, the plant is using up its reserves prematurely. Check the pH and consider a light dose of organic nitrogen (bat guano, blood meal) to slow down the process without reactivating vegetative growth.<\/p>\n\n\n\n NPK profile: Zero nutrients. Just water with an adjusted pH.<\/strong><\/p>\n\n\n\n The final flush \u2014 watering with clean, pH-adjusted water only for the last 7\u201314 days before harvest \u2014 aims to wash away excess mineral salts accumulated in the growing medium and the plant\u2019s tissues. The result is a bud that burns cleanly, without the chemical taste or harsh smoke that indicate an insufficient or absent flush.<\/p>\n\n\n\n During the flush, the yellowing of the leaves accelerates significantly. This is normal and to be expected: the plant is using up all its internal reserves. A plant that is completely green at the time of harvest, following a proper flush, usually indicates an excess of residual nitrogen in the tissues.<\/p>\n\n\n\n Flushing in organic cultivation?<\/strong> If you have been using organic fertilisers (guano, bone meal, compost) throughout the season, flushing is less critical because organic fertilisers do not cause mineral salts to build up in the same way as synthetic fertilisers. A 5\u20137-day flush is sufficient.<\/p>\n\n\n\n Most of the nutrient deficiencies you see in your plants aren\u2019t what they seem. Before adding any nutrients, check the pH. In 70 % of cases, correcting the pH resolves the deficiency without adding anything else.<\/p>\n\n\n\n The correct diagnostic protocol:<\/strong><\/p>\n\n\n\n Autoflowering seeds react differently to nutrients. Their growth cycle is shorter, their roots are more sensitive, and there\u2019s no room for recovery if you make the mistake of over-fertilising: you can\u2019t extend the vegetative stage to give them time to recover.<\/p>\n\n\n\n Three key settings for automatic transmission cars:<\/p>\n\n\n\n \u2192 View 00 Seeds\u2019 automatic strains with detailed product descriptions: catalogue of automatic machines<\/a>.<\/p>\n\n\n\n \u2192 Return to the Complete Cultivation Guide<\/a> to see the rest of the process: training techniques, pest control and harvesting.<\/p>\n\n\n\n\n
\n\n\n\nWhat is NPK and why does it matter?<\/h2>\n\n\n\n
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\n\n\n\npH before nutrients: the rule that changes everything<\/h2>\n\n\n\n
Culture medium<\/th> Optimal pH range<\/th> Out-of-range result<\/th><\/tr><\/thead> Soil \/ organic substrate<\/td> 6,0 - 7,0<\/strong><\/td> Iron and manganese lock-up (high pH); aluminium and manganese toxicity (low pH)<\/td><\/tr> Coco coir<\/td> 5,8 - 6,3<\/strong><\/td> Calcium and magnesium levels frequently fall outside the normal range<\/td><\/tr> Hydroponics \/ aeroponics<\/td> 5,5 - 6,2<\/strong><\/td> Severe and rapid blockages; the system lacks the \u00abbuffer\u00bb provided by the soil<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStage 1: Germination and seedling stage (weeks 1\u20132)<\/h2>\n\n\n\n
Parameter<\/th> Recommended value<\/th><\/tr><\/thead> Added nutrients<\/td> None. The growing medium provides enough.<\/td><\/tr> pH of irrigation water<\/td> 6.0 \u2013 6.5 (clay)<\/td><\/tr> EC of irrigation water<\/td> 0.4 \u2013 0.8 mS\/cm<\/td><\/tr> Signal to move on to the next stage<\/td> 3\u20134 pairs of fully developed true leaves<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStage 2: Vegetation (approximately weeks 3\u20138)<\/h2>\n\n\n\n
Parameter<\/th> Weeks 3\u20134 (start of the growing season)<\/th> Weeks 5\u20138 (full-grown)<\/th><\/tr><\/thead> Approximate NPK ratio<\/td> 3-1-2<\/td> 3-1-2 to 4-2-3<\/td><\/tr> Dosage (relative to the manufacturer\u2019s maximum)<\/td> 25\u201350 %<\/td> 50\u201375 %<\/td><\/tr> pH of irrigation water (soil)<\/td> 6,0 \u2013 6,8<\/td> 6,0 \u2013 6,8<\/td><\/tr> EC target in the growing medium<\/td> 0.8 \u2013 1.2 mS\/cm<\/td> 1.2\u20131.8 mS\/cm<\/td><\/tr> Signalling the start of flowering<\/td> For feminised strains, switch to 12\/12; for autoflowers, the first white pistils signal the start<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStage 3: Early flowering (weeks 1\u20133 of flowering)<\/h2>\n\n\n\n
Parameter<\/th> Recommended value<\/th><\/tr><\/thead> Approximate NPK ratio<\/td> 1-3-2<\/td><\/tr> Dose relative to the maximum<\/td> 60\u201375 %<\/td><\/tr> pH of irrigation water (soil)<\/td> 6,2 \u2013 7,0<\/td><\/tr> EC target<\/td> 1.4 \u2013 2.0 mS\/cm<\/td><\/tr> Useful supplements<\/td> CalMag if the water is soft (conductivity < 0.4); enzymes if you\u2019re using organic soil<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStage 4: Peak flowering (weeks 4\u20137 of flowering)<\/h2>\n\n\n\n
Parameter<\/th> Weeks 4\u20135<\/th> Weeks 6\u20137<\/th><\/tr><\/thead> NPK ratio<\/td> 0-3-3<\/td> 0-3-3 to 0-2-3<\/td><\/tr> Dose relative to the maximum<\/td> 75\u2013100 %<\/td> 50\u201375 % (to be gradually reduced)<\/td><\/tr> pH of irrigation water (soil)<\/td> 6,2 \u2013 7,0<\/td> 6,2 \u2013 7,0<\/td><\/tr> EC target<\/td> 1.6 \u2013 2.2 mS\/cm<\/td> 1.4 \u2013 2.0 mS\/cm<\/td><\/tr> Frequently asked questions<\/td> PK 13\/14 or equivalent (phosphorus and potassium boost); amino acids to enhance terpene synthesis<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nStage 5: Maturation and final flush (last 1\u20132 weeks)<\/h2>\n\n\n\n
Parameter<\/th> Value<\/th><\/tr><\/thead> Added nutrients<\/td> None. Just water.<\/td><\/tr> water pH<\/td> 6.0 \u2013 6.5 (clay)<\/td><\/tr> Duration<\/td> 7 days in organic soil; 10\u201314 days in mineral soil or coconut fibre<\/td><\/tr> EC of the drainage water at the start<\/td> High (indicates salt build-up)<\/td><\/tr> EC of the drainage water at the end<\/td> Close to the inlet water EC: the substrate is clean<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\nThe most common shortcomings: identifying them and taking action<\/h2>\n\n\n\n
Visual symptom<\/th> Probable deficiency<\/th> Most common cause<\/th> Correction<\/th><\/tr><\/thead> Yellowing of the lower leaves, spreading upwards<\/td> Nitrogen (N)<\/td> pH too low, depleted growing medium, or the natural onset of flowering<\/td> Adjust the pH; during the vegetative stage, increase the nitrogen dose<\/td><\/tr> Brown spots, burnt tips, curled leaves<\/td> Calcium (Ca)<\/td> Soft water, low pH, excess K or Mg<\/td> CalMag; raise the pH to 6.5+<\/td><\/tr> Yellowing between the veins (green veins, yellow leaf)<\/td> Magnesium (Mg)<\/td> Soft water, incorrect pH, excess Ca or K<\/td> Magnesium sulphate (Epsom salt): 1\u20132 g\/L<\/td><\/tr> New leaves that are pale yellow or white<\/td> Iron (Fe)<\/td> High pH (> 7.0), excess manganese<\/td> Lower the pH to 6.0\u20136.5; foliar iron chelate<\/td><\/tr> Scorched leaf margins and tips, with a generalised brown colour<\/td> Toxicity caused by an excess of nutrients (nutrient burn)<\/td> Dosage too high, salt build-up<\/td> Flush with clean water; halve the dose<\/td><\/tr> Leaves with irregular yellow patches, premature leaf drop<\/td> Potassium (K)<\/td> High or low pH, excess calcium<\/td> Adjust pH; fertiliser with a high potassium content<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n \n
\n\n\n\nAutoflowers: specific adjustments for feminised strains<\/h2>\n\n\n\n
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\n\n\n\nSummary: the NPK cycle at a glance<\/h2>\n\n\n\n
Phase<\/th> Approximate NPK<\/th> Main character<\/th> Mistake to avoid<\/th><\/tr><\/thead> Seedling<\/td> 0-0-0<\/td> Nothing. The growing medium does it all.<\/td> Applying fertiliser too early<\/td><\/tr> Vegetation<\/td> 3-1-2<\/td> Nitrogen<\/td> Overdosing from the outset<\/td><\/tr> Early flowering<\/td> 1-3-2<\/td> F\u00f3sforo takes the lead<\/td> Cut the N off in one go<\/td><\/tr> Peak flowering<\/td> 0-3-3<\/td> Phosphorus + Potassium<\/td> Keep the N level high (fluffy buds)<\/td><\/tr> Final flush<\/td> Just water<\/td> correct pH<\/td> Skipping the flush or doing it too soon<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n