Nutrients

Macro Nutrients

NPK (Nitrogen, Phosphorus, Potassium) typically come from the soil and are crucial throughout the plant's lifespan, although in differing ratios. Vegetating plants require more Nitrogen and less Phosphorus and Potassium, while flowering plants demand much less Nitrogen and much more Phosphorus and Potassium during the flowering stage.

Cannabis also obtains Oxygen, Carbon and Hydrogen from water and air.

Deficiencies

Most deficiencies present as a colour change on the leaves, moving from the centre outwards or from the edge of the leaf moving inwards. Deficiencies can appear on older leaves first or newer leaves first. Mobile nutrient (such as NPK) deficiencies will show on older leaves first as the plant moves nutrients to the younger leaves. Deficiencies will remain in leaves that have already finished growing even after it is remedied in fresher leaves. This means you must be patient in remedying mobile deficiencies

The most common deficiencies that I have come across are Magnesium and Calcium, fortunately, there is a product called CalMag that provides both. This should be added to the soil before planting if possible. Note that some nutrients will change the pH of the water.

Using too many nutrients can create a nutrient burn on the leaves, primarily on the tips

Foliar Spray

Nutrients can be delivered to the plant when dissolved in water via a fine mist. The size of the droplets appears to be the limiting factor to efficacy[citation]. If possible the spray should be applied to the underside of leaves as this is where the most stomata reside. Lights should be turned off before foliar spraying to prevent the light lensing through droplets burning the leaf surface. Foliar spray should not be used to provide emergency nutrients to the plant