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[Food For Thought] Primary NPK Macronutrients (Part 2 of 3)

[Food For Thought] Primary NPK Macronutrients (Part 2 of 3)

Alex Grady

You're reading part 2 of a 3 part series into plant nutrition. Here you'll find out about Nitrogen, Phosphorus and Potassium (NPK). 

What are N, P & K?

You must've heard of NPK nutruents.

They mostly contain Nitrogen (N), Phosphorus (P) and Potassium (K) - the three macros plants need.

The problem is, it's easy to underfeed or overfeed.

If this happens, don't just tweak your nutrient strength! It's not that simple. You may need to tweak your NPK ratio.

Here's how to spot and treat NPK overfeeding and underfeeding.

Nitrogen (N)

Nitrogen is most important in vegetative growth. Without it, plants can't carry out basic functions.

However, too much of it can affect the sugars and vitamins content in fruits and vegetables, making them taste different.

Key for

  • Formation and use of proteins, carbohydrates, hormones, chlorophyll, vitamins, enzymes & aminos
  • Pushes new leaf growth and
  • Stem growth (helps establish the shape and size of the plant)

Signs of deficiencies

  • Slowed/stunted growth of overall plant

  • Poorly formed roots and weak stems and stalks

  • Older leaves turn pale green/yellow, and some dark spots and burned edges may appear

  • Leaves curl and bend in the worst cases

  • Disease and bugs due to lack of strength

Nitrogen deficiency

Signs of excess

  • Weak, stunted plants

  • Leaves that are thick, brittle and dark green

  • Disease and bugs due to lack of strength

  • Spindly, frail stems

  • Vegetative growth instead of flowering

  • A deficiency of Potassium (K)

  • Leaf burn, causing them to turn yellow or brown

First things first, stop using any foliar feeds. Next, weaken feeds or alter your NPK ratio.

If that doesn't do it, flush out your plant, media and system for a few days.

When you next feed plants, use a weaker nutrient solution than before, and slowly build up your nutrient strength as plants recover.

If you’re actively trying to promote flowering, use a bloom feed.


At the end of the flowering, leaves might start turning yellow. This is normal and only happens because your plants need to use nitrogen reserves for fruit and flower development.

Phosphorus (P)

Phosphorus is key to fruit and flower formation.

Key for

  • Helps plants use and form essential oils, sugars and starches
  • Key to photosynthesis
  • Aids respiration, energy transfer, storage and use of water
  • Boosts root growth
  • Increases the size and numbers of fruits and flowers
  • Improves seed quality
  • Strengthens plant defenses

Signs of deficiencies

Look on older leaves for signs of:

  • Slowed/stunted growth leading to small leaves

  • Small and thin stems that can go purple

  • Dark blue-green foliage (tends to occur in older leaves first)

  • Purpling foliage, starting on the veins (undersides of leaves), and then yellow when on the brink of dying

  • Dead leaf tips - they die before the leaf changes colour

  • Purple/black spots on leaves, later spreading to stems

  • Delayed plant maturity (no/slow flowering) and limited fruit and seed production

Phosphorous Deficiency

Don't just add more phosphorus - the problem could be caused by having too much of another element.

Instead, flush your plants, media and system, and feed plants only water for a few days.

Slowly reintroduce a suitable strength feed - make sure there's enough phosphorus (Mammoth P and Bat Guano are great for this). Remember that adding too much Iron (Fe) or Zinc (Zn) can cause a phosphorus deficiency.

Signs of excess

Phosphorus is notoriously tough to absorb. Good luck feeding plants too much of it!

If, by magic, you've managed to do this, your plant will probably be deficient in Iron (Fe), Magnesium (Mg), Copper (Cu) and Zinc (Zn). A sign of one of these deficiencies is a dead give away.

  • Pale green/yellowing between the veins of higher, new leaves (Fe, Cu, Zn) or older leaves (Mg)

  • Leaf blades that die and curl upwards (deficiency of Mg, Cu, Zn)

  • Leaves that turn white (deficiency of Mg or Fe)

  • Purple stems (deficiency of Mg)

  • Small leaves (deficiency of Zn)

Immediately flush your plant with a pH adjusted solution or water. Feed with water for 2 - 4 days - or until you see signs of recovery. At this point, you can reintriduce your feed. Start off with a weak mix and work your way up.


Plants react to freezing cold conditions and a phosphorus deficiency in the same way! So make sure you're sure of the cause before taking action.

Phosphorus deficiency is most common when your nutrient pH is above 7 or below 5.5

Potassium (K)

Potassium is key to fruit and flower formation.

Key for

  • Activates enzymes and produces proteins and sugars for building cells and tissue
  • Plays a key role in the process of photosynthesis and plant metabolism
  • Strengthens stems and plant structure
  • Aids the control of water and chemicals in plants
  • Improves the flavour and colour of fruit

Signs of deficiencies

A Potassium deficiency can be caused by underfeeding or by having too much Nitrogen (N) and Sodium (Na) in your nutrient solution.

  • Slowed/stunted growth of overall plant

  • Poorly formed roots and weak stems and stalks

  • Older leaves turn pale green/yellow, and some dark spots and burned edges may appear

  • Leaves curl and bend in the worst cases

  • Disease and bugs due to lack of strength


Potassium Deficiency

There’s no quick fix here! Adding more Potassium can very easily cause you to overfeed another element.

First of all, flush your whole system with a pH-adjusted solution or water. Do this for 3 or 4 days. Then start with an appropriate feed (with enough Potassium). Keep a close eye on your pH.

Remember that excess Sodium (Na) can cause a Potassium deficiency, so choose your boosters wisely.

Signs of excess

Too much potassium can cause deficiencies of Calcium (Ca), Magnesium (Mg) and Nitrogen (N). So, to check for excess potassium, look out for one of those deficiencies.

  • Short, restricted plants as a result of reduced growth (deficiency of Ca or N)

  • Pale green/yellow leaves that can turn purple/brown (deficiency of N)

  • Thin, spindly, purple/brown stems (deficiency of N)

  • Decaying roots, flowers and insides of plants (deficiency of Ca)

  • Yellow/bright green discolouring of the edges of leaves (deficiency of Mg)

  • Crispy/Crinkled leaves (deficiency of Ca or Mg)

  • Brown spots on leaves (deficiency of Ca)

Flush for 3 - 4 days or until plants reover. Then add a suitable strength NPK feed, with enough Potassium.

Since excess potassium often causes a calcium and magnesium deficiency, use a good CalMag once plants have recovered.

What Next?

Now you know what N, P & K are all about, don't forget to check out the rest of the articles in the series.

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