Aquarium plants - Nutrient deficiency

In a planted tank with high levels of growth (usually with the aid of CO2 supplementation and good light intensity), it is likely that the plants will strip the nutrients from the water column faster than they can be "naturally" replaced. Naturally in this case referring to say Nitrogen being being proved by the fish themselves (if present) and other elements replaced by the regular water changes, feeding etc.
In this case it's preferable to supplement the tank with your fertiliser and dosing regime of choice.
Even then you may observe your plants displaying the characteristics of nutrient deficiency and the better able we are to spot the tell tale signs, the better able we are to correct the problem......

How to spot the signs

The first step is do determine if the deficiency is showing in the established growth or the new. This aids us in determining the cause.
It's possible for a plant to reuse some nutrients from old growth to sustain the new if they are in short supply. These nutrients are classed as "mobile". Other nutrients are classed as "immobile", in this case the plant isn't able to transport nutrients from old to new, so the signs will show in the new growth.
Mobile nutrients include: Nitrogen (N), Phosphorous (P), Potassium (K), Magnesium (Mg) and Zinc (Zn).
Immobile nutrients include: Boron (B), Calcium (Ca), Copper (Cu), Iron (Fe), Manganese (Mn) and Sulphur (S).

Visually determining nutrient deficiencies is by no means an exact science. Some signs can be indicative of several nutrients deficiencies and a lack of a particular nutrient can prevent a plant from effectively using another one!
In cases of extreme deficiency the signs may not match the signs of a plant with slight deficiency.

In conclusion - What to look for can (and should) only be used as a guide.

• Nitrogen (N)
  
  • Old leaves show first
    
    • Leaves turn yellow/green with older leaves appearing more yellow.
      Leaves rarely die completely in the aquarium
• Cure - Increase Nitrate to 10-20 ppm - Dose Potassium Nitrate (KNO3)

• Phosphorous (P)
  
  • Old leaves show first
    
    • Plant stops growing.
      It may become darker green
      In some cases a build of a pigment called anthocyanin may occure and turn the plant purple
      
• Cure - Increase Phosphate to 0.5-2.0 ppm - Dose Mono Potassium Phosphate (KH2PO4)

• Potassium (K)
  
  • Old leaves show first
    
    • Pinholes appear in leaves, increasing in size with time while the leaf  often remains green.
      In some cases older leaves may turn yellow, but the veins will not stay green as with Magnesium deficiency.
      New leaves may grow with withered or 'burnt' edges.
      
• Cure - Increase Potassium to 10-20 ppm - Dose Potassium Nitrate (KNO3)

• Iron (Fe)
  
  • New leaves show first
    
    • Leaves start pale or yellow from new.
      In severe cases new leaves grow with no chlorophyll and soon die
      
• Cure - Increase available Iron to 0.1 ppm - Dose Ferrous Sulphate ( Fe(II)SO4 )

• Calcium (Ca)
  
  • New leaves show first
    Mild deficiency
    • Smaller Distorted new leaf growth
      Reduced leaf tissue with prominent central vein.
      Leaves often grow cupped rather than flat
      
    Moderate deficiency
    
    • Leaves bend and twist randomly
      Reduced leaf size
      Short, twisted roots. Tips may die off
      White streaks or white edges to new growth
    Severe deficiency
    
    • New growth almost completely white
      Leaves appear as deformed stumps
      Growing points at both the shoots and root die off
• Cure - Increase Calcium to 10-30 ppm - Dose Calcium Sulphate (CaSO4)

• Magnesium (Mg)
  
  • Old leaves show first
    
    • Yellowing of older leaves starting at edges. Larger veins remain green.
      Yellow Spots
      Symptons often similar to Iron deficiency as a lack of Magnesium prevents the plant using Iron.
      
• Cure - Increase Magnesium to 2-5 ppm - Dose Magnesium Sulphate (MgSO4)

• Boron (B)
  
  • New leaves show first
    
    • Similar to Calcium deficiency
      New growth distorted and small
      Growth tips of both roots and shoots will die back
      Possibly brittle stems
      
• Cure - Make sure your trace element mix contains boron - Possibly increase trace dose

• Sulphur (S)
  
  • New leaves show first
    
    • Similar to Nitrogen deficiency
• Cure - There are no established guidelines for dosing sulphur. However adequate dosing of elements such as Magnesium, and Potassium via sulphates should prevent a lack of Sulphur
  

• Manganese (Mn)
  
  • New leaves show first
    
    • Dead yellowish tissue between leaf nerves
• Cure - Make sure your trace element mix contains Manganese - Possibly increase trace dose

• Copper (Cu)
  
  • New leaves show first
    
    • Tips of leaves die
      Edges wither
      
• Cure - Make sure your trace element mix contains Copper - Possibly increase trace dose.
  Be aware of the danger to inverts in your tank (shrimps etc) with increased levels of copper.
  (This shouldn't normally be a problem with a good quality trace mix such as Fluidsensor Trace)
  

• Zinc (Zn)
  
  • Old leaves show first
    
    • Yellow tissue between nerves, starts at leaf extremities
      
• Cure - Make sure your trace element mix contains Zinc - Possibly increase trace dose

• Molybdenum (Mo)
  
  • Old leaves show first
    
    • Yellow spots between nerves.
      Brown colour to leaf edges
      Deficiency will inhibit flowering
      
• Cure - Make sure your trace element mix contains Molybdenum - Possibly increase trace dose