Read the leaves: nutrient deficiency symptoms

Nutrients come in three groups: macronutrients (nitrogen N, phosphorus P, potassium K), secondary nutrients (calcium, magnesium, sulphur) and micronutrients or trace elements (boron, iron, manganese, zinc, copper, molybdenum, chlorine, nickel). Carbon, hydrogen and oxygen come from air and water.

First question: old leaves or new leaves?

Some nutrients move around inside the plant, so a shortage shows first on the oldest, lowest leaves: nitrogen, phosphorus, potassium and magnesium. Others cannot move, so a shortage shows first on the newest growth at the tip: calcium, iron, sulphur, boron, zinc, manganese and copper. That one observation cuts the list in half.

What each one looks like

• Nitrogen: stunted growth and very pale colour, the older leaves yellowing first; in extreme cases they look burnt.
• Phosphorus: a short, dark-green plant; in extreme cases the leaves turn brown or black, with a bronze or purple tint underneath.
• Potassium: small spots at the tips and edges of pale leaves, turning rusty, with the tips folding over.
• Calcium: a dark-green plant but pale, soft young leaves drying from the tips, and growing points that die. In fruit it shows as blossom-end rot in tomato and pepper.
• Magnesium: paleness starting from the leaf edges with no spots, edges that may fold like a cup, and leaves that drop in extreme cases.
• Iron: pale or yellow new leaves with the main veins staying green, and no spots.
• Sulphur: light-green leaves with pale-green veins, and no spots.
• Manganese: pale leaves with dark-green veins forming a net pattern.
• Zinc: pale, narrow, short leaves with dark-green veins and dark spots on the leaves and edges.
• Boron: discolouration, breaking and dropping of the leaf buds.
• Copper: pale pink between the veins, then wilting and dropping.
• Molybdenum: light-green, lemon-yellow or orange leaves, spots across the whole leaf except the veins, and sticky secretions underneath.

The colours are indicative and can vary from plant to plant.

In Qatar it is usually not what you think

In Qatar’s alkaline soil (pH 7.5–8.5) iron, zinc and manganese are present in the ground but locked up where roots cannot take them in. The classic sign is yellow new leaves with green veins in a rich soil. More fertiliser will not fix it. Use chelated iron of the Fe-EDDHA type (the only chelate that holds at high pH) or a micronutrient foliar spray, and keep adding compost, which lowers pH around the roots over time.

Salty water also mimics a deficiency: stunted growth, dark thick leaves and burnt tips (see Irrigating with salty water). Waterlogged roots yellow leaves too. Check watering and salt before blaming nutrients.

Quick organic fixes

• Nitrogen: fish amino acid, compost tea or composted poultry manure.
• Potassium: the banana-peel flowering booster or potassium sulphate. Avoid wood ash, which raises alkalinity further.
• Calcium: agricultural gypsum (calcium without raising pH) and steady watering, which is usually the real cause of blossom-end rot. Powdered egg shell is slow to act.
• Magnesium: Epsom salt, about a teaspoon per litre as a foliar spray, once.
• Phosphorus: bone meal and compost.
• Iron, zinc, manganese: Fe-EDDHA chelated iron, and seaweed extract or a micronutrient foliar spray.