How to Identify and Correct Tomato Nutrient Deficiencies

13 June 2008, written by Jeremy Dore

This week I noticed that two of my tomato plants had yellow-brown leaves at the bottom and certainly didn’t look as perky as I expected given the attention I have been giving them. Admittedly these were two of the many surplus plants I raised from seed and so they ended up outdoors in rather poor soil usually reserved for less fussy plants. But I love tomatoes and can’t bear to see plants die, so out came my reference books to find out the cause.

Just as we need a full range of vitamins from a wide variety of food, so plants need essential minerals to survive and grow well. There are a whole range of these which are usually supplied by rich soil and compost. The main ones are:

Nitrogen (N): this is essential for plant cell growth and chlorophyll – used in leaves and all the green parts of the plant – and is therefore essential for all vegetables. In particular vegetables grown for their leaves need a plentiful supply. Lack of nitrogen results in slow or spindly growth and leaves can yellow, often the older ones first.
Phosphorus (P): essential for healthy roots and also for fruit to ripen. Lack of phosphorus shows up when growth is poor, leaves begin to have a blue/green tint or fall off and fruit and flowers are disappointingly small or late.
Potassium (K): vital for flower and fruit growth. When lacking, fruiting plants are unproductive and older leaves can show signs of ‘scorching’, turning brownish and rolling up inwards and downwards.
Magnesium (Mg): A lack of magnesium shows up as discolouring of the leaves between the veins: from a healthy green to a pale yellow and eventually brown – a sort of mottled appearance called ‘intervein chlorosis’.
Calcium (Ca): A lack of calcium shows up as young leaves curling inwards and lacking colour, and is often a problem in acid soils. ‘Blossom end rot’ in tomatoes is caused by this condition.
Many other nutrients – Zinc, Copper, Manganese, Iron, Sulphur and Boron are also required in small quantities. Several of these can also give rise to intervein chlorosis (see Magnesium above)

It seemed most likely to me that my tomatoes were suffering from magnesium deficiency – it’s a common problem with tomatoes and potatoes, especially on light sandy soils such as mine. Now, the traditional agricultural approach to deficiencies is to identify the missing mineral and spread the corresponding fertiliser to deal with the problem, or an NPK one containing the top three minerals listed above. I’m not keen on taking that approach because it goes against the organic principles I follow when gardening. Plants need a full range of nutrients, preferably from natural sources, and overdosing them on one (even if it’s in response to a deficiency) can often reduce the availability of others. So for organic alternatives the following are good:

Seaweed Liquid Feed: Quite expensive but full of all the required nutrients, particularly potassium which is great as tomato plants mature. You simply dilute a capful in a watering can, best applied to the leaves (a ‘foliar feed’) once a week, where it is better absorbed than being washed into the soil
Comfrey Leaf Tea: Comfrey is a great plant to have – it grows quickly bringing up nutrients from deep down in the soil and its leaves can be cut back, packed into a container with water (and perhaps some urine) to make a foul-smelling liquid that is rich in almost everything that developing plants require. It is then diluted like seaweed feed. Just make sure you place comfrey plants away from your main beds (shade is fine) as it spreads easily and is almost impossible to get rid of.
Leaf Feeds: useful for green crops that are harvested for their leaves as they contain plenty of nitrogen. Borage tea is made in a similar way to comfrey and works well for hungry plants. Urine can also be used, well diluted, or at least added to the compost heap once in a while.
Mulches: - see my recent GrowBlog article about this – well-rotted compost or comfrey leaves make an excellent mulch which gradually releases nutrients to the plant.

Of course, it’s worth mentioning that vegetables shouldn’t be harvested for a few days after a feed has been applied and even if it is organic they should be washed well.

Prevention is better than cure – for most areas of my garden the solution is to mix in plenty of organic compost a little while before planting. But I still rely on foliar feeds such as seaweed and I’m convinced my plants look better for it and produce far more. Those tomato plants should be recovering in no time…