Sean:

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: (my favourite) 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: Comfry 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: 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…