Foliar feeding tomatoes, what a leaf spray can and can't do

September 29, 2025

Foliar feeding sounds like a secret the serious tomato growers keep to themselves, and it's actually a pretty simple idea, which is that a plant can take up a little bit of nutrient through its leaves as well as through its roots, so a diluted liquid feed sprayed onto the foliage gets in quickly.

What a leaf spray can actually do

The leaves only take in a small amount at a time, so a spray is a top up and a quick correction rather than the main meal. The roots still do most of the feeding, and a tomato growing in good soil with compost dug through it and a sensible feed at planting doesn't strictly need a leaf spray at all. Where it earns its place is when a plant is looking hungry and you want to see a response quickly, or when the soil is cold early in the season and the roots aren't taking much up.

The yellow leaves it's good for

The classic case is magnesium. If the older, lower leaves on your tomatoes go yellow between the veins while the veins themselves stay green, that's often a magnesium shortage, and a weak magnesium sulphate solution sprayed onto the leaves is a common way to green them back up. Mix it to the rate on the pack and don't be tempted to make it stronger, because a strong mix scorches leaves.

The problem a spray won't fix

Blossom end rot is the one people most often try to spray their way out of, and it doesn't work. That's the dark, sunken patch on the bottom of the fruit, and it comes from calcium not reaching the fruit, which nearly always comes back to uneven watering rather than a lack of calcium in the soil. A spray on the leaves doesn't get calcium into a fruit that's already forming, so the fix is steady, deep watering and a good layer of mulch to stop the soil swinging between bone dry and soaked.

How to spray without burning the leaves

Spray early in the morning or late in the afternoon, never in the heat of the day, because droplets sitting on leaves in full sun can scorch them and the spray dries before much of it gets in. Wet the tops and the undersides of the leaves until they're just damp, and stop before it's dripping off. Always check the label first, because a lot of liquid feeds are made to be watered onto the soil, and only some list spraying onto leaves as a use. If it's watered onto the soil around the plant, that works perfectly well too.

Tonic or feed

It's worth knowing the difference between the two kinds of bottle on the shelf. A seaweed concentrate is low in the main plant nutrients and works more as a tonic that helps plants cope with stress and transplanting, while a liquid fertiliser carries the actual nutrients that make leaves and fruit. Plenty of people use both, a seaweed tonic when seedlings go in and a liquid feed once the plants are flowering.

Shop related products

Manutec Magnesium Sulphate 1kg. For yellowing between the veins on older leaves, mixed to the rate on the pack.
Seasol seaweed concentrate 600ml. A seaweed tonic for transplanting and stressed plants.
PowerFeed 600ml. A liquid fertiliser for once the plants are growing and flowering.
Neutrog GOGO Juice 2L RTU. A ready to use liquid for the soil around the plant.

Did you know we also keep pump up pressure sprayers and the 1 litre Seasol concentrate in the shop, and neither is on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf.

The rest of the feeding range is in our garden centre, and if your tomatoes are looking sorry for themselves, bring a leaf in and we'll have a look at it with you.