Watering & Feeding Tomatoes

Watering and feeding tomato plants can be a tricky business – especially when grown in containers.

Plants require:

  • Access to water 24/7 but not be over-watered
  • Access to nutrients 24/7 without being over-fed

The usual advice is to keep compost or soil just moist and not too wet.
However, because roots need both moisture and air, it is a good idea to have a wet/dry cycle, where after watering, the soil is allowed to almost dry out – but not completely, which allows air back into the root zone.

One way to achieve this cycle is to use a Smart Valve.

  • Plants are grown in large pots stood in a tray.
  • A valve is placed in the tray and attached to a tank or water container.
  • A certain level of water (2cm for example) is allowed into the tray through the valve.
  • Plants absorb all the water before the tray is refilled to the same level.

This means that plants will receive water whenever they need it, but the valves make sure that plants are never stood constantly in water which would cause the soil in the pots to become saturated and the lack of oxygen in the root zone would cause:

  • Root Rot
  • Poor nutrient absorption
  • A lack of oxygen in soil slows growth of plants (respiration grinds to a halt) and they die!

That’s the result when roots are constantly saturated with water.

A great way to improve the potting soil is to add perlite.

Perlite holds both moisture and air, so is ideal for keeping roots healthy and plants growing at a consistent rate.

Feeding – Soil Grown Plants
Plants should only be fed with tomato food when plants are beginning to fruit because the nutrient levels of tomato food are mixed for flowering and fruiting plants.

Before fruiting, it’s normal practice to feed with a grow food or balanced feed up to the time of flowering, then switch to a bloom/tomato feed when flowers begin to set fruit.

Of course, plant food is normally applied at the time of watering and most basic “Tomato Food” manufacturers usually recommend an application of feed, from once a week to every other watering.

The problem is, every time water only is applied to the roots, it washes away the nutrients in the top few inches of soil – where most of the nutrient absorbing roots live!

This is one reason for the success of the Grow Pot and “ring culture”.

Plants are fed in the inner ring and watered in the outer ring. This avoids the problem of pure water removing nutrients from around the fine roots close to the stem base.

Using Grow Pots is a great way to optimize a grow bag. Adding perlite will also improve the soil structure and increase oxygen to the roots.

For watering, two more factors are:

  • Temperature
  • Size of plant and amount of leaves

The warmer the day – the more water a plant needs. That’s just common sense, but also, take into account the amount of leaves there are.

A plant with few leaves will need less water than one that has a lot of leaves – like Tumbling Tom for example.

What happens if I over-water?
Roots need air as well as water, so too much water will prevent roots (and plants) developing to their full potential.

Also, sitting for long periods in cold water is likely to make plants vulnerable to fungal diseases such as tomato blight. Roots become unhealthy without air (oxygen) so it’s good to choose a growing medium that won’t compact.

If you use peat, add perlite to keep the soil loose and easier to re-wet when it becomes dry. A drop or two of washing up liquid can be used as a wetting agent (surfactant) but only use a small amount.

How to water safely
Of course it’s difficult keeping the soil moist because on a hot day it will dry-out quickly. It is better to give a little extra to thoroughly soak the compost, then let it dry-out slowly.

Just before it becomes dry to the touch and the pot or container is much lighter in weight, give it another good watering.

The problem with giving little and often when growing in soil, is that water will always find the quickest escape route. That may leave patches or pockets of dry soil beneath the surface.

Roots in dry soil cannot absorb water or nutrients. This can lead to nutrient deficiency and result in Blossom End Rot which is caused by calcium deficiency. At such times a foliar feed may be necessary.

For feeding soil grown plants, two factors are:

  • Age of Plant
  • State of Compost

It’s very easy to overfeed tomato plants – especially when they are young. The problem with feeding young plants is that their roots are very sensitive and may be damaged if fed a solution of feed that is too strong – such as the same strength that a fruiting adult plant would have.

If you use good compost (already containing food) there is very little need to feed until the plant is beginning to fruit – assuming that it has been potted-on with new compost every four weeks or so. After about four or five weeks in the same pot, the nutrients in the soil will have been used-up, so extra feeding may be necessary.

What happens if I overfeed?
The roots of young plants may be damaged and older plants may show signs of abnormal growth such as growing more leaves and fewer flowers than expected. Leaf curl may also be caused by over feeding.

It is possible to block one nutrient by giving too much of another – so my motto is … less is more and if in doubt, don’t! That’s two motto’s!

How to feed safely.
If you feed young plants, do so immediately after transplanting with just half the recommended dosage on the packet (that’s half strength).

Tomato plants, until they begin to fruit, should be fed with general purpose plant food (a balanced feed) as tomato food is mainly for the fruiting stage.

Water SprayerIt’s very easy to become obsessed with feeding and how much should be given at each stage of a plant’s growth. Generally, I try to avoid feeding until the fruiting stage … then I give them feed at every watering, but usually at a lower strength than recommended on the box – little and often is the secret.

If your tomato plants are in desperate need, foliar feeding is a good way to get the food in fast!

Be careful to check your box instructions because not all plant food is suitable for foliar feeding.

See Also: Feeding Tomato Plants

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