Nutrient Strength for Tomatoes

This week I would like to look at nutrient strength for tomatoes but first, here is a list of some of the physiological problems, caused by erratic temperatures, inconsistent watering and strong direct sunlight, rather than caused by diseases.

Physiological disorders include:

  • Blossom End Rot – most of us have experienced BER. Tomatoes inedible owing to a dark leathery patch on the underside of the fruit.
  • Blotchy ripening – this happens either because not enough potassium was given or the harsh direct sunlight prevented lycopene from being made under the skin of the fruit.
  • Greenback – shows as green shoulders around top edge of fruit. This can also be caused by the same reasons for blotchy ripening above.
  • Fruit splitting – soil condition from very dry to very wet. Happens to outside plants, often when it rains. Tomatoes in self watering plants usually avoid this problem.
  • Catfacing – underside disfigured caused by poor pollination. Mainly affects larger tomatoes that are formed in segments like an orange.
  • Rough and sometimes thick skin caused by wide temperature swings. As skins expand and contract owing to erratic watering and temperatures, skins may grow more thickly to cope.

It is also the case that over and under feeding can cause some fruit disorders too.

Measuring nutrient strength for tomatoes
The way nutrient strength is measured is with an EC meter – EC stands for “electrical conductivity”. This measures the amount of mineral salts in water after the feed has been added.

In the photo below I’m taking the EC of my water tank that supplies a series of auto pots. The plants are growing in 50/50 soil/perlite medium.

Nutrient Strength for Tomatoes

Nearly 2.3 – that’s about right for the moment!

An EC of between 2.0 to 2.5 is moderate, but the tomato grower may decide to increase or decrease the strength of the feed depending on:

  • The growth stage of plants and fruit.
  • Weather conditions – high or low temperatures and light levels.
  • The tomato variety – often, large tomato varieties do better with a slightly stronger nutrient feed.

If the EC is kept below 1.5 or above 3.0 for a long period of time …
Fruit size may be effected, more tomatoes may suffer from physiological disorders especially blotchy ripening and blossom end rot owing to a lack of potassium and calcium respectively.

So how does this effect the average home-grower with nothing more than a bottle of Tomorite and its instructions to feed his or her toms?

The EC of your tomato food for soil-grown plants
When growing tomatoes in soil (rather than other media), it is standard practice to reduce the nutrient strength to half.
Most brands of tomato food take this into account already, and when Tomorite or similar type feeds are diluted to the recommended dose, they provide an EC of between 1.0 to 1.5 – that’s around half the strength you would give tomatoes growing perlite, coir or rockwool in a commercial greenhouse.

The reason why the strength of Tomorite and liquid type foods is reduced is because there is plenty of food in the soil already – at least in the beginning!

Tomatoes are hungry feeders
We know that tomato plants are hungry feeders, so by the middle of August, the soil is almost depleted of nutrients – especially if growing in containers and grow bags.

It’s at this point, with only a couple of months left of the season, that I increase the recommended strength (or double the frequency) to the level that tomatoes receive when growing in media other than soil (AKA hydroponically).

You are in control
Once you know the strength of your nutrients, you can make adjustments as necessary depending on growth stage and weather conditions.

A higher nutrient strength is often given in cloudy conditions and a lower strength when the weather is hot and bright – when plants need more water.

Feeding more regularly
It is true that feeding little and often is the best way to feed tomato plants. One reason for feeding at every other watering (at least), is because whenever water only is given, nutrients are washed away from the plant’s roots leaving just fresh water – not good for growth if plants are fed just once a week or once a fortnight and watered every day. A good reason to use grow pots in grow bags – ring culture.

However, too much Tomorite, or any other standard type tomato food, will eventually lead to a premature demise of the plant. A high build up of nutrient salts may also cause Blossom End Rot in medium and large varieties.

Tomato taste and nutrients
Too much nitrogen as fruit reaches maturity can produce a watery taste.
Too much potassium is considered to increase the acid level in some varieties and also reduce the magnesium uptake – a good reason to add magnesium (Epsom salts) as a foliar spray.

One of the issues when feeding organically is how much food to give. The problem is, organic feed can’t usually be measured by an EC meter which measures mineral salt levels.

There is a growing trend to use both organic and mineral feeds – the organic supplying the great soil benefits and the mineral (Tomorite) feeds supplying more control over the nutrient strength. Organic feed, especially green teas, such as comfrey and seaweed, don’t have a negative effect if given too generously, so are ideal as a partner with mineral nutrients.

The amazing thing about all of this, is that every tomato variety has its own peculiarities and each one does best with a slightly different nutrient strength. One of the advantages of growing the same variety each season is that we can get used to its peculiarities and achieve the best possible results.

In Conclusion
To sum up, here is my advice:

When plants are fruiting, feed half the recommended dose at every other watering. You won’t go too far wrong with this method.

If the weather is very warm, feed half the recommended dose at every three waterings.

Self Watering Planters
If you are using self watering planters, you could adjust the reservoir so that the pH is around 6.5 and the EC is between 2.0 and 2.5.

If your tap water isn’t high in calcium your pH is probably OK but you will need to give plants extra calcium (see last two weeks newsletters).

If you increase the recommended dose of your liquid feed by 50% to 75% (that’s 1.5 to 1.75 times) the EC for your reservoir will be roughly correct for feeding after the nutrients in the soil have been used up.

The above applies to Tomorite and other similar liquid feeds. Nutri Grow feed that comes with the Quadgrow Planter is already calculated for EC, so stick to their instructions.

That’s about it for this week – I hope your tomatoes taste as good as they look!


  1. brian
    | Reply

    hi Nick
    i use pots which i have in trays so i put water in the tray which i then add tomorite or shoujd i add tomorite in to the pot yours brian

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