Nutrient Strength for Tomatoes Later In The Season

July was a month of high temperatures and wet weather, but now that we’ve entered August, let’s hope the weather settles down a bit!

Many of the problems we now experience as tomato growers are physiological, caused by temperatures and direct sunlight, rather than caused by diseases.

Physiological disorders include:

  • Blossom End Rot
  • Blotchy ripening
  • Greenback – shows as green shoulders around top edge of fruit.
  • Fruit splitting – soil condition from very dry to very wet.
  • Catfacing – underside disfigured caused by poor pollination
  • Rough and sometimes thick skin caused by wide temperature swings.

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

Nutrient Strength for Tomatoes Later In The Season

Measuring nutrient strength
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 foods are diluted, 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 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 soilless media.

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 it is warm and bright – when plants need more water.

It’s true that feeding little and often is the best way to feed tomato plants. One reason for feeding at every watering 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!

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 will most likely cause Blossom End Rot in medium and large varieites.

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 slighltly 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.

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


This article, Nutrient Strength for Tomatoes Later In The Season, is from Nick’s Newsletter





8 Responses

  1. dzikamai

    hi nick, thanks for this article. i had a problem in keeping the size of my tomatoes big.thanks

  2. richard

    patience,Valerie Patience.

  3. John Bowtell

    Hi Nick, My Sweet Aperitif , cocktail size, are very tasty and hanging like bunches of grapes. The Red Alert are filling out nicely both varieties greenhouse grown this year to avoid blight. Thanks for all the tips.

    • Nick

      You are welcome!

  4. Allan Ingram

    Hi Nick.
    I am in Fort William in West Highlands. We have not had much warm weather, it has been wet most of the time. I find I am not watering my plants much and they not getting fed much as the soil is always wet and I don’t want to over water and I feel this is a problem, as all my plants have some green fruit but they are very small so far. It does not appear that our weather is going to improve either. I have 1 red tomato but that one is tiny.
    My of my plant leaves in all my plants developed brown blotches and I removed them. The leaves at the top of each plant are now a healthy green. I used a seaweed spray early season but not in last 6 weeks. I only use tomorite, should I use something else as well.
    So far I don’t have many green toms but I have plenty of flowers, I hope I get a good amount of reds later in season.
    This is my fifth season growing tomatoes and I think I am still a novice and this is so far the worst results I have had in the 5 years.

    • Nick

      Sorry to hear that you are having such a bad season. When the weather is against us and the summer is dull and wet, growing tomatoes outdoors or even in a greenhouse is very challenging.

  5. Valerie

    Good morning Nick.Just to let you know my toms are looking fine after I took your advice.However they are not ripening.Any advice? Or are we just dependent on the weather.
    I find it most frustrating to see a good yield of green tomatoes!

    • Nick

      Hi Valerie, Sometimes they stay green no matter how hard we try to encourage them to ripen!
      Once they start turning colour, they should ripen reasonably quickly.