I remember some seasons ago commenting on the subject of calcium and tomatoes, and the lack of calcium in liquid tomato food.
What I didn’t know at the time was that calcium precipitates (won’t mix) when added as a liquid with (some) other mineral nutrients. The calcium drops out of the solution and solidifies, often creating a white film over the bottom of watering cans and containers.
Two part feeds
To avoid the problem, undiluted liquid nutrients are often kept in two separate containers A and B as in the case of the Quadgrow Planter.
When you buy a Quadgrow Planter, they provide two containers with a powder in each to add your own water.
One is the main nutrient formula and the other is calcium. These are added separately to the watering can or water tank. Because the strength of the minerals have been diluted, precipitation doesn’t occur – hopefully!
You may wonder why this is relevant?
Well, most liquid type feeds such as Tomorite do not contain calcium – you will need to make sure that whatever medium you are using has calcium in it.
Compost and Limestone
Most good quality potting compost will contain enough calcium in the form of ground limestone, which is used to raise the pH of compost, to offset the low pH of peat – and end up with a pH somewhere between 6.0 and 7.0. The best soil pH for tomatoes is around 6.8.
Calcium also raises the pH of water – hence hard water areas where kettles rattle because of a calcium deposit.
The importance of calcium
Calcium isn’t just important in order to avoid Blossom End Rot, it is used in cell wall formation and is required in many aspects of growth.
Because it is immobile (doesn’t move around a plant’s system as nitrogen does for example), deficiency symptoms show first in new growth. Young leaves can lose their colour and plants become stunted, stiff and woody.
Important tip … If buying peat free compost next season, be aware that limestone, which is normally added to compost that contains peat, to counteract the low pH of peat (see above), may not have been added. It is always worth adding calcium as a foliar spray through the season – just in case!
Soil, Compost and Peat
There is often a misunderstanding when it comes to compost, peat and soil in its true sense.
- Compost is not true soil
- Peat is not true soil
- Clay, sand and silt is true soil – also called loam soil
Compost usually contains decomposed green and organic material plus peat, lime and nutrients. Peat is decomposed matter too.
True soil contains clay, sand, silt and humus plus a few extras and is usually called “loam soil” or mineral based soil.
What is Humus?
Humus is made by the action of soil organisms – worms for example – from organic matter.
Humus and Clay – The Bees Knees and The Real Thing!
Humus particles attach themselves to clay particles and become a storehouse for nutrients as well as holding good amounts of moisture. John Innes is a real soil mix and contains humus and clay, as opposed to a soilless mix which is normally how you might describe potting compost that is largely composed of peat without humus and clay.
We often see bags of compost “with added John Innes” on the front.
This provides the benefit of both organic composted material with humus and clay, which in the right amounts, will provide the best of both mixes.
Why is humus/clay so important for plants – especially hungry and thirsty tomato plants?
There is a term called “cation exchange capacity” – a term that could make anyone run for the hills!
It is simply an exchange of nutrients – roots exchange the hydrogen ions in their roots for other nutrient ions held in soil particles. That’s how roots are able to absorb calcium, magnesium and potassium for example.
The fact is, real soil – loam, containing humus and clay – can hold more nutrients and moisture than basic potting compost.
The benefits of this ability, to store nutrients and moisture better than ordinary potting compost, is that plants are less likely to suffer from blossom end rot and fruit split, to name just two problems, if grown in loam soil.
Humus and wormeries
Worm humus is one of the best additions we can add to any mix and the reason why wormeries are becoming so popular. Worm humus added to a clay based mix, like John Innes, will be more effective than when added to a soilless mix because of the cation exchange capacity of humus/clay particles.
The end in sight already!
I can’t believe the season is in its final stages – during the spring and early summer we could have thrown a seed out of the kitchen window and be picking tomatoes in July. The past few weeks have been a bit more challenging, but if you are growing tomatoes in the UK, you are used to a challenge!
This article “Calcium and Tomatoes” is taken from one of Nick’s Newsletters.