The pH of soil and water is a measure of the amount of acid or alkaline they contain. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is alkaline.
Nutrient deficiencies can occur if the pH is outside of the range that a particular plant likes – tomato plants are happiest with a pH around 6.8, just on the acid side.
If you grow tomatoes on an allotment or directly in your garden soil, measuring the pH could be very useful and will give you the opportunity to make adjustments for the following season. To change the pH value of soil may take some time for organic additives such as egg shells to break down in the soil.
Lime raises pH and ammonium sulphate lowers soil pH.
If you use grow bags and multi-purpose compost to grow your tomatoes, it may be better to adjust the pH of the water you use than the soil. You will probably only use that soil for tomatoes for one season, unless you have a method of avoiding disease when re-used.
Saving rain water is ideal as the pH is usually slightly acidic and ideal for tomato plants and many other plants too!
Here’s a graph that shows the accessibility of nutrients to plants at various pH levels.
You can see that iron, manganese and zinc are available at low levels when the pH is slightly acidic. This isn’t a problem because tomato plants only need very small amounts of these nutrients.
However the macro nutrients of nitrogen, potassium and phosphorus are highly available – a good thing!
Calcium, magnesium and sulphur are also required at good levels, but notice how little calcium is available when the pH is below 7.0.
Hard water usually has a high pH and contains a lot of calcium. Soft water usually has a low pH and is low in calcium. If you live in a hard water area, you will know by shaking your kettle – if there are bits in it that rattle, you have hard water.
This means that if we live in a soft water area, or mainly use rain water from a water butt, we may need to give calcium as a supplement to our plants.
However, it’s better to give water to tomato plants that is slightly acid (below a pH of 7.0) because they will be able to absorb a wider range and larger amount of nutrients – calcium can be added as a foliar feed.
Tap water also contains chlorine so it is best to fill up the watering can the evening before you use the water – the chlorine will reduce or dissipate when left overnight.