Hard or soft water – what does it mean
Andrew Wilson, ICL’s technical manager for Ireland and the UK, looks at the implications of hard or soft irrigation water. While he is often asked about the effect of irrigation water pH on the crop, he explains that when it comes to growing media it’s the bicarbonate level (hardness or softness) in the irrigation water that is the most important factor.
So what is pH?
Technically pH is the negative logarithm of the hydrogen ions concentration. Put more simply, it is a measure of how acidic or basic (alkaline) a solution is - measured on a scale of 0 (most acidic) to 14 (most basic) with 7 being neutral. As it’s a logarithmic scale, a small change has a big effect, moving 1 pH unit , from 6.0 to 5.0 increases the hydrogen ions 10 x, moving 2 pH units to 4.0 increases the hydrogen ions 100 x.
Growers are aware of the need for growing media to have the correct pH, as this directly impacts on nutrient availability and therefore crop growth and vigour. The important thing to grasp is that it is the bicarbonate level in the water – the hardness or softness – that is important to the crop and the growing media.
The pH of the water is important, but this is in relation to the efficacy of plant protection products and some foliar fertilisers.
Hard water supply
So, back to bicarbonates in the water supply. A water analysis carried out by a laboratory will determine the bicarbonate level – this is a measure of how ‘hard’ or ‘soft’ the water is. Most of Ireland has hard mains water – a quick look inside the nursery’s kettle will confirm this - basically, the more ‘furring-up’, the higher the bicarbonate level and the harder the water.
It is important that the irrigation water contains an adequate level of bicarbonate as this stabilises and acts as a buffer, it is only when levels are too high, or too low, that remedial action should be taken.
When there is a hard water supply on the nursery, over a reasonable period of time the bicarbonates in the water supply can react and increase the pH of the growing media making it more alkaline. The extent and rate of change will depend on how hard the water is and the quantity applied to the crop.
As the growing media pH rises some nutrients, such as iron, become locked up and less available to the plant. Iron sensitive crops, such as ericaceous species, will often be the first to show signs of deficiency - resulting in chlorotic and pale leaves.
Some growers harvesting rainwater for irrigation may conversely have a soft water supply. This will generally have low levels of magnesium and calcium as well as a low buffering capacity. Insufficient magnesium availability is likely to produce interveinal chlorosis in older leaves, while low calcium levels will lead to poor development and chlorosis in rapidly growing tips and fruit expansion.
A water analysis will give an indication of the levels of bicarbonates present and so what, if any, action should be considered.
For the vast majority of growers with hard water, several different approaches can be taken. Rainwater is inherently ‘soft’ and can be collected to dilute other sources. Acid injection can be looked at to neutralise the bicarbonates in the hard water prior to use. Alternatively, a liquid feed programme can be used employing acidifying water soluble feeds, such as the Peters Excel and Universol Hard Water ranges.
For the minority with soft water, it may be appropriate to increase the levels of calcium and magnesium available to the crop. This can be achieved with a specifically designed water soluble fertiliser, such as Universol Soft Water or by incorporating Osmocote CalMag into the growing media.