Most of the water in the south and south-east of the country contains substantial amounts of calcium, giving it its characteristic "hardness". These calcium levels can result in several problems, all of which can be alleviated by acidifying the water.
Although pH 7 is considered "neutral" (neither acidic nor alkaline), it is not the optimum crop pH for irrigation waters on outdoor and container stock.
The recommended range of irrigation water pH and substrate solution pH for production depends on the crop being grown. In general, the pH should range from 5.2 to 6.8 for irrigation water and from 5.4 to 6.3 for substrate solution. Should the pH and alkalinity be high, then your water may need acid treatment prior to use on crops.
A pH reading is a measurement of the hydrogen ion concentration of a solution (how acidic or alkaline a solution is) and readings range from 0 (most acidic) to 14 (most alkaline). Availability of micronutrients such as iron, manganese, zinc, copper and boron, and future plant growth can be severely reduced by high substrate and irrigation water pH. High pH water can cause salts to precipitate out of fertiliser stock tanks. Water with a high pH can also reduce the efficacy of pesticides. Many products are active longer in low pH solutions.
The calcium is largely in the form of calcium bicarbonate, which precipitates out as the familiar white deposits on foliage and water fittings. Continued use of hard water for irrigation can also lead to an accumulation of calcium in composts. It is a particular problem with long-season protected container crops when irrigation eventually leads to a high pH.
Many species of nursery stock are sensitive to lime and a build-up of the pH in the container compost can be harmful. Mist and fog propagation of nursery stock can cause the development of a white scale on leaf surfaces, reducing photosynthesis. Blocked nozzles as well as stiffened capillary matting are also a result of deposits building up from the use of hard water.
Alkalinity is a measure of water's capacity to neutralise acids.
Dissolved bicarbonates such as calcium bicarbonate, sodium bicarbonate and magnesium bicarbonate, along with carbonates such as calcium carbonate, are the key contributors to alkalinity in irrigation water.
Since bicarbonates and carbonates are the major components of water alkalinity, most laboratories assume that total carbonates (TC = carbonates + bicarbonates) equals alkalinity. In most cases, this is a safe assumption. For most waters in the South East, bicarbonates account for more than 90% of all alkalinity present. Check whether your water analysis shows total carbonates or separate figures for both.
The term "alkalinity" should not be confused with "alkaline", which describes situations where pH levels exceed 7.0. Alkalinity establishes the buffering capacity of water and affects how much acid is required to change the pH.
There are various methods to soften water. The safest but also the most expensive involve ion-exchange resins, which completely remove the calcium and bicarbonate. However, they exchange carbonates for other ions, which if regenerated with salt is a chloride substance that can be antagonistic to some crops and is unsuitable for use on nurseries. For horticulture, the calcium bicarbonate is best neutralised by adding small quantities of concentrated acid to the water supply.
Acidification of water
By adding the correct amount of acid, the pH of water supplies can be reduced to 6.0 - suitable for most crops, except ericaceous, where it can be lower. If you are in a hard-water area and decide to acidify your water, you may need to change the lime content of your growing media to match the new levels of alkalinity. If you lower the pH you may need to raise the lime content.
Concentrated nitric acid (60% w/w) also provides some nitrogen, helping to offset the cost of applying the acid. For each 100ml of concentrated nitric acid added to 1,000 litres of water you are supplying 22mg/litre (parts per million) of nitrogen. From a number of hard-water samples tested, the amount of acid required was found to vary from between 50ml and 200ml of concentrated nitric acid per 1,000 litres of water supply. Groundwater from a borehole tends to have a higher alkalinity than surface waters.
It is essential to accurately assess the amount of acid required for your particular water supply. If a small quantity of acid is added in excess of that required, the water supply will become very acid (see graph). The quantity of acid required is best assessed by the process of titration. Using the graph as an example, a water sample containing 100mg/l of calcium requires 275ml of concentrated nitric acid to bring the pH to 5.9. Note that this graph is an example only and your water sample should be assessed individually.
Nutrients from acids
Apart from citric acid, some acids used for water acidification also supply a plant nutrient in conjunction with supplying H+. The nutrient supplied can be beneficial to plant growth - if not supplied in excess - but it can also react with fertiliser salts in concentrated stock solutions or with pesticides if mixed into spray solutions.
Adjust the fertiliser programme if your water is being acidified. For example, if using phosphoric acid reduce phosphate levels to account for the phosphate being supplied from the acid. If water is very high in alkalinity then it is not feasible to acidify with phosphoric acid because the amount of phosphorous would risk crop damage.
If using nitric acid, account for the additional N supplied from the acid. Using 67% nitric acid to acidify water containing 6.0 milli-equivalents (meqs)/lt of alkalinity would supply 67mg/lt N at each irrigation - a significant quantity of nitrogen. Sulphuric acid treatment for 6.0meq of alkalinity would supply 75mg/lt S, more than sufficient sulphur for plant production, where 20-30mg/lt of S is suggested for most ornamental crops.
Citric acid is ideal as an acidifier for nutrient stock solutions and pesticide solutions because it is much less likely to react with fertiliser salts or pesticides than the other three acids. Although much safer to use, the cost may make it less desirable for large irrigation systems.
Health and safety
Concentrated acids are dangerous chemicals and must always be handled with care. Staff working with them must be adequately trained, have all the necessary personal protection equipment and ideally work in pairs. Respirators and face masks are advisable where fumes can present a hazard.
Acid suppliers provide the safety data sheets for their products and staff should familiarise themselves with the details. Always add acid to water, not water to acid. Acidified water is corrosive and may eat away the metallic components of your irrigation system.