What you need to know about the water you use for watering

Many growers do not realise how crucial the chemical composition of the water they use to water their plants is. Alkalinity, pH and the content of dissolved salts are important factors that determine whether water is suitable for plant cultivation. In this article, we will introduce you to several facts about water that every grower should know in order to achieve optimal results in their garden.

Whether you grow hydroponically or in growing media, water is one of the basic resources your plants need. However, few growers know that the chemical composition of water largely affects how suitable it is for plant cultivation. The main characteristics of water in this context are alkalinity, pH and EC.  

Alkalinity expresses the water’s ability to resist acidification and is affected by the concentration of carbonates (H2₂CO₃) and bicarbonates (HCO). Other compounds also influence alkalinity, for example hydroxides, sulphides, phosphates, silicates or borates, but these are usually present in water only in minimal amounts. The higher the alkalinity of the water, the less its pH is affected by acidic substances, such as water-soluble fertilisers with a high content of ammonium or urea. Conversely, the pH of water with low or zero alkalinity will drop sharply after fertilisers are added, because it does not contain enough buffers to neutralise acids. You can determine the alkalinity of water from a laboratory analysis, and it is usually expressed as the amount of CaCO₃ or Ca(HCO₃)₂.

Alkalinity above 150 ppm CaCO₃ is considered high, and such water will increase the pH of the substrate. If the water’s alkalinity is not too high and is between 150-250 ppm, this can be addressed by using a fertiliser that creates an acidic reaction in the substrate. Such fertilisers contain more than 20% of the total nitrogen in the form of ammonium (NH₄+) or urea (the greater the proportion, the more acidic the reaction). By contrast, fertilisers that contain nitrogen only in the form of nitrates cause a neutral reaction.

• Water with an alkalinity of 60-150 ppm is considered ideal. Combine it with fertilisers that contain 20-30% of the total nitrogen in the form of NH₄+/urea.
• Water with an alkalinity below 60 ppm has low buffering capacity. Use fertilisers that contain most of the nitrogen in the form of nitrates (more than 90%) and only a small amount of ammonium or urea.
• If the water’s alkalinity is extremely high (above 250 ppm), it is necessary to reduce it first by adding acid to approximately 100 ppm and then combine the water with fertilisers with a low content of ammonium and urea.

Water pH expresses the concentration of hydrogen ions (H+) and hydroxide ions (OH-) and is an indicator of the acidity or alkalinity of a solution. We measure pH on a scale from 0 to 14, with values below 7 indicating an acidic solution, above 7 an alkaline solution, and pH 7 being neutral. The ideal pH range of water intended for irrigation is between 5 and 7, because it is close to the optimal pH value for growing most plant species (5.8-6.2). The importance of pH lies in the fact that acidity affects the availability of nutrients to plants, and at extreme values problems with nutrition occur.

• If you grow in organic growing media (soil, coco, peat), regularly monitor the pH of the substrate and keep it within the range of 5.5 to 6.5.
• If you grow hydroponically, monitor the pH of the nutrient solution and keep it consistently within the pH range of 5.6 to 6.4.

You can adjust the pH of water by adding an acid or an alkali. To lower pH, nitric, phosphoric, sulphuric or citric acid is most commonly used, with citric acid being the only organic one and therefore safer to handle. To raise pH, products containing soluble magnesium or calcium are used. When adjusting water pH, be patient, add the acid or alkali gradually, and each time mix the solution thoroughly and measure it again. You can use litmus paper to measure pH, but if you are serious about growing, it is worth investing in a digital pH meter.

EC is an abbreviation for electrical conductivity, which indicates how well a solution conducts electric current. In the context of plant cultivation, EC is used to assess the concentration of dissolved salts (ions) in the water or nutrient solution that plants use. These dissolved salts contain mineral elements and nutrients such as nitrogen, phosphorus, potassium, calcium, magnesium and others. Digital EC meters or combined EC and pH meters are used to measure EC.

The lower the EC of water intended for watering, the better, especially if you use fertilisers containing mineral salts. Adding fertilisers increases the EC of the water, and too high a salt content will cause the solution to burn the plant roots, which is reflected in burnt leaf tips (overfeeding). Rainwater, distilled water or water filtered through a reverse osmosis filter contains no dissolved salts and has an EC of 0 mS/cm. Tap water always contains dissolved salts, and its EC depends on the specific region.

• If the EC of water intended for watering is higher than 1 mS/cm (before adding fertilisers), it is necessary to first filter it through reverse osmosis or mix it with distilled water in such a ratio that the EC value falls below 0.5 mS/cm.
• Water with an EC of 0.5 – 1 mS/cm (before adding fertilisers) can be used for watering plants and in hydroponic systems, but over time salts may accumulate in the substrate, which then needs to be flushed more often with clean water.

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