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pH of Nutrient Solution and plant nutrition

The term pH is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter. The pH of a solution can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the concentrations of H+ and OH- are equal, and the solution is said to be neutral.

When growing plants hydroponically, the pH range commonly found in hydroponic nutrient solutions is between 5.5 and 6.2. In soil, the optimum range is from about 6.0 to 6.8. The reason that the pH of the nutrient solution is so important is that it affects nutrient solubility. Solubility is the ability of a nutrient to stay dissolved in a solution (water). Using Figure 1 as an example, the solubility of micronutrients (iron, manganese, zinc, copper, and boron) and phosphorus decrease as pH increases.

For plant roots to be able to absorb nutrients, the nutrients must be dissolved in solution. The process of precipitation (the reverse of dissolving) results in the formation of solids (salts) in the nutrient solution, making nutrients unavailable to plants. Not all precipitation settles to the bottom of the tanks, some precipitates occur as very fine suspension and is invisible to the naked eye.

Plants can tell us their problems through leaf symptoms (e.g. Iron deficiency) when it's too late. Iron is one essential plant nutrient whose solubility is affected by pH which is why it is added in a chelated form (or daily). Iron deficiency symptoms occur readily. At pH values over 7, less than 50% of the Iron is available to plants. At pH 8.0, no Fe is left in solution due to iron hydroxide precipitation (Fe(OH)3 - which eventually converts to rust). As long as the pH is kept below 6.5, over 90% of the Iron is available to plants. This is why higher quality hydroponic nutrients add iron is chelated forms. In Figure 3, you can see that 100% of iron EDDHA is available to plants between a pH of 4 and 10. EDTA and DTPA are also great chelating agents and allow iron to be available to plants at higher pH values. Blue Planet Nutrients uses iron EDDHA, EDTA, and DTPA in our formula.

Nitrogen is the essential inorganic nutrient required in the largest quantity by plants. Most plants are able to absorb either nitrate (NO3-) or ammonium (NH4+) or both. NH4+ as the sole source of nitrogen or in excess is deleterious to the growth of many plant species. Some plants yield better when supplied with a mixture of NH4+ (ammonium) and NO3- (nitrate) compared to NO3- alone. A combination of NH4+ and NO3- can be used to buffer against changes in pH.

Plants grown in nutrient solution containing only NO3- as the sole nitrogen source tend to increase solution pH - hence the need to add acid. But when approximately 10%-20% of the total nitrogen is supplied as NH4+, the nutrient solution pH is stabilized at pH 5.5. NH4+ concentration needs to be monitored as it has been shown recently that micro-organisms growing on plant root surfaces can convert the NH4+ to NO3-. Since hand-held ion-selective electrodes for measuring both NH4+ and NO3- are now available, it should be possible to accurately monitor and maintain a predetermined NO3-/NH4+ ratio throughout the life of the crop. Figure 2 visually illustrates the relationship between NH4+ and NO3- and pH. For proper pH management, Blue Planet Nutrients GROW provides approximately 20% Nitrogen as NH4+ and 80% as NO3- and Blue Planet MICRO provides 10% Nitrogen as NH4+ and 90% as NO3-.

pH also routinely changes throughout the day. Daylight photosynthesis produces hydrogen ions which can cause the nutrient acidity to increase (lowering the pH). At dusk photosynthesis stops and the plants increase their rate of respiration and this coupled with the respiration of micro organisms and the decomposition of organic matter uses up the hydrogen ions so the acidity of the solution tends to decrease (pH rises).

pH Management

Phosphoric acid and Potassium hydroxide and potassium carbonate are used to modify the pH of nutrient solutions and these chemicals are what are typically found in products pH UP and pH Down. Phosphoric acid is used to lower pH. Potassium hydroxide and potassium carbonate are used to raise pH.

Figure 1.

Figure 2. Relationship between NH4+ and NO3-  ratio and pH Figure 3. Chelated iron availability


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