Septembre 2007.
CONTENTS Introduction The Hydroponic Nutrient Solution Nutrient Solution Management (i) Electrical Conductivity (E.C.) How to measure E.C. of the nutrient solution Recommended values of E.C. Tips for accurate E.C. measuring using a portable E. C. meter (ii) ph How to measure ph of the nutrient solution Recommended values of ph Tips for accurate ph measuring How to get the desired E.C and ph values. Adjusting for ph values of hydroponic nutrient solution.
Introduction Hydroponics is the science of growing plants in medium other than soil. The plants grown in hydroponics are fed with a nutrient solution which consists of water into which are dissolved all the essential elements needed for their growth and development. The Hydroponic Nutrient Solution Plants require 16 essential nutrient elements in proper proportions for their optimum growth and development. These essential elements are divided into macro elements (required in relatively larger quantities) and micro elements (required in smaller quantities). The macro elements are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur and the micro elements consist of iron, chlorine, boron, manganese, copper, zinc and molybdenum. In hydroponics, all nutrients are supplied to the plants in the nutrient solution. Nutrient Solution Management Proper management of the nutrient solution is the key to success in hydroponic gardening. The grower can adjust the concentration of the nutrient solution to suit the plant growth stage and at the same time, provide the essential nutrient elements in balanced amounts. The nutrient solution is prepared by dissolving exact amounts of inorganic fertilizers given in the' recommended formulation. This will ensure that the plants are getting the right balance of essential elements. While optimum nutrition is easy to achieve in hydroponics, incorrect management of the nutrient solution may hamper plant growth and lead to poor yield. To ensure that the nutrient solution is providing the
right balance of fertilizer, its E.C. (Electrical Conductivity) and ph (acidity) should be measured and monitored. (i) Electrical Conductivity (E.C.) When inorganic fertilizers are dissolved in water, they dissociate into ions which conduct electricity. This ability of the solution to conduct electricity gives its Electrical Conductivity (E.C.) which indicates the concentration of fertilizers solution. III the nutrient Portable phiec meter Although a measure of electrical conductivity does not give any indication of which specific nutrient ions are pre ent or any information about undissolved nutrients, it gives a useful measure of the total concentration of nutrient elements. E.C. is most commonly expressed in microsiemens per centimetre ( S/cm) or millisiemens per centimetre (ms/cm). How to measure E.C. of the nutrient solution From one dripper, collect a Portable EC meter 2
+ small amount ( 20-30cc) of hydroponic nutrient solution in a small clean container Remove the protective cap of the E.C. Meter and switch on the latter. Immerse the tip of the E.C. meter in the nutrient solution collected, taking care to completely immerse the electrodes in the solution. Allow to stabilize for a few seconds. Read the displayed E.C. value. Remove the E.C. meter from the nutrient solution. Switch off the E.C. meter. Rinse the tip of the E.C. meter with water. Replace the protective cap. Recommended values Recommended values of E.C. for hydroponic nutrient solution for the different crops at different crop stages are as follows: At One week Flowering / End of crop Adjustment for season transplanting after Fruiting cycle (Winter) transplanting to first flower Flowering / Fruiting End of crop cycle Tomato 1.2 1.2-1.5 1.8-2.0 2.0 2.0-2.2 2.2 Cucumber 1.2 1.2-1.5 1.8 1.8 2.0 2.0 Melon* 1.2 1.2-1.5 1.8 2.0-2.8 2.0 2.2-2.8 Sweet 1.2 1.2-1.5 1.2-2.2 2.2-2.4 2.0-2.4 2.4-2.8 Pepper Lettuce 1.2 l.2 1.2 1.2 1.2 1.2 * For melon it is important to adjust the E.C. to a higher level at the end of the crop cycle to increase the brix content of the fruit so that the fruit is sweeter. 3
Note: Adjustment in winter is made with respect to evapotranspiration in plants. Bench type and portable E.C. meter are available commercially for measuring the E.C. of the nutrient solution. Tips for accurate E.C. measuring using a portable E. C. meter Calibrate the E.C. meter regularly using standard solutions available commercially. Check the battery status; low batteries in portable E.C. meters often causes error. Do not immerse the E.C. meter above the immersion level. Always take measurements at the same time each day; this will allow easier interpretation of the result. When not in use, switch off the meter and replace the protective cap as per recommendation of the supplier Store the E. C. meter in a cool and dry place. (ii) ph The ph is a measure of the concentration of hydrogen ions (H+) in a solution. ph ranges from 1 to 14 and a ph of 7 is neutral, below 7 is acid and above 7 is alkaline. Pure water has a ph of 7, i.e it is neutral. ph is an important measure of nutrient solution and media as it strongly affects absorption of certain nutrients or elements (particularly the micronutrients) by the plant. Most plants grow at their best in a nutrient solution having a ph between 5.5 and 6.5. 4
When ph rises above 6.5 some of the micro-nutrients (including iron, manganese, copper, zinc and boron) begin to precipitate out of solution and are no longer available to the plants. At low ph,.. excessive uptake of certain elements can be stimulated and the solubility of elements like calcium and manganese drops sharply. Therefore, careful control of the ph is important in order to keep all the essential elements in solution and to prevent toxicity due to excessive uptake. Portable ph meter The ph of a nutrient solution is measured using a ph meter. How to measure ph of the nutrient solution Collect a small amount of the hydroponic nutrient solution in a small container, made of plastic or glass, from the drippers. Remove the protective cap. Switch on the ph meter. Immerse the tip of the ph meter in the nutrient solution collected, taking care to completely immerse the electrodes in the solution. Allow to stabilize for a few seconds. Read the displayed ph value. Remove the ph meter from the nutrient solution. Switch off the ph meter. Rinse the tip of the ph meter with water. Replace the protective cap. 5
Recommended values Recommended values of ph for hydroponic nutrient solution for the different crops are as follows: Tomato 5.5-6.5 Cucumber 5.8-6.0 Sweet Pepper 5.5-6.5 Lettuce 6.0-6.5 Tips for accurate ph measuring Calibrate the ph meter regularly using standard solutions. Check the battery status; low batteries in portable ph meters are often a cause of error. Crystals may appear around the cap. This condition is normal. The crystals will dissolve when rinsed with water. After use, rinse the electrode with water to minimize contamination. Always ensure the ph probe is immersed in a few drops of storage solution or ph 7 solution in the protective cap. Always replace the protective cap after use. Store the meter in a cool dry place. How to get the desired E.C and ph values. Concentrated hydroponic nutrient solution (or stock solution) can be obtained as follows: 1. Already prepared mixtures of nutrient solution A and B separately obtained from Chemco Ltd. 6
2. Individual fertilizer salts which are mixed in different proportions to prepare the nutrient solution A and B using the CIRAD formulation. Once the concentrated hydroponic nutrient solutions A and B have been prepared, these have to be diluted before being fed to the plants. This is achieved by adjusting the pump injector to the recommended level which is 0.6% for the CIRAD formulation and 1.0% for the Chemco solution. At these settings of the injector pump, the nutrient solution at the dripper will have an E.C. of around 1.8-2.0 ms/cm and a ph around 5.8-6.2. If a lower E.C. value is required, the injector pump is calibrated at a lower value while a higher E.C. value is obtained with the injector pump set at higher values. As for the ph, if the nutrient solution is too acidic it will give a low ph and then it is recommended to adjust the pump injector of the concentrated solution B (which is more acidic) to a lower percentage. Plate 4: Injection pumps Plate 5: Adjusting injection pump at 0.6% 7
Adjusting for ph values of hydroponic nutrient solution. Please note that: (i) The ph of the hydroponic nutrient solution can be adjusted using either an acid (phosphoric acid) or an alkali (sodium hydroxide) only if the ph of the water being used to prepare the nutrient solution is found to be alkaline or acidic respectively. (ii) The ph of the diluted nutrient solution may also not fall in the recommended range due to irregular functioning of the fertilizer injector pumps or they have not been properly calibrated. It is recommended to ensure that the fertilizer injector pumps are in good working conditions. 8
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