ABSTRACT
Specific crops have different optimum levels of each nutrient� These levels are measured in milligrams per liter (mg/L) or parts per million (ppm)� One part per million is one part of one substance in a million parts of another� Water is the solvent in hydroponics as it is in soil for the elements (solutes)� An optimum formulation depends on a number of factors�
Plant: Different plants like different levels of the essential elements� Stage of Plant Growth: When plants are young developing seedlings, they need
lower levels of macroelements� As they mature and start forming fruit some elements, such as potassium, calcium, and iron will be in more demand by the plant� When plants are growing, initially we add more phosphorous to promote root growth� As fruiting crops of tomatoes, peppers, and eggplants develop flowers and fruit, we can help the plant shift into a more generative flowering-fruiting phase from its initial rapid, leafy growth of a vegetative state� This can be done by the overall concentration of the nutrient solution, frequency and duration of irrigation cycles, and temperature control� With hydroponics we can observe these characteristics and assist the plant to be more productive by altering these factors, especially when growing indoors or in greenhouses�
Weather: Light levels and day length have a great effect on plant growth� With indoor and greenhouse gardening, we can supplement with artificial lighting, although that still is not as efficient as the natural sunlight� During short days of lower light, we can slow plant growth by adjusting the irrigation cycles, temperatures, and nutrient formulation�
Total dissolved solutes instruments measure the solutes in water by electrical conductivity (EC) (Figure 9�1)� When solutes are dissolved in water, the solution will conduct electricity� The quantity and nature of solutes determines its EC� This is expressed as millimhos per unit volume (mMho)� By monitoring the EC of a nutrient solution, one can determine when changes occur and know when to add elements or change the nutrient solution� Nutrient solutions having adequate essential elements possess an EC range between 1�5 and 2�5 mMho or slightly higher�
Plants take up much more water than nutrients, and at a greater rate� The volume of solution should be maintained relatively constant� That can be done by the use of an automatic float valve�
The total concentration of the nutrient solution elements should be between 1000 and 1500 ppm to facilitate uptake by the plant roots� Conductivity readings of these concentrations would correspond to 1�5 and 3�5 mMhos� Cucumbers prefer lower values (1�5-2�0 mMho), whereas tomatoes do better at higher values (2�5-3�5 mMho)�
Overall, plants harvested for their leaves (lettuce, herbs) prefer high nitrogen levels because it promotes vegetative growth� On the other hand, fruit-bearing crops should have lower N and higher P, K, and Ca levels�
You may find many nutrient formulations online or in books, such as my book Hydroponic Food Production� Formulae for different crops are available from these sources� The following is a general formulation of macronutrients plus iron for a 20-U�S�-gal tank� Because weights are small, use grams instead of ounces or pounds� You will need a gram scale that can weigh accurately within 0�1 g�
• Calcium (Ca): 180 ppm Nitrogen (N): 140 ppm • Phosphorus (P): 50 ppm Potassium (K): 352 ppm • Magnesium (Mg): 50 ppm Sulfur (S): 168 ppm • Iron (Fe): 5 ppm
• Calcium Nitrate: 62 g Potassium Nitrate: 8 g • Potassium Sulfate: 46 g Magnesium Sulfate: 38 g • Monopotassium Phosphate: 17 g Iron Chelate (10% Fe): 4 g
If you wish to use a larger or smaller volume, just use a ratio as a factor to multiply the weight� For example, if you want to make up only 10-U�S� gallons, multiply each weight of compound by the factor: 10/20 = 0�5� So, for calcium nitrate it would be 0�5 × 62 g = 31 g�
With the micronutrients because their weight is very small, you can make up a concentrated “stock solution” and then use a small volume of it to add to the 20-gallon tank� Use a 5-or 10-gallon water tank to store the micronutrient stock solution� Keep it in the dark to prevent algae growth� In this case, make up a 300 times normal strength stock solution as outlined for a 10-gallon volume�
• Manganese (Mn): 0�8 ppm Copper (Cu): 0�07 ppm • Zinc (Zn): 0�2 ppm Boron (B): 0�3 ppm • Molybdenum (Mo): 0�03 ppm
• Manganese Sulfate: 41 g Copper Sulfate: 3�2 g • Zinc Sulfate: 11 g Boric Acid: 20�5 g • Ammonium Molybdate: 0�6 g
Now add a portion of this stock solution to the nutrient solution� Once again for using a 20-U�S�-gal tank add 20 × (1/300) = 0�066 U�S� gallons of the micronutrient stock solution to a 20-U�S�-gal tank� The factor 1/300 is to dilute the concentrate from 300 times back to normal one-time strength� Once again it is better to measure this small volume using milliliters (1 mL = 1/1000 L)� The conversion to liters from U�S� gallons is 3�785 L per gallon� The conversion is 0�066 × 3�785 = 0�250 L or 250 mL� It is best to measure this volume with a 100-mL graduated cylinder� Scales and graduated cylinders should be available at a hydroponics shop or if not go online to a science laboratory supply distributor�
Now test the pH of the nutrient solution in the stock tank and adjust it either up or down using an acid or base as described in Chapter 7� Of course, the easiest method is to purchase a “pH Up” or “pH Down” solution from a hydroponic store (Figure 9�2)� Add a small volume of the pH adjuster solution slowly while stirring to get good mixing� Check the pH with an indicator paper or pH meter as mentioned also in Chapter 7� Do not be afraid of exceeding the desired pH value as you can always adjust it in the opposite direction using the opposite solution from which you were using� If that happens, you are adding too much at any time between checking it�
Making up your own nutrient formulation is more involved than just purchasing a ready-made concentrate solution, but it is more fun� With commercial stock solutions, simply follow directions� They often tell you to use a number of teaspoons of their concentrate to each gallon of the tank solution� To be more accurate, use a small graduated cylinder� Have fun, this goes back to your school chemistry classes!
