Monitoring a Hydroponic / Aeroponic Reservoir
Young Plants
Young plants tend to use more water than nutrient. Therefore, for the first 2 to 4 weeks, adding plain water to a reservoir is probably all that is needed, because solution will become saline (i.e. 1,800PPM) when a plant takes in water without much nutrient. Until plants start to use a decent amount of nutrient, it is not necessary to do complete reservoir changes because there are nutrients that have not been used by the plants. In most cases, 600 to 1,000PPM is adequate for seedlings and vegetative growth.
Aging Plants
It doesn’t hurt to make a solution on the weak side (i.e. 1,000PPM) until plants start to use equal parts nutrient and water, because PPM will rise as water is used by the plants.
When plants use equal parts nutrient and water, adding 1,000 to 1,500PPM concentrated solution is recommended.
When plants use more nutrient than water, adding a concentrated solution about 1,500PPM is recommended, unless a stronger concentration is needed to keep PPM at optimal levels. One should try to keep the PPM near 1,500PPM in the reservoir when liquid is added. This way, the solution will stay within the 1,000 to 1,500PPM boundaries, even as PPM slowly drops as plants use more nutrient than water.
When a reservoir needs changing (i.e. every 1 to 2 weeks), it is a good idea to allow the solution to run low. For example, if a full reservoir is at 1,500PPM, it is possible to allow water and nutrient to lower to a level such as 800PPM. This will give a little flush since the solution is a little on the weak side. And now there is only a little liquid to pump out of the reservoir before a new solution is added, thus minimizing maintenance.
A major factor worth noting is that larger reservoirs will have less fluctuations in PPM and pH and will keep maintenance down.
Two gallons of solution per plant in a top-feeding system is a good amount to put in a reservoir. This size compensates only for minor daily fluctuations in PPM and pH.
Plants that are close to intense light will use nutrient up more quickly than plants that receive less intense light.
When plants do not get the correct doses of food, nutrient deficiencies occur. When a deficiency occurs, plants normally change color from green to green-yellow to yellow. Deficiencies are often a sign that the reservoir needs a change, or specific elements need to be added to the reservoir.
When a deficiency occurs, it is recommended to give the plants the food they crave (i.e. nitrogen or calcium). Nitrogen is the most common deficiency.
Deficiencies should change within a day or two after the proper fertilizer is applied, and plants should go back to a healthy green, unless the deficiency caused serious damage.
How to Use and Clean a TDS Meter
A. The TDS meter electrodes should be rinsed with clean or distilled water, or isopropyl alcohol and water. A Q-tip helps scrub the electrodes free of debris.
B. Now the meter should be submersed in a calibrating solution (ie. 1,000PPM). A small container or the protective cap around the electrodes serves as a place to hold the calibrating solution.
C. The dial should be turned until the reading shows that of the calibrating solution (i.e. 1,000PPM).
D. The electrodes should then be cleaned again with clean or distilled water.
E. The meter should be dipped in the reservoir after the fertilizer is added.
F. Fertilizer (chemical fertilizers) or water should be added to adjust the reading between 1,000 to 1,500PPM.
If the PPM is higher than wanted, adding water can dilute the solution to the desired PPM range. The meter is actually useless in determining proper amounts of an organic or chemical-organic solution, but it is a good reference meter. There is a section in this chapter showing organic formulations and chemical formulations, and a section showing how to get the desired PPM of a specific element (i.e. nitrogen) in a fertilizer on pages 88 to 90.
Hopefully the water source is near 0PPM so that water is not stocked with unwanted PPM that can put limits on the amount of fertilizer added to a solution. There are many relatively inexpensive machines such as reverse osmosis machines and distillation devices that remove unwanted dissolved solids from a water supply.
Note: The PPM readings should only be used as a reference as to when to change solutions because they do not read the actual parts per million of a solution. Calculating parts per million of a particular fertilizer or element is best done with a little math and chemistry calculation as explained on pages 88 to 90.
Most meters are priced under $100. These measure PPM on a scale of 100 (i.e. 100, 500, 1000, 1100). For most people, these meters do the deed. However, there are expensive meters that measure a wide variety of elements in a solution. These tools are for an experienced hydroponic farmer.
Determining PPM without a Meter
Step 1
The percentage of the elements in a fertilizer (i.e. 20-20-20) is needed in order to determine the PPM.
The fertilizer packs are listed as NPK. N is all nitrogen, but phosphorous is listed as a compound (P2O5), and potassium is listed as (K2O).
Phosphorous (P) is 44% of phosphoric acid (P2O5), potassium (K) is 83% of potash (K2O).
To get the PPM from a 15-30-15 fertilizer, the first step is to take all three numbers and move the decimal one decimal place over to the right. In the case of nitrogen, the number would be 200. This number will give the parts per million of nitrogen when 1 gram is added to each quart or liter.
For phosphorous, a grower should multiply the 300 by .44. For example, 300 x .44 = 132PPM.This number will give the parts per million of phosphorous when 1 gram is added to each quart or liter.
For potassium, a grower should multiply the 150 by .83. For example, 150 x .83 = 124.5PPM. This number will give the parts per million of potassium when 1 gram is added to each quart or liter.
More Advanced Note: Some growers make their own plant food with 5 to 7 basic salts, as described on pages 95 to 96.
How to get the percentage of an element (i.e. K = potassium) in a compound (i.e. K2SO4).
Here is how to get desired parts per million of sulphur (S) and potassium (K) in potassium sulphate (K2SO4).
A. The periodic table of elements should be referenced in order to get the atomic numbers of each atom. For example, potassium has an atomic number of 19, sulphur has an atomic number of 16, and oxygen has an atomic number of 8.
B. Now, to determine the percentage of each element, all the elements must have their atomic numbers multiplied by the number of ions in a compound. In the case of K2SO4, the atomic number of potassium, which is 19, is multiplied by 2 to give 38, because there are 2 potassium ions. Since there is only one sulphur ion, 16 is multiplied by 1 to give 16. Oxygen has 4 ions in the compound, therefore 8 is multiplied by 4 to give 32.
C. Now all of the atomic numbers are multiplied by the number of ions, then all of the atoms multiplied by their atomic weights are added up. For example, the total number in potassium sulphate is (2 x 19) +16 + (4x 8) = 86.
D. To get the percentage of each element, the amount of ions is multiplied by the element’s atomic number. For example, in the case of potassium, 2 x 19 = 38.
E. The amount of ions multiplied by the element’s atomic number is divided by the sum of all the elements multiplied by their atomic numbers. In the case of potassium, the 38 (number of ions x atomic number) is divided by 86 = .44.
F. The number multiplied by 100 gives the percentage. For potassium, .44 x 100 = 44%.
Step 2
Finally, the percentage number should have the decimal place moved over one place to the right. In the case of potassium, the number would be 440. This number will give the parts per million of an element when 1 gram is added to each quart or liter. In the case of potassium , 1 gram of potassium sulphate in a quart of liquid will give 440PPM of potassium. Using half a gram per quart will give 220PPM of potassium and 95PPM of Sulphur.
Available PPM
The level of solubility (and purity) in water will make the final say. For example, some solutions and powders will completely dissolve into usable ions, while others will not be soluble in water, hence the elements will not be readily available to plants. For example, gypsum (CaSO4) is not very soluble in water, which makes it almost useless for hydroponic usage. However, gypsum does break down slowly in soil where it works fine. All formulas in this chapter are nearly 100% soluble in water.
How to pH a Solution
PH is the measure of the hydrogen ion concentration in a solution or other medium. There are more hydrogen ions in an acid solution than in a basic solution. On a scale, a pH of 7.0 is neutral, under 7.0 is acidic, and over 7.0 is alkaline. A plant’s intake of certain elements is greatly affected by pH. A pH of 5.5 to 6.5 is the standard for this organic hydroponic technique. A pH of 6.0 to 6.5 works well for vegetative growth and early flowering, while a pH of 5.5 to 6.3 works well during flowering.
A. The pH of plain water should be checked before adding the fertilizers. That pH number should be written down where it can easily be found. If the water pH is the same in the future, it is easier to make a quick formula using the same fertilizers without having to measure.
B. All of the fertilizers can be added and mixed well. The quantities should be written down for future reference.
C. A clean pH pen should be calibrated at 7.0, which is the pH reading of the calibrating solution.
D. The pen should be dipped into the solution and pH up or pH down should be added until the reading is in the preferred 5.5 to 6.5 range.
Examples of organic pH up are baking soda (sodium bicarbonate), Earth Juice® Natural Up and wood ashes. There are endless pH up solutions available anywhere garden supplies are available. Baking soda should be used carefully, because too much sodium is not wanted. Fortunately, sodium can be flushed out with plain water. The plants can absorb improper amounts of sodium if the potassium levels are not sufficient. Using feeding combinations that don’t rock the pH level means little or no pH up is needed.
An example of organic pH down is the addition of Earth Juice® Natural Down, and Greenfire® Earth Juice Grow. White flour and vinegar have been reported to work fine. There are many brands of pH down available.
Writing down the quantity of pH up or pH down that is added (for future reference) is a good method for putting together an identical solution in the future.
E. The electrodes should be rinsed in clean water before the meter is turned off.
pH Drift
The pH should be checked daily and adjusted if necessary because many fertilized solutions will drift significantly upward or downward in pH in less than 24 hours. Organic fertilizers tend to drift upward in pH after mixed, and may continue to do so a day or up to a few days after the solution is mixed. Adding molasses and avoiding certain fertilizers can keep the upward pH drift in a solution (organic or chemical-organic) to a minimum.
For fertilizer solutions with organic nutrients, pH drift is most common after the solution is mixed, and when certain nutrients in the reservoir run low.
Making an organic (or chemical-organic) solution a day or two in advance, with molasses (1.5ml per gallon of water) is a good starting point.
How to Use and Clean the pH Pen
If a pH pen is not cleaned after each day it is used, it can be hard to get accurate readings and it may not calibrate to the correct reading, especially if organic fertilizers are used. Using a cleaning solution before it is put away helps calibrate the pen accurately. The pen should be allowed to stay moist when it is put away. A few drops of calibrating solution in the bottom cap helps the electrodes stay moist. Cleaning the pH pen with clean tap water and a Q-tip works too, and saves money. When a Q-tip is used, it is recommended to gently pull the fluff away from the stick so that the soft cotton-batting can be moved between tough spaces. Care should always be taken with the glass, because it can break and start to give weird readings without a grower noticing the error.
After the pen is rinsed, pH buffer 7.0 solution is used to calibrate the pen. The meter should read 7.0. It may take several seconds to reach a stable reading. The pen can read 7.2 for a few seconds, and then it can slowly move down before it reads a constant 7.0.
A cheap bottle of pH 4.0 should be used once in a while to determine the condition of the pH meter in order to see that the pen calibrates at two different numbers, 4.0 and 7.0.
After the pH pen is calibrated, it should be rinsed well with clean water before taking a reading. The pen should be rinsed well with clean water after each reading. If the water is good, clean running cold tap water, the pen will often stay at one number when it is rinsed.
If the pH pen is really clean and calibrated properly, it should stay calibrated for several readings.
When the calibration does read differently it is probably because the pen needs a cleaning. If the meter is not clean and it is calibrated, all readings can be inaccurate. Weak batteries can also throw off the readings and make the pH pen function at a slower speed.
