I found this and copied it from another thread. Mogie was the one to write this I take no credit what so ever I just found that it is a very good explanation
If you have any desire to mix your own fertilizers, it is important that you have at least basic understanding of these terms
Electricity is conducted due to the presence of ions (electrically charged) in a solution. The ions get there by introduction of salts via our ferts!
EC (electric conductivity) is a representation of how much potential a solution has to conduct electricity...SO, by testing the ability of a solution to conduct electricity, we can indirectly determine the amount of salts present....thereby knowing if we have the right concentration of ferts.
E.C. is a measure of salinity by measuring its conductivity. You want an E.C. under 2.0.....anything around 4.0 signifies an extreme excess of salt which calls for immediate leeching. There are devices for measuring...honestly, I haven't bought one because I am pretty comfortable with this....but if you experiment may be worth a look!
Despite my natural aversion for ppm measurement, it would be a good idea when mixing ferts as a beginner. Check your nursery or Home Depot. I've F-ed up a plant or two in my day, always because I THOUGHT I was more accurate with eye measurement than I really was!
CF (Conductivity Factor) basically represents the same information but expressed differently. 1 ms/cm is equal to a CF of 10. I don't use this factor and haven't seen many people use it...but just in case, you now have a conversion!
TDS is a measurement, by weight, of the Total Dissolved Solids in a solution and is measured in PPMs (Parts Per Million). Basically, when you hear someone say they introduce nutrients at a rate of 500 PPMs, it means that they have 500 milligrams of solid dissolved in a Liter of water.
You can figure the PPM of your ferts one of two ways.....you can precisely measure the ferts and water you mix together...or you can make a solution and measure the PPM of IT. The shortcut lies in hand-held meters which measure the EC of a solution and then apply mathematical conversion to the EC figure to arrive at PPMs. Keep in mind, even this is an approximate measurement...but plenty accurate for growers!
Some meters actually display the EC AND the PPM readings. Some only express the EC value and this can be tricky because European and American made meters measure at different rates.
An American device that displays an E.C. of 1.0 has 500PPMs.
A European device that displays an E.C. of 1.0 has 640PPMs.
(I believe Australian manufacturers have a different conversion factor also...but not sure.)
So if your American device only displays E.C., use that old algebra and set the two given equations equal to each other. For example, let's say your E.C. reading is 1.2.
Using the known ratio given just above, we'll cross multiply to solve for x to get the unknown ppms.
1.0/500ppm=1.2/x
x=600
Your solution has 600PPMs. (remember, this would be with the American device...a European device would produce a different result!)
If you want to figure out the PPMs yourself, its pretty easy. Each 1 gram of fertilizer per liter of water gives 10PPMs of each given nutrient per percentage point. Sounds clumsy, wish I could state that better....here's an example.
Use 1 gram of SuperPhosphate 0-20-0 in a liter of DISTILLED water. The solution would have 0ppm of N, 200ppm of P, and 0ppm of K. Also keep in mind that tap water already has dissolved solids...most likely anywhere between 200-400 PPMs. Use distilled water when possible...0 PPMs!!
A guideline for NPK strengths
Now, I have seen different parameters for acceptable PPMs. Here is a decent guideline for the N-P-K standards...play around, but make only gradual changes up! Approximate tolerance range of PPMs per nutrient .... most micronutes are tolerated by plants within the same ranges...but the plant just doesn't need nearly that much!
N 200-400
P 200-600
K 200-600
Approximate tolerance range of TOTAL PPMs in soil/medium
(Obviously the plant can tolerate more as it gets larger and has more root area to uptake nutes and leaf area to transpire water.)
Seedling 500-600
Vegetative 800-1000
Flowering 1000-1500
Flushing 500
PPMs for each growth phase
You may desire more N during veg stages for example. The key is NOT to obsess over the exact numbers. Too many experienced growers give advice in exact parameters to appear 'scientific' or something...but there are too many other factors involved in the actual UPTAKE of these nutrients by roots to claim specificity. These are just general parameters that you can tweak under your own conditions. And again, the plant can tolerate more as it gets larger and has more root area to uptake nutes and leaf area to transpire water...start light, gradually increase with each feeding as your lant can handle it.
Keep in mind, my estimates are given for soil mediums which can tolerate higher levels because the soil components will bond with many elemental ions....a hydro system needs to be more precise....I'm not very familiar with those systems. You've read the dislaimer! Also remember that these parameters are based on the assumption of using DISTILLED water. Tap water will add another 200-400 PPMs, so you would have to adjust accordingly...especially with Cl, Ca, Na, and in rare case, S.
SEEDLING (2-3 weeks...look for 4-5 sets of alternating nodes before moving to veg)
N 150
P 100
K 150
Ca 75
Mg 75
S 25-50
Fe 15
Cl 15
Mn 7
Cu 9
Zn 3
Total: 600ppm
VEGETATIVE/PRE-FLOWER (2-4 weeks)
N 300
P 100
K 200
Ca 100
Mg 100
S 50-75
Fe 25
Cl 25
Mn 10
Cu 15
Zn 5
Total: @1000pm
FLOWER (strain dependent-usually 6-8 weeks)
N 300-400
P 400
K 200
Ca 100-150
Mg 100-150
S 100-150
Fe 25+
Cl 25+
Mn 10
Cu 15
Zn 5
Total: @1300-1500ppm
FLUSH/HARVEST (2 weeks)
N 0
P 75
K 75
Ca 50
Mg 50
S 50
Fe 25
Cl 25
Mn 10
Cu 15
Zn 5
Total: @400ppm
I cannot stress enough that these are estimates...conservative ones due to the fact that chemical ferts allow less room for error. In addition, different strains and conditions will result in different ratios....experiment often and use caution always
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