Found this in a article this talks of the differant problems with following the instructoins on the back of the nute bottles.the postman.

Canna and other Dutch companies generally recommend even higher values. So the necessity of being aware of this formula is quite apparent. Although one really must ask how the plants should survive long term on values of 2,0 – 2,2 EC, most customers actually use such strong dosages in their grow because they are unaware of this essential information. The result is not only a waste of fertilizer in hydroponic set-ups. Plant care also becomes much more complicated and problematic. Regular flushings with water are necessary at such high EC levels. On the one hand the plants grow very fast (they stretch, get leafy, etc.), and on the other hand they develop complex symptoms of over- and underfertilization from the high salt content in the substrate. In the end massive overdoses of nutrients lead to a high risk of mold in the flowering phase, reduced levels of resistance against pests, lower yields, and possibly pose a health risk from radioactive traces in the cannabis (from phosphate).

Soil

For the cultivation on soil there is a common rule among cultivators to use 50% of the recommended dosages. Growers follow this rule without actually knowing what salt concentration they are feeding their plants with. For soil cultivation an EC meter is the most important tool to find out what the proper dosage is. This is why everyone who values their plants should not save on this rather moderate investment of 50,- Euros!

To demonstrate how important the EC value is we have tested 5 popular types of fertilizer. Only the fertilizer for flowering was selected because during this phase growers fertilize the most (after the motto: more fertilizer = more yield) and during this part of the life cycle plants become increasingly sensitive to excess salts.

Before we look at the results it is important to understand one thing: generally one should never fertilize more than 0,8 EC on soil.
That's why the first important step is to find out what the EC of your tapwater is. In this example we are using a 50-50 mix of osmosis water and normal tapwater to get 0,43 EC. Obviously, if you have a value over 0,8 in your tapwater you have to lower the EC value through filtration. For small gardens it´s sufficient to use a Britt Filter if you do not have the money to buy a household osmosis filter (approx. 150,- Euros). The Britt filter can reduce the EC value by 0,15-0,20. Another alternative would be to buy 5L bottles of mineral water when you fertilize. Good mineral water has approx. 0,25 EC.

Here are the test results on our water with 0,43 EC (measured on Hanna Dist 3 meter).


General Hydroponics
One Part Bloom
(N-P-K: 2-4-7)

Recommended min. dosage (Hydro): 8ml/1L
or 1,5 EC
Recommended dosage on soil: "less" (?!)
Frequency on soil: every second watering

2ml/1L = 1,30 EC
4ml/1L = 2,00 EC
0,5ml/1L= 0,70 EC

The amazing thing about these instructions is the inaccuracy of the amount to fertilize on soil. What does "less" mean? If we were to use the common rule and mix 50% of the recommended dosage we still get an EC of 2,0! That would lead to immediate symptoms of overfertilization. Even at 25% of the dosage we are still in the danger zone. With 0,5ml/L or 6% of the recommended dosage we finally have a safe solution! This example shows how important it is to have an EC meter at hand.

Canna
Terra Flores
(N-P-K: 10-9-19)

Recommended dosage: 5ml/1L
Frequency: 1-3 x weekly

5ml/1L = 1,83 EC
2ml/1L = 1,05 EC
1ml/1L = 0,76 EC