EC & TDS meters work by measuring electrical conductivity, no actual chemical information. Many different salts beyond just nitrates and phosphates will contribute to electrical conductivity. You really need to know a lot more about your water, and your fertilizer, before you can extrapolate an EC into a fertilizer or nutrient concentration. There is background work to do before EC data is meaningful. Once you know what the EC of water straight from the tap is, and the EC of your ''standard'' fertilizer solution is, then you can create a calibration curve so that you can convert an EC measured to an approximation of fertilizer concentration.
More work than I would like to do. But I'm lazy.
I measure the amount of fertilizer I use, by the volume as teaspoons (roughly 5 ml in volume). and I measure this into a fixed volume of water. My set up I fill a 55 gallon barrel with water, mix up the fertilizer. Then water the collection using a pump to push this water through the garden hose.)
This way I control my fertilizer concentration and don't have to use any meters to know my fertilizer concentration.
For my preferred 12-1-2 fertilizer with Ca++, Mg++ and micronutrients I know that
1/4 teaspoon per gallon = 40 ppm as N
1/2 teaspoon per gallon = 79 ppm as N
1 teaspoon per gallon = 158 ppm as N
I choose dose based on season and how rapidly or slowly trees are growing at the time.
Useful link
Fertilizer calculator - this is how I got the ppm as N per teaspoon.
http://firstrays.com/free-informati...ion-of-fertilizers/fertilizer-tds-calculator/
Really useful references put together with Orchids in mind, BUT remember, trees, vegetable crops and orchids need all the same nutrients in roughly the same ratios. Fertilizer information is pretty much applicable to all species of vascular plants.
Orchids are grown in synthetic substrates - often bark chunks
Bonsai are grown in synthetic substrates - usually a pumice based potting mix.
Trust me, orchid nutrition and bonsai nutrition have much in common.
Many articles in the link below, please dive in and read as time permits.
http://firstrays.com/free-information/feeding-and-watering/