Hey GC! Quick question, on estimate how many grams of N, P, K do i need to give my seedlings when i fertilize? I need to know in grams.
Why do you need to know in grams? Ferts you buy usually come with instructions on mixing based on volume (teaspoons, tablespoons, gallons, etc). Why don't you tell us more about the situation you are facing instead of making us guess?
Better question, how do you translate fertilizer formulas (20-20-20) to literally how much of that nutrient is in the mixture, again in grams.
Basically those numbers represent the relative amounts of the three elements. I say basically because there is an adjustment that needs to be made to the P and K values. This wikipedia entry explains it... Fertilizer - Wikipedia, the free encyclopedia (see sections : 'Reporting of NPK' and 'Mass Fraction Conversion to Elemental Values') So basically, each element would be a percentage of the total. So, if you have 10 grams of a 20-20-20 fertilizer, you (in theory) have 2 grams of each element. Again, these aren't exact amounts and this is not an exact example. Just something to point you in the right direction. Good luck.
side note: Your seedlings should not need fertilizer. Most soils are going to prove all the nutrients that you seedlings need for the first 2-3 weeks of their life. People who grow in coco coir (a lifeless/soiless medium - like hydro) often don't add nutrients to their grows until 2 weeks in... which means their seedlings survive in their own power without any problems. Be VERY careful when feeding seedlings - they can burn very easily, and burn is not reversible. Once the nutrients are in your soil - you'll have to flush the soil with 3x the water as the capacity of the pot their in. So -- more than likely -- you don't need to feed them, at all, right now.
Those numbers are the percentage of the substance that is that particular nutrient, so a fert that is 20-10-15 is 20% nitrogen, 10% phosphorous, 15% potassium, and 55% other stuff. If you put for example a tablespoon of the fert into your water then you have 20% of a tablespoon of nitrogen...etc. I suppose you could weigh the amount of fert you feed and break out the percentages accordingly.