Normally I'd make my own potting compost but our precious home-grown compost heap has been added to the garden soil this year so I have no choice but to buy something. He brought back 3 bags of potting compost from the shopping trip and on the bags it said 'enough fertiliser to last for 1 month'. Admittedly it said that in French and he didn't understand it but he then got his face in a knot because I said that I wouldn't use it. I said that I didn't want to grow our seeds and therefore food in artificial chemicals even if it was just to start them off. But he said that nitrogen was nitrogen whether it came from a blue pellet or from decaying plants etc etc. He understands the organic principle but said that I had no idea how the other 'organic' compost was made, didn't know its provenance, that it probably wasn't produced in a sustainable fashion and was therefore no better than the bag that wasn't organic etc etc. He said that I had just been taken in by 'green-wash' without understanding the basics.
So I ask here, because you folk come from such diverse walks of life and have such wonderful knowledge, what is the difference? Is it bad for your seeds to get their initial nutrients from artificial chemicals? Do they know the difference? I get the feeling that its bad because the chemicals are produced in an unsustainable way but is it actually worse for the plant? He's a scientist and I want to be able to stand my ground on this one.
I'm sorry to hear that. I hope everything will be better.
Nitrogen is nitrogen, whether it's from synthetic chemicals or organic sources, but the way that nitrogen is packaged, delivered, and used is another entirely different matter. Synthetic nitrogen fertilizers give your plants a heavy and fast dose of N, possibly cause root/leaf burn, make them excessively leafy, generally more disease and pest prone, kill soil microbes and earthworms, and cause faster breakdown of organic matter/humus. This is what I call a "soda pop" diet for plants. Also, nitrogen fertilizers typically come as NPK and in the form of N "packaging" as ammonia/ammonium which isn't always what a plant may prefer for its N uptake (some like ammonia, some like nitrates, etc.).
Well-prepared compost will have lower N than synthetics obviously, but all that N is available in more diverse and complex forms to supply a wide diversity of plant and soil life needs, in balanced proportion with other critical soil nutrients and trace elements, in chemical and mineral forms that can be absorbed and used more efficiently, resulting in more even, measured, and robust, hardy growth from plants. Good compost should also inoculate and help maintain soil microbe diversity and suppress disease and pests. Much of the N in compost is locked up in the compost microbes and soil life, which will gradually die and release nutrients and also produce certain nutrients for plants. Synthetics usually do the opposite. It is the diversity, complexity, and richness of compost that brings out the same in the soil and plants. Complete nutrition and health for plants goes way, way, way beyond simple NPK.
To more directly answer your questions, yes, your seeds probably will know the difference and respond accordingly. Small amounts of synthetic fertilizer may not affect them or their vitality too much. Too much can burn them or kill them or make them grow too fast and spindly. I would test it straight, in a 50/50 mix of compost + something else, or compost it a bit more and let it "age" before use to bleed off excessive N. A week or two may be sufficient. It's been my experience that store-bought potting soils either have too much fertilizer or not enough N in them because they're only partially composted, and aging and mixing with other materials will solve most of my issues. Seeds usually come with all the nutrients they need to sprout in the seed and do well for a little while. What they really need is a suitable sprouting environment and friendly microbes to make sure they don't die from damping off or rot.