First of all, simply because the substances used in synthetic fertilizers aren't AS harmful as pesticides does not mean they are not harmful substances. I realize the flaw in this argument- organic doesn't mean harmless. Chicken poo is a great organic fertilizer, but even it could make a person very, very ill if you were to accidentally ingest it. But the type of harm caused by synthetic chemicals is different than that caused by organic substances.
Secondly, most synthetic fertilizers are at least made partly of petroleum byproducts. No thank you. I'm glad they're using as much as they can, but I still don't want petroleum byproducts feeding my food.
Third, and I think the biggest argument agains synthetic fertilizers, is that they do nothing for the soil long term, and can in fact throw the entire ecosystem that is your soil completely out of balance. Have you ever read the label on a bag of Miracle Grow (I'm talking about the liquifeed stuff for garden vegetables). You have to apply that stuff every two weeks to reap the full benefits. That's because the nutrients are water soluble, so two weeks is all the longer they last- your plants use what they can absorb from the soil, and the rest washes away. Organic fertilizers last longer in the soil because they are not completely water soluble until broken down, and they break down slowly. And the best organic fertilizers work with the soil by adding organic matter as well and nutrients, which attracts worms, which make MORE fertilizer... and so on.
But what about the cost- aren't conventional fertilizers a lot cheaper? I don't really think so, but it depends on a few factors. 1) how much do you have to buy? Well, if you're starting a new garden RIGHT NOW and haven't had time to properly prepare, then quite a bit (depending on the condition of your soil and your gardening expectations- I didn't add much last year and had a totally respectable harvest). But, if you plan ahead, you can virtually fertilize your garden for free/cheap, depending on your resources. If you have much of a yard, trees, or can get either grass clippings or leaves from elsewhere, you have a great source of compost. Add your kitchen scraps (produce, egg shells, coffee grounds, etc) and you're on your way. If you have chickens or goats, even better (but if you have chickens or goats, you probably already know that!). If you live in or near a rural area you could also reach out to farmers in your area via a community bulletin board or craigslist for a source of manure. There was even a great article in Mother Earth News (link forthcoming) about FREE fertilizer teas- they evaluated "teas" (liquid fertilizer made by steeping something) made from lawn clippings, chicken manure, seaweed (for gardeners near the coast), and human urine... I know I know, ew, BUT human urine, diluted 1:20 with water, was the most well balanced fertilizer listed, with a 1:1:1 NPK ratio. Yeah, if we didn't have a house guest this summer, I'd probably have a jug of my own pee in the bathroom.
Since I'm still working on improving the soil quality in my own garden and didn't have enough compost to go around I did purchase some fertilizer this year- I built up some of the beds with composted cow manure I bought at the hardware store, and I used some root vegetable and tomato fertilizers from Gardens Alive (and I do have bigger potatoes, onions, and tomatoes than all my friends... not that I'm competitive or anything...). But those kinds of products ARE pricey. I don't know if I'll buy them again, unless skipping on them next summer makes my crop significantly smaller. But even then, both products only have to be applied twice per growing season- once at planting and once at blossoming. I'll have to work out what the price per season for Miracle Grow applied every two weeks for the same crops would be. I'll bet it's not much cheaper.
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