As somebody else said: it depends. Some fertilizers are shit and contain harmful shit but most are fine.
Here’s some things to know tho:
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If fertilizer gets on the leaves you must wash them before eating (which should be done anyway) as the residue may have toxic heavy metals (which may be there for plant growth).
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Fertilizer is developed for maximum profits, only things that are needed for fast growth are added. this causes the plants to grow fast but with little nutrients needed for healthy long term growth. Its like plant fast food, it’s cheap to make and will make you fat fast but your going to need more than just carbs fat and suger to be healthy.
So the plant will have less nutrients for us, and more importantly less flavour, for herbs like basil and celery the flavour difference is incredible.
for spinach flavour doesn’t matter much. spinach also has basically no nutrients for us in the first place, in fact the oxalic acid in Spinach, when eaten, removes calcium from your body. So fast growing spinach has basically no downsides.
For a more complete diet I would recommend a seeweed based fertilizer (make sure it’s sustainably harvested!).
- dont let any fertilizer get in a river/lake any fertilizer will unbalance its ecosystem.
Also sweet potato (Kumara) leaves are so so good. And easer to grow, and you get sweet potatoes, and they are healthier, and have much less oxalic acid. Only problem with them is they are not very frost resistant and require more light, if you want tubers, then spinach, (in low light it may not produce tubers but will still have leaves).
I would choose a nettle, as Admiral Butterflies do. There flowers aren’t extravagant but the poison tipped spines are very good protection.
Which nettle you may ask (as there are many)
Ideally Urtica sykesii for its beautiful triangular leaves perfectly embellished with triangular edges which itself have beautiful spikes (one of my favorite plants).
For added protection the most dangerous nettle Urtica ferox a nettle so big it has become a woody tree with thick spikes as long as your nose, loaded with enough poison to kill a dog or leave a man limp for weeks (I would make sure no dogs are harmed). The beautiful red admirals favorite choice.