It is plain to see that pesticides kill off a few soil organisms (micro and macro) along with above-ground bugs. After all, they are poisons. No one will dispute that fact. Unfortunately, the deadly effects of synthetic chemical fertilizers are not quite as well known. In practically every instance, these chemicals actually reduce soil fertility instead of improving it. Most of these so-called fertilizers are soluble salts.
Salts are compounds consisting of positive and negative electrically charged ions. Instead of combining to form molecules, these ions arrange themselves boy-girl-boy-girl in a crystalline structure. When soluble salts dissolve, their ions separate individually from the crystal, and are then free to interact independently with whatever.
The formula for the salt fertilizer ammonium nitrate is NH4NO3. Its crystals are made up of equal numbers of ammonium ions (NH4+) and nitrate ions (NO3-). In the presence of water, these two ions disassociate and conduct business independently. Plants use these nitrogen-containing ions in chlorophyll. In nature, without synthetic chemical fertilizers, plants are supplied these ions (mostly nitrate) by the metabolic processes of symbiotic microorganisms etching them out of complex organic compounds. Plants take up each ion as it is made available.
When exposed to ammonium nitrate, most plants will favor the ammonium ion, automatically taking up all they can of what is available to them. Ammonium ions start becoming toxic to plants even when stored at extremely low levels. Ammonium nitrate almost always feeds ammonium to plants at toxic levels. Plants have not yet evolved to refuse excess ammonium because only a hundred years ago, they were almost never exposed to more than they needed.
Just as the human body has defenses against high salt levels, so do plants. Plants which have been fed ammonium will often suddenly start growing rapidly and erratically as a way to quickly use up these excess ions and reduce toxicity. Plants are under extreme stress when experiencing ammonium toxicity, making them extremely susceptible to diseases and insect attacks.
Let me tell a story about aphids. While worm farming in an 8000 sq ft barn in Acme WA, I tore off the rotted roof, built and installed trusses, and covered them with greenhouse-grade plastic film. I rebuilt the loft and used it for testing soils on plants. I started about 30 bell pepper plants from seed in a mixture of peat moss, vermicompost, and perlite. Then I transferred them to 3-gallon pots of the same soil, except I tested two commercially available potting soils in two pots each. I was amazed by the extreme rapid growth of the pepper plants growing in the commercial potting soil that happened to be locally manufactured. The leaves were huge and the plants were twice as tall as the others. Then I started noticing aphids on those two plants. I tried controlling the bugs with some environmentally friendly insecticidal soap. But it did not help. Within a couple days, the bottoms of the leaves were completely coated with a layer of aphids, but the other pepper plants right next to them were never bothered. The infested plants’ soil was a mixture of mink manure, coir, and forest products. While visiting the place where it was made, I learned it had been composted for only a few days, and then dried. It became apparent it was rich in ammonium due to insufficient composting of the mink manure. It might as well have contained ammonium nitrate. I also tested my soil and those same commercial soils on basil in 4-inch pots. I had the same problem of not being able to keep aphids off the basil grown in mink manure. But the other basil plants were not attacked. After finally removing the infested plants, I continued to grow the remaining basil for a couple weeks, and the remaining peppers for a couple months without seeing any more aphids in the barn.
One more quick story about aphids. The yard behind the house I moved into in Gardnerville NV had a lawn in the center surrounded by rocks spread on top of plastic. I removed the rocks and plastic from a 200 sq ft area, made a raised bed, dumped in 6 inches of finished compost, and planted a garden. A few feet from the garden, there were a couple weeds growing through holes in the plastic. Those weeds were stressed from trying to grow in smothered soil. And their plight became even more obvious when I noticed they were infested with aphids. The aphids did not bother the garden though. I pulled the infested weeds and got rid of them. Even though the food the aphids were feeding on was gone, they still did not land on any of the plants in the garden. Unhealthy plants attract plant-eating insects, and healthy plants do not.
The nitrate in ammonium nitrate will be taken up by plants if it is still around after the ammonium has been used up. But by then it has usually been leached into the subsoil, displaced by runoff, and/or converted to nitrogen gas while oxidizing (burning) soil organisms and other organic matter. Both of the ions in ammonium nitrate kill beneficial micro and macro soil organisms.
Urea (Miracle-Gro), decomposes into ammonium ions, force-feeding plants similarly to ammonium nitrate. In the process of forming those ammonium ions, anhydrous ammonia is given off. Anhydrous ammonia is extremely toxic, capable of killing nearly any soil organism.
Now here is where it really get insane: Most people who use ammonium nitrate do not even need nitrogen-containing ions because they are already supplied by organic matter via soil life. Some people think they are accomplishing something when they try these chemicals and see plants growing faster. They do not realize they are looking at sick plants desperately trying to grow fast enough to save their own lives. And they do not realize they have just created a need for toxic rescue insecticides. They get excited about something they misunderstand. They are hooked. And they use the stuff again next year.
A combination of toxicity and the loss of symbiotic organisms causes plants to have difficulty getting enough mineral nutrition. This situation becomes apparent when fruits and veggies start tasting bland. It becomes even more apparent when repeated use of these chemicals stops having an effect. At that point, after wasting money on toxic so-called fertilizer, a large amount of time and money needs to be spent on remediating the soil while missing the benefit of having that soil.