This could be a very long answer. I’ll try to summarize without writing a book, and if you want elaborations on one in particular we can focus on that one and expand as needed on any others.
First, there’s the environmental damage and impact on human health caused by chemical N-P-K fertilizers.
Then, there’s the problems of overuse. Excessive doses of some nutrients is a direct cause of other nutrient deficiencies.
And there’s the build-up of chemical Salts. Because these fertilizers are by definition SALTS. Everyone knows Salt is BAD for agriculture.
On top of all this is the terrible toll that fertilizer manufacturing takes on the environment and the people who live near the factories. They pollute; they’re dangerous. Remember the Bhopal fertilizer plant explosion in India in 1984? The Toulouse fertilizer plant explosion in France in 2001? On our own shores, the worst accident involving fertilizer took place in 1947 in Texas, when 600 people were killed and 3,500 people were injured; it was part of the testimony presented in July 2005 before the Senate Homeland Security Committee, which was studying national security risks:And that’s just the N.
Unlike Nitrogen, Phosphorus and Potassium are immobile in Soil.
Feldspars and Micas contain most of the Potassium in our Soil. K in fertilizers is almost always applied as Potassium Chloride from mines in Canada.
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K dissolves in Water. P does not.
Instead, it binds to Soil particles and stays put until some nice microbe comes along and un-locks it. Clay Soils tend to keep the strongest grip on it; Sandy Soils are looser and let it drizzle through.
Phosphate and Potash fertilizers don’t just raise the chemical P and K levels; they also add damaging Soil Salts. And although most Soils in the U.S. have perfectly adequate levels of Phosphorus and Potassium, and even though they don’t need any more from your fertilizer, people use them anyway. This is a problem because too much Phosphorus locks other nutrients OUT of plants.
Iron and Zinc deficiencies are common in Soil over-loaded with non-dissolving P. A fatal disease in livestock called ‘Grass Tetany’ — a complex condition linked to Mg deficiency in cows and other ruminants — is examined by French author André Voisin: ‘Excessive and repeated dressings of Potassium fertilizers cause Magnesium deficiencies in plants, particularly Grasses…’ It’s even bad for the animals that depend on them; they too develop Mg deficiency: