Old school ppl just poured these chemicals on without any concern for over spray and concentration and amount ingredient per area.. as I mentioned above... and still do...
Classic example are the contractors spraying kerb edges on burns...way too much and why too wide.. and bet they just dump rather than measure .....And never read the full agricultural manual l to realise they should only be mixing at 3/4 full dose.
Secondly is most soils will lack in certain trace elements... and yes arsenic, now banned is one, NZ soils tend to lack in many trace elements ..Selenium, iodine, chromium.. think fluorine is another.. the history of central nth island is interesting... couple bags , think was selenium dropped over the whole area and suddenly grass for farming was possible.
Others like Epsom salts, a spoon full in a bucket water every couple yrs around citrus trees sorts a lot issues.
Others like copper, iron etc also get added to soils to create good balance for crops.. crops that produce healthy foods.
Just like animals, humans require a balanced diet , so do plants...
An other classic was lack iodine in NZ soils.. lack that in our diet results in goiter or gout (cant rem which. Upto the 1920s NZ had a pandemic of the deases.. then we added it to salt.. pandemic ended withing a yr. Then around the 1980s, government decided free choice.. and the deasie has re emerged in our hospitals.
So yeah there is a strong case for many of these sprays etc.. on the other hand very often its 'throwing the baby out with the bathwater.' due to miss use , and far too commonly on the extreme side.
And how do I know this crap? (like other crap)
At school in the 50s and 60s we learnt about the history of the central nth island and issues returning solders had on the land granted to them after the WW1.. Add to that 20 odd yrs breeding Austrailan and NZ parrots...where diet etc is critical to good results...And research that stuff one comes across other interesting stuff.. throw in The NZ agricultural chemical manual on my manuals book shelf in my office.