depends what you mean by modern. 6000 bc first confirmed use of fertilizer. 1000 ad crop rotation replaces fallowing in europe. 1790 gypsum discovered to help plant growth. 1850 fertilizer identified to contain inorganic materials that can be made artificially. 1870 molecules identified as ammonium and phosphorous.

many developments since. but i think 1870 would be a good date for modern. if we split too many hairs. the date would be 2014. we are always discovering New information about agriculture.