What is Modern Agriculture?

For simple terms, agronomy as a science really began in the middle of the 1800, so that we could properly describe modern, science based agriculture as modern agriculture and agriculture that predates the use of science in agriculture as pre-modern. Now not all of agriculture has ever become science based, so this is not based on a date in time when all of agriculture changed.

The worst consequences of pre-modern agriculture would include tillage up and down steep slopes that produced severe erosion, the practice of discarding manure instead of spreading it back on the fields to retain fertility, Repeatedly growing the same crops on any given field until that field would fail to produce that crop, then abandoning that field as fallow for a decade or so, and going right back to growing the same crop. Pre-modern agriculture did not understand why certain crops did very poorly on certain fields, but they did observe it, and they tried something different. Crops that would never succeed on an acidic soil, or an alkaline soil could have been predicted with a high degree of confidence today, but pre-modern agriculture did have the sense to use certain rule of thumb approaches... you can grow certain crops on land that grows an oak tree, other crops on lands that grow pine trees. Knowledge of the needs of given crops for soil drainage was developing pre-science. Farmers learned that grain would die if the ground stayed too wet for too long. This rule of thumb reasoning of course is the starting base of the science of agriculture. We should understand that a lot of observation took place well before serious scientific study of agriculture began.